Effects of nighttime heterogeneous reactions on the formation of secondary aerosols and ozone in the Pearl River Delta

Fine particulate matter (PM2.5) and ozone (O-3) are currently the most concerning air pollutants. Long-term exposure to PM2.5 and O-3 can induce cardiovascular diseases, presbyopia, and preterm birth. Acute exposure to PM2.5 and O-3 can also result in hypertension and decreased heart rate variability. In recent years, PM2.5 pollution abatement in China has achieved remarkable results, but O-3 pollution issues have emerged. In the troposphere, O-3 and PM2.5 have a close association, i.e., O-3 can oxidize gaseous precursors to form sulfate, nitrate, ammonium and secondary organic aerosols in PM2.5, and PM2.5 affects the generation and removal of O-3 by providing surfaces for heterogeneous reactions. Recent studies show that PM2.5 and O-3 have a significant positive correlation in the photochemically active season in the Pearl River Delta (PRD) region. However, it is still difficult to achieve cooperative control for PM2.5 and O-3 due to our limited knowledge of their coupling relationship. To date, most studies have focused on the photochemical mechanism in the daytime, and our understanding of the chemical processes at night is insufficient. As an important reactive intermediate, N2O5 mainly accumulates through the nocturnal reaction of NO2 and O-3 and is removed by heterogeneous reactions to produce HNO3. These processes directly affect ozone production and particulate matter formation. In addition to losing NOx, the heterogeneous reaction of N2O5 would lead to particulate nitrate formation and photolabile species (i.e., ClNO2) release. ClNO2 is a reservoir of NO2 and chlorine, providing a connection between nitrogen oxide pollution and halogen activation. The formation of ClNO2 represents an activation process of chlorine from the aerosol phase to the gas phase. ClNO2 photolysis releases chlorine radicals and oxidizes VOCs, which could increase radical levels and promote ozone formation. Moreover, NO2 released via ClNO2 photolysis also promotes O-3 formation in the morning. In coastal areas, sufficient particle chlorine in aerosols might be driven by sea salt chloride interactions, which lead to the formation of ClNO2. Therefore, with high concentrations of nitrogen oxides, nighttime heterogeneous N2O5 reactions would make a substantial contribution to PM2.5 and O-3 in coastal regions. Although several studies have shown that heterogeneous N2O5 reactions play an important role in the formation of PM2.5 and O3, related observations in China are limited. In this study, N2O5 and ClNO2 were measured in Shenzhen during the season with severe ozone pollution in October 2018. Iodine chemical ionization time-of-flight mass spectrometry (TOFCIMS, Aerodyne Inc.) was used to measure N2O5 and ClNO2. Based on collocated measurements of gas- and particle-phase pollutants, we quantitatively calculated the nighttime heterogeneous reactions and assessed their reactivities. The results show that the highest concentrations of N2O5 and ClNO2 could be up to 1524 and 477 pptv (5 min average, 1 pptv = 1 pptv=10(-3) mm(3)/m(3))), respectively, and they were affected by precursors. The concentrations of N2O5 and ClNO2 had pronounced peaks from sunset to sunrise. The nitrate formed via N2O5 reactions at night contributed 24%-60% of the total nighttime nitrate particles. In particular, the concentration of ClNO2 had a noticeable peak in the morning, and the photolysis of ClNO2 generated Cl radicals, which quickly removed alkanes, and the maximum removal efficiency was 2-3 times higher than that of OH radicals. In addition, the diurnal variation in VOCs with different reactivities also implies that chlorine radicals participate in the VOC oxidation cycle in the morning. This reflects that Cl radical-induced oxidation reactions could promote the generation of ozone and secondary organic aerosols through the HOx radical cycle. Our study shows that active nighttime heterogeneous reactions can significantly promote the formation of PM2.5 and O-3 in the coastal areas of the PRD region. Thus, it is urgent to call for more studies in the future to coordinate the control of PM2.5 and O-3 in the PRD region.