Hydroxymethanesulfonate as a tracer for fog processing of individual aerosol particles

Hydroxymethanesulfonate (HMS), an important component in fog and cloud systems, is identified in the negative ion mass spectra of individual particles sampled with an aerosol time-of-flight mass spectrometer (ATOFMS). The peak assignment at mass/charge (m/z)–111 is confirmed by analyzing HMS particles produced in the laboratory. Individual particle mass spectra from a field campaign in Bakersfield, California reveal the presence of HMS in the ambient aerosol during isolated time periods near the dissipation phase of fog suggesting fog processing of the aerosol. The lifetime of HMS in the aerosol is limited to less than 12 h. Using the presence of HMS as a tracer for aqueous-phase fog processing, the size distribution indicates that particles with aerodynamic diameters greater than 0.7 μm are most affected by the fog and associated high relative humidity. HMS is predominantly associated with particles containing carbon, ammonium, sulfate, and nitrate. Examination of elemental carbon particles containing HMS shows the presence of organic carbon which most likely enhances the hygroscopicity of these particles. This study demonstrates the ability for real-time measurements of fog processing at the single particle level using a chemical tracer for aqueous-phase reactions and highlights the possibilities for using high temporal resolution measurements to gain unique insights into atmospheric processes.