Isotope modeling of nitric acid formation in the atmosphere using ISO-RACM: testing the importance of NO oxidation, heterogeneous reactions, and trace gas chemistry

Abstract. Here we present ISO-RACM, an isotope mass balance model that utilizes the Regional Atmospheric Chemistry Mechanism to predict Δ¹⁷O values in atmospheric nitrate. A large number of simulations were carried out that varied atmospheric parameters that are important in altering the magnitude and range of Δ¹⁷O values generated in photochemically produce nitrate. These parameters included temperature, relative humidity, actinic flux, aerosol surface area and chemical speciation, and three different N₂O₅ uptake parameterizations. Trace gas mixing ratios were also varied including CH₄, CO, NO_x, O₃, volatile organic compounds and biogenic organic compounds. The model predicts that there are seasonal, latitudinal and diurnal variations in Δ¹⁷O values due to changes in actinic flux with lower values corresponding to higher actinic fluxes. There was also a minor positive correlation between higher Δ¹⁷O values and increased temperature. There were distinct differences in Δ¹⁷O depending on which N₂O₅ parameterization was used, mostly the result of changing relative humidity being a factor in two of the parameterization schemes. Changing CO and CH₄ mixing ratios had negligible impact on Δ¹⁷O values but significant variation in magnitude and range were predicted with NO_x, O₃, and organic loading. High NO_x and O₃ generated high Δ¹⁷O with a narrow (10 ‰) range, while high organics led to low Δ¹⁷O values and a wider range of possible values. Implications for using Δ¹⁷O to evaluate NO_x-NO_y chemistry and aerosol formation processes are discussed, as is needed future research.