Response of the Reaction Probability of N2O5 with Authentic Biomass-Burning Aerosol to High Relative Humidity

N2O5 and ClNO2, important oxidant reservoirs, were recently demonstrated to be produced in simulated nocturnal aging of biomass-burning smoke. However, the heterogeneous kinetics of N2O5(g) reactive uptake, gamma(N2O5), and ClNO2(g) product yields, phi(ClNO2), are still under investigation. Our previous experiments on biomass-burning aerosol (BBA) revealed unexpectedly low and consistent N2O5 reaction probabilities despite often large chloride aerosol mass fractions. This could be explained by the inaccessibility of N2O5 to chloride due to the lack of chloride salt deliquescence or inhibition from organic coatings. In this work, an entrained aerosol flow tube system was deployed to examine the reaction probability of dinitrogen pentoxide and the nitryl chloride yield at 86% relative humidity (RH) for four types of BBA sampled from combustion emissions. At 86% RH, gamma(N2O5) ranged from 3.4 x 10(-3) on longleaf pine needle BBA to 16 x 10(-3) on black needlerush BBA with a 100-300% increase in gamma(N2O5) for high-chloride fuels and little change in low-chloride fuels compared to previous determinations of gamma(N2O5) at <75% RH. These trends demonstrate how aqueous chloride phases drive N2O5 reactive uptake and that organic coatings do not limit gamma(N2O5) in high-chloride fuels at high RH. phi(ClNO2) was substantial in experiments with high-chloride BBA, where phi(ClNO2) approached 100% at 86% RH. We conclude that the complex chemical composition and morphology of BBA along with the solid phase state of chloride salts in BBA at RH < similar to 80% limit the ability for N2O5 to heterogeneously react with BBA and produce ClNO2(g).

Automated summary

The study investigated the heterogeneous kinetics of N2O5(g) reactive uptake and ClNO2(g) product yields in biomass-burning smoke, revealing that high chloride fuels drive N2O5 reactive uptake and organic coatings do not limit the reaction in high-chloride fuels at high relative humidity. The experiments showed substantial nitryl chloride yield in high-chloride biomass-burning aerosol, approaching 100% at 86% relative humidity. The complex chemical composition and morphology of biomass-burning aerosol, as well as the solid phase state of chloride salts at relative humidity below 80%, limit the ability for N2O5 to react with the aerosol and produce ClNO2(g).