Recently, realistic simulation of nitrous acid (HONO) based on theHONO / NO ratio of 0.02 was found to have a significant impact on theglobal budgets of HO (OH + HO) and gas phase oxidationproducts in polluted regions, especially in winter when other photolyticsources are of minor importance. It has been reported thatchemistry-transport models underestimate sulphate concentrations, mostlyduring winter. Here we show that simulating realistic HONO levels cansignificantly enhance aerosol sulphate (S(VI)) due to the increased formationof HSO. Even though in-cloud aqueous phase oxidation of dissolvedSO (S(IV)) is the main source of S(VI), it appears that HONO relatedenhancement of HO does not significantly affect sulphate becauseof the predominantly S(IV) limited conditions, except over eastern Asia.Nitrate is also increased via enhanced gaseous HNO formation andNO hydrolysis on aerosol particles. Ammonium nitrate is enhancedin ammonia-rich regions but not under ammonia-limited conditions.Furthermore, particle number concentrations are also higher, accompanied bythe transfer from hydrophobic to hydrophilic aerosol modes. This implies asignificant impact on the particle lifetime and cloud nucleating properties.The HONO induced enhancements of all species studied are relatively strong inwinter though negligible in summer. Simulating realistic HONO levels is foundto improve the model-measurement agreement of sulphate aerosols, mostapparent over the US. Our results underscore the importance of HONO for theatmospheric oxidizing capacity and corroborate the central role of cloudchemical processing in S(IV) formation.
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