Measurement report: Intensive biomass burning emissions and rapid nitrate formation drive severe haze formation in Sichuan basin, China: insights from aerosol mass spectrometry
Abstract. Haze pollution is a severe environmental problem, caused by elevation of fine particles (aerodynamic diameter < 2.5 μm, PM2.5), which is related to secondary aerosol formation, unfavourable synoptic conditions, regional transport, etc. The regional haze formation in basin areas, along with intensive emission of precursors, high relative humidity and poor dispersion conditions, is still limitedly understood.In this study, a field campaign was conducted to investigate the factors resulting in haze formation in Sichuan Basin (SCB) during winter in 2021. The fine aerosol chemical composition was characterised by using a time-of-flight aerosol chemical speciation monitor (ToF-ACSM) with the aim of inorganic and organic aerosol characterisation and source apportionment. The average concentration of non-refractory fine particles (NR-PM2.5) was 98.5 ± 38.7 μg/m3, and organics aerosols (OA), nitrate, sulphate, ammonium, and chloride occupied 40.3, 28.8, 10.6, 15.3 and 5.1 % of PM2.5. Three factors, including a hydrocarbon-like OA (HOA), a biomass burning OA (BBOA), and an oxygenated OA (OOA), were identified by applying the positive matrix factorisation (PMF) analysis, and they constituted 24.2, 24.2 and 51.6 % of OA on average, respectively. Nitrate formation was promoted by gas-phase and aqueous-phase oxidation, while sulphate was mainly formed through aqueous-phase. OOA showed strong dependence on Ox, demonstrating the contribution of photooxidation to OOA formation. OOA concentration increased as aerosol liquid water content (ALWC) increased within 200 μg/m3 and kept relatively constant when ALWC > 200 μg/m3, suggesting the insignificant effect of aqueous-phase reactions on OOA formation. Among the three haze episodes identified during the whole campaign, the driving factors were different: the first haze episode (H1) was driven by nitrate formation through photochemical and aqueous-phase reactions, and the second haze episode (H2) was mainly driven by the intense emission of primary organic aerosols from biomass burning and vehicle exhaust, while the third haze episode (H3) was mainly driven by reactions involving nitrate formation and biomass burning emission. HOA and BBOA were scavenged, while OOA, nitrate, and sulphate formation were enhanced by aqueous-phase reactions during fog periods, which resulted in the increase of O:C from pre-fog to post-fog periods. This study revealed the factors driving severe haze formation in SCB, and implied the benefit of controlling nitrate as well as intense biomass burning and vehicle exhaust emission to the mitigation of heavy aerosol pollution in this region.
