Abstract
Abstract. Recently, realistic simulation of nitrous acid (HONO) based on the HONO / NOx ratio of 0.02 was found to have a significant impact on the global budgets of HOx (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.
Highlights
Despite improvements in the control strategies of air quality in urban areas during the last decades, reducing air pollution is still a major challenge due to the very complex chemical mechanisms and the large number of species emitted into the atmosphere
We focus on the HONO impacts during the boreal winter season, while the summer and/or annual results are presented in the Supplement
The enhancement of HONO leads to increased oxidation rates by OH leading to enhancements of O3, N2O5 and H2O2 and HNO3 and H2SO4 levels, especially during winter (Fig. 2) while in summer much smaller changes are simulated
Summary
Despite improvements in the control strategies of air quality in urban areas during the last decades, reducing air pollution is still a major challenge due to the very complex chemical mechanisms and the large number of species emitted into the atmosphere. Li et al (2010) found that HONO sources play an important role in the formation of secondary aerosols in Mexico City, substantially enhancing their concentrations by a factor of 2 on average in the morning They found that the simulated particle-phase nitrate and ammonium are substantially enhanced in the morning, though the effect on sulphate aerosol was much smaller, being in good agreement with the measurements. Elshorbany et al (2012b) have shown that HONO levels can be realistically parameterized, being about an order of magnitude higher compared to the reference simulations that consider the reaction of OH + NO as the sole HONO source They showed that HONO photolysis significantly enhances the gas phase mixing ratios of H2O2, HNO3, O3 and H2SO4, which play an important role in the formation of aerosols. We investigate the impact of HONO on the aerosol physical and chemical properties and compare the simulation results with measurement data from monitoring networks
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
