Measurement report: The importance of biomass burning in light extinction and direct radiative effect of urban aerosol during the COVID-19 lockdown in Xi'an, China

Abstract

Abstract. Due to the complexity of emission sources, a better understanding of aerosol optical properties is required to mitigate climate change in China. Here, an intensive real-time measurement campaign was conducted in an urban area of China before and during the COVID-19 lockdown in order to explore the impacts of anthropogenic activities on aerosol light extinction and the direct radiative effect (DRE). The mean light extinction coefficient (bext) decreased from 774.7 ± 298.1 Mm−1 during the normal period to 544.3 ± 179.4 Mm−1 during the lockdown period. A generalised additive model analysis indicated that the large decline in bext (29.7 %) was due to sharp reductions in anthropogenic emissions. Chemical calculation of bext based on a ridge regression analysis showed that organic aerosol (OA) was the largest contributor to bext in both periods (45.1 %–61.4 %), and the contributions of two oxygenated OAs to bext increased by 3.0 %–14.6 % during the lockdown. A hybrid environmental receptor model combined with chemical and optical variables identified six sources of bext. It was found that bext from traffic-related emissions, coal combustion, fugitive dust, the nitrate and secondary OA (SOA) source, and the sulfate and SOA source decreased by 21.4 %–97.9 % in the lockdown, whereas bext from biomass burning increased by 27.1 %, mainly driven by the undiminished need for residential cooking and heating. An atmospheric radiative transfer model was further used to illustrate that biomass burning, rather than traffic-related emissions, became the largest positive effect (10.0 ± 10.9 W m−2) on aerosol DRE in the atmosphere during the lockdown. Our study provides insights into aerosol bext and DRE from anthropogenic sources, and the results imply the importance of controlling biomass burning for tackling climate change in China in the future.

High-time-resolution chemical composition and source apportionment of PM2.5 in northern Chinese cities: implications for policy

“…The secondary sources resolved by HERM are different among the three cities. In Xi'an and Shijiazhuang, this factor is characterized by high EV values for SO 2− 4 (62 %-75 %), NO − 3 (55 %-53 %), and NH + 4 (60 %-56 %) and a medium EV value for OA (23 %-29 %), which showed good correlations with SO 2− 4 (R 2 = 0.85-0.90) and NO − 3 (R 2 = 0.85-0.92) (Dai et al, 2020;Tian et al, 2022). In addition, the OA concentration of this factor was calculated by the EV value of OA, which was close to the secondary OA (SOA) concentration estimated by the BC-trace method (see Sect.…”

“…This is likely because biomass burning is an open source, which makes it more difficult to control compared to other primary sources. Biomass used for residential heating in rural areas is still common (Ren, 2021;Tian et al, 2022;. Hence, the clean-energy revolution should be promoted urgently, especially in northwestern China.…”

“…S1). Meanwhile, it is still unclear whether the actual causes of the pollution are topography, meteorological conditions, or local emissions (Chen et al, 2021;Tian et al, 2022;Wang et al, 2015;. In this study, we conducted intensive real-time observation of PM 2.5 chemical components in Xi'an, Shijiazhuang, and Beijing during wintertime.…”