Photochemical Evolution of the 2013 California Rim Fire: Synergistic Impacts of Reactive Hydrocarbons and Enhanced Oxidants

Large wildfires markedly alter regional atmospheric composition, but chemical complexity challenges model predictions of downwind impacts. Here, we elucidate key facets of gas-phase photochemistry and assess novel chemical processes via a case study of the 2013 California Rim Fire plume. Airborne in situ observations, acquired during the NASA Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) mission, illustrate the evolution of volatile organic compounds (VOC), oxidants, and reactive nitrogen over 12 hours of atmospheric aging. Measurements show rapid formation of ozone and peroxyacyl nitrates (PNs), sustained peroxide production, and prolonged enhancements in oxygenated VOC and nitrogen oxides (NOx). Measurements and Lagrangian trajectories constrain a 0-D puff model that approximates plume photochemical history and provides a framework for evaluating key processes. Simulations examine the effects of 1) previously-unmeasured reactive VOC identified in recent laboratory studies, and 2) emissions and secondary production of nitrous acid (HONO). Inclusion of estimated unmeasured VOC leads to a 250 % increase in OH reactivity and a 70 % increase in radical production via oxygenated VOC photolysis. HONO amplifies radical cycling and serves as a downwind NOx source, although two different HONO production mechanisms (particulate nitrate photolysis and heterogeneous NO2 conversion) exhibit markedly different effects on ozone, NOx, and PNs. Analysis of radical initiation rates suggests that oxygenated VOC photolysis is a major radical source, exceeding HONO photolysis when averaged over the first 2 hours of aging. Ozone production chemistry transitions from VOC-sensitive to NOx-sensitive within the first hour of plume aging, with both peroxide and organic nitrate formation contributing significantly to radical termination. To simulate smoke plume chemistry accurately, models should simultaneously account for the full reactive VOC pool and all relevant oxidant sources.

Prediction of Secondary Organic Aerosol from the Multiphase Reaction of Gasoline Vapor by Using Volatility–Reactivity Base Lumping

The secondary organic aerosol (SOA) formation from photooxidation of gasoline vapor was simulated by using the UNIfied Partitioning Aerosol phase Reaction (UNIPAR) model, which predicted SOA growth via multiphase reactions of hydrocarbons. The Carbon Bond 6 (CB6r3) mechanism was incorporated with the SOA model to estimate the hydrocarbon consumption and the concentration of radicals (i.e., RO2 and HO2), which were closely related to atmospheric aging of gas products. Oxygenated products were lumped according to their volatilities and reactivity and linked to stoichiometric coefficients and their physicochemical parameters, which were dynamically constructed at different NOx levels and degrees of gas aging. To assess the gasoline SOA potential in ambient air, model parameters were corrected for gas–wall partitioning (GWP), which was predicted by a qualitative structure activity relationship for explicit products. The simulated gasoline SOA mass was evaluated against observed data obtained in the UF-APHOR chamber under ambient sunlight. The influence of environmental conditions on gasoline SOA was characterized under varying NOx levels, aerosol acidity, humidity, temperature, and concentrations of aqueous salts and gasoline vapor. Both the measured and simulated gasoline SOA formation was sensitive to seeded conditions (acidity and hygroscopicity) and NOx levels. A considerable difference in SOA mass appeared before and after efflorescence relative humidity in the presence of salted aqueous solution. SOA growth in the presence of aqueous reactions was more impacted by temperature than that in absence of seed. The impact of GWP on SOA formation was generally significant, and it appeared to be higher in the absence of wet salts. We conclude that the SOA model in the corpus with both heterogeneous reactions and the model parameters corrected for GWP is essential to accurately predict SOA mass in ambient air.

Particle-phase processing of α-pinene NO₃ secondary organic aerosol in the dark

The NO3 radical represents a significant night-time oxidant present in or downstream of polluted environments. There are studies that investigated the formation of secondary organic aerosol (SOA) from NO3 radicals focusing on yields, general composition, and hydrolysis of organonitrates. However, there is limited knowledge about how the composition of NO3-derived SOA evolves as a result of particle phase reactions. Here, SOA was formed from the reaction of α-pinene with NO3 radicals generated from N2O5, and the resulting SOA aged in the absence of external stimuli. The initial composition of NO3-derived α-pinene SOA was slightly dependent upon the concentration of N2O5 injected (excess of NO3 or excess of α-pinene), but was largely dominated by dimer dinitrates (C20H32N2O8-13). Oxidation reactions (e.g. C20H32N2O8 C20H32N2O9 C20H32N2O10 etc...) accounted for 60–70 % of the particle phase reactions observed. Fragmentation reactions and dimer degradation pathways made up the remainder of the particle-phase processes occurring. The exact oxidant is not known, though suggestions are offered (e.g. N2O5, organic peroxides, or peroxy-nitrates). Hydrolysis of −ONO2 functional groups was not an important loss term during dark aging under the relative humidity conditions of our experiments (58–62 %), and changes in the bulk organonitrate composition were likely driven by evaporation of highly nitrogenated molecules. Overall, 25–30 % of the particle-phase composition changes as a function of particle-phase reactions during dark aging representing an important atmospheric aging pathway.

Atmospheric aging enhances the ice nucleation ability of biomass-burning aerosol

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.

Vegetable waxes as modifiers of surface properties of polymeric composites

We investigated the possibility of a complex use of vegetable waxes as modifiers of surface properties of polymeric composites. The wax samples were extracted from vegetable raw materials and then introduced into the rubber formulations used for the manufacture of the sidewalls of passenger tires and pigmented paints as a dispersant additive. Physical-mechanical properties of the obtained compositions were determined. The introduction of the investigated wax into serial rubber formulations does not impair the technical characteristics of the mixtures. The investigated waxes ensure a good protection of the elastomeric compositions against atmospheric aging. The waxy substances can be effectively used as additives that increase the ability of inorganic pigments to dispersion. The results showed that the introduction of the wax increased the hardness of the film coatings of paint-and-lacquer materials, the gloss of the prepared coatings being not substantially decreased.

Proteome-wide effects of naphthalene-derived secondary organic aerosol in BEAS-2B cells are caused by short-lived unsaturated carbonyls

Exposure to air pollution causes adverse health outcomes, but the toxicity mechanisms remain unclear. Here, we investigated the dynamic toxicities of naphthalene-derived secondary organic aerosol (NSOA) in a human bronchial epithelial cell line (BEAS-2B) and identified the chemical components responsible for toxicities. The chemical composition of NSOA was found to vary with six simulated atmospheric aging conditions (C1–C6), as characterized by high-resolution mass spectrometry and ion mobility mass spectrometry. Global proteome profiling reveals dynamic evolution in toxicity: Stronger proteome-wide impacts were detected in fresh NSOA, but the effects declined along with atmospheric aging. While Nrf2-regulated proteins (e.g.,NQO1) were significantly up-regulated, the majority (78 to 97%) of proteins from inflammation and other pathways were down-regulated by NSOA exposure (e.g.,Rho GTPases). This pattern is distinct from the reactive oxygen species (ROS)-mediated toxicity pathway, and an alternative cysteine reaction pathway was revealed by the decreased abundance of proteins (e.g.,MT1X) prone to posttranslational thiol modification. This pathway was further validated by observing decreased Nrf2 response in reporter cells, after preincubating NSOA with cysteine. Ethynyl-naphthalene probe was employed to confirm the alkylation of cellular proteome thiols on the proteome-wide level by fresh NSOA via in-gel fluorescence imaging. Nontarget analysis identified several unsaturated carbonyls, including naphthoquinones and hydroxylated naphthoquinones, as the toxic components responsible for cysteine reactivity. Our study provides insights into the dynamic toxicities of NSOA during atmospheric aging and identifies short-lived unsaturated carbonyls as the predominant toxic components at the posttranslational level.

The effects of morphology, mobility size and SOA material coating on the ice nucleation activity of black carbon in the cirrus regime

There is evidence that black carbon (BC) particles may affect cirrus formation and hence global climate by acting as potential ice nucleating particles (INPs) in the troposphere. Nevertheless, the ice nucleation (IN) ability of bare BC and BC coated with secondary organic aerosol (SOA) material remains uncertain. We have systematically examined the IN ability of 100–400 nm size-selected BC particles with different morphologies and different SOA coatings representative of anthropogenic (toluene and n-dodecane) and biogenic (β-caryophyllene) sources in the cirrus regime (−46 to −38 °C). Several aerosolized BC proxies were selected to represent different particle morphologies and oxidation levels. Atmospheric aging was further replicated with exposure of SOA-coated BC to OH. The results demonstrate that the 400 nm hydrophobic BC types nucleate ice only at or near the homogeneous freezing threshold (−42 to −46 ˚C). Deposition IN, as opposed to purely homogeneous freezing, was observed to occur for some BC types between 100–200 nm within the investigated temperature range. More fractal BC particles did not consistently act as superior deposition INPs over more spherical ones. SOA coating generated by oxidizing β-caryophyllene with O3 did not seem to affect BC IN ability. However, SOA coatings generated from OH oxidation of various organic species did exhibit higher IN onset supersaturation ratio with respect to ice (SSi) compared with bare BC particles, with toluene SOA coating showing an increase of SSi by 0.1–0.15 while still below the homogeneous threshold. n-dodecane and β-caryophyllene-derived SOA only froze in the homogeneous regime. We attribute the inhibition of IN ability to the filling of the pores on the BC surface by the SOA material coating. OH exposure levels of all SOA coating experiments, from an equivalent atmospheric 10 days to 90 days, did not render significant differences in IN potential. Our study suggests that BC particles with large sizes and/or oxidized surfaces generally exhibit better IN ability, and that the organic coating materials can inhibit ice formation.