A CAPRAM Modeling Study on the Role of Heterogeneous Reactions on Dust in Tropospheric Chemistry

A heterogeneous dust chemistry module was developed and coupled toward the near-explicit mechanism MCM. Model simulations were performed for different dust concentrations (low, high, and very high dust), and the most significant changes were modeled in HNO₃, by 99% for very high dust, forming surface nitrates. Surface photolysis of nitrates resulted in an increase in HONO. Chemical rate analyses revealed that the direct uptake on dust played only a minor role for most species, except N₂O₅, HNO₃, H₂O₂, and SO₂. Average SO₂ oxidation rates of 3 μg m^–3 h^–1 and 0.3 μg m^–3 h^–1 were modeled for dust loads of 196 μg m^–3 and 19.6 μg m^–3, which are higher than the reported aqueous-phase oxidation rates. Sensitivity simulations considering the uptake of only one compound revealed that the N₂O₅ and HNO₃ uptake had the strongest effects. The uptake of only HNO₃ with surface photolysis resulted in major increases in NO_x, OH, and HONO by 20%, 36%, and 5110%, respectively. Further sensitivity simulations with higher uptake coefficients showed that the uptake of O₃ could act as an indirect source of HONO because of higher NO_x and OH concentrations. The minimum required uptake coefficient for direct effects to occur (γ_min) was also determined for the main inorganic species and had values of 10^–6 for O₃ and SO₂ and 10^–4 for NO₂, thus suggesting that direct uptake of NO₂ on dust is an unimportant sink under urban conditions. Overall, this study shows the importance of photoenhanced uptake and uptake coefficient values in heterogeneous chemistry.