Abstract
Abstract. In-cloud production of sulfate modifies aerosol size distribution, with important implications for the magnitude of indirect and direct aerosol cooling and the impact of SO2 emissions on the environment. We investigate which sulfate sources dominate the in-cloud addition of sulfate to different particle classes as an air parcel passes through an orographic cloud. Sulfate aerosol, SO2 and H2SO4 were collected upwind, in-cloud and downwind of an orographic cloud for three cloud measurement events during the Hill Cap Cloud Thuringia campaign in autumn 2010 (HCCT-2010). Combined SEM and NanoSIMS analysis of single particles allowed the δ34S of particulate sulfate to be resolved for particle size and type. The most important in-cloud SO2 oxidation pathway at HCCT-2010 was aqueous oxidation catalysed by transition metal ions (TMI catalysis), which was shown with single particle isotope analyses to occur primarily in cloud droplets nucleated on coarse mineral dust. In contrast, direct uptake of H2SO4 (g) and ultrafine particulate were the most important sources modifying fine mineral dust, increasing its hygroscopicity and facilitating activation. Sulfate addition to "mixed" particles (secondary organic and inorganic aerosol) and coated soot was dominated by in-cloud aqueous SO2 oxidation by H2O2 and direct uptake of H2SO4 (g) and ultrafine particle sulfate, depending on particle size mode and time of day. These results provide new insight into in-cloud sulfate production mechanisms, and show the importance of single particle measurements and models to accurately assess the environmental effects of cloud processing.
Highlights
Sulfate-containing atmospheric particles have a significant but uncertain climatic effect through their role in radiative forcing (IPCC, 2013)
Hill cap cloud measurements (FCE; “Full Cloud Event”) were taken when the following conditions were met: the liquid water content at Schmücke was > 0.1 g m−3, the wind direction was between 200◦ and 250◦, the wind speed was between 2 and 12 m s−1, the valley stations were free of fog and all sites were free of precipitation, the temperature was > 0◦C, and the local meteorological conditions were stable
The sulfur cycle observed during the HCCT-2010 campaign was complex, with different reactions responsible for adding sulfate to the different classes of particulate as they passed through the cloud (Fig. 6 and Table 5)
Summary
Sulfate-containing atmospheric particles have a significant but uncertain climatic effect through their role in radiative forcing (IPCC, 2013). Incloud SO2 oxidation and production of sulfate aerosol mass results in significant modification of the aerosol size distribution and particle hygroscopicity, which is important in controlling the lifetime and climatic effect of aerosol, health impacts, and the availability of trace metals. Scattering is most efficient for particles in the size range of 0.3–0.8 μm, the wavelength of visible light, sulfate produced in clouds on pre-existing particles has a greater direct aerosol effect than the ultrafine particles formed from gas-phase SO2 oxidation (Hegg, 1994). The indirect aerosol effect refers to the increase in cloud condensation nuclei (CCN) number concentration due to anthropogenic activities, which results in smaller, more numerous cloud droplets for the same liquid water content (LWC), Atmos. Changes in pH, hygroscopicity and other parameters are important for aerosol lifetime, health effects and trace metal availability (Nel, 2005; Pope and Dockery, 2006; Jickells et al, 2005)
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