Impacts of springtime biomass burning in Southeast Asia on atmospheric carbonaceous components over the Beibu Gulf in China: Insights from aircraft observations

Limited by the scarcity of in situ vertical observation data, the influences of biomass burning in Southeast Asia on major atmospheric carbonaceous compositions in downwind regions have not been thoroughly studied. In this study, aircraft observations were performed to obtain high time-resolved in situ vertical distributions of black carbon (BC) as well as carbon monoxide (CO) and carbon dioxide (CO₂). Four types of profiles were revealed: Mode I (from 2000 to 3000 m, the BC, CO and CO₂ concentrations were enhanced), Mode II (with increasing altitude, the BC, CO and CO₂ concentrations almost decreased), Mode III (inhomogeneous vertical BC, CO and CO₂ profiles with BC peaks were observed from 2500 to 3000 m) and Mode IV (the BC, CO and CO₂ concentrations increased above 1500 m). Furthermore, simulations were conducted to calculate radiative forcing (RF) caused by BC and study the heating rate (HR) of BC in combination with the vertical BC profiles. A larger BC distribution in the atmosphere resulted in a sharp RF change from negative to positive values, imposing a nonnegligible influence on the atmospheric temperature profile, with maximum HR values ranging from 0.4 to 5.8 K/day. The values of the absorption Ångström exponent (AAE) were 1.46 ± 0.11 and 1.48 ± 0.17 at altitudes from 1000 to 2000 and 2000–3000 m, respectively. The average BC light absorption coefficient at the 370 nm wavelength (α _{BC (370)}) accounted for 50.3 %–76.8 % of the α ₍₃₇₀₎, while the brown carbon (BrC) light absorption coefficient at the 370 nm wavelength (α _{BrC (370)}) contributed 23.2 %–49.7 % to the α ₍₃₇₀₎ at altitudes of 1000–2000 m. At altitudes of 2000–3000 m, α _{BC (370)} and α _{BrC (370)} contributed 43.8 %–88.2 % and 11.8 %–56.2 % to the α ₍₃₇₀₎, respectively. These findings show that calculations that consider the surface BC concentration but ignore the vertical BC distribution could result in massive uncertainties in estimating the RF and HR caused by BC. This study helped achieve a deeper understanding of the influences of biomass burning over the region of Southeast Asia on the profiles of atmospheric carbonaceous compositions and atmospheric BC absorption and its warming effect.