A 1-D marine stratocumulus cloud model has been supplemented with a comprehensive and up-to-date aqueous phase chemical mechanism for the purpose of assessing the impact that the presence of clouds and aerosols has on gas phase HO x , NO x and O 3 budgets in the marine boundary layer. The simulations presented here indicate that 5 cloud may act as a heterogeneous source of HONO g via the conversion of HNO 4(g) at moderate pH (∼4.5). The photolysis of nitrate (NO − 3 ) has also been found to

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... ribute to this simulated increase in HONO g by ∼5% and also acts as a minor source of NO 2(g) . The effect of introducing deliquescent aerosol on the simulated increase of HONO g is negligible. The most important consequences of this elevation in HONO g 10 are that, in the presence of cloud, gas phase concentrations of NO x species increase by a factor of 2, which minimises the simulated decrease in O 3(g) , and results in a regeneration of OH g . This partly compensates for the removal of OH g by direct phase transfer into the cloud and has important implications regarding the oxidising capacity of the marine boundary layer. The findings presented here also suggest that previous 15 modelling studies, which neglect the heterogeneous HNO 4(g) reaction cycle, may have over-estimated the role of clouds as a sink for OH g and O 3(g) in unpolluted oceanic regions, by ∼ 10% and ∼ 2%, respectively. 25 and biogenic DMS and SO 2 . Many of these particles, which reside above the ocean, 278 ACPD 1Abstract Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion c EGS 2001 are hygroscopic in nature and become deliquescent at relative humidities above 60-70%. Turbulent mixing above the ocean results in the diffusion of the smallest size fraction of these particles (< 10µM) throughout the entire MBL. Once such particles enter supersaturated air (RH > 100%) they may become activated into cloud droplets, thus serving as cloud condensation nuclei (CCN). Although the volume fraction of the 5 aqueous phase is relatively small compared to that of the gas phase, the rates of chemical processes which occur on and in deliquescent aerosol and in cloud droplets can be much faster than the corresponding gas phase processes. Thus, heterogeneous production and loss of key gas phase oxidants may have an appreciable effect on the gas phase composition of the MBL, as demonstrated by a host of previous modelling 10 25 279 Abstract ACPD 1, 277-335, 2001 Abstract ACPD 1, 277-335, 2001 Abstract ACPD 1, 277-335, 2001 Abstract ACPD 1, 277-335, 2001 Abstract Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion c EGS 2001 important to note that cloud droplets nucleated on e.g. sea-salt or sulphate, retain their chemical identity, therefore allowing for size resolved chemistry. The chemistry-transport module describes the emission/dry deposition fluxes, vertical transport by turbulence, exchange between the gas and aqueous phase, the chemical processes in each phase and the chemical processes at the surface of the aqueous 5 Abstract ACPD 1, 2001 Abstract 25 the aerosol chemistry due to computational limitations, (vi) the equilibrium constant for CO 2 was described using a single equilibrium step (Lelieveld and Crutzen, 1991) (vii) the reaction of HNO 4 with HSO − 3 replaced the reaction of HONO aq with OH aq (see elec-287 Abstract ACPD 1, 277-335, 2001 Abstract ACPD 1, 277-335, 2001 Abstract Clouds, HO x and NO x in the MBL Abstract 1, 277-335, 2001 Abstract ACPD 1, 277-335, 2001 Abstract ACPD 1, 277-335, 2001 Abstract Clouds, HO x and NO x in the MBL Abstract ACPD 1, 277-335, 2001 Abstract Clouds, HO x and NO x in the MBL Abstract ACPD 1, 277-335, 2001 Abstract ACPD 1, 277-335, 2001 Abstract ACPD 1, 277-335, 2001 Abstract Clouds, HO x and NO x in the MBL Abstract 1, 277-335, 2001 Abstract Clouds, HO x and NO x in the MBL Abstract 1, 277-335, 2001 Abstract Clouds, HO x and NO x in the MBL Abstract 1, 277-335, 2001 Abstract ACPD 1, 277-335, 2001