Reactive Nitrogen Multiphase Chemistry and Chlorine Activation on Authentic Biomass Burning Aerosol

Abstract

Biomass burning is both a major source of particulate matter and reactive nitrogen oxide radicals, yet the rate and products of the multiphase chemistry between the two has not been well studied. Nitryl chloride (ClNO$_2$), formed by the multiphase reaction of dinitrogen pentoxide (N$_2$O$_5$) on chloride-containing particles, is known to occur in polluted marine environments where it contributes significantly to the oxidant budget\footnote{Finlayson-Pitts, B. et al. Nature 337, 241-244 (1989).}. Since the observation of ClNO$_2$ levels in a moderately polluted region far from the coast, there has been motivation to determine the source of particulate chloride, postulated as the limiting ingredient to ClNO$_2$ formation\footnote{Thornton, J. et al. Nature 464, 271-274 (2010).}. Using a smog chamber reactor at Carnegie Mellon University coupled to a biomass combustion chamber with controlled particle and gas injection, we simulated the nocturnal evolution of N$_2$O$_5$ in biomass burning plumes mixed with ozone. A range of authentic fuel types, including saw grass, cut grass, and white European birch were burned in the combustion chamber and a portion of the smoke plume was then mixed into the $12 m^3$ chamber, where it was exposed to 70-150 ppb of ozone and relative humidity ranging from 0-60\% in the dark. Gaseous N$_2$O$_5$, nitric acid (HNO$_3$), ClNO$_2$, Cl$_2$, and HCl were monitored using a high resolution time of flight iodide adduct chemical ionization mass spectrometer, and particle size distributions and composition were monitored continuously with an SMPS and a Soot-Particle Aerosol Mass Spectrometer among other instruments. With chemical mechanism modeling I attempt to abstract N$_2$O$_5$ uptake coefficients as well as molar yields of ClNO$_2$ and report correlated changes in Cl$_2$ and HCl. I analyze dependences on combustion fuel type. These results suggest a potentially important impact of chlorine atom initiated oxidation in biomass burning plumes. The uptake of N$_2$O$_5$ and yields of ClNO$_2$, Cl$_2$, and HCl determined from this study will allow for more robust parameterizations of these compounds in atmospheric models.