Aerosol Ion Characteristics During the Big Bend Regional Aerosol and Visibility Observational Study

摘要

The ionic compositions of particulate matter with aerodynamic diameter ≤2.5 μm (PM_(2.5)) and size-resolved aerosol particles were measured in Big Bend National Park, Texas, during the 1999 Big Bend Regional Aerosol and Visibility Observational study. The ionic composition of PM_(2.5) aerosol was dominated by sulfate (SO_4~(2-)) and ammonium (NH_4~+). Daily average SO_4~(2-) and NH_4~+ concentrations were strongly correlated (R~2 = 0.94). The molar ratio of NH_4~+ to SO_4~(2-) averaged 1.54, consistent with concurrent measurements of aerosol acidity. The aerosol was observed to be comprised of a submicron fine mode consisting primarily of ammoniated SO_4~(2-) and a coarse particle mode containing nitrate (NO_3~-). The NO_3~- appears to be primarily associated with sea salt particles where chloride has been replaced by NO_3~-, although formation of calcium nitrate (Ca(NO_3)_2) is important, too, on several days. Size-resolved aerosol composition results reveal that a size cut in particulate matter with aerodynamic diameter ≤1 μm would have provided a much better separation of fine and coarse aerosol modes than the standard PM_(2.5) size cut utilized for the study. Although considerable nitric acid exists in the gas phase at Big Bend, the aerosol is sufficiently acidic and temperatures sufficiently high that even significant future reductions in PM_(2.5) SO_4~(2-) are unlikely to be offset by formation of particulate ammonium nitrate in summer or fall.