Contribution of methane sulfonate (MSA-) in optics
Methane sulfonic acid (MSA) condenses onto exisiting particles, mainly in the soluble accumulation and coarse modes (Saltzmann et al., JGR, 1983; Pzenny et al., J. Atmos. Chem., 1992). The contribution of the resulting particulate methanesulfonate (MSA-) can be taken into account in the optics calculations in a similar way as done for ammonium nitrate. In remote regions, especially outside the tropics, the contribution of MSA- to the total particulate sulfur burden cannot be neglected. It seems reasonable to use the refractive index of sulfuric acid (H2SO4) for MSA, as is done by Kinne et al. (2003). The similarity between the refractive index for MSA and H2SO4 is also discussed by Myhre et al. (Applied Optics, 2004).
Assuming all MSA condenses into the accumulation mode, the impact on the annual global mean aerosol optical depth (AOD) is estimated to be 7e-3. In absolute sense, the MSA contribution to the AOD reaches maximum values over the Southern Ocean (between 0.001 and 0.002 in the annual mean and between 0.002 and 0.005 in austral winter). In relative sense, the contribution maximizes at 5-10% over the Antarctic in austral summer (see figures).
#2 Updated by Twan van Noije over 6 years ago
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