Abstract
Abstract. The potential for chemistry occurring in cloud droplets to impact atmospheric composition has been known for some time. However, the lack of direct observations and uncertainty in the magnitude of these reactions led to this area being overlooked in most chemistry transport models. Here we present observations from Mt Schmücke, Germany, of the HO2 radical made alongside a suite of cloud measurements. HO2 concentrations were depleted in-cloud by up to 90% with the rate of heterogeneous loss of HO2 to clouds necessary to bring model and measurements into agreement, demonstrating a dependence on droplet surface area and pH. This provides the first observationally derived assessment for the uptake coefficient of HO2 to cloud droplets and was found to be in good agreement with theoretically derived parameterisations. Global model simulations, including this cloud uptake, showed impacts on the oxidising capacity of the troposphere that depended critically on whether the HO2 uptake leads to production of H2O2 or H2O.
Highlights
Clouds occupy around 15 % of the volume of the lower troposphere and can impact atmospheric composition through changes in transport, photolysis, wet deposition and incloud oxidation of sulfur
The potential for chemistry occurring in cloud droplets to impact atmospheric composition has been known for some time
We present observations from Mt Schmücke, Germany, of the HO2 radical made alongside a suite of cloud measurements
Summary
Clouds occupy around 15 % of the volume of the lower troposphere and can impact atmospheric composition through changes in transport, photolysis, wet deposition and incloud oxidation of sulfur. The general consensus, until recently, was that these reactions would produce H2O2 (Jacob, 1996), but the significance of the reactions depends critically on whether this is the case or whether, instead, H2O is produced (Macintyre and Evans, 2011) This is significant, as H2O2 can photolyse to return odd hydrogen (HOx = OH + HO2) to the gas phase, whilst cloud uptake of HO2 to form H2O provides a terminal sink for HOx. Recent work by Mao et al (2013) postulates that a catalytic mechanism involving the coupling of the transition metal ions Cu(I)/Cu(II) and Fe(II)/Fe(III) may rapidly convert HO2 to H2O, rather than H2O2 in aqueous aerosols. This paper presents the impact of cloud droplets on measured gas-phase OH and HO2 and uses these observations to assess the proposed aqueous-phase mechanisms and determine the global impact of clouds on the tropospheric oxidising capacity
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