The influence of cloud chemistry on HOx and NOx in the Marine Boundary Layer: a 1-D modelling study
1IMAU, University of Utrecht, Utrecht, The Netherlands
2Current Address: FOM-AMOLF, Kruislaan 107, Amsterdam, The Netherlands
3Joint Research Center, Environment Institute, Ispra(Va), Italy
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 HOx, NOx and O3 budgets in the marine boundary layer. The simulations presented here indicate that cloud may act as a heterogeneous source of HONOg via the conversion of HNO4(g) at moderate pH (~4.5). The photolysis of nitrate (NO3-) has also been found to contribute to this simulated increase in HONOg by ~5% and also acts as a minor source of NO2(g). The effect of introducing deliquescent aerosol on the simulated increase of HONOg is negligible. The most important consequences of this elevation in HONOg are that, in the presence of cloud, gas phase concentrations of NOx species increase by a factor of 2, which minimises the simulated decrease in O3(g), and results in a regeneration of OHg. This partly compensates for the removal of OHg 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 modelling studies, which neglect the heterogeneous HNO4(g) reaction cycle, may have over-estimated the role of clouds as a sink for OHg and O3(g)in unpolluted oceanic regions, by ~10% and ~2%, respectively.