Heterogeneous Oxidation of Biomass Burning Aerosol Surrogates by O3, NO2, NO3, and N2O5

Biomass burning is a major source of gases and particles to the atmosphere with a source strength of similar magnitude to fossil fuel burning. The particulate matter (PM) fraction of remote biomass burning events has been shown to significantly impact local air quality. Furthermore, biomass burning plumes can reach the upper troposphere and lower stratosphere (UT/LS). Consequently, biomass burning aerosol (BBA) can perturb atmospheric radiation directly through its effect on light extinction and indirectly by altering cloud properties. During transport, BBAs can react by gas-to-particle, termed heterogeneous, reactions with trace gases such as O3, NO2, NO3, and N2O5. It has been previously shown that high nighttime NO3, concentrations can render the NO3 radical a similar if not even more important oxidizer compared to OH. These oxidation reactions can result in the chemical transformation of the particles and thus significantly alter their physical and chemical properties. This in turn can have important implications for the particles’ role in cloud formation processes but will also impact the ability to apportion the source strength of BBAs if the molecular marker for biomass burning is altered during transport. Here we present a study employing a newly custom-built chemical ionization mass spectrometer coupled to a temperature-controlled rotating wall flow reactor to determine the heterogeneous kinetics between major organic compounds found in BBAs and O3, NO2, NO3. O3 is produced by passing O2 over an Hg lamp at 254 nm. N2O5 is produced by reacting an excess amount of O3 with NO2 and then stored at 193 K. NO3 is produced by thermal dissociation of N2O5. Detection of the reactant gases is achieved by using SF6- and I- as reagent ions. Our experimentally determined reactive uptake coefficients of O3, NO3, and N2O5 by oleic acid, and NO3 by unconjugated linoleic acid and n-hexadecane show agreement with previous studies. The major organic species determined in BBA particles is levoglucosan (1,6-anhydro-beta-glucopyranose), ≈50 wt.%, which also serves as molecular marker for biomass burning commonly applied in source apportionment studies. Employed in this study are thin films of levoglucosan and mixtures of levoglucosan/BBA compounds (e.g. substituted guaiacols, resin acids, and alkenoic acids) to act as surrogates for BBA surfaces. Levoglucosan shows negligible uptake of O3, however, the reactive uptake coefficient of NO3 is significantly greater, ≈10-4. This result suggests nighttime chemistry may have important implications on source apportionment of BBA. The atmospheric findings of our study are discussed further.