Enhanced daytime secondary aerosol formation driven by gas-particle partitioning in downwind urban plumes

Anthropogenic emissions from city clusters can significantly enhance secondary organic aerosol (SOA) formation in the downwind regions, while the mechanism is poorly understood. To investigate the effect of pollutants within urban plumes on organic aerosol (OA) evolution, a field campaign was conducted at a downwind site of the Pearl River Delta region of China in the fall of 2019. A time-of-flight chemical ionization mass spectrometer coupled with a Filter Inlet for Gases and Aerosol (FIGAERO-CIMS) was used to probe the gas- and particle-phase molecular composition and thermograms of organic compounds.  For air masses influenced by urban pollution, strong daytime SOA formation through gas-particle partitioning was observed, resulting in higher OA volatility. The obvious SOA enhancement was mainly attributed to the equilibrium partitioning of non-condensable (10^0.5 μg m^-3) organic vapors. We speculated that the elevated NO_x concentration could suppress the formation of highly oxidized products, resulting in a smooth increase of condensable (10^0.5 μg m^-3) organic vapors. Evidence showed that urban pollutants (NO_x and VOCs) could promote daytime SOA formation by increasing the OH production rate. Our results highlight the important role of urban anthropogenic pollutants in SOA control in the suburban region.