Optical and Microphysical Properties of the Aerosol Field over Sofia, Bulgaria, Based on AERONET Sun-Photometer Measurements
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atmosphere
Article
Optical and Microphysical Properties of the Aerosol Field over Sofia,
Bulgaria, Based on AERONET Sun-Photometer Measurements
Tsvetina Evgenieva *, Ljuan Gurdev, Eleonora Toncheva and Tanja Dreischuh *
Institute of Electronics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Blvd, 1784 Sofia, Bulgaria;
lugurdev@ie.bas.bg (L.G.); etoncheva@ie.bas.bg (E.T.)
* Correspondence: tsevgenieva@ie.bas.bg (Ts.E.); tanjad@ie.bas.bg (T.D.)
Abstract: An analysis of the optical and microphysical characteristics of aerosol passages over Sofia
City, Bulgaria, was performed on the basis of data provided by the AErosol RObotic NETwork
(AERONET). The data considered are the result of two nearly complete annual cycles of passive
optical remote sensing of the atmosphere above the Sofia Site using a Cimel CE318-TS9 sun/sky/lunar
photometer functioning since 5 May 2020. The values of the Aerosol Optical Depth (AOD) and the
Ångström Exponent (AE) measured during each annual cycle and the overall two-year cycle exhibited
similar statistics. The two-year mean AODs were 0.20 (±0.11) and 0.17 (±0.10) at the wavelengths
of 440 nm (AOD440 ) and 500 nm, respectively. The two-year mean AEs at the wavelength pairs
440/870 nm (AE440/870 ) and 380/500 nm were 1.45 (±0.35) and 1.32 (±0.29). The AOD values
obtained reach maxima in winter-to-spring and summer and were about two times smaller than
those obtained 15 years ago using a hand-held Microtops II sun photometer. The AOD440 and
AE440/870 frequency distributions outline two AOD and three AE modes, i.e., 3 × 2 groups of aerosol
events identifiable using AOD–AE-based aerosol classifications, additional aerosol characteristics,
and aerosol migration models. The aerosol load over the city was estimated to consist most frequently
of urban (63.4%) aerosols. The relative occurrences of desert dust, biomass-burning aerosols, and
Citation: Evgenieva, Ts.; Gurdev, L.; mixed aerosols were, respectively, 8.0%, 9.1% and 19.5%.
Toncheva, E.; Dreischuh, T. Optical
and Microphysical Properties of the Keywords: atmospheric aerosols; sun photometer; aerosol optical depth; Ångström exponent; aerosol
Aerosol Field over Sofia, Bulgaria, characterization; AERONET; Bulgaria
Based on AERONET Sun-Photometer
Measurements. Atmosphere 2022, 13,
884. https://doi.org/10.3390/
atmos13060884
1. Introduction
Academic Editor: Begoña Artíñano The Earth’s atmosphere contains an immense quantity of aerosol particles known also
Received: 20 April 2022
as atmospheric particulate matter. These particles, with their abilities to scatter and/or
Accepted: 26 May 2022
absorb the incoming solar radiation and to act as cloud condensation nuclei, are of primary
Published: 29 May 2022
importance for the Earth’s radiative budget, thus influencing significantly the climate and
living conditions on Earth [1,2]. Because of their numerous impacts combined with high
Publisher’s Note: MDPI stays neutral
spatial and temporal variability due to their short lifetime, the aerosols are the object of
with regard to jurisdictional claims in
comprehensive global and local monitoring and studies of their sources, type, transport,
published maps and institutional affil-
microphysical, and optical properties using a variety of experimental active (lidars) and
iations.
passive (photometers, radiometers) ground-based, ship-borne, air-borne, and space-borne
remote sensing facilities and networks [3–11]; weather information (e.g., [12]); and powerful
data processing and interpretation program products and forecasting models [13–21]. The
Copyright: © 2022 by the authors.
ultra-fine (submicron) fraction of the near-surface atmospheric aerosols, the so-called
Licensee MDPI, Basel, Switzerland. fine dust particles, may dangerously affect the ecosystems and human health, causing
This article is an open access article harmful mechanical, chemical, radiological, and microbiological impacts, e.g., on the
distributed under the terms and human respiratory and cardiovascular systems [22,23]. Therefore, the remote active and
conditions of the Creative Commons passive and the in situ monitoring of the atmospheric aerosol load is of considerable not
Attribution (CC BY) license (https:// only scientific but also societal importance, allowing policymakers and local authorities to
creativecommons.org/licenses/by/ be timely informed to undertake measures [24,25] to reduce the negative effects of the air
4.0/). pollutions on the climate and human health and improve air quality.
Atmosphere 2022, 13, 884. https://doi.org/10.3390/atmos13060884 https://www.mdpi.com/journal/atmosphereAtmosphere 2022, 13, 884 2 of 29
The AErosol RObotic NETwork (AERONET) [6] is a worldwide network comprising
numerous ground-based sun-photometer sites [26–35]. The photometric data obtained au-
tomatically following predefined scenarios of measuring the sun and moon irradiance and
sky radiance in several spectral bands allow one to retrieve, through automatic AERONET
data processing, a series of important columnar aerosol characteristics [6,36–39]. The
values of the retrieved characteristics may correspond to three quality levels of data
processing—without cloud screening (level 1.0), with cloud screening (level 1.5) [40], and
with cloud screening and quality assurance (level 2.0) [41]. The knowledge of the aerosol
characteristics would allow one to estimate the Earth’s radiative budget [25,36,42–44] and
the air quality [45,46] and to validate the results about the aerosol optical depth (AOD)
measured by satellite-borne apparatuses [47–50].
Regular lidar monitoring of the aerosol stratification over Sofia City, Bulgaria, is per-
formed by the Sofia Station at the Institute of Electronics, Bulgarian Academy of Sciences
(IE-BAS), which is involved in the activities of the European Aerosol Research LIdar NET-
work (EARLINET) [5] and the Aerosol, Clouds and the Trace gasses Research Infrastructure
(ACTRIS) [51]. In May 2020, the station’s equipment was completed with a Cimel CE318-
TS9 sun/sky/lunar photometer [52,53] involved in AERONET that provides almost the
whole set of column-integrated or averaged characteristics of the aerosol field over Sofia
at different optical wavelengths [54]. As almost two annual cycles have already been
completed since the beginning of the photometer operation, we deemed it appropriate to
perform an initial analysis of the accumulated data.
Different aerosol classification methods have been developed to categorize the aerosol
types when using AERONET observations. AOD and the Ångström exponent (AE) are
typically used for aerosol type classification [27,29,31–33,55,56]. There are also methods that
use other combinations of aerosol characteristics, such as AE and single scattering albedo
(SSA) [50]; SSA and fine-mode fraction (FMF) [57]; or AE, SSA, and FMF [58,59]. Using
also additional information for possible source regions or transport trajectories [17,18,21], a
conclusion can be drawn about the predominant aerosol type.
The present work is aimed at: tracking the climatology of the aerosol optical depth [60,61]
and Ångström exponent [62,63] over Sofia at some radiation wavelengths and comparing
the results obtained with similar ones obtained at other AERONET sites; comparing the
values of AOD being obtained now with those obtained 15 years ago and estimating
the efficiency of the measures undertaken to improve the air quality; and estimating
the typology of the aerosol events over Sofia on the basis of AOD, AE, other aerosol
characteristics and additional sources of information, and their relative weight in the
overall climatology picture.
The paper is organized as follows: in the next section, Section 2, we briefly describe
the location of the Sofia Site (IE-BAS) and some peculiarities of its environment; the
performance and capabilities of the Cimel CE318-TS9 sun/sky/lunar photometer; and the
research approach, procedures and forecasting models. The results of the work concerning
the climatology peculiarities and the typical aerosol events observed at the Sofia Site are
described and discussed in Section 3. The main conclusions drawn from the results obtained
are summarized in Section 4.
2. Site, Instrumentation and Research Approach
2.1. Sofia Site
The Cimel CE318-TS9 sun/sky/lunar photometer is installed on the roof of the IE-BAS
building, 42.65 N, 23.38 E, 620 m above sea level (ASL), in the southeast part of Sofia
City, which is situated in a heavily urbanized mountain valley. Sofia Valley (average
550 m ASL) is surrounded by mountains: Vitosha and Plana in the south, Lyulin in the
west, the Balkan Mountains in the north, and Lozen in the east (Figure 1). The complex
orography, the intricate air-flow pattern caused by the diurnal mountain winds, and
high urbanization [64,65], as well as the temperature inversions, complicate the natural
ventilation and trap the air pollutants [66,67]. Since the largest Bulgarian metalworkingage 550 m ASL) is surrounded by mountains: Vitosha and Plana in the south, Lyulin in
the west, the Balkan Mountains in the north, and Lozen in the east (Figure 1). The com-
Atmosphere 2022, 13, 884 plex orography, the intricate air-flow pattern caused by the diurnal mountain winds, and 3 of 29
high urbanization [64,65], as well as the temperature inversions, complicate the natural
ventilation and trap the air pollutants [66,67]. Since the largest Bulgarian metalworking
plantlocated
plant locatedatat a distance
a distance of of about
about 20 20
kmkm northeast
northeast fromfrom
the the
citycity center
center stopped
stopped func-
functioning
tioning in 2009, the prevalent air pollutant sources have been mainly traffic,
in 2009, the prevalent air pollutant sources have been mainly traffic, domestic heating, domestic
heating, industries,
industries, thermalstations,
thermal power power stations, dusty biomass
dusty roads, roads, biomass
burning,burning, etc. [68,69].
etc. [68,69]. Model
Model analyses indicate the predominant contribution being of the local sources
analyses indicate the predominant contribution being of the local sources of pollutants of pol-
lutants
except inexcept in the
the cases cases of long-range-transported
of long-range-transported secondarysecondary
aerosols aerosols
and desertanddust
desert dust
[69]. Sofia
[69]. Sofia has a continental climate with a mean annual temperature◦ of 10 °C and a mean
has a continental climate with a mean annual temperature of 10 C and a mean annual
annual precipitation of 576 mm [70]. The air temperature normally reaches a minimum in
precipitation of 576 mm [70]. The air temperature normally reaches a minimum in January
January and a maximum in July. The wind rose in Sofia shows predominant western and
and a maximum in July. The wind rose in Sofia shows predominant western and eastern
eastern winds with a mean annual wind speed of 2.4 m/s [70,71].
winds with a mean annual wind speed of 2.4 m/s [70,71].
Figure1.1.Map
Figure MapofofSofia
SofiaValley
Valley(Google
(GoogleMaps
Mapsimage)
image)with
withIE-BAS
IE-BASlocation
locationmarked
markedbybyred
redasterisk
asterisk(a)(a)and
Com
and photo of the sun photometer at Sofia Station
photo of the sun photometer at Sofia Station (b). (b).
2.2.Cimel
2.2. CimelCE318-TS9
CE318-TS9 Operation
Operation and AERONET
AERONETCapabilities
Capabilities
TheCE318-TS9
The CE318-TS9 sun/sky/lunar
sun/sky/lunar photometer
photometercomprises
comprises optical
opticalchannels
channels at at
thethe
wave-
wave-
lengths λ = 340, 380,
lengths λ = 340, 380, 440, 440, 500, 675, 870, 937, 1020, and 1640 nm with a field of view of 1.3◦
675, 870, 937, 1020, and 1640 nm with a field of view of 1.3°
and performs automatically daytime
and performs automatically daytime measurements measurementsofofsun sunirradiance
irradianceand and skysky radiance
radiance and
and night-time measurements of moon irradiance according to
night-time measurements of moon irradiance according to predefined scenarios [52,53]. predefined scenarios
[52,53].
Direct sunDirect sun measurements
measurements are carried areout
carried
every out
3 orevery
5 min3atorall5 the
minavailable
at all thewavelengths,
available
wavelengths,
while whilein
sky radiances sky radiances principal
almucantar, in almucantar,
plane,principal
and hybridplane, and hybrid
scenarios scenarios in
are measured,
are measured,
general, in general,
every hour every
at several hour at several
wavelengths (380,wavelengths
440, 500, 675,(380,
870,440, 500,
1020, and 675, 870,
1640 1020,
nm). The
and 1640
overall scannm). The overall and
configuration scanschedule
configuration and schedule
are chosen in such are chosen
a way as toinprovide
such a way as to
a sufficient
provideofa accurate
amount sufficientraw
amount
dataof accuratethe
ensuring rawunambiguous
data ensuringand the accurate
unambiguous andof
retrieval accurate
the char-
retrieval of the characteristics of interest [6,28,36]. The corresponding
acteristics of interest [6,28,36]. The corresponding automatic AERONET data processing automatic AERO-
NET one
allows datato processing allows
retrieve a large set of one to retrieve
atmospheric a large set of atmospheric
aerosol-characteristic parameters, aero- such as
sol-characteristic parameters, such as AOD, AE, AOD of
AOD, AE, AOD of the fine (AODf500 ), and coarse (AODc500 ) aerosol fractions the fine (AOD f500), and coarse
at λ = 500 nm,
(AODc500
volume ) aerosol
size fractions
distribution at λ complex
(VSD), = 500 nm,refractive
volume sizeindexdistribution
(CRI, nr +in(VSD), complex re-
im ), SSA, scattering
fractive index (CRI, n r+inim), SSA, scattering phase function, precipitable water content,
phase function, precipitable water content, etc. [6,28,36,38,39,41,72]. AODf500 and AODc500
etc. [6,28,36,38,39,41,72].
indicate the contribution AOD andand
of thef500fine AOD c500 indicate the contribution of the fine and the
the coarse aerosol fractions to the aerosol optical
coarse aerosol fractions to the aerosol optical characteristics. To provide an idea of the
characteristics. To provide an idea of the order of magnitude of the errors in measuring or
order of magnitude of the errors in measuring or retrieving some variables or aerosol
retrieving some variables or aerosol characteristics, it is estimated, for instance [6,40], that
characteristics, it is estimated, for instance [6,40], that the absolute error in determining
the absolute error in determining the AOD is 440 nm andAtmosphere 2022, 13, 884 4 of 29
2.3. Research Approach and Procedures
The research approach in the work consists first in studying the AOD and AE climatol-
ogy by tracking their variations during the days, months, seasons, and years and revealing
the peculiarities of their evolution and statistics [29]. The frequency distributions were
analyzed as well of AOD and AE for each annual cycle and the overall two-year cycle,
along with important statistical characteristics of the time series, such as mean values, stan-
dard deviations characterizing the range of fluctuations (the variability) of the quantities,
least and largest values, medians, and distribution skewness. The frequency distributions
turned out to have a more complex multimode structure with modes indicating a possible
grouping of the aerosol events (daily aerosol situations) according to the specific daily mean
values of their AOD and AE. After distinguishing the different groups (zones) and outlining
their boundaries on AOD–AE scatter plots, recognition is performed of the possibly specific
aerosol events in each zone on the basis of appropriate sets of AERONET-provided aerosol
characteristics that are usually known for key aerosol types. The characteristics we have
chosen for this work are the particle VSD, CRI, and SSA as well as the particle sphericity fac-
tor (SF) and linear depolarization ratio (DR). After the aerosol types are identified and their
AOD–AE boundaries outlined, the latter are compared with such boundaries (AOD–AE
classification thresholds) obtained by other authors (e.g., [27–29,31,33,34,56,73–77]). The
comparison shows near threshold positions.
The AERONET data employed in the paper are of Version 3.0 algorithm products and
quality level 1.5 [41]. Note that sometimes the raw data from the photometer measurements
are not regularly provided because of unfavorable (cloudy) weather, technical problems,
and a time gap for calibrating the instrument.
The information about the backward trajectories was obtained through the National
Oceanic and Atmospheric Administration (NOAA) Hybrid Single-Particle Lagrangian
Integrated Trajectory model (HYSPLIT) [17,18] and that about the Saharan dust intrusions
through the Barcelona Supercomputing Center Dust Regional Atmospheric Model (BSC-
DREAM8b v2.0) [19,20]. The data about the fires in Bulgaria and adjacent regions used
in the paper were provided by the NASA’s Fire Information for Resource Management
System (FIRMS), part of NASA’s Earth Observing System Data and Information System
(EOSDIS) [78] on the basis of the satellite observation from the Moderate Resolution
Imaging Spectroradiometer (MODIS) aboard the Aqua and Terra satellites, and the Visible
Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi NPP and NOAA 20 satellites.
Information on the weather conditions [79] was also used as provided by the Bulgarian
National Institute of Meteorology and Hydrology.
3. Results and Discussion
The total number of individual measurements performed by the Cimel CE318-TS9
sun/sky/lunar photometer at the Sofia (IE-BAS) Site from 5 May 2020 to 28 February
2022 and selected as representative is 25,609, of which 8196 were during 2020, 15,212 were
during 2021, and 2201 were during 2022. The corresponding total number of active days of
measurements is 374, including 144 days in 2020, 191 days in 2021, and 39 days in 2022. A
time gap exists from 12 May 2021 to 15 July 2021, when the instrument had to be calibrated.
Two more gaps, from 7 July to 17 August 2020 and from 11 November to 30 November
2021, are due to technical issues. In fact, the period of measurements under consideration
here consists of two nearly complete annual cycles including partly or entirely the four
seasons—spring (March, April, and May, MAM), summer (June, July, and August, JJA),
autumn (September, October, and November, SON), and winter (December, January, and
February, DJF). The first cycle lasts from 5 May 2020 to 28 February 2021 and the second
one from 1 March 2021 to 28 February 2022. Then, the seasonal distribution of the number
of measurements and active days is as follows: for the first cycle—650 measurements
within 17 days in the spring (MAM), 2955 measurements within 44 days in the summer
(JJA), 4285 measurements within 71 days in the autumn (SON), and 1788 measurements
within 49 days in the winter (DJF); for the second cycle—2543 measurements withingust, JJA), autumn (September, October, and November, SON), and winter (December,
January, and February, DJF). The first cycle lasts from 5 May 2020 to 28 February 2021
and the second one from 1 March 2021 to 28 February 2022. Then, the seasonal distribu-
tion of the number of measurements and active days is as follows: for the first cycle—650
Atmosphere 2022, 13, 884 measurements within 17 days in the spring (MAM), 2955 measurements within 44 days 5 of 29
in the summer (JJA), 4285 measurements within 71 days in the autumn (SON), and 1788
measurements within 49 days in the winter (DJF); for the second cycle—2543 measure-
ments within 43 days in the spring (MAM), 6737 measurements within 47 days in the
43summer
days in(JJA),
the spring (MAM), 6737 within
4093 measurements measurements
50 days inwithin 47 days
the autumn in theand
(SON), summer (JJA),
2558 meas-
4093 measurements within 50 days in the autumn
urements within 53 days in the winter (DJF). (SON), and 2558 measurements within
53 days in the winter (DJF).
3.1. Climatology Features of AOD and AE
3.1. Climatology Features of AOD and AE
3.1.1.Temporal
3.1.1. TemporalBehavior
Behavior
Figure2 2presents
Figure presents sequences
sequences of of
AODAOD values
values forfor years
years 2020,
2020, 2021,
2021, andand
20222022 (in dif-
(in different
ferent colors) at the wavelengths λ = 440 nm (AOD 440) and λ = 500 nm (AOD500). Consid-
colors) at the wavelengths λ = 440 nm (AOD440 ) and λ = 500 nm (AOD500 ). Considering the
ering the three graphs there, one can notice that they have similar behavior and mutually
three graphs there, one can notice that they have similar behavior and mutually complete
complete the lacking sections. Additionally, it is seen in Figure 2 that the AOD is a
the lacking sections. Additionally, it is seen in Figure 2 that the AOD is a strongly variable,
strongly variable, non-stationary random function of the time, reaching maxima on av-
non-stationary random function of the time, reaching maxima on average within the periods
erage within the periods around March (February, March, and April) and August (June,
around March (February, March, and April) and August (June, July, August, and Septem-
July, August, and September). Similar behavior has also been observed at other Balkan
ber). Similar behavior has also been observed at other Balkan and European AERONET
and European AERONET sites and has been explained as due to intense Saharan dust
sites and has been explained as due to intense Saharan dust (SD) intrusions [29,33]. Cer-
(SD) intrusions [29,33]. Certainly, smoke from fires and dust from road pavement and the
tainly, smoke from fires and dust from road pavement and the Vitosha Mountain (Figure 1)
Vitosha Mountain (Figure 1) should also have contributed to the increase of the atmos-
should also have contributed to the increase of the atmospheric turbidity in the summer. In
pheric turbidity in the summer. In late autumn and winter, the atmosphere should be
late autumn and winter, the atmosphere should be clearer because of the frequent snow
clearer because of the frequent snow and rain precipitations that clean the air and mois-
and rain precipitations that clean the air and moisten the ground. A seasonal pollutant in
ten the ground. A seasonal pollutant in this case is the smoke from domestic heating [68].
this case is the smoke from domestic heating [68].
Figure2.2.Variations
Figure VariationsofofAOD
AOD (a) and
440 (a)
440 and AOD500
500 (b)
(b)obtained
obtainedinin2020,
2020,2021,
2021,and
and2022.
2022.
Sequences
Sequencesof of AE
AE values for years
values for years2020,
2020,2021,
2021,andand2022
2022(in(in different
different colors
colors again)
again) at
atthe
thewavelengths
wavelengthspairspairsλ1λ
/λ12/λ of 440/870
of2 440/870 nm 440/870
nm (AE (AE)440/870 ) and 380/500
and 380/500 nm (AEnm (AE
380/500) are pre- )
380/500
are presented
sented graphically
graphically in 3.
in Figure Figure 3. Itthat
It is seen is seen that the
the three three
graphs of graphs of AEseparately
AE vs. time, vs. time,
separately and together,
and together, represent represent almost stationary,
almost stationary, stronglyrandom
strongly variable variablefunctions
random functions
with ap-
with approximately
proximately the same theconstant
same constant
mean valuemean andvalue
rangeand range of fluctuations,
of fluctuations, the only
the only exception
exception being a hard-to-notice
being a hard-to-notice increase inincrease
AE frominJune
AE from June to
to October October
that that is
is probably probably
due to the
due to the increased fine particles fraction arising from biomass burning. Let us also
note the periodical decreases of AE440/870 and AE380/500 to levels of the order of 0.3–0.4.
Such decreases should be mainly due to Saharan dust intrusions during the different
seasons [32–34,77]. Low values of AE440/870 , well below unity, are known to be intrinsic to
aerosol events with prevailing coarse fraction, such as SD intrusions and marine aerosols
passages. This is seen as well from the AERONET data. The desert dust may also increase
the AOD. The carbonaceous aerosols resulting from biomass burning or domestic heating
are usually accepted to have a prevailing fine-particle fraction with an AE440/870 well above
unity (e.g., AE440/870 > 1.4). In the process of ageing, however, they undergo various
chemical, morphological, and microphysical changes including increasing the particle
size and decreasing AE440/870 down to unity and below unity [80,81]. According to someLow values of AE440/870, well below unity, are known to be intrinsic to aerosol events with
prevailing coarse fraction, such as SD intrusions and marine aerosols passages. This is
seen as well from the AERONET data. The desert dust may also increase the AOD. The
carbonaceous aerosols resulting from biomass burning or domestic heating are usually
accepted to have a prevailing fine-particle fraction with an AE440/870 well above unity (e.g.,
Atmosphere 2022, 13, 884 AE440/870 > 1.4). In the process of ageing, however, they undergo various chemical, mor- 6 of 29
phological, and microphysical changes including increasing the particle size and de-
creasing AE440/870 down to unity and below unity [80,81]. According to some experimental
observations [81] and simulations
experimental observations [82], a probability
[81] and simulations exists that exists
[82], a probability the AE 440/870 of aerosol
that the AE440/870
ensembles containing carbonaceous particles could be even as low as 0.2–0.3.
of aerosol ensembles containing carbonaceous particles could be even as low as 0.2–0.3.
Figure 3. Variation of AE440/870 (a) and AE380/500 (b) obtained in 2020, 2021, and 2022.
Figure 3. Variation of AE440/870 (a) and AE380/500 (b) obtained in 2020, 2021, and 2022.
Themonthly
The monthlymean meanvalues
values ofof AOD
AOD440, ,AOD 500, AE440/870, and AE380/500 during the years
440 AOD500 , AE440/870 , and AE380/500 during the
2020, 2021, and 2022 are plotted in Figure
years 2020, 2021, and 2022 are plotted in Figure 4. 4. The diagrams of AODofatAOD
The diagrams λ = 440 at nm
λ = (Figure
440 nm
(Figure 4a) and λ = 500 nm (Figure 4b) are similar in shape and confirm, in general,the
4a) and λ = 500 nm (Figure 4b) are similar in shape and confirm, in general, the
above-mentionedconclusions
above-mentioned conclusionsbasedbased onon Figure
Figure 2; 2; namely,
namely, thethe values
values of AOD
of AOD are are some-
somewhat
what during
higher higher during
February, February,
March, andMarch,
Apriland April
and have and have a maximum
a maximum around Julyaround July
(June, (June,
July, and
July, and August). The peak seen in November 2021 should be due
August). The peak seen in November 2021 should be due to SD intrusions (see Section 3.2.2, to SD intrusions (see
SectionDust
Saharan 3.2.2,Intrusions)
Saharan Dust andIntrusions)
warm weather and warm
[79] inweather
the first [79]
decadein the firstmonth.
of the decade of the
month.
The passive remote sensing measurements of the aerosol optical and microphysical
The passive
characteristics overremote sensing
Sofia began measurements
in IE-BAS more than of 15
theyears
aerosol
agooptical
using aand microphysical
spectroradiometer
characteristics over Sofia began in IE-BAS more than 15 years
and a Microtops II hand-held sun photometer [83,84]. The Microtops II sun photometer ago using a spectroradi- has
five optical channels at λ = 380, 500, 675, 936, and 1020 nm and a field of viewIIofsun
ometer and a Microtops II hand-held sun photometer [83,84]. The Microtops 2.5◦pho-
[85].
Wetometer
foundhas five opticalto
it interesting channels
compare at the
λ = 380, 500,for
results 675,
the936,
AOD andobtained
1020 nm at andλ a= field
500 nmof view
with
of 2.5° [85]. We found it interesting to compare the results
the Cimel CE318-TS9 sun/sky/lunar photometer with those obtained in Sofia in 2006 for the AOD obtained at λ=
500 nm with the Cimel CE318-TS9 sun/sky/lunar photometer with
and 2007 [86,87] with the Microtops II sun photometer. Such a comparison is considered those obtained in Sofia
in 2006 and
reasonable 2007 [86,87]
because of the with the Microtops
fact that the relativeIIdifference
sun photometer.
between Such
the aresults
comparison
obtained is
considered reasonable because of the fact that the relative difference
simultaneously by the sun photometers Microtops II and Cimel CE318 (calibrated according between the results
toobtained
the AERONETsimultaneously
protocols) byduring
the sun photometers
a joint Microtops
experimental II andcarried
campaign Cimelout CE318 (cali-at
in 2007
brated according to the AERONET protocols) during a joint experimental
the Central Geophysical Observatory of the Polish Academy of Sciences in Belsk, Poland, campaign car-
ried out in 2007
was about 6% [88]. at the Central Geophysical Observatory of the Polish Academy of Sci-
ences in Belsk, Poland, was about 6% [88].
The comparison showed that the values measured 15 years ago are about twice as high
The comparison showed that the values measured 15 years ago are about twice as
(see Figure 4b) as the results obtained in years 2020, 2021, and 2022. This could be taken as
high (see Figure 4b) as the results obtained in years 2020, 2021, and 2022. This could be
evidence for the improvement of the city air quality due to the measures undertaken by the
Bulgarian government [89] and Sofia Municipality [90–92] to implement the European air
quality policy [93] and to the largest metalworking company in Bulgaria located near Sofia
ceasing its functioning. The Moderate Resolution Imaging Spectroradiometer (MODIS)
data also confirm the tendency of a decreasing AOD over Sofia from 2000 to 2019 [94,95].
The diagrams in Figure 4c,d of the monthly means of AE440/870 and AE380/500 show
that they vary irregularly, in general. In 2021, they seem to oscillate around some aver-
age value, which corresponds to the behavior seen in Figure 3. The data series of 2020
and 2022, however, are too short to allow any concrete conclusions. Nevertheless, one
may notice increased AE means from June to August 2020 and from August to Octo-
ber 2021, which also corresponds to the slight upward shift (Figure 3) of the AE in the
summer-to-autumn season.Atmosphere 2022, 13, 884 7 of 29
Atmosphere 2022, 13, x FOR PEER REVIEW 8 of 34
Figure 4. Monthly mean values of AOD440 (a), AOD500 (b), AE440/870 (c), and AE380/500 (d) for
different years. The values for 2006 and 2007 were obtained by the Microtops II sun photometer.
Seasonal mean values of AOD440 (e), AOD500 (f), AE440/870 (g), and AE380/500 (h) for the periods
2020–2021, 2021–2022, and 2020–2022.
The seasonal means of AOD440 , AOD500 , AE440/870 , and AE380/500 concerning the above-
described two annual cycles and the overall two-year cycle are illustrated in Figure 4e–h,
where it is seen that the dependencies AOD vs. season and AE vs. season have similar
shapes at both wavelengths and wavelength pairs, respectively. Expectedly, for the three
cycles, the seasonal means of AOD have maxima in the summer (JJA). The comparison
performed with results of similar earlier investigations at different wavelengths [33,77]Atmosphere 2022, 13, 884 8 of 29
based on one to two decades of measurements across Europe shows that the behavior of
AOD vs. the season obtained here is similar to those obtained for southeast Europe at
AERONET sites in Bucharest, Thessaloniki, and Athens. Additionally, the AOD results
in [33,77] exceed, to a certain extent, the ones obtained here.
The seasonal means of AE440/870 and AE380/500 during the period 5 May 2020–28
February 2022 (2020–2022) are practically constant (Figure 4g,h), which supports the sup-
position about AE as a stationary random function of time; also, they are close to those
obtained in [33,77].
3.1.2. Statistics of AOD and AE
Figures 5 and 6 present the frequency distributions of individual-measurement results
(realizations)
Atmosphere 2022, 13, x FOR of AOD and AE obtained during the periods (measurement cycles)
PEER REVIEW 10 of 34 2020–2021,
2021–2022, and 2020–2022, respectively, at the wavelengths λ = 440 nm and 500 nm.
Atmosphere 2022, 13, x FOR PEER REVIEW
Figure 5. Frequency distributions of AOD440 and AOD500 for the periods 5 May 2020 to 28 11 of 34
February
Figure 5. Frequency distributions of AOD440 and AOD500 for the periods 5 May 2020 to 28 February
2021 (a,d), 1 March 2021 to 28 February 2022 (b,e), and 5 May 2020 to 28 February 2022 (c,f).
2021 (a,d), 1 March 2021 to 28 February 2022 (b,e), and 5 May 2020 to 28 February 2022 (c,f).
The histograms in Figure 6 exhibit asymmetric AE distributions having negative
skewness. They seem to be bimodal at the wavelength pair of 380/500 nm, and
three-modal at the wavelength pair of 440/870 nm. The peak positions of the major and
minor modes for the pair 440/870 nm for the periods 2020–2021, 2021–2022, and 2020–
2022 were, respectively, 1.58, 1.28, and 0.93; 1.68, 1,25, and 0.63; and 1.68, 0.95, and 0.65.
The corresponding peak positions for the pair 380/500 nm were 1.48 and 0.73; 1.48 and
0.75; and 1.48 and 0.75. The multimode frequency distributions of AOD and AE suggest a
possible aerosol event grouping in the AOD–AE space, each group corresponding pos-
sibly to some aerosol type having specific optical and microphysical properties. One may
expect to distinguish six such groups on the AOD440–AE440/870 scatter plots and four such
groups, on the AOD500–AE380/500 scatter plots. After the different groups are revealed and
outlined, the corresponding aerosol events can be characterized and specified on the ba-
sis of some suitable set of their optical and microphysical characteristics provided by
AERONET sun-photometer measurements. Such a procedure for the characterization
and classifications of the aerosol events, or more precisely, daily aerosol situations, is
further described in Section 3.2.
6. Frequency
Figure Figure distributions
6. Frequency of AE
distributions of AE andAE
440/870 and
440/870 AE
380/500 forperiods
for the
380/500 the periods 5 May
5 May 2020 2020
to 28 to 28 February 2021
February
2021 (a,d), 1 March 2021 to 28 February 2022 (b,e), and 5 May 2020 to 28 February 2022 (c,f).
(a,d), 1 March 2021 to 28 February 2022 (b,e), and 5 May 2020 to 28 February 2022 (c,f).
Other important statistical characteristics of the distributions and populations of
AOD and AE are given in Table 1, which summarizes the total number of measurements;
the minimum, maximum, and mean values and the standard deviations of AOD and AE;
and the skewness and the median of the frequency distributions. The percentages of re-
alizations with AOD < 0.05, 0.1, 0.2, and 0.3 and AE < 0.8 and 1.4 are shown in Table 2.
The mean values of AOD440 and AOD500 for the period 2020–2021 were 0.18 ± 0.09Atmosphere 2022, 13, 884 9 of 29
The histograms in Figure 5 outline asymmetric AOD distributions with positive
skewness. The distribution of data from the first annual cycle (2020–2021) seems unimodal
as well, with no pronounced embryos of second minor modes. However, in the distributions
of data from the second annual cycle (2021–2022) and from the two-year cycle (2020–2022),
second weaker minor modes were already present and well distinguishable, especially
at λ = 440 nm. The positions of their peaks were at AOD440 = 0.37 and AOD500 = 0.31.
The major peaks for the periods (2020–2021), (2021–2022), and (2020–2022) were located,
respectively, at AOD440 = 0.15, 0.09, and 0.12 and at AOD500 = 0.13, 0.07, and 0.10.
The histograms in Figure 6 exhibit asymmetric AE distributions having negative
skewness. They seem to be bimodal at the wavelength pair of 380/500 nm, and three-
modal at the wavelength pair of 440/870 nm. The peak positions of the major and minor
modes for the pair 440/870 nm for the periods 2020–2021, 2021–2022, and 2020–2022
were, respectively, 1.58, 1.28, and 0.93; 1.68, 1,25, and 0.63; and 1.68, 0.95, and 0.65. The
corresponding peak positions for the pair 380/500 nm were 1.48 and 0.73; 1.48 and 0.75;
and 1.48 and 0.75. The multimode frequency distributions of AOD and AE suggest a
possible aerosol event grouping in the AOD–AE space, each group corresponding possibly
to some aerosol type having specific optical and microphysical properties. One may expect
to distinguish six such groups on the AOD440 –AE440/870 scatter plots and four such groups,
on the AOD500 –AE380/500 scatter plots. After the different groups are revealed and outlined,
the corresponding aerosol events can be characterized and specified on the basis of some
suitable set of their optical and microphysical characteristics provided by AERONET sun-
photometer measurements. Such a procedure for the characterization and classifications
of the aerosol events, or more precisely, daily aerosol situations, is further described in
Section 3.2.
Other important statistical characteristics of the distributions and populations of AOD
and AE are given in Table 1, which summarizes the total number of measurements; the
minimum, maximum, and mean values and the standard deviations of AOD and AE; and
the skewness and the median of the frequency distributions. The percentages of realizations
with AOD < 0.05, 0.1, 0.2, and 0.3 and AE < 0.8 and 1.4 are shown in Table 2.
Table 1. Statistical characteristics of the aerosol optical depths AOD440 and AOD500 and the Ångström
exponents AE440/870 and AE380/500 .
Period Parameter Total Number Mean Standard Deviation Min Median Max Skewness
AOD440 9678 0.18 0.09 0.03 0.16 0.66 1.20
5 May 2020–28 AE440/870 9678 1.41 0.32 0.29 1.49 2.04 −1.01
February 2021 AOD500 9678 0.15 0.08 0.02 0.13 0.61 1.27
AE380/500 9678 1.29 0.31 0.19 1.34 2.10 −0.95
AOD440 15931 0.22 0.12 0.04 0.18 0.77 0.87
1 March 2021–28 AE440/870 15931 1.48 0.37 0.25 1.61 2.08 −1.24
February 2022 AOD500 15931 0.18 0.11 0.03 0.15 0.64 0.88
AE380/500 15931 1.34 0.27 0.36 1.40 1.95 −1.09
AOD440 25609 0.20 0.11 0.03 0.17 0.77 1.03
5 May 2020–28 AE440/870 25609 1.45 0.35 0.25 1.56 2.08 −1.13
February 2022 AOD500 25609 0.17 0.10 0.02 0.14 0.64 1.05
AE380/500 25609 1.32 0.29 0.19 1.38 2.10 −1.07
The mean values of AOD440 and AOD500 for the period 2020–2021 were 0.18 ± 0.09
and 0.15 ± 0.08, respectively, while the mean AE440/870 and AE380/500 were 1.41 ± 0.32 and
1.29 ± 0.31. The corresponding means for the period 2021–2022 were 0.22 ± 0.12 and 0.18 ± 0.11
along with 1.48 ± 0.37 and 1.34 ± 0.27. The means for the overall two-year period 2020–2022
were 0.20 ± 0.11 and 0.17 ± 0.10 along with 1.45 ± 0.35 and 1.32 ± 0.29, respectively.Atmosphere 2022, 13, 884 10 of 29
Table 2. Percentage of realizations with AOD < 0.05, 0.1, 0.2, or 0.3 or AE < 0.8 or 1.4.
Parameter AOD < 0.05 AOD < 0.1 AOD < 0.2 AOD < 0.3 Parameter AE < 0.8 AE < 1.4
Period (%) (%) (%) (%) (%) (%)
5 May 2020–28 February 2021 AOD440 2.72 19.49 67.48 89.59 AE440/870 6.19 37.74
AOD500 5.56 27.93 78.79 93.83 AE380/500 8 57.76
1 March 2021–28 February 2022 AOD440 0.98 18.28 54.4 77.07 AE440/870 9.46 27.46
AOD500 2.41 28.89 65.23 83.18 AE380/500 6.92 49.68
5 May 2020–28 February 2022 AOD440 1.64 18.74 59.34 81.8 AE440/870 8.22 31.34
AOD500 3.6 28.53 70.35 87.21 AE380/500 7.33 52.73
It is interesting to note, for instance, that the AOD440 and AE440/870 mean values obtained
over Sofia were near the average AERONET AOD440 = 0.22 ± 0.17 and AE440/870 = 1.42 ± 0.29
values obtained in the period 2002–2019 over Minsk (Belarus) [56].
The relative frequencies of events with AOD440 < 0.2 and AE440/870 > 1.4 were
50–70% and 60–70% (Table 2), respectively, which suggests that the prevalent aerosol
situations over Sofia are characterized mainly by fine-fraction particles and relatively low
atmospheric turbidity.
The lower mean value of AOD during 2020 compared to 2021 gives rise to questions
about how the COVID-19 lockdown affected the atmospheric aerosol load and air quality.
The COVID-19 lockdown in Bulgaria lasted from 13 March to 13 May 2020, ending about
a week following the beginning of the AERONET activities at the Sofia Site. According
to Copernicus Atmosphere Monitoring Service (CAMS) [96], a large dust intrusion in the
period 26–29 March 2020 disturbed the effect of the lockdown emission reduction on the
average mass concentration of particulate matter with an aerodynamic diameter of less
than 10 µm (PM10 ) in Sofia. Moreover, intense Saharan dust intrusions took place in May
2020 [97]. Thus, there may have been some aftereffect of the lockdown, but it is difficult to
estimate its duration. Judging by the seasonal and monthly averages of AOD (Figure 4), it
can be assumed that the aftereffects, if any, lasted until autumn. On the other hand, in the
summer of 2021, there were intense wildfire activity and SD intrusions, so it is difficult to
unambiguously determine the cause of the lower AOD in the summer of 2020.
3.2. Aerosol Typology
The type of aerosol particles and situations can be estimated using passive (photo-
metric) or active (lidar) optical remote sensing approaches providing information about
some specific optical and microphysical aerosol characteristics (see Section 2.3). The estima-
tion is based on empirically found correspondences between the aerosol types and their
characteristic parameters [27–29,32–34,76]. Such correspondences have been established by
long-term aerosol remote sensing at sites with different climates and geographies under
known aerosol environments [27,28,32,34,73–75]. One may also conduct remote sensing
and contact probing in parallel accompanied by laboratory investigations of the aerosol
species [98–100]. The aerosol identification would be more reliable, in principle, when
based on a wider set of parameters and prognostic models (Section 2.3). At the beginning
of the process of recognition, however, it would be useful to use a minimum number of
appropriate parameters ensuring a reasonable initial orientation in the analysis. AOD and
AE are two such parameters, and we shall use them below in the process of deciphering
the aerosol situations over Sofia.
3.2.1. AOD–AE Scatter Plots
The daily averaged pairs of AOD440 and AE440/870 during the annual cycles 2020–2021
and 2021–2022 are presented as scatter plots in Figure 7a,b, respectively. The days belonging
to different seasons are denoted by different signs and colors. One may distinguish on the
scatter plots six characteristic areas (zones) separated naturally by boundary bands (not
sharp borders) of no events or lower-density events. The different zones are outlined by hor-
izontal and vertical orange lines and numbered clockwise from one to six, beginning from
the upper right-hand corner of the plots. These characteristic zones are in fact a projection ofAtmosphere 2022, 13, 884 11 of 29
the 3×2 modal structures of the AOD440/AE440/870 frequency distributions (Section 3.1.2). Their
boundaries for the annual cycle 2020–2021 are: AOD440 > 0.3, AE440/870 > 1.3; AOD440 > 0.3,
1.0 < AE440/870 < 1.3; AOD440 > 0.3, AE440/870 < 1.0; AOD440 < 0.3, AE440/870 < 1.0; AOD440 < 0.3,
1.0 < AE440/870 < 1.3; and AOD440 < 0.3, AE440/870 > 1.3. For the annual cycle 2021–2022,
the zone boundaries are: AOD440 > 0.3, AE440/870 > 1.4; AOD440 > 0.3, 0.8 < AE440/870 < 1.4;
Atmosphere 2022, 13, x FOR PEER REVIEW440 > 0.3, AE440/870 < 0.8; AOD440 < 0.3, AE440/870 < 0.8; AOD440 < 0.3, 0.8 < AE440/870
AOD 14 of 1.4. The percentages of days (aerosol events) falling within
the boundaries of each zone are, respectively, 8.3%, 2.8%, 0.6%, 8.8%, 16.0%, and 63.5%,
during the cycle 2020–2021, and 9.9%, 3.6%, 3.6%, 3.1%, 16.6%, and 63.2% during the cycle
seasons of snowfalls and rainfalls. The meteorological parameters on the days considered
2021–2022. The corresponding percentages concerning the overall two-year cycle 2020–2022
in the work (Section 3.2.2 below) are presented in Table 3.
are: 9.1%, 3.2%, 2.1%, 5.9%, 16.3%, and 63.4%.
Figure
Figure 7. Scatterplots
7. Scatter plotsofofAOD
AOD vs.vs.
440440 AE440/870
AE440/870 for thefor the periods
periods 5 May
5 May 2020 2020
to 28 to 28 February
February 2021
2021 (a) and
(a) and 1 March
1 March 2021 to2021 to 28 February
28 February 2022(b).2022 (b).
TableThe lines we traced
3. Meteorological indicate the approximate positions of the boundary bands and may
parameters.
serve as classification thresholds when they distinguish groups of events of different types.
groupingTemperature
Such aDate Dew
allows at least for Pointconsistent
a more Wind Speed Visibility
and clear analysis Precipitation
and characterization
of the aerosol events(°C) (°C)
based on comparing (m/s) and microphysical
their optical (km) 24 h (l/m2)
characteristics
14 September
and behavior with known 21.2 such characteristics
11 and 1.1
behavior, previously
18–25 established 0 experi-
mentally2020under different aerosol conditions using active and passive remote sensing and
12 October
contact (in situ) probing
15.3 approaches (Section 3.2, introductory
11.2 1 paragraph).
12–20 0
2020
Studying the aerosol properties group by group revealed the specificity of the aerosol
1 November
events within the different AOD440 –AE440/870 groups (zones). Thus, we found one zone of
8.1 2.7 0.8 8–25 2.7
2020
biomass-burning aerosols (zone one), another zone of urban aerosols (zone six), two zones
(three and2021
19 July four) of Saharan
22.5 dust events,17.2and possibly1.6marine aerosols
12–20for AOD440 0.3 or
2021
AE440/870 < 1.0 (Figure 7a) or 0.8 (Figure 7b) are sparsely populated. The occurrences
8 November
of these cases in the12.8 first and the second
8.7 annual1.3cycles are about
12–2511.7% and 17.1%
0 for
2021
3.2.2. Aerosol Situations
Biomass-Burning AerosolsAtmosphere 2022, 13, 884 12 of 29
AOD440 > 0.3 and 9.4% and 6.7% for AE440/870 < 1.0 or 0.8, respectively. This means that
biomass-burning aerosols (zone one), Saharan dust intrusions (zones three and four), and
marine aerosols (zone four [29,31,56,77]) have not been frequent aerosol events over Sofia.
The typical zone of marine aerosols with AOD440 < 0.15 and AE440/870 < 1.0 [29,31] is almost
empty. The most densely populated zone six involves almost evenly all seasons. It should
characterize continental and anthropogenic urban aerosols, including aerosol pollutants.
It is seen as well that zone one is occupied mainly by events occurring in summer and
autumn—the seasons of fire activity. Another interesting conclusion is that the cases of
a clear atmosphere with AOD440 < 0.1 and 0.05 occur mostly in winter and autumn—the
seasons of snowfalls and rainfalls. The meteorological parameters on the days considered
in the work (Section 3.2.2 below) are presented in Table 3.
Table 3. Meteorological parameters.
Temperature Dew Point Wind Speed Visibility Precipitation
Date
(◦ C) (◦ C) (m/s) (km) 24 h (l/m2 )
14 September 2020 21.2 11 1.1 18–25 0
12 October 2020 15.3 11.2 1 12–20 0
1 November 2020 8.1 2.7 0.8 8–25 2.7
19 July 2021 22.5 17.2 1.6 12–20 0
5 August 2021 27.7 13.5 1.5 10–20 0
14 August 2021 23.5 13.4 1.6 20–25 0
18 August 2021 20.9 12.9 4 15–25 1.5
23 August 2021 23.3 11.1 1.9 20–25 0
25 August 2021 19.3 14.6 1.3 9–25 0
17 September 2021 20.8 14.2 1.1 16–28 0
8 November 2021 12.8 8.7 1.3 12–25 0
3.2.2. Aerosol Situations
Biomass-Burning Aerosols
Biomass-burning smoke in the air over the Balkan Peninsula is a seasonal phenomenon
occurring mainly in the summer and early autumn, when the wildfire activity is most
intensive. The main sources of smoke over Sofia are fires in Bulgaria and neighboring
countries [78] producing smoke of a similar age and type of combustion material under
similar weather conditions. The trans-boundary aerosols from distant regions would
differ in combustion material, age, and density. In general, the microphysical and optical
properties of the biomass-burning smoke depend on various factors, such as the geographic
location of the fire, the combustion material and its moisture, the type of combustion (more
or less intense flaming or smoldering), the ambient temperature and humidity, the smoke
age, etc. [27,28,73,74]. The smoke particles increase the atmospheric turbidity and the fine
aerosol fraction. Correspondingly, AOD440 and AE440/870 increase and are usually well
above 0.3 and 1.4 [28], respectively, for fresh aerosols produced minutes to hours before.
Thus, during the days with AOD440 –AE440/870 pairs falling within the boundaries of zone
one (Figure 7a,b), the air over Sofia City must have contained biomass-burning smoke
particles. Some increase of the aerosol sizes may have taken place as a result of smoldering-
type combustion or ageing accompanied by particle coagulation, condensation, etc. This
could lower AE440/870 down to values of about 1.2–1.4 [28,74]. The presence is also possible
of small amounts of larger-in-size desert dust particles with a lower AE440/870 . The desert
dust particles would also lead to lowering the particle sphericity factor [101–103], thus
increasing the linear depolarization effect [104] of the aerosol ensembles. In the case of a
strong prevalence of fine-mode smoke particles, the single-scattering aerosol albedo should
diminish with the wavelength λ as a consequence of the rapidly decreasing light scattering
as compared with absorption. The values of SSA are smaller for a higher elemental (black)-
carbon content and the related absorption [98,99]. Note as well that the biomass-burning
aerosol particles should have a spherical shape [28,33].
As is shown by using additional AERONET data and supported by HYSPLIT and
BSC-DREAM8b models predictions, almost all daily mean aerosol situations falling withinAtmosphere 2022, 13, 884 13 of 29
zone one (Figure 7a,b) exhibit similar (above-described) peculiarities of their characteristics
that are intrinsic to biomass-burning aerosol ensembles. Such peculiarities, concerning
23 August 2021, are illustrated in Figure 8 and Table 4. Table 4 contains information on
the daily mean aerosol optical depth and Ångström exponent, and on the range of the
particle sphericity factor, linear depolarization ratio, and real part of the refractive index,
during the specific days considered. It is seen there that AODf500 is 28 times larger than
AODc500 ; i.e., the optical impact of the fine aerosol fraction is strongly prevailing, which is
in accordance with the VSD shape plotted in Figure 8a. AOD440 = 0.36, and AE440/870 = 1.90
(Table 4). The real part of the refractive index at λ = 440 nm (nr440 ) varies from 1.40 to
1.57, and the SSA decreases with λ (Figure 8b) as that of biomass-burning aerosols, as
described in [28]. The SF varies from 91.7% to 99.0%, being most frequently near 99.0%,
and the DR at λ = 440 nm (DR440 ) varies from 0.002 to 0.005. The 120-h back trajectories
revealed by the HYSPLIT model arriving over Sofia at 12:00 UTC at heights of 500 m, 1500
m, and 3000 m above ground level (AGL) are shown in Figure 8c. The air mass arriving
at a height of 3000 m begins in the Atlantic Region and reaches Bulgaria after traveling
over continental Europe. It should contain mainly continental and urban aerosols because
most of the Atlantic marine particles would have sedimented during the long travel to
Bulgaria. The BSC-DREAM8b model does not predict Saharan dust intrusions over Sofia at
12:00 UTC (Figure 8d). A three-day map of the fires within Bulgaria and adjacent regions
obtained
Atmosphere 2022,via
13, xNASA’s FIRMS [78] for the period 21–23 August 2021 is shown in Figure
FOR PEER REVIEW 16 of 9a.
34
Strong fire activity is seen in northwestern Bulgaria and near Sofia.
Figure 8. Volume size distributions (a) and single-scattering albedos (b) of the aerosol particles;
Figure 8. Volume size distributions (a) and single-scattering albedos (b) of the aerosol particles;
HYSPLIT model 120-hour back trajectories of the air masses arriving over Sofia at 12:00 UTC at
HYSPLIT model 120-h back trajectories
heights of m,
of 500 m, 1500 theand
air3000
masses
m AGLarriving over Sofia at
(c) and BSC-DREAM8b 12:00Saharan
forecast UTC dust
at heights
profiles
overmSofia
of 500 m, 1500 m, and 3000 AGL at 12:00 UTC (d)
(c) and on 23 August 2021.forecast Saharan dust profiles over Sofia
BSC-DREAM8b
at 12:00 UTC (d) on 23 August
Table 4.2021.
AERONET daily mean values of the aerosol optical depths AOD440, AOD500, AODf500, and
AODc500, and of the Ångström exponents AE440/870 and AE380/500. Range of the retrieved values of the
particle sphericity factor (SF), linear depolarization ratio (DR), and refractive index real part nr440.
Date AOD440 AOD500 AODf500 AODc500 AE440/870 AE380/500 SF (%) DR440 nr440
14 Sep-
tember 0.36 ± 0.09 0.30 ± 0.08 0.24 ± 0.07 0.06 ± 0.01 1.52 ± 0.04 1.40 ± 0.08 0.6–67.2 0.022–0.133 1.35–1.56
2020Atmosphere 2022, 13, x FOR PEER REVIEW 17 of 34
19 July 2021 0.46 ± 0.07 0.39 ± 0.05 0.29 ± 0.07 0.10 ± 0.02 1.34 ± 0.19 1.29 ± 0.09 3.7–28.9 0.086–0.132 1.46–1.51
Atmosphere
5 August2022, 13, 884 14 of 29
0.43 ± 0.02 0.39 ± 0.02 0.17 ± 0.02 0.22 ± 0.01 0.67 ± 0.10 0.79 ± 0.12 1.0–4.4 0.091–0.139 1.40–1.49
2021
14 August
0.29 ± 0.04 0.24 ± 0.03 0.22 ± 0.03 0.01 ± 0.002 1.85 ± 0.02 1.51 ± 0.03 93.2–99.0 0.002–0.005 1.44–1.50
2021 Table 4. AERONET daily mean values of the aerosol optical depths AOD440 , AOD500 , AODf500 , and
18 August AODc500 , and of the Ångström exponents AE440/870 and AE380/500 . Range of the retrieved values of
0.43 ± 0.07 0.37 ± 0.05 0.27 ± 0.05 0.09 ± 0.01 1.37 ± 0.10 1.29 ± 0.07 3.0–20.3 0.104–0.131 1.51–1.54
2021 the particle sphericity factor (SF), linear depolarization ratio (DR), and refractive index real part nr440 .
23 August
Date 0.36 ±AOD
0.02440 0.30 ±AOD
0.02500 0.28 ±AOD
0.02f5000.01 ± AOD
0.002
c500
1.90 ±AE0.03 1.47 ±AE0.04 91.7–99.0
SF (%) 0.002–0.005
DR440 1.40–1.57
nr440
2021 440/870 380/500
14 September 2020 0.36 ± 0.09 0.30 ± 0.08 0.24 ± 0.07 0.06 ± 0.01 1.52 ± 0.04 1.40 ± 0.08 0.6–67.2 0.022–0.133 1.35–1.56
25October
12 August 2020 0.64 0.23 ± 0.010.53 0.19 ± 0.010.51 0.17 ± 0.01 0.01 ± 0.011.83 1.74 ± 0.121.37 1.37 ± 0.04 98.6–99.0 0.002 1.36–1.43
± 0.09
0.05 ± 0.01
± 0.07
0.04 ± 0.01
± 0.08
0.03 ± 0.01
0.02 ±0.01
0.004
± 0.002
± 0.03
1.25 ± 0.11
± 0.07
1.12 ± 0.14
99.0 0.002 1.53
2021 2020
1 November 60.3–99.0 0.004–0.033 1.43–1.58
19 July 2021 0.46 ± 0.07 0.39 ± 0.05 0.29 ± 0.07 0.10 ± 0.02 1.34 ± 0.19 1.29 ± 0.09 3.7–28.9 0.086–0.132 1.46–1.51
5 17 Sep-
August 2021 0.43 ± 0.02 0.39 ± 0.02 0.17 ± 0.02 0.22 ± 0.01 0.67 ± 0.10 0.79 ± 0.12 1.0–4.4 0.091–0.139 1.40–1.49
14 August 2021 0.29 ± 0.04 0.24 ± 0.03 0.22 ± 0.03 0.01 ± 0.002 1.85 ± 0.02 1.51 ± 0.03 93.2–99.0 0.002–0.005 1.44–1.50
tember
18 August 2021
0.29 ± 0.02
0.43 ± 0.07
0.27 ± 0.02 0.10
0.37 ± 0.05
± 0.004
0.27 ± 0.05
0.17 ± 0.02
0.09 ± 0.01
0.52 ± 0.04
1.37 ± 0.10
0.65 ± 0.05
1.29 ± 0.07
0.4–20.7
3.0–20.3
0.107–0.141
0.104–0.131
1.49–1.51
1.51–1.54
2021 2021
23 August 0.36 ± 0.02 0.30 ± 0.02 0.28 ± 0.02 0.01 ± 0.002 1.90 ± 0.03 1.47 ± 0.04 91.7–99.0 0.002–0.005 1.40–1.57
25 August 2021 0.64 ± 0.09 0.53 ± 0.07 0.51 ± 0.08 0.02 ± 0.004 1.83 ± 0.03 1.37 ± 0.07 99.0 0.002 1.53
178September
Novem- 2021 0.29 ± 0.02 0.27 ± 0.02 0.10 ± 0.004 0.17 ± 0.02 0.52 ± 0.04 0.65 ± 0.05 0.4–20.7 0.107–0.141 1.49–1.51
8 November
0.30 ± 0.05
0.30 ± 0.05
0.26 ± 0.05
0.26 ± 0.05
0.13 ± 0.02
0.13 ± 0.02
0.13 ± 0.04
0.13 ± 0.04
0.78 ± 0.13
0.78 ± 0.13
0.77 ± 0.09
0.77 ± 0.09
5.9–99.0 0.002–0.148 1.47–1.51
ber 20212021 5.9–99.0 0.002–0.148 1.47–1.51
Figure9.9.NASA’s
Figure NASA’sFIRMS
FIRMS fire
fire maps
maps for
for Bulgaria
Bulgariaand
andadjacent
adjacentregions
regionsfor the
for periods
the 21–23
periods August
21–23 August
2021 (a), 12–14 September 2020 (b), and 23–25 August 2021
2021 (a), 12–14 September 2020 (b), and 23–25 August 2021 (c) (https://firms.modaps.eosdis.nasa. (c)
(https://firms.modaps.eosdis.nasa.gov/, accessed on 15 May 2022).
gov/, accessed on 15 May 2022).
Such
The a situation occurred
biomass-burning on over
aerosols 14 September
Sofia with2020, when~1.6–1.9
AE440/870 the meanandAE 440/870 = 1.52,
above should be
AOD 440 = 0.36, and the SSA decreased with λ; however, AODf500 is only four times larger
the result of wildfires within Bulgaria and the neighboring countries. The aerosol ensem-
than AODc500, the SF varies from low (0.6%) to moderate (67.2%) values, and the DR440
bles with AE440/870 < 1.6 may be aged (of trans-boundary origin) and due to smoldering
varies from 0.022 to 0.133 (Table 4). The BSC-DREAM8b model predicts the presence of
combustion or containing desert dust or/and marine aerosols.
an SD layer at an altitude of about 4500 m with a maximum concentration of 10 µg/m3. At
Such a situation occurred on 14 September 2020, when the mean AE440/870 = 1.52,
the same time, according to the HYSPLIT back-trajectories model, the trans-boundary air
AOD440 = 0.36, and the SSA decreased with λ; however, AODf500 is only four times larger
masses that arrived over Sofia had mainly followed trajectories over Europe, Turkey, and
than AODc500 , the SF varies from low (0.6%) to moderate (67.2%) values, and the DR440
Egypt. Intensive fire activity in northern and central Bulgaria and adjacent regions in the
varies from 0.022 to 0.133 (Table 4). The BSC-DREAM8b model predicts the presence of an
period 12–14 September 2020 was observed (Figure 9b). Thus, the aerosol load over Sofia
SD layer at an altitude of about 4500 m with a maximum concentration of 10 µg/m3 . At
on 14 September 2020 could have contained a mixture of biomass-burning, Saharan dust,
the same time, according to the HYSPLIT back-trajectories model, the trans-boundary air
and marine aerosols. Another interesting example is the situation on 25 August 2021,
masses that arrived over Sofia had mainly followed trajectories over Europe, Turkey, and
when active fires in the period 23–25 August 2021 concentrated mainly in the western
Egypt. Intensive fire activity in northern and central Bulgaria and adjacent regions in the
part of the country were seen (Figure 9c), and all aerosol characteristics but one indicate
period
the presence September
12–14 2020 wassmoke
of biomass-burning observed (Figure
(Table 9b).
4). The Thus, the
exception aerosol
is the SSA load
that wasovertoo
Sofia
on 14 September 2020 could have contained a mixture of biomass-burning,
high and slowly varying—from nearly 0.99 at λ = 440 nm to nearly 0.98 at λ = 1020 nm. Saharan dust,
and marine aerosols. Another interesting example is the situation on 25 August
This is, perhaps, a rare case of a low content (You can also read
