Brown carbon (BrC) affects the Earth's radiative balance due to its strong light absorption at short wavelengths. A custom‐made combustion chamber was used to simulate biomass and coal burning and to investigate the emission characteristics of BrC absorption. Absorption Ångström exponents (AAEs) at the wavelength pair of 370 and 880 nm ranged from 1.19 to 3.25, suggesting the possible existence of BrC in biomass‐ and coal‐burning emissions. Based on the assumption that AAE_BC = 1.0, BrC from biomass burning contributed to 41–85% of the total particles light absorption at 370 nm, which is much higher than that from coal burning (15–18%). The estimated absorption emission factors of BrC at 370 nm for biomass and coal burning were 15–47 and 2–13 m²/kg, respectively. A 10 × 10 km gridded BrC optical emission inventory for biomass and coal burning in China for 2015 was developed based on the measured absorption emission factors of BrC values and high‐resolution activity data. The total annual BrC absorption cross section emissions from biomass and residential coal burning were 4,194 Gm² (relative uncertainty at the 95% confidence level of −33.2, 41.2%) and 615 Gm² (−39.3, 40.1%), respectively. These results should be useful for improving estimates of the radiative effects of BrC in China.

Restrictions on human activities were implemented in China to cope with the outbreak of the Coronavirus Disease 2019 (COVID-19), providing an opportunity to investigate the impacts of anthropogenic emissions on air quality. Intensive real-time measurements were made to compare primary emissions and secondary aerosol formation in Xi'an, China before and during the COVID-19 lockdown. Decreases in mass concentrations of particulate matter (PM) and its components were observed during the lockdown with reductions of 32-51%. The dominant contributor of PM was organic aerosol (OA), and results of a hybrid environmental receptor model indicated OA was composed of four primary OA (POA) factors (hydrocarbon-like OA (HOA), cooking OA (COA), biomass burning OA (BBOA), and coal combustion OA (CCOA)) and two oxygenated OA (OOA) factors (less-oxidized OOA (LO-OOA) and more-oxidized OOA (MO-OOA)). The mass concentrations of OA factors decreased from before to during the lockdown over a range of 17% to 58%, and they were affected by control measures and secondary processes. Correlations of secondary aerosols/ΔCO with Ox (NO2 + O3) and aerosol liquid water content indicated that photochemical oxidation had a greater effect on the formation of nitrate and two OOAs than sulfate; however, aqueous-phase reaction presented a more complex effect on secondary aerosols formation at different relative humidity condition. The formation efficiencies of secondary aerosols were enhanced during the lockdown as the increase of atmospheric oxidation capacity. Analyses of pollution episodes highlighted the importance of OA, especially the LO-OOA, for air pollution during the lockdown.

Black carbon (BC) aerosol has important effects on the climate and hydrology of the Tibetan Plateau (TP). An intensive measurement campaign was conducted at Lulang (∼ 3300 m a.s.l. – above sea level), southeastern TP, from September to October 2015, to investigate the sources and physicochemical characteristics of refractory BC (rBC) aerosol. The average rBC mass concentration was 0.31 ± 0.55 µg m⁻³, which is higher than most prior results for BC on the TP. A clear diurnal cycle in rBC showed high values in the morning and low values in the afternoon. A bivariate polar plot showed that rBC loadings varied with wind speed and direction, which also reflected the dominant transport direction. The estimated net surface rBC transport intensity was +0.05 ± 0.29 µg s⁻¹ m⁻², indicating stronger transport from outside the TP compared with its interior. Cluster analysis and a concentration-weighted trajectory model connected emissions from north India to the high rBC loadings, but the effects of internal TP sources should not be overlooked. The average mass median diameter (MMD) of rBC was 160 ± 23 nm, with smaller MMDs on rainy days (145 nm) compared with non-rainy days (164 nm). The average number fraction of thickly coated rBC (F_rBC) was 39 ± 8 %, and it increased with the O₃ mixing ratios from 10:00 to 14:00 LT, indicating that photochemical oxidation played a role in forming rBC coatings. The average rBC absorption enhancement (E_abs) was estimated to be 1.9, suggesting that light absorption by coated rBC particles was greater than for uncoated ones. The E_abs was strongly positively correlated with the F_rBC, indicating an amplification of light absorption for internally mixed rBC. For rBC cores < 170 nm, E_abs was negatively correlated with MMD, but it was nearly constant for rBC cores > 170 nm. Our study provides insight into the sources and evolution of rBC aerosol on the TP, and the results should be useful for improving models of the radiative effects of carbonaceous aerosols in this area.