Nitric acid and the origin and size segregation of aerosol nitrate aloft during BRACE 2002

摘要

As part of the BRACE 2002 May field intensive, the NOAA Twin Otter flew 21 missions over terrestrial, marine, and mixed terrestrial and marine sites in the greater Tampa, Florida, airshed including over Tampa Bay and the Gulf of Mexico. Aerosols were collected with filter packs and their inorganic fractions analyzed post hoc with ion chromatography. Anion mass dominated both the fine- (particle diameters ≤ 2.5 μm) and coarse-mode (particle diameters 10.0-2.5 μm) inorganic fractions: SO_4~(2-)in the fine fraction, 3.7 μgm~(-3) on average and Cl~- and NO_3~- in the coarse fraction, 0.6 μg m~(-3) on average and 1.4 μgm~(-3) on average, respectively. Ammonium ion dominated the inorganic fine-mode cation mass, averaging 1.2 μg m~(-3), presumably in association with SO_4~2. Coarse-mode cation mass was dominated by Na~+, but the concentrations of Ca~(2+) and K~+ together often equaled or exceeded the Na~+ mass which was, on average, 0.6 μgm~(-3). Nitrate appeared predominantly in the coarse rather than the fine fraction, as expected, and the fine fraction never contributed > 15% of the total NO_3 concentration. Nitric acid dominated the NO_3~- contribution from both aerosol size fractions, and constituted at least 45% of the total NO_3 in all samples. Coarse-mode Cl~- depletion, and hence NO_3~-replacement, reached 100% within the first 4h of plume travel from the urban core in some samples, although it was most often less than 100% and slightly below the expected 1:1 ratio with coarse-mode NO_3~- concentration: the slope of the regression line of NO_3~- concentration to Cl~- depletion was 0.9 in the coarse fraction. In addition, terrestrial samples were markedly lower in Cl~- depletion, and thus in substituted NO_3~-, than were marine and mixed samples: 15-25% depletion in terrestrial samples vs. 50-65% in marine samples with the same air mass age. Thus, we conclude that NO_3~- and its progenitor compound HNO_3 were present in the Tampa airshed in insufficient amounts to titrate fully the slightly alkaline coarse-mode particles there, and to replace completely the Cl~- from the coarse-mode NaCl.