An Investigation into the Ionic Chemical Composition and Mixing State of Biomass Burning Particles Recorded During TRACE-P P3B Flight#10

In this study bulk airborne aerosol composition measured by the PILS-IC (integration time of 3 min 24 s) during TRACE-P P3B Flight 10 are used to investigate the ionic chemical composition and mixing state of biomass burning particles. A biomass burning plume, roughly 3–4 days old, moderately influenced by urban pollution aerosols recorded in the Philippine Sea is investigated. Focusing on the fine particle NO₃⁻, SO₄²⁻, K⁺, NH₄⁺, and water-soluble organics, the observed correlations and nearly 1-to-1 molar ratios between K⁺ and NO₃⁻ and between NH₄⁺ and (SO₄²⁻+ inferred Organics) suggest the presence of fine-mode KNO₃, (NH₄)₂SO₄, and NH₄(Organics) aerosols. Under the assumption that these ion pairs existed, and because KNO₃ is thermodynamically less favored than K₂SO₄ in a mixture of NO₃⁻, SO₄²⁻, K⁺, NH₄⁺, and Organic anions, the measurements suggest that aerosols could be composed of biomass burning particles (KNO₃) mixed to a large degree externally with the (NH₄)₂SO₄ aerosols. A “closed-mode” thermodynamic aerosol simulation predicts that a degree of external mixing (by SO₄²⁻ mass) of 60 to 100% is necessary to achieve the observed ionic associations in terms of the existence of KNO₃. However, the degree of external mixing is most likely larger than 90%, based on both the presence of KNO₃ and the amounts of NH₄NO₃. Calculations are also shown that the aerosol mixing state significantly impacts particle growth by water condensation/evaporation. In the case of P3B Flight #10, the internal mixture is generally more hygroscopic than the external mixture. This method for estimating particle mixing state from bulk aerosol data is less definitive than single particle analysis, but because the data are quantitative, it may provide a complementary method to single particle chemical analysis.