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

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 (NH₄⁺) volatilization was suppressed, which reduced NH₄⁺ 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.