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

Abstract

Inorganic salts are a significant component of biomassburning 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 ClNO 2 (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 N 2 O 5 (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.

Biomass burning and marine aerosol processing over the southeast Atlantic Ocean: a TEM single-particle analysis

“…Relative humidity at filter collection times according to back trajectories is in the 60 %-70 % range, which would facilitate aerosol deliquescence and the subsequent uptake of HCl. Previous work (Semeniuk et al, 2007;Jahn et al, 2021) has also shown BBA to uptake water in this RH range. Hydrochloric acid partitioning into aerosol water has also been inferred in urban atmospheres, where the highly water absorbing and soluble chloride in the aqueous phase enhances aerosol water uptake through co-condensation and particle growth, causing haze to form (Gunthe et al, 2021).…”

“…UV exposure can work to break down oligomers and lowvolatility components in organic aerosol (Wong et al, 2015;Lignell et al, 2014) and may account for lower amounts and/or higher volatility of organic aerosol present on CLAR-IFY filters. Photooxidation can also lead to fragmentation of organic chains, and oxidation has been observed to change biomass burning organic aerosol (BBOA) volatility in laboratory studies (Jahn et al, 2021) as well as physical properties (Jahl et al, 2021). Our results of less organic aerosol present for aged samples are consistent with the findings of Dobracki et al (2022) and Sedlacek III et al (2022) of loss of organic aerosol and organic coating with age, although TEM results are caveated by preferential loss of volatile organic material.…”

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

“…Previous studies found that organic coatings or surface films decrease the reactive uptake of N 2 O 5 on particles. ,,,, Transmission electron microscopy (TEM) analysis of BBA generated during separate experiments that utilized several of the biomass fuels on which we performed kinetics experiments revealed that many particles did indeed contain organic coatings that concealed inorganic salt phases (Figure ). We unfortunately could not obtain TEM/EDX compositional analysis of BBA from all of the fuels used in these kinetics experiments, however, given our extensive microscopy analysis on other BBA samples produced from similar fuels, and previous research on chloride salt phases in BBA, we expect similar core–shell and other complex particle morphologies to be present. ,, Such organic coatings prevalent in BBA would explain the low γ­(N 2 O 5 ) values even in high-chloride fuels in our previous work . However, conducting these experiments above the deliquescence relative humidities of chloride salts expected in BBA seems to allow for the presence of accessible aqueous chloride phases that drive N 2 O 5 hydrolysis.…”

“…These are typical of the types of organic aerosol coatings we observed in the BBA we studied and would limit the ability of N 2 O 5 to react with chloride salts. More details and analysis can be found in Jahn et al…”

“…TEM/EDX analysis of several other types of BBA revealed that soot and salts in the aerosol produced from some types of biomass fuels are coated in a stable organic coating (i.e., one that did not decompose during exposure to the electron beam or evaporate under vacuum), while others appeared to have more liquid-like organic coatings that partially evaporated under vacuum . The TEM/EDX analysis showed chloride displacement in one area of a collected chamber-aged BBA particle but not in another region of the same particle which may be due to differing organic coating thicknesses or other morphological features; see Jahn et al for further discussion . Additional microscopic analysis of the specific fuels used in this work would be needed to determine if such morphology plays a role in the observed differences in γ­(N 2 O 5 ) and φ­(ClNO 2 ) between the fuels and BBA studied.…”