Non‐Negligible Uptake of Nitrous Acid in Present‐Day Clouds

Abstract Cloudwater acidity has continuously decreased over the eastern United States due to anthropogenic emission control. Cloud uptake of weak acids, which is more effective under less acidity conditions, has attracted increasing interest. This study assessed how and to what extent nitrous acid (HONO) uptake impacted present‐day cloud chemistry. A gas‐cloudwater partitioning scheme with subsequent oxidation reactions for HONO was developed and implemented, and HONO heterogeneous chemistry was updated in the Community Multiscale Air Quality Modeling System (CMAQ). The modified CMAQ was employed to quantify the effect of HONO uptake on cloudwater acidity and acidity‐dependent chemical processes during the June 2021 Michigan‐Ontario Ozone Source Experiment campaign. Our model results indicated that HONO uptake could lower cloudwater pH, especially in clouds with pH > 5, by 0.1 or larger and produce non‐negligible changes in both total ion concentrations and ion composition. These results were corroborated for a cloud event by available measurements at the Whiteface Mountain (WFM) monitoring site in New York State. Simulations indicated cloud uptake of 10% of gaseous HONO (HONO (g) ) during the event, suggesting a potential sink for HONO (g) and likely resulting in a 0.1 unit decrease in cloudwater pH. Subsequently, ammonium (NH 4 + ) volatilization was suppressed, which reduced NH 4 + underestimation by up to 10%. In addition, simulated nitrite concentrations were averaged 10 μeq/L, in agreement with previous measurement studies. This study highlighted the importance of accurately representing weak acids in chemical transport models to improve understanding of present‐day cloud chemistry. Plain Language Summary Nitrous acid (HONO) is a weak acid and can be effectively taken up by less acidic clouds. It then breaks down into hydrogen ions and nitrite ions. Less acidic clouds have become more common in recent years over the eastern United States, mainly due to pollution controls that have reduced sulfur dioxide and nitrogen oxide emissions. This change may allow more gaseous HONO taken up by clouds, potentially lowering cloudwater pH and affecting cloud chemistry. Until now, these effects of HONO cloud uptake have not been looked into. In this work, we estimated HONO cloud uptake and its effects on today's cloud chemistry using an improved computer model. We found that HONO uptake could lower cloudwater pH by 0.1 units or more, with a meaningful influence on the amount and mix of ions, especially ammonium underestimation reduced by up to 10%. This study explains why it is important to include weak acids correctly in computer models that help us understand cloud chemistry. Key Points Uptake of nitrous acid (HONO) with subsequent in‐cloud oxidation and updated heterogenous reactions were implemented in CMAQ The improved CMAQ simulated 10% of HONO taken up by clouds, which could lower cloudwater pH by 0.1 and affect ion balance and composition The improved CMAQ reduced underestimation of in‐cloud ammonium cloud water loadings by up to 10%