Morphology of Organic Carbon Coatings on Biomass-Burning Particles and Their Role in Reactive Gas Uptake

Abstract

Inorganic salts are a significant component of biomass-burning aerosol (BBA) and have inconsistently been observed to undergo chemical reactions with strong acids and other reactants during atmospheric aging, altering particle hygroscopicity and further reactivity while also liberating reactive halides such as ClNO2(g) and HCl(g) and recycling or removing nitrogen oxides. The condensation of organic carbon to BBA coemitted by wildfires and other biomass combustion processes can affect aerosol particle reactivity with trace gases. These organic coatings along with deliquescence of chloride salts requiring high relative humidities >80% were recently proposed to explain the low observed reaction probability of N2O5(g) with BBA. We performed a series of single-particle analyses to characterize the morphology and composition of laboratory-generated BBA from authentic fuels using transmission and scanning electron microscopies (T/SEM) to test this hypothesis. Stable organic coatings that appear thicker or more oxidized than the particle bulk (likely tar balls) were observed to form on some spherical BBA particles but only when photooxidation was not applied. Inorganic salt components were inconsistently observed to react during simulated photooxidative atmospheric aging, sometimes undergoing chloride displacement reactions with strong acid vapors to produce sulfate and nitrate salts. Particles were also observed where chloride-salt regions were not completely depleted by reaction with strong acids. Organic carbon particle coatings plus the physical phase of chloride salts and deliquescence limitations appear to play a significant role in determining in which particles and fuel types these chloride displacement reactions can occur and the extent of these reactions with acidic vapors.