Brown carbon absorption and radiative effects under intense residential wood burning conditions in Southeastern Europe: New insights into the abundance and absorptivity of methanol-soluble organic aerosols.

Biomass burning is a major source of Brown Carbon (BrC), strongly contributing to radiative forcing. In urban areas of the climate-sensitive Southeastern European region, where strong emissions from residential wood burning (RWB) are reported, radiative impacts of carbonaceous aerosols remain largely unknown. This study examines the absorption properties of water- and methanol-soluble organic carbon (WSOC, MeS_OC) in a city (Ioannina, Greece) heavily impacted by RWB. Measurements were performed during winter (December 2019 - February 2020) and summer (July - August 2019) periods, characterized by RWB and photochemical processing of organic aerosol (OA), respectively. PM2.5 filter extracts were analyzed spectrophotometrically for water- and methanol-soluble BrC (WS_BrC, MeS_BrC) absorption. WSOC concentrations were quantified using TOC analysis, while those of MeS_OC were determined using a newly developed direct quantification protocol, applied for the first time to an extended series of ambient samples. The direct method led to a mean MeS_OC/OC of 0.68 and a more accurate subsequent estimation of absorption efficiencies. The mean winter WS_BrC and MeS_BrC absorptions at 365 nm were 13.9 Mm-1 and 21.9 Mm-1, respectively, suggesting an important fraction of water-insoluble OA. Mean winter WS_BrC and MeS_BrC absorptions were over 10 times those observed in summer. MeS_OC was more absorptive than WSOC in winter (mean mass absorption efficiencies - MAE365: 1.81 vs 1.15 m2 gC-1) and especially in summer (MAE: 1.12 vs 0.27 m2 gC-1) due to photo-dissociation and volatilization of BrC chromophores. The winter radiative forcing (RF) of WS_BrC and MeS_BrC relative to elemental carbon (EC) was estimated at 8.7 % and 16.7 %, respectively, in the 300-2500 nm band. However, those values increased to 48.5 % and 60.2 % at 300-400 nm, indicating that, under intense RWB, BrC forcing becomes comparable to that of soot. The results highlight the consideration of urban BrC emissions in radiative transfer models, as a considerable climate forcing factor.

Apportionment of black and brown carbon spectral absorption sources in the urban environment of Athens, Greece, during winter.

This study examines the spectral properties and source characteristics of absorbing aerosols (BC: Black Carbon; BrC: Brown Carbon, based on aethalometer measurements) in the urban background of Athens during December 2016-February 2017. Using common assumptions regarding the spectral dependence of absorption due to BC (AAEBC = 1) and biomass burning (AAEbb = 2), and calculating an optimal AAEff value for the dataset (1.18), the total spectral absorption was decomposed into five components, corresponding to absorption of BC and BrC from fossil-fuel (ff) combustion and biomass burning (bb), and to secondary BrC estimated using the BC-tracer minimum R-squared (MRS) method. Substantial differences in the contribution of various components to the total absorption were found between day and night, due to differences in emissions and meteorological dynamics, while BrC and biomass burning aerosols presented higher contributions at shorter wavelengths. At 370 nm, the absorption due to BCff contributed 36.3% on average, exhibiting a higher fraction (58.1%) during daytime, while the mean BCbb absorption was estimated at 18.4%. The mean absorption contributions due to BrCff, BrCbb and BrCsec were 6.7%, 32.3% and 4.9%, respectively. The AbsBCff,370 component maximized during the morning traffic hours and was strongly correlated with NOx (R2 = 0.76) and CO (R2 = 0.77), while a similar behavior was seen for the AbsBrCff,370 component. AbsBCbb and AbsBrCbb levels escalated during nighttime and were highly associated with nss-K+ and with the organic aerosol (OA) components related to fresh and fast-oxidized biomass burning (BBOA and SV-OOA) as obtained from ACSM measurements. Multiple linear regression was used to attribute BrC absorption to five OA components and to determine their absorption contributions and efficiencies, revealing maximum contributions of BBOA (33%) and SV-OOA (21%). Sensitivity analysis was performed in view of the methodological uncertainties and supported the reliability of the results, which can have important implications for radiative transfer models.

Long-term brown carbon spectral characteristics in a Mediterranean city (Athens).

This study analyses 4-years of continuous 7-λ Aethalometer (AE-33) measurements in an urban-background environment of Athens, to resolve the spectral absorption coefficients (babs) for black carbon (BC) and brown carbon (BrC). An important BrC contribution (23.7 ± 11.6%) to the total babs at 370 nm is estimated for the period May 2015-April 2019, characterized by a remarkable seasonality with winter maximum (33.5 ± 13.6%) and summer minimum (18.5 ± 8.1%), while at longer wavelengths the BrC contribution is significantly reduced (6.8 ± 3.6% at 660 nm). The wavelength dependence of the total babs gives an annual-mean AAE370-880 of 1.31, with higher values in winter night-time. The BrC absorption and its contribution to babs presents a large increase reaching up to 39.1 ± 13.6% during winter nights (370 nm), suggesting residential wood burning (RWB) emissions as a dominant source for BrC. This is supported by strong correlations of the BrC absorption with OC, EC, the fragment ion m/z 60 derived from ACSM and PMF-analyzed organic fractions related to biomass burning (e.g. BBOA). In contrast, BrC absorption decreases significantly during daytime as well as in the warm period, reaching to a minimum during the early-afternoon hours in all seasons due to photo-chemical degradation. Estimated secondary BrC absorption is practically evident only during winter night-time, implying the fast oxidation of BrC species from RWB emissions. Changes in mixing-layer height do not significantly affect the BrC absorption in winter, while they play a major role in summer.

Sources of atmospheric aerosol from long-term measurements (5 years) of chemical composition in Athens, Greece.

To identify the sources of aerosols in Greater Athens Area (GAA), a total of 1510 daily samples of fine (PM 2.5) and coarse (PM 10-2,5) aerosols were collected at a suburban site (Penteli), during a five year period (May 2008-April 2013) corresponding to the period before and during the financial crisis. In addition, aerosol sampling was also conducted in parallel at an urban site (Thissio), during specific, short-term campaigns during all seasons. In all these samples mass and chemical composition measurements were performed, the latest only at the fine fraction. Particulate organic matter (POM) and ionic masses (IM) are the main contributors of aerosol mass, equally contributing by accounting for about 24% of the fine aerosol mass. In the IM, nss-SO4(-2) is the prevailing specie followed by NO3(-) and NH4(+) and shows a decreasing trend during the 2008-2013 period similar to that observed for PM masses. The contribution of water in fine aerosol is equally significant (21 ± 2%), while during dust transport, the contribution of dust increases from 7 ± 2% to 31 ± 9%. Source apportionment (PCA and PMF) and mass closure exercises identified the presence of six sources of fine aerosols: secondary photochemistry, primary combustion, soil, biomass burning, sea salt and traffic. Finally, from winter 2012 to winter 2013 the contribution of POM to the urban aerosol mass is increased by almost 30%, reflecting the impact of wood combustion (dominant fuel for domestic heating) to air quality in Athens, which massively started in winter 2013.

Simulated air quality and pollutant budgets over Europe in 2008.

Major gaseous and particulate pollutant levels over Europe in 2008 have been simulated using the offline-coupled WRFCMAQ chemistry and transport modeling system. The simulations are compared with surface observations from the EMEP stations, ozone (O3) soundings, ship-borne O3 and nitrogen dioxide (NO2) observations in the western Mediterranean, tropospheric NO2 vertical column densities from the SCIAMACHY instrument, and aerosol optical depths (AOD) from the AERONET. The results show that on average, surface O3 levels are underestimated by 4 to 7% over the northern European EMEP stations while they are overestimated by 7-10% over the southern European EMEP stations and underestimated in the tropospheric column (by 10-20%). Particulate matter (PM) mass concentrations are underestimated by up to 60%, particularly in southern and eastern Europe, suggesting underestimated PM sources. Larger differences are calculated for individual aerosol components, particularly for organic and elemental carbon than for the total PM mass, indicating uncertainty in the combustion sources. Better agreement has been obtained for aerosol species over urban areas of the eastern Mediterranean, particularly for nss-SO4(2), attributed to the implementation of higher quality emission inventories for that area. Simulated AOD levels are lower than the AERONET observations by 10% on average, with average underestimations of 3% north of 40°N, attributed to the low anthropogenic emissions in the model and 22% south of 40°N, suggesting underestimated natural and resuspended dust emissions. Overall, the results reveal differences in the model performance between northern and southern Europe, suggesting significant differences in the representation of both anthropogenic and natural emissions in these regions. Budget analyses indicate that O3 and peroxyacetyl nitrate (PAN) are transported from the free troposphere (FT) to the planetary boundary layer over Europe, while other species follow the reverse path and are then advected away from the source region.

Cesium-137 in grass from Chernobyl fallout.

Grass ecosystem was monitored for 137Cs, a relatively long-lived radionuclide, for about 16 years since the Chernobyl reactor accident occurred on April 26, 1986. Cesium-137 in grass gramineae or poaceae the species, ranged from 122.9 Bq kg(-1) (September 4, 1986) to 5.8 mBq kg(-1) (October 16, 2001) that is a range of five orders of magnitude. It was observed that there was a trend of decreasing 137Cs with time reflecting a removal half-time of 40 months (3 1/3 years), which is the ecological half-life, T(ec) of 137Cs in grassland.

Coloured rain dust from Sahara Desert is still radioactive.

A coloured rain event originating from the Sahara Desert occurred on April 9, 2000 at Thessaloniki, Northern Greece (40 degrees 38'N, 22 degrees 58'E). The radioactive nuclides that were determined in a coloured rain dust sample were 137Cs of Chernobyl origin, 7Be of cosmogenic origin and 40K of terrestrial origin. Cesium-137 still remained 14 years after the Chernobyl accident, reaching 26.6 Bq kg-1 in the coloured rain dust.