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

Abstract

Abstract. To mitigate climate change in China, a better understanding of optical properties of aerosol is required due to the complexity in emission sources. Here, an intensive real-time measurement was conducted in an urban area of China before and during the lockdown of Coronavirus Disease 2019 (COVID-19), to explore the impacts of anthropogenic activities on aerosol light extinction and direct radiative effect (DRE). The mean light extinction coefficient (bext) reduced from 774.7 ± 298.1 Mm−1 during the normal period to 544.3 ± 179.4 Mm−1 during the lockdown period. The generalized addictive model analysis indicated that the large decline of bext (29.7 %) was entirely attributed to the sharp reductions in anthropogenic emissions. Chemical calculation of bext based on the ridge regression analysis showed that organic aerosol (OA) was the largest contributor to bext in both periods (45.1–61.4 %), and contributions of two oxygenated OAs to bext increased by 3.0–14.6 % during the lockdown. A hybrid environmental receptor model combining with chemical and optical variables identified six sources of bext. It was found that bext from traffic-related emission, coal combustion, fugitive dust, nitrate plus secondary OA (SOA) source, and sulfate plus SOA source decreased by 21.4–97.9 % in the lockdown, whereas bext from biomass burning increased by 27.1 % mainly driven by undiminished needs of residential cooking and heating. The atmospheric radiative transfer model was further used to illustrate that biomass burning instead of traffic-related emission 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 implied the importance of biomass burning for tackling climate change in China in the future.

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

“…Data availability. The data used to support the findings of this study are archived at the Institute of Earth Environment, Chinese Academy of Sciences: https://doi.org/10.5281/zenodo.6457841 (Tian et al, 2022).…”

“…The fact that b ext from traffic-related emissions showed the largest reduction (97.9 %) emphasised the effectiveness of controlling private gasoline cars and commercial and construction diesel trucks during the lockdown (Y. C. . For the two secondary sources, although previous studies have reported an enhancement of secondary aerosol formation efficiencies due to an increase in the atmospheric oxidation ca- pacity during the lockdown Le et al, 2020;Tian et al, 2021), the decreases in gas and organic precursors (e.g. NO 2 , SO 2 , and volatile organic compounds) led to 47.5 % and 21.4 % reductions in b ext from the nitrate and SOA and the sulfate and SOA sources respectively.…”

“…This could be explained by the increased mass diameter of OA with increasing OA oxidation level (Wang et al, 2021;. The MSE of MO-OOA was larger during the lockdown period than that during the normal period, probably re- lated to the enhanced atmospheric oxidation capacity during the lockdown period (Tian et al, 2021), which is found to result in a higher ageing state of OA with larger diameter (Hu et al, 2016;. The dependence of OA MAEs on the oxidation level presented a more complex trend.…”

“…All instruments were placed on the rooftop of an office building (∼ 10 m above the ground), approximately 30 m from the nearest road used by vehicular traffic. A detailed description of the sampling site can be found in Tian et al (2021). In this study, the sampling campaign consisted of two distinct periods: the normal period (1 to 23 January 2020) and the COVID-19 lockdown period (27 January to 7 February 2020).…”

“…The Q-ACSM measured concentrations of non-refractory species in PM 1 (NO − 3 , SO 2− 4 , NH + 4 , Cl − , and OA), and OA was further resolved into POA as well as less-oxidised and more-oxidised oxygenated OA (LO-OOA and MO-OOA respectively). Detailed information on the Q-ACSM data process and source apportionment of OA can be found in our previous paper (Tian et al, 2021). The Xact 625i quantified hourly element concentrations in PM 2.5 through X-ray fluorescence analysis, including Si, K, Ca, Cr, Mn, Fe, Zn, As, Se, Ba, Hg, and Pb.…”