The influence of cloud chemistry on HO_x and NO_x in the moderately polluted marine boundary layer: a 1-D modelling study

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 has on gas phaseHO_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. The conversion of HNO_4(g) at moderate pH (~ 4.5) is responsible for this, and, to a lesser extent, the photolysis of nitrate (NO₃^-). 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 may have important implications regarding the oxidising capacity of the marine boundary layer.