Quantification of the enhancement of PM concentration by the downward transport of ozone from the stratosphere.

The downward transport of ozone (O) stemming from the stratosphere-to-troposphere exchange (STE) can be a significant contributor to background O. Such enhancement of background O may also influence ground-level PM, particularly in polluted regions which have abundant precursor emissions. In this study, we quantified the STE impact on tropospheric O and its subsequent influence on surface PM across the northern hemisphere. The sensitivity analyses was conducted by using a comprehensive hemispheric atmospheric modeling system. Results suggest the surface PM concentration can be considerably enhanced by the STE in polluted regions including East China, East US, and Europe, mostly in winter and spring. In winter, the PM concentrations in East China, East US, and Europe are estimated to be enhanced by 1.3%, 3.5% and 5.5% due to the STE. The STE-enhanced PM concentrations are typically higher on high pollution days suggesting likely increasing contributions in regions with the growing pollution levels. During the heavy polluted days, the PM concentrations in East China can be enhanced by 2.289 μg/m in winter and 2.034 μg/m in spring due to the STE. The STE-enhanced PM also exhibits strong diurnal variations following a pattern similar to the total PM concentration, with high increasing ratio in the morning and low at afternoon, suggesting that the enhancement is most pronounced during peak pollution events. The STE-enhanced PM is exclusively contributed by the increase of nitrate, ammonium, and secondary organic aerosol which in-turn are strongly influenced by the atmospheric oxidation capacity.