Comment on acp-2022-830

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> To curb the spread of a novel coronavirus pandemic (COVID-19), a preventive lockdown (LCD) policy was first implemented across China in early 2020, resulting in a substantial drop-off in anthropogenic pollutant emissions and thus the amelioration of air quality. Unexpectedly, several haze events driven by enhanced secondary organic aerosols (SOA) still took place in the eastern China during the LCD. To investigate the effect of LCD measures on the formation and evolutionary process of SOA, PM_2.5 samples were collected before and during the LCD in Jinan, East China. The samples were measured for dicarboxylic acids (diacids) and related compounds, water-soluble inorganic ions, carbonaceous species, as well as the stable carbon isotopic compositions (&delta;¹³C) of major diacids. Our results show that despite the sharp decrease of primary pollutants (e.g., CO, SO₂, NO₂, and element carbon) during the LCD, the O₃ concentration, proportion of secondary inorganic aerosols, concentration levels, and relative abundance of diacid homologues in water-soluble organic compounds (WSOC) were still 2&ndash;4 times higher than those before the LCD. The ratios of oxalic acid (C₂) to diacids (C₂/diacids) and to total detected organic components were higher during the LCD than those before the LCD, suggesting the more aged organic aerosols during the LCD under the clearer sky conditions. The temporal changes, diurnal variations in major diacids, and their higher concentrations and contributions during the LCD than before the LCD are mainly due to the enhanced photochemical oxidation by the higher O₃ and the stronger solar radiation during the LCD. Interestingly, compound-specific stable carbon isotope ratios (&delta;¹³C) of C₂ and other major diacids show higher values in the nighttime than the daytime before the LCD, which indicate a significant contribution of organic acids via aqueous phase oxidation at night. Source apportionments using the molecular characteristics of organic compounds and positive matrix factorization (PMF) model suggest that the aqueous oxidation (45.2 %) and coal combustion (16.7 %) were the major sources before the LCD but the photochemical oxidation lunched by the higher O₃ concentration (48.8 %) and aqueous oxidation (17.7 %) were the dominant source during the LCD. The increased &delta;¹³C values of oxalic acid and other major organic acids along with the high ratios of C₂/Gly, C₂/mGly, and C₂/diacids before and during the LCD confirm an isotopic fractionation effect during the precursor oxidation processes. Furthermore, more positive &delta;¹³C values of diacids are observed in the daytime than the nighttime during the LCD, which suggest an enhanced photochemical oxidation in the urban atmosphere during this period.