Estimation of elevated black carbon episode over Ukraine using Enviro-HIRLAM

Abstract. Biomass burning is one of the biggest sources of black carbon concentrations which negatively impacts human health and contribute to climate forcing. In this work we explore horizontal and vertical variability of black carbon concentrations over Ukraine during a wildfire episode in August 2010. Using Enviro-HIRLAM modelling framework the black carbon atmospheric transport was modelled for coarse and accumulation mode aerosol particles emitted by the wildfire. Elevated pollution levels were observed within the boundary layer. The influence of the black carbon emissions by the wildfire was identified up to 550 hPa level and at distances of about 2000 km from the fire areas. Strong temperature inversions at nighttime resulted in subsiding air mass movement and lead to increased concentrations of black carbon in the atmospheric boundary layer. Ground-based measurements of dust showed increase of concentration up to 73 % in comparison to average values. The fingerprint of local fires was found in the areas with local maxima of summary black carbon values for coarse mode. The findings of the case study could help to understand the behaviour of black carbon distribution during anticyclonic conditions which often observed in mid-latitudes in the summer and lead to wildfires occurrence.


The vertical structure includes 40 model levels with more detailed resolution in the boundary layer. In general, main weatherforming layer (i.e., up to 500 hPa) includes 22 model levels. This provides a great opportunity for studying the BC vertical transport. The model was run as the following: at first, the reference (or control) run and then, the run with direct aerosol effect 100 included. However, for the purpose of the BC spatio-temporal analysis during severe forest fire episode (7-17 August 2010) we decided to use the output of only a reference run.
The spatial analysis of modelling results was carried out considering all grid cells without spatial averaging and interpolation.
It allowed detecting concentrations changes within each grid caused by anticyclonic air movements. Evaluation of accumulated 105 BC impact was performed by time integration of concentration for studied period. Therefore, the summed values represent the total amount (for both the accumulation and coarse modes) transported by air movements through grid cells.

Additional data for analysis
The mass concentration of dust in PM10 size fraction is only aerosol species that are measured at 129 monitoring sites in Ukraine. "These measurements contain all coarse aerosol particles regardless of their origin (Nadtochii et al., 2019)". 110 Therefore, it is complex to compare accurately results with ground-based measurements. The Ukrainian air pollution https://doi.org/10.5194/acp-2022-103 Preprint. Discussion started: 22 February 2022 c Author(s) 2022. CC BY 4.0 License. monitoring network was established several decades ago, and under continuous development and expansion. Therefore, majority of the monitoring sites are situated in cities. Such cities have large number of anthropogenic sources such as factories, thermal power stations, roads, etc. Hence, it is reasonable to use daily averaging and to calculate integral value for each city based on several monitoring stations. Such approach can improve signal-to-noise ratio in a time series. In our study, dust mass 115 concentration data was selected from year 2010 in 20 Ukrainian cities, which are approximately geographically equally distributed within the country. It was done for purposes of intercomparison between western-eastern-southern-northern territories.
The upper air soundings data from the Wyoming University database (http://weather.uwyo.edu/upperair/sounding.html) were 120 used to detect temperature inversions. The air temperature vertical profiles were analyzed for 2-18 August 2010 at the following soundings stations: Kyiv (station code 33345), Kharkiv (34300), Odesa (33837), Rostov-na-Donu (34731), Kalac (34247) and Voronezh (34122). Voronezh and Kalac are situated near the areas where the forest fires occurred. Kharkiv and Rostov-na-Donu were selected for estimation of inversion impact at some distance from the fires. Kyiv and Odesa were chosen as relatively distant to sources of the emissions. The temperature vertical profiles were taken into consideration up to 3-3.5 125 km above the ground surface at 00 UTC times during the studied period.
Note, that different additional fire related emissions of local origin could also take place. These can influence the observed levels of the BC concentrations in the lowest layer near the surface. "For that, we used the Global Fire Emissions Database (GFED4) (Giglio et al., 2013) from which analysis of burned areas was made". This database has a spatial resolution of 130 0.25°x0.25° of latitude vs. longitude. The burning fraction selected for August 2010 represented the fraction of the burned area within 0.25°x0.25°cell.

135
According to the Climate Forecast System (CFS) Reanalysis (source: www.wetterzentrale.de) of the 500 hPa geopotential maps over Europe, the blocking anticyclone, which caused severe hot weather, lack of precipitation and occurrence of wildfires, lasted from the end of June to the second half of August 2010 over the East Europe and south-western regions of Russia (see example Fig. 2). Hot air masses from Central Asia penetrated into the north-west, and anticyclone was detected throughout the whole troposphere before the highest pollution levels distributed out of burning cells. Continuous extreme 140 weather and clear sky conditions together with highest insolation in the middle latitudes caused domination of high temperature and low humidity regimes. These were the most favorable conditions for drought formation that played crucial role in emerging fires and their rapid distribution.

Dispersion of wildfire emissions
The main source of wildfire emissions was located outside of the Ukraine's territory and consisted of several burning areas (see Fig. 3b). Observed anticyclonic conditions influenced on formation and development of spatio-temporal patterns for BC 150 atmospheric transport and dispersion. The time-series for each grid point consist of two maxima. These are connected with observed dominated atmospheric circulation patterns. Typical clockwise air movement for anticyclones in the Northern Hemisphere caused intensive atmospheric transport towards Ukraine during two episodes: 7-8 and 13-16 August 2010. For these episodes, elevated concentrations were also observed in the northern regions of Ukraine (as shown in Fig. 3).

Diurnal variability of BC in the region 170
During 3-18 August at nighttimes the whole European territory of Russia, central and eastern territories of Ukraine were characterized by presence of surface air temperature inversions. Therefore, it is well seen through BC diurnal variations in the lowest 500-meter layer. The deepest and strongest (up to 655 m depth and up to 12.5℃) inversions were observed during 4-7 and on 16 August (see Fig. 4). At 500 m level the air temperature was warmer by 10-12℃ than at 2 m. For other nights the 175 inversions were weaker with an average difference of 3-4℃ in the 2-500 m layer.

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In our study, diurnal variations of BC content in the boundary layer were well detected. Maxima more often were observed at night and morning hours, whereas daytime pollution levels in the lower troposphere were low. These diurnal variations appeared due to radiative surface cooling on summer nights, and especially in case of blocking anticyclones. Such anticyclones cause forming of air temperature surface inversions. Together with intensification of downward air movement at nighttimes, BC from the whole lower and middle tropospheric levels is accumulated within the boundary layer reaching elevated values 185 there. During daytime, the processes of less intensive air descending and turbulence increasing are resulted in more homogeneous vertical distribution. Hence, it can lead to decrease of near-surface BC content. As it happened throughout the Ukraine's territory with anticyclonic weather, BC spatial distribution at the separate level in the lower troposphere looks more The vertical distribution of BC particles was well detected in the lowest 3-km layer with the maximum observed in the boundary layer. Approximately at 700 hPa level, BC concentration for both accumulation and coarse modes started to decrease very rapidly (Fig. 5). The levels of 630 hPa for the coarse mode and 590 hPa for the accumulation mode were identified as the highest altitudes where the influence of wildfire emissions was detected constantly during daytime. Rarely, during the midday elevated BC concentrations were detected at 590 hPa (for the coarse mode) and at 550 hPa (for the accumulation mode) 195

Figure 5: BC vertical profiles for coarse (a,b) and accumulation (c,d) modes over Kalac, Kharkiv and Odesa at 00UTC (a,c) and 12UTC (b,d) on 14th August.
It is well seen from the fig. 5 that BC rather equally distributed in the 1000-700 hPa layer during day hours. But at nightime, 200 BC was observed mostly in the boundary layer, especially the coarse mode. Temperature inversions and air descending during night hours caused more intense coarse mode deposition, therefore it is hard to find coarse particles at the distance more than 1000 km out from the active fires (represented by Odesa at fig. 5). However, accumulation mode could be transported at such long distances and the transportation was observed at the lower 3-km layer.

Spatial variability in BC in Ukraine during the wildfire episode
In general, the wildfire emissions have large accumulative effect in the near-surface layer. Total accumulated amount of BC for the period 3-18 August 2010 reached 13500 ppbm (for accumulation mode) and 2200 ppbm (coarse mode) in the lower tropospheric layer near the burning areas/ cells (Fig. 6). A large amount of combustion products was transported through the 210 atmosphere to the south-west and deposited over territories of the Eastern Ukraine, the Azov and Black Seas. The integral values of BC on these territories exceeded 800 and 150 ppbm for accumulation and coarse modes, respectively. This is well seen from Fig. 6 where the regions were affected by intensive deposition processes. Due to smaller sizes of the particles, the accumulation mode has larger spatial coverage than the coarse mode. Large difference in dust content between August and other months was observed on the seashore of the Azov Sea and in the central part of Ukraine. The concentrations were higher on 0.05-0.25 μg/m3 than usually in the same month. Majority of cities in the central part of Ukraine showed 17-73% higher dust concentrations than average in 2010 and 8-45% higher than usually 235 in August.
In the western parts of Ukraine, the integral values were lower than 100 and 500 ppbm for coarse and accumulation modes, respectively (Fig. 6). In these regions the dust concentration difference between August and other months was very low and it did not exceed 0.05 μg/m3. However, dust content in the region is low and differences up to 0.05 μg/m3 correspond to 28-240 58% concentration increase over average values.
It should be noted that in addition to the BC atmospheric transport from remote regions (i.e. in particular, due to forest fires occurred outside of Ukrainian territory), local fire emissions could influence levels of near-surface concentrations. During the studied period of elevated pollution episodes in August 2010, there were observed a large number of local fires (Fig. 7). Of 245 course, these could not impact large pollution levels throughout whole boundary layer and the middle troposphere, and moreover, were not transported far from original burned areas. But these fires may influence the total BC deposition and accumulation near the surface layer. The largest area of local fires was observed in the Eastern Ukraine, where burned fraction reached 0.13 (i.e. that on these territories local fires could contribute up to 10-13%).