Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 1 2 2001 10.5194/acpd-1-277-2001 http://www.atmos-chem-phys-discuss.net/1/277/2001/ http://www.atmos-chem-phys-discuss.net/1/277/2001/acpd-1-277-2001.html http://www.atmos-chem-phys-discuss.net/1/277/2001/acpd-1-277-2001.pdf 277 335 2001-10-25 The influence of cloud chemistry on HO_x and NO_x in the Marine Boundary Layer: a 1-D modelling study J. E. Williams F. J. Dentener A. R. van den Berg IMAU, University of Utrecht, Utrecht, The Netherlands Current Address: FOM-AMOLF, Kruislaan 107, Amsterdam, The Netherlands Joint Research Center, Environment Institute, Ispra(Va), Italy 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 O₃ budgets in the marine boundary layer. The simulations presented here indicate that 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 (NO₃^-) 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 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 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.