Clouds, HOx and NOx in the MBL

Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion Abstract Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion Abstract 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 contribute 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 re-generation 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 overestimated the role of clouds as a sink for OH g and O 3(g) in unpolluted oceanic regions, by ∼ 10% and ∼ 2%, respectively.