Organic Acids and Cloud Droplet Acidity in Recent Years at Whiteface Mountain, with Focus on Wildfire Smoke Influence

Abstract. Clouds facilitate the transformation of atmospheric gases and particles, yet the impact of cloud processing on organic compounds remains poorly understood due to the paucity of routine measurements within aqueous samples (e.g., cloud water and precipitation). This study presents seven years (2018–2024) of routine summertime measurements of three major Low Molecular Weight Organic Acids (LMWOA: formate, acetate and oxalate) in cloud water samples collected from the summit of Whiteface Mountain in the northeastern United States, including their contributions to Dissolved Organic Carbon (DOC), ion balance, and cloud water acidity, with critical evaluation of sample handling procedures to minimize volatilization, microbial degradation and contamination. Formate and acetate were the dominant monocarboxylic acids, exhibiting seasonal variability consistent with changes in biogenic emissions, whereas oxalate showed higher concentrations in smoke-impacted clouds. A growing fraction of samples exhibit surplus ammonium relative to sulfate and nitrate concentrations, which previous research hypothesized results from unmeasured organic acids, consistent with the positive relationship between surplus ammonium and LMWOA concentrations observed in the current study. An observed correlation between oxalate and DOC, with higher slope at higher ozone concentrations, supports enhanced in-cloud secondary production of oxalate under high oxidant levels. A comparison of "Aged" versus "Fresh" wildfire smoke-influenced samples suggest that aging can enhance ammonium, with heavier organic acids dominating the DOC pool and acidity, whereas fresh plumes were primarily influenced by directly emitted LMWOA. This study highlights the need for continued monitoring of the evolving cloud water chemistry to better understand the broader impacts on atmospheric chemistry.