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

Abstract. 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 the Filter Inlet for Gases and Aerosols (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 gas–particle partitioning of high-volatility (semi-volatile organic compounds + intermediate volatility organic compounds + volatile organic compounds, C∗>0.3 µg m−3) organic vapors. Using the equilibrium equation could underestimate the contribution of high-volatility organic vapors, since the volatility of these species in the particle phase was lower than that in the gas phase. We speculated that the elevated NOx concentration could suppress the formation of highly oxidized products, resulting in a smooth increase of low-volatility (extremely low volatility organic compounds + low volatility organic compounds, C∗≤0.3 µg m−3) organic vapors. Evidence has shown that urban pollutants (NOx and VOCs) could enhance the oxidizing capacity, while the elevated VOCs were mainly responsible for promoting daytime SOA formation by increasing the RO2 production rate. Our results highlight the important role of urban anthropogenic pollutants in SOA control in the suburban region.