Aerosol liquid water driven by anthropogenic nitrate: implications for lifetimes of water-soluble organic gases and potential for secondary organic aerosol formation

Aerosol liquid water (ALW) is ubiquitous and important in many atmospheric processes. It is well known that ALW influences aerosol optical properties, visibility, and radiative impacts. A substantial body of research now highlights the role that ALW can play in the formation of secondary organic aerosol (SOA) through the partitioning of gas-phase water-soluble organic compounds (WSOCg) to the condensed phase and subsequent aqueous-phase reactions. A recent modeling study drew attention to the anthropogenic nature of ALW in the southeastern United States, where predicted ALW is driven by regional sulfate. This work presents field evidence of a mechanism by which ALW driven by anthropogenic ammonium nitrate can influence the lifetimes and concentrations of WSOCg and the potential for SOA formation. Aerosol composition and gas-phase glyoxal concentrations were measured at the San Pietro Capofiume (SPC) ground site in the Po Valley, Italy as part of the Pan-European Gas-Aerosols Climate Interactions Study. Hourly ALW mass concentrations were then calculated with two equilibrium thermodynamic models: (1) the Extended Aerosol Inorganics Model (E-AIM) model and (2) the Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model. Locally formed, anthropogenic ammonium nitrate was the main driver of variability in ALW concentrations at SPC and this ammonium nitrate resulted in a 42% enhancement, on average, in ALW concentrations at the SPC site as compared to a nitrate-poor, upwind site. Because the ammonium nitrate enhancement at SPC can be attributed to anthropogenic sources, we refer to the associated ALW as anthropogenic and controllable. We observed an exponential decrease in gas-phase glyoxal with increasing ALW, suggesting that this controllable ALW served as a sink for reactive organic gases. Notably, the glyoxal-ALW relationship varied with particle surface area, with the strongest relationship between ALW and glyoxal observed in the upper quartile of ALW. This adds support to previous work that has shown that both bulk and surface processes can be important determinants of the reactive uptake of glyoxal and SOA formation in wet aerosols. Finally, during a stagnation event, the partitioning potential of gas-phase glyoxal to ALW, which accounts for the temporal co-location of elevated ALW and glyoxal concentrations, explained a substantial fraction of variability in particle-phase WSOC (WSOCp) concentrations (R² = 0.57), suggesting that elevated WSOCp concentrations could be explained, in part, by the partitioning of WSOCg to ALW and subsequent aqueous-phase reactions. Ammonium nitrate is expected to increase in importance under future climate and emissions scenarios due to increased emissions of precursors, as well as policies aimed at reducing sulfur emissions. The impacts of increased particulate nitrate in future climate and air quality scenarios may be under predicted because they do not account for the increased potential for SOA formation in nitrate-derived ALW. The ongoing effort to understand the precursors, products, and mechanisms of SOA formation through aqueous chemistry is key to the incorporation of these processes into global climate models and will aid in elucidating the broader impacts of increases in ammonium nitrate and ALW.