Observations of methane sulfonic acid in tropical tropospheric aerosol

Methane sulfonic acid (MSA) was observed in tropical tropospheric aerosol by the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument during the 2006 Costa Rica Aura Validation Experiment. Atmospheric MSA is an oxidation product of biogenic dimethyl sulfide emitted from the ocean's surface. Due to its low vapor pressure, MSA, like sulfuric acid, partitions efficiently to the condensed phase and can promote small particle growth. Maximum particulate MSA concentrations were observed in a layer from 7-12 km. Both MSA and sulfate constituents were markedly less common in tropospheric aerosol above 12 km. Historically, the ratio of particulate MSA to non-sea salt sulfate (NSS) has been used as a tracer for biogenic influence, although previous measurements have largely been limited to the boundary layer. Estimates of the MSA:NSS ratio based on single particle mass spectra increased with altitude to a maximum of ~0.3 in the mid-troposphere, a factor of 5-10 higher than values typical of tropical boundary layer. Possible sources of mid-tropospheric MSA and implications for tropical aerosol formation are discussed.