No abstract

Secondary organic aerosol (SOA) constitutes a large fraction of organic aerosol worldwide, however, the formation mechanisms in polluted environments remain poorly understood. Here we observed fast daytime growth of oxygenated organic aerosol (OOA) (with formation rates up to 10 μg m −3 h −1 ) during low relative humidity (RH, daytime average 38 ± 19%), high RH (53 ± 19%), and fog periods (77 ± 13%, fog occurring during nighttime with RH reaching 100%). Evidence showed that photochemical aqueous-phase SOA (aqSOA) formation dominantly contributed to daytime OOA formation during the periods with nighttime fog, while both photochemical aqSOA and gas-phase SOA (gasSOA) formation were important during other periods with the former contributing more under high RH and the latter under low RH conditions, respectively. Compared to daytime photochemical aqSOA production, dark aqSOA formation was only observed during the fog period and contributed negligibly to the increase in OOA concentrations due to fog scavenging processes. The rapid daytime aging, as indicated by the rapid decrease in m,p-xylene/ethylbenzene ratios, promoted the daytime formation of precursors for aqSOA formation, e.g., carbonyls such as methylglyoxal. Photooxidants related to aqSOA formation such as OH radical and H 2 O 2 also bear fast daytime growth features even under low solar radiative conditions. The simultaneous increases in ultraviolet radiation, photooxidant, and aqSOA precursor levels worked together to promote the daytime photochemical aqSOA formation. We also found that biomass burning emissions can promote photochemical aqSOA formation by adding to the levels of aqueous-phase photooxidants and aqSOA precursors. Therefore, future mitigation of air pollution in a polluted environment would benefit from stricter control on biomass burning especially under high RH conditions.

Wintertime ammonium nitrate aerosol pollution is a severe air quality issue affecting both developed and rapidly urbanizing regions from Europe to East Asia. In the United States, it is acute in western basins subject to inversions that confine pollutants near the surface. Measurements and modeling of a wintertime pollution episode in Salt Lake Valley, Utah, demonstrate that ammonium nitrate is closely related to photochemical ozone through a common parameter, total odd oxygen, Ox,total. We show that the traditional nitrogen oxide and volatile organic compound (NOx‐VOC) framework for evaluating ozone mitigation strategies also applies to ammonium nitrate. Despite being nitrate‐limited, ammonium nitrate aerosol pollution in Salt Lake Valley is responsive to VOCs control and, counterintuitively, not initially responsive to NOx control. We demonstrate simultaneous nitrate limitation and NOx saturation and suggest this phenomenon may be general. This finding may identify an unrecognized control strategy to address a global public health issue in regions with severe winter aerosol pollution.