Cloud-Aerosol Chemistry Observations at Whiteface Mountain: Organic Acids and the Growing Importance of Ammonium

Organic compounds are vital to atmospheric chemistry, with clouds playing a key role in their formation and transformation. Di-carboxylic organic anions, such as oxalate, act as tracers for aqueous-phase chemical processes. This study presents summer measurements of three organic acids (formic, acetic, oxalic), inorganic anions, and cations in cloud water, aerosol, and cloud droplet residual samples obtained 2018-2024 from the summit of Whiteface Mountain (WFM), a forested site in the Adirondack Mountains of northern New York State. Contributions of these acids to dissolved organic carbon (DOC), ion balance, and acidity are assessed in both cloud and aerosol samples. The current study builds on prior studies linking oxalate-to-DOC ratios with ozone concentrations, from which inferences have been made about biogenic volatile organic carbon (BVOC) contributions to secondary organic aerosol (SOA) formation, and we present new insights based on comparisons between cloud water and aerosol phases. We further expand upon the findings of Lawrence et al. (2023), which showed that more than half of the cloud water samples at WFM exhibit excess ammonium (i.e. exceeding sulfate plus nitrate concentrations) in recent years, by evaluating the relationship between excess ammonium and organic acids in both the cloud and aerosol phases.  These findings provide new insights into multi-phase chemistry and SOA formation processes at a remote forested site downwind of many natural and anthropogenic sources, and a site frequently influenced by wildfire smoke.