Global and regional impacts of HONO on the chemical composition of clouds and aerosols

Abstract. Recently, realistic simulation of nitrous acid (HONO) based on the HONO / NO_x ratio of 0.02 was found to have a significant impact on the global budgets of HO_x (OH + HO₂) and gas phase oxidation products in polluted regions, especially in winter when other photolytic sources are of minor importance. It has been reported that chemistry-transport models underestimate sulphate concentrations, mostly during winter. Here we show that simulating realistic HONO levels can significantly enhance aerosol sulphate (S(VI)) due to the increased formation of H₂SO₄. Even though in-cloud aqueous phase oxidation of dissolved SO₂ (S(IV)) is the main source of S(VI), it appears that HONO related enhancement of H₂O₂ does not significantly affect sulphate because of the predominantly S(IV) limited conditions, except over eastern Asia. Nitrate is also increased via enhanced gaseous HNO₃ formation and N₂O₅ hydrolysis on aerosol particles. Ammonium nitrate is enhanced in ammonia-rich regions but not under ammonia-limited conditions. Furthermore, particle number concentrations are also higher, accompanied by the transfer from hydrophobic to hydrophilic aerosol modes. This implies a significant impact on the particle lifetime and cloud nucleating properties. The HONO induced enhancements of all species studied are relatively strong in winter though negligible in summer. Simulating realistic HONO levels is found to improve the model-measurement agreement of sulphate aerosols, most apparent over the US. Our results underscore the importance of HONO for the atmospheric oxidizing capacity and corroborate the central role of cloud chemical processing in S(IV) formation.