ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-2-39-2002 The influence of cloud chemistry on HO<sub>x</sub> and NO<sub>x</sub> in the moderately polluted marine boundary layer: a 1-D modelling study Williams J. E. 1 2 Dentener F. J. 3 van den Berg A. R. 1 IMAU, University of Utrecht, Utrecht, The Netherlands Current Address: FOM-AMOLF, Kruislaan 107, Amsterdam, The Netherlands Joint Research Center, Environment Institute, Ispra(Va), Italy 05 02 2002 2 1 39 54 Copyright: © 2002 J. E. Williams et al. 2002 This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Generic License. To view a copy of this licence, visit https://creativecommons.org/licenses/by-nc-sa/2.5/ This article is available from https://acp.copernicus.org/articles/2/39/2002/acp-2-39-2002.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/2/39/2002/acp-2-39-2002.pdf

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 has on gas phaseHO<sub>x</sub>, NO<sub>x</sub> and O<sub>3</sub> budgets in the marine boundary layer. The simulations presented here indicate that cloud may act as a heterogeneous source of HONO<sub>g</sub>. The conversion of HNO<sub>4(g)</sub> at moderate pH (~ 4.5) is responsible for this, and, to a lesser extent, the photolysis of nitrate (NO<sub>3</sub><sup>-</sup>). The effect of introducing deliquescent aerosol on the simulated increase of HONO<sub>g</sub> is negligible. The most important consequences of this elevation in HONO<sub>g</sub> are that, in the presence of cloud, gas phase concentrations of NO<sub>x</sub> species increase by a factor of 2, which minimises the simulated decrease in O<sub>3(g)</sub>, and results in a regeneration of OH<sub>g</sub>. This partly compensates for the removal of OH<sub>g</sub> by direct phase transfer into the cloud and may have important implications regarding the oxidising capacity of the marine boundary layer.