Evolution of atmospheric age of particles and its implications for the formation of a severe haze event in eastern China

Atmospheric age reflects how long particles have been suspended in the atmosphere, which is closely associated with the evolution of air pollutants. Severe regional haze events occur frequently in China, influencing air quality, human health, and regional climate. Previous studies have explored the characteristics of mass concentrations and compositions of fine particulate matter (PMinline-formula_2.5) during haze events, but the evolution of atmospheric age remains unclear. In this study, the age-resolved University of California, Davis/California Institute of Technology (UCD/CIT) model was developed and applied to simulate the concentration and age distribution of PMinline-formula_2.5 during a severe regional haze episode in eastern China. The results indicated that PMinline-formula_2.5 concentrations in the North China Plain (NCP) gradually accumulated due to stagnant weather conditions during the beginning stage of the haze event. Accordingly, the atmospheric age of elemental carbon (EC), primary organic aerosol (POA), sulfate (inline-formula $M4inlinescrollmathmlchem{\mathrm{normal\; SO}}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 29pt17ptsvg-formulamathimg544235742d8f4a0f97153436b699bab8 acp-23-10563-2023-ie00001.svg29pt17ptacp-23-10563-2023-ie00001.png ), and secondary organic aerosol (SOA) gradually increased. The subsequent PMinline-formula_2.5 concentration growth was driven by the local chemical formation of nitrate (inline-formula $M6inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimga33a7d42b70ca1fe513ac92c5832eec2 acp-23-10563-2023-ie00002.svg25pt16ptacp-23-10563-2023-ie00002.png ) under high relative humidity. The newly formed inline-formula $M7inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimga02883d0956e7dc256b9fe9fffa70b09 acp-23-10563-2023-ie00003.svg25pt16ptacp-23-10563-2023-ie00003.png particles led to a decrease in the mean atmospheric age of inline-formula $M8inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg8a872e45f44a0fc3c08e466e371cfb3a acp-23-10563-2023-ie00004.svg25pt16ptacp-23-10563-2023-ie00004.png particles. During the regional transport stage, aged particles from the NCP moved to the downwind Yangtze River Delta (YRD) region, leading to a sharp increase in PMinline-formula_2.5 concentrations and the average age of EC, POA, inline-formula $M10inlinescrollmathmlchem{\mathrm{normal\; SO}}_{\mathrm{normal\; 4}}^{\mathrm{normal\; 2}-}$ 29pt17ptsvg-formulamathimg8c898138530c760447165fe6cdc920bb acp-23-10563-2023-ie00005.svg29pt17ptacp-23-10563-2023-ie00005.png , and SOA in YRD. In contrast, the average age of inline-formula $M11inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimgeb51cd45ba2a21283d090226a04e61ba acp-23-10563-2023-ie00006.svg25pt16ptacp-23-10563-2023-ie00006.png and ammonium remained unchanged or even slightly decreased due to continuous local formation in the YRD region. Different evolution of the atmospheric age among these components provides a unique perspective on the formation of PMinline-formula_2.5 components during the regional haze event. The information can also be used for designing effective control strategies for different components of PMinline-formula_2.5.