Biomass burning is a source of both particulate chloride and nitrogen oxides, two important precursors for the formation of nitryl chloride (ClNO), a source of atmospheric oxidants that is poorly prescribed in atmospheric models. We investigated the ability of biomass burning to produce NO(g) and ClNO(g) through nocturnal chemistry using authentic biomass-burning emissions in a smog chamber. There was a positive relationship between the amount of ClNO formed and the total amount of particulate chloride emitted and with the chloride fraction of nonrefractory particle mass. In every fuel tested, dinitrogen pentoxide (NO) formed quickly, following the addition of ozone to the smoke aerosol, and ClNO(g) production promptly followed. At atmospherically relevant relative humidities, the particulate chloride in the biomass-burning aerosol was rapidly but incompletely displaced, likely by the nitric acid produced largely by the heterogeneous uptake of NO(g). Despite this chloride acid displacement, the biomass-burning aerosol still converted on the order of 10% of reacted NO(g) into ClNO(g). These experiments directly confirm that biomass burning is a potentially significant source of atmospheric NO and ClNO to the atmosphere.

Ice nucleation and the resulting cloud glaciation are significant atmospheric processes that affect the evolution of clouds and their properties including radiative forcing and precipitation, yet the sources and properties of atmospheric ice nucleants are poorly constrained. Heterogeneous ice nucleation caused by ice-nucleating particles (INPs) enables cloud glaciation at temperatures above the homogeneous freezing regime that starts near −35 °C. Biomass burning is a significant global source of atmospheric particles and a highly variable and poorly understood source of INPs. The nature of these INPs and how they relate to the fuel composition and its combustion are critical gaps in our understanding of the effects of biomass burning on the environment and climate. Here we show that the combustion process transforms inorganic elements naturally present in the biomass (not soil or dust) to form potentially ice-active minerals in both the bottom ash and emitted aerosol particles. These particles possess ice-nucleation activities high enough to be relevant to mixed-phase clouds and are active over a wide temperature range, nucleating ice at up to −13 °C. Certain inorganic elements can thus serve as indicators to predict the production of ice nucleants from the fuel. Combustion-derived minerals are an important but understudied source of INPs in natural biomass-burning aerosol emissions in addition to lofted primary soil and dust particles. These discoveries and insights should advance the realistic incorporation of biomass-burning INPs into atmospheric cloud and climate models. These mineral components produced in biomass-burning aerosol should also be studied in relation to other atmospheric chemistry processes, such as facilitating multiphase chemical reactions and nutrient availability.

The reactive uptake kinetics of nitrogen pentoxide (N2O5) to authentic biomass-burning aerosol and the production of nitryl chloride (ClNO2) was determined using an entrained aerosol flow tube reactor.

Ice-nucleating particles (INPs) in biomass-burning aerosol (BBA) that affect cloud glaciation, microphysics, precipitation, and radiative forcing were recently found to be driven by the production of mineral phases. BBA experiences extensive chemical aging as the smoke plume dilutes, and we explored how this alters the ice activity of the smoke using simulated atmospheric aging of authentic BBA in a chamber reactor. Unexpectedly, atmospheric aging enhanced the ice activity for most types of fuels and aging schemes. The removal of organic carbon particle coatings that conceal the mineral-based ice-active sites by evaporation or oxidation then dissolution can increase the ice activity by greater than an order of magnitude. This represents a different framework for the evolution of INPs from biomass burning where BBA becomes more ice active as it dilutes and ages, making a larger contribution to the INP budget, resulting cloud microphysics, and climate forcing than is currently considered.

Prediction of ice formation in clouds presents one of the grand challenges in the atmospheric sciences. Immersion freezing initiated by ice-nucleating particles (INPs) is the dominant pathway of primary ice crystal formation in mixed-phase clouds, where supercooled water droplets and ice crystals coexist, with important implications for the hydrological cycle and climate. However, derivation of INP number concentrations from an ambient aerosol population in cloud-resolving and climate models remains highly uncertain. We conducted an aerosol-ice formation closure pilot study using a field-observational approach to evaluate the predictive capability of immersion freezing INPs. The closure study relies on co-located measurements of the ambient size-resolved and single-particle composition and INP number concentrations. The acquired particle data serve as input in several immersion freezing parameterizations, that are employed in cloud-resolving and climate models, for prediction of INP number concentrations. We discuss in detail one closure case study in which a front passed through the measurement site, resulting in a change of ambient particle and INP populations. We achieved closure in some circumstances within uncertainties, but we emphasize the need for freezing parameterization of potentially missing INP types and evaluation of the choice of parameterization to be employed. Overall, this closure pilot study aims to assess the level of parameter details and measurement strategies needed to achieve aerosol-ice formation closure. The closure approach is designed to accurately guide immersion freezing schemes in models, and ultimately identify the leading causes for climate model bias in INP predictions.

The morphology and composition of laboratory-generated biomass-burning aerosol (BBA) and bottom ash particles from authentic fuels were determined using transmission and scanning electron microscopies (TEM/SEM) and single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-ICP-ToF-MS). BBA particles with mineral material identified through elemental analysis using SEM represented 3−25% of the individual BBA particle numbers analyzed. This percentage varied depending on the fuel, with BBA from grass fuels containing more mineral particles than BBA from ponderosa pine wood. TEM analysis showed that these particles typically consist of carbonaceous material and a small (50−500 nm) region rich in nonvolatile elements. We also performed SEM/EDX analysis on soil, mineral BBA, and ash particles to measure Si:Al:Fe ratios and show that each of these particle classes possesses a different average bulk composition. However, individual particles within each population possess varying Si:Al:Fe ratios that may not be sufficiently unique to consistently determine particle sources from single-particle analysis. Mineral regions in BBA particles were similar in composition to residual bottom ash particles but were more likely to contain mixtures of nonvolatile elements, suggesting that ash particles typically underwent more complete combustion and mineralization. Multielement sp-ICP-ToF-MS analysis confirmed the presence of mineral particles in BBA, ash, and soil samples, with the most prevalent elements being the common crustal elements Al, Si, and Fe. Zn-and Ti-bearing particles were identified in both ash and soil samples, with more Zn present in ash particles and more Ti present in soil particles, suggesting that both of these types of particles would be prevalent in ambient measurements of BBA and biomass-burning impacted air masses. The mineral phases present in combustion-derived mineral phases are likely distinct from those present in soil-derived particles and may significantly affect the bulk properties of biomass-burning smoke. Both mineral BBA particles and lofted ash are likely sources of bioavailable iron and phosphorus that have been measured in biomass-burning emissions. These combustion-generated mineral phases are also important sources of ice-nucleating particles that have recently been reported in biomass-burning aerosol and bottom ash.

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, γ­(N2O5), and ClNO2(g) product yields, φ­(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, γ­(N2O5) ranged from 3.4 × 10–3 on longleaf pine needle BBA to 16 × 10–3 on black needlerush BBA with a 100–300% increase in γ­(N2O5) for high-chloride fuels and little change in low-chloride fuels compared to previous determinations of γ­(N2O5) at <75% RH. These trends demonstrate how aqueous chloride phases drive N2O5 reactive uptake and that organic coatings do not limit γ­(N2O5) in high-chloride fuels at high RH. φ­(ClNO2) was substantial in experiments with high-chloride BBA, where φ­(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 < ∼80% limit the ability for N2O5 to heterogeneously react with BBA and produce ClNO2(g).

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.