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Abstract. To better understand the effects of wildfires on air quality andclimate, it is important to assess the occurrence of chromophoric compoundsin smoke and characterize their optical properties. This study explores themolecular composition of light-absorbing organic aerosol, or brown carbon(BrC), sampled at the Missoula Fire Sciences laboratory as a part of theFIREX Fall 2016 lab intensive. A total of 12 biomass fuels from different planttypes were tested, including gymnosperm (coniferous) and angiosperm(flowering) plants and different ecosystem components such as duff, litter,and canopy. Emitted biomass burning organic aerosol (BBOA) particles werecollected onto Teflon filters and analyzed offline using high-performanceliquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer(HPLC–PDA–HRMS). Separated BrC chromophores were classified by theirretention times, absorption spectra, integrated absorbance in the near-UVand visible spectral range (300–700 nm), and chemical formulas from theaccurate m∕z measurements. BrC chromophores were grouped into the followingclasses and subclasses: lignin-derived products, which include lignin pyrolysisproducts; distillation products, which include coumarins and flavonoids;nitroaromatics; and polycyclic aromatic hydrocarbons (PAHs). The observedclasses and subclasses were common across most fuel types, although specific BrCchromophores varied based on plant type (gymnosperm or angiosperm) andecosystem component(s) burned. To study the stability of the observed BrCcompounds with respect to photodegradation, BBOA particle samples wereirradiated directly on filters with near UV (300–400 nm) radiation, followedby extraction and HPLC–PDA–HRMS analysis. Lifetimes of individual BrCchromophores depended on the fuel type and the corresponding combustioncondition. Lignin-derived and flavonoid classes of BrC generally hadthe longest lifetimes with respect to UV photodegradation. Moreover,lifetimes for the same type of BrC chromophores varied depending on biomassfuel and combustion conditions. While individual BrC chromophoresdisappeared on a timescale of several days, the overall light absorption bythe sample persisted longer, presumably because the condensed-phasephotochemical processes converted one set of chromophores into anotherwithout complete photobleaching or from undetected BrC chromophores thatphotobleached more slowly. To model the effect of BrC on climate, it isimportant to understand the change in the overall absorption coefficientwith time. We measured the equivalent atmospheric lifetimes of the overallBrC absorption coefficient, which ranged from 10 to 41 d, with subalpinefir having the shortest lifetime and conifer canopies, i.e., juniper, havingthe longest lifetime. BrC emitted from biomass fuel loads encompassingmultiple ecosystem components (litter, shrub, canopy) had absorptionlifetimes on the lower end of the range. These results indicate thatphotobleaching of BBOA by condensed-phase photochemistry isrelatively slow. Competing chemical aging mechanisms, such as heterogeneousoxidation by OH, may be more important for controlling the rate of BrCphotobleaching in BBOA. 

Abstract. Chamber oxidation experiments conducted at the Fire Sciences Laboratory in 2016 are evaluated to identify important chemical processes contributing to the hydroxy radical (OH) chemistry of biomass burning non-methane organic gases (NMOGs). Based on the decay of primary carbon measured by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS), it is confirmed that furans and oxygenated aromatics are among the NMOGs emitted from western United States fuel types with the highest reactivities towards OH. The oxidation processes and formation of secondary NMOG masses measured by PTR-ToF-MS and iodide-clustering time-of-flight chemical ionization mass spectrometry (I-CIMS) is interpreted using a box model employing a modified version of the Master Chemical Mechanism (v. 3.3.1) that includes the OH oxidation of furan, 2-methylfuran, 2,5-dimethylfuran, furfural, 5-methylfurfural, and guaiacol. The model supports the assignment of major PTR-ToF-MS and I-CIMS signals to a series of anhydrides and hydroxy furanones formed primarily through furan chemistry. This mechanism is applied to a Lagrangian box model used previously to model a real biomass burning plume. The customized mechanism reproduces the decay of furans and oxygenated aromatics and the formation of secondary NMOGs, such as maleic anhydride. Based on model simulations conducted with and without furans, it is estimated that furans contributed up to 10 % of ozone and over 90 % of maleic anhydride formed within the first 4 h of oxidation. It is shown that maleic anhydride is present in a biomass burning plume transported over several days, which demonstrates the utility of anhydrides as markers for aged biomass burning plumes. 

Abstract. Anthropogenic secondary organic aerosol (ASOA), formed from anthropogenicemissions of organic compounds, constitutes a substantial fraction of themass of submicron aerosol in populated areas around the world andcontributes to poor air quality and premature mortality. However, theprecursor sources of ASOA are poorly understood, and there are largeuncertainties in the health benefits that might accrue from reducinganthropogenic organic emissions. We show that the production of ASOA in 11urban areas on three continents is strongly correlated with the reactivityof specific anthropogenic volatile organic compounds. The differences inASOA production across different cities can be explained by differences inthe emissions of aromatics and intermediate- and semi-volatile organiccompounds, indicating the importance of controlling these ASOA precursors.With an improved model representation of ASOA driven by the observations,we attribute 340 000 PM2.5-related premature deaths per year to ASOA, which isover an order of magnitude higher than prior studies. A sensitivity casewith a more recently proposed model for attributing mortality to PM2.5(the Global Exposure Mortality Model) results in up to 900 000 deaths. Alimitation of this study is the extrapolation from cities with detailedstudies and regions where detailed emission inventories are available toother regions where uncertainties in emissions are larger. In addition tofurther development of institutional air quality management infrastructure,comprehensive air quality campaigns in the countries in South and CentralAmerica, Africa, South Asia, and the Middle East are needed for furtherprogress in this area. 

Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary aerosols that affect Earth’s radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH 2 SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate.