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

Abstract. 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 (PM_2.5) during haze events, but the evolution of atmospheric age remains unclear. In this study, the age-resolved UCD/CIT model was developed and applied to simulate the concentration and age distribution of PM_2.5 during a severe regional haze episode in eastern China. The results indicated that PM_2.5 concentrations in the North China Plain (NCP) gradually accumulated due to stagnant weather conditions at the beginning stage of the haze event. Accordingly, the atmospheric age of elemental carbon (EC), primary organic aerosol (POA), sulfate (SO₄²⁻), and secondary organic aerosol (SOA) gradually increased. The subsequent PM_2.5 concentration growth was driven by the local chemical formation of nitrate (NO₃⁻) under high relative humidity. The newly formed NO₃⁻ particles led to a decrease in the mean atmospheric age of the NO₃⁻ 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 PM_2.5 concentrations and the average age of EC, POA, SO₄²⁻, and SOA. In contrast, the average age of NO₃⁻ 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 PM_2.5 components during the regional haze event. The information can also be used for designing effective control strategies for different components of PM_2.5.