Characterizing Atmospheric Brown Carbon and Its Emission Sources during Wintertime in Shanghai, China

Effects of aerosol water content and acidity on the light absorption of atmospheric humic-like substances in winter

Atmospheric humic-like substances (HULIS) could affect regional climate due to their strong light-absorbing capacity. Daily fine particulate matter (PM2.5) samples were collected from December 18, 2016 to January 8, 2017 at an urban site in Chongqing, Southwest China. The mean concentration of HULIS in terms of carbon (HULIS-C) was 6.4 ± 3.4 μg m-3, accounting for 72% of water-soluble organic carbon. The mass absorption efficiency at 365 nm (MAE365) and absorption Ångström index (AAE) of atmospheric HULIS were 2.8 ± 0.30 m2 g-1 C and 4.6 ± 0.37, respectively. Good correlations between the light absorption coefficients of HULIS at 365 nm (Abs365) and the concentrations of K+, elemental carbon, NO3-, and NH4+ were observed, with correlation coefficients higher than 0.83, indicating that biomass burning and secondary formation were potential sources of light-absorbing HULIS, as evidenced by abundant fluorescent components related to less-oxygenated HULIS. Comparing the changes in Abs365 values, concentrations of major water-soluble inorganic ions and carbonaceous compounds in PM2.5, and environmental factors during the clean and pollution periods, we found that extensive biomass burning during the pollution period contributed significantly to the increase of Abs365 values. Moreover, the aerosol pH during the pollution period was close to 4, and NO2 concentration and aerosol water content were about 1.6 and 2.7 times higher than those during the clean period, respectively, which were favorable to form secondary HULIS through aqueous phase reactions in the presence of high NOx, resulting in an evident increase in its light absorption. Knowledge generated from this study is critical for evaluating the regional radiative forcing of brown carbon in southwest China.

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. PM_2.5 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.