A study on the extent of neutralization of sulphate aerosol through laboratory and field experiments using an ATOFMS and a GPIC

Extent of neutralization (EoN) of atmospheric aerosol is an important parameter in understanding related nucleation mechanisms, acid-catalyzed reactions and gas–aerosol partitioning. Ion m/z   −195 (HSO4H2SO4−) detected by the Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) has been used as an indicator of incompletely-neutralized sulphate aerosol, but there are no laboratory data to support this assumption. In this study, experiments using artificially generated sulphuric acid nucleated aerosol and metal sulphate aerosol across a range of EoN found that the peak area ratio and hit ratio of ion m/z   −195 (HSO4H2SO4−) to ion m/z   −97 (HSO4−) detected by the ATOFMS increased with decreasing EoN. Area ratio and hit ratio are sensitive to EoN at the low and high value zones, respectively. In ambient air measured by the ATOFMS and a Gas Particle Ion Chromatograph (GPIC) in Toronto, Canada, ion m/z   −195 was always detected in ammonium sulphate aerosol, and its hit number and peak area varied widely, regardless of EoN indicated by the equivalent ratio of NH4+ to (SO42−+NO3−). Thus, ion m/z −195 alone is not an indicator of acidic sulphate aerosol. The combined approach using the ATOFMS and the GPIC found that cloud-processing formed incompletely-neutralized acidic sulphate aerosol in 2 out of 35 days sampled in winter in Toronto, Canada. It is interesting that the two episodes both occurred at night. Formation of incompletely-neutralized acidic sulphate aerosol caused a decrease in the concentration of particulate nitrate. This can be explained by acidic sulphate aerosol reacting with ammonium nitrate, leading to the release of HNO3 to the gas phase. It was also found that the GPIC results occasionally suffered a positive artifact of NH4+ concentration caused by the clogging-induced high back-pressure in the instrument.

► This work provides laboratory data on incompletely-neutralized sulphate aerosol and m/z −195 ion.
► Field measurements are used to evaluate reliability of using m/z −195 ion as a marker of acidity.
► This study improves reliability in interpreting atmospheric processes associated with acid aerosols.