N2O5 reactive uptake kinetics and chlorine activation on authentic biomass-burning aerosol†
We examined the reactive uptake of dinitrogen pentoxide (N2O5) to authentic biomass-burning aerosol (BBA) using a small chamber reservoir in combination with an entrained aerosol flow tube. BBA was generated from four different fuel types and the reactivity of N2O5 was probed from 30 to 70% relative humidity (RH). The N2O5 reactive uptake coefficient, γ(N2O5), depended upon RH, fuel type, and to a lesser degree on aerosol chloride mass fractions. The γ(N2O5) ranged from 2.0 (±0.4) ×10−3 on black needlerush derived BBA at 30% RH to 6.0 (±0.6) ×10−3 on wiregrass derived BBA at 65% RH. Major N2O5 reaction products were observed including gaseous ClNO2 and HNO3 and particulate nitrate, and used to create a reactive nitrogen budget. Black needlerush BBA had the most particulate chloride, and the only measured ClNO2 yield > 1%. The ClNO2 yield on black needlerush decayed from an initial value of ∼100% to ∼30% over the course of the burn experiment, suggesting a depletion of BBA chloride over time. Black needlerush was also the only fuel for which the reactive nitrogen budget indicated other N-containing products were generated. Generally, the results suggest limited chloride availability for heterogeneous reaction for BBA in the RH range probed here, including BBA with chloride mass fractions on the higher end of previously reported values (∼17–34%). Though less than fresh sea spray aerosol, ∼50%. We use these measured quantities to discuss the implications for nocturnal aerosol nitrate formation, the chemical fate of N2O5(g), and the availability of particulate chloride for activation in biomass burning plumes.