Evolution of brown carbon in wildfire plumes

Particulate brown carbon (BrC) in the atmosphere absorbs light at subvisible wavelengths and has poorly constrained but potentially large climate forcing impacts. BrC from biomass burning has virtually unknown lifecycle and atmospheric stability. Here, BrC emitted from intense wildfires was measured in plumes transported over 2 days from two main fires, during the 2013 NASA SEAC4RS mission. Concurrent measurements of organic aerosol (OA) and black carbon (BC) mass concentration, BC coating thickness, absorption Ångström exponent, and OA oxidation state reveal that the initial BrC emitted from the fires was largely unstable. Using back trajectories to estimate the transport time indicates that BrC aerosol light absorption decayed in the plumes with a half-life of 9 to 15 h, measured over day and night. Although most BrC was lost within a day, possibly through chemical loss and/or evaporation, the remaining persistent fraction likely determines the background BrC levels most relevant for climate forcing

Pathways to Highly Oxidized Products in the Delta 3-Carene + OH System

Oxidation of the monoterpene Delta 3-carene (C10H16) is a potentially important and understudied source of atmospheric secondary organic aerosol (SOA). We present chamber-based measurements of speciated gas and particle phases during photochemical oxidation of Delta 3-carene. We find evidence of highly oxidized organic molecules (HOMs) in the gas phase and relatively low-volatility SOA dominated by C-7-C-10 species. We then use computational methods to develop the first stages of a Delta 3-carene photochemical oxidation mechanism and explain some of our measured compositions. We find that alkoxy bond scission of the cyclohexyl ring likely leads to efficient HOM formation, in line with previous studies. We also find a surprising role for the abstraction of primary hydrogens from methyl groups, which has been calculated to be rapid in the alpha-pinene system, and suggest more research is required to determine if this is more general to other systems and a feature of autoxidation. This work develops a more comprehensive view of Delta 3-carene photochemical oxidation products via measurements and lays out a suggested mechanism of oxidation via computationally derived rate coefficients.Peer reviewe

Molecular composition and volatility of isoprene photochemical oxidation secondary organic aerosol under low- and high-NOx conditions

Here, we present measurements of secondary organic aerosol (SOA) formation from isoprene photochemical oxidation in an environmental simulation chamber at a variety of oxidant conditions and using dry neutral seed particles to suppress acid-catalyzed multiphase chemistry. A high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) utilizing iodide-adduct ionization coupled to the Filter Inlet for Gases and Aerosols (FIGAERO) allowed for simultaneous online sampling of the gas and particle composition. Under high-HO 2 and low-NO conditions, highly oxygenated (O : C ≥ 1) C 5 compounds were major components (~50%) of SOA. The SOA composition and effective volatility evolved both as a function of time and as a function of input NO concentrations. Organic nitrates increased in both the gas and particle phases as input NO increased, but the dominant non-nitrate particle-phase components monotonically decreased. We use comparisons of measured and predicted gas-particle partitioning of individual components to assess the validity of literature-based group-contribution methods for estimating saturation vapor concentrations. While there is evidence for equilibrium partitioning being achieved on the chamber residence timescale (5.2 h) for some individual components, significant errors in group-contribution methods are revealed. In addition, >30% of the SOA mass, detected as low-molecular-weight semivolatile compounds, cannot be reconciled withmore » equilibrium partitioning. These compounds desorb from the FIGAERO at unexpectedly high temperatures given their molecular composition, which is indicative of thermal decomposition of effectively lower-volatility components such as larger molecular weight oligomers.« les

Molecular composition and volatility of isoprene photochemical oxidation secondary organic aerosol under low- and high-NOx conditions

Here, we present measurements of secondary organic aerosol (SOA) formation from isoprene photochemical oxidation in an environmental simulation chamber at a variety of oxidant conditions and using dry neutral seed particles to suppress acid-catalyzed multiphase chemistry. A high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) utilizing iodide-adduct ionization coupled to the Filter Inlet for Gases and Aerosols (FIGAERO) allowed for simultaneous online sampling of the gas and particle composition. Under high-HO 2 and low-NO conditions, highly oxygenated (O : C ≥ 1) C 5 compounds were major components (~50%) of SOA. The SOA composition and effective volatility evolved both as a function of time and as a function of input NO concentrations. Organic nitrates increased in both the gas and particle phases as input NO increased, but the dominant non-nitrate particle-phase components monotonically decreased. We use comparisons of measured and predicted gas-particle partitioning of individual components to assess the validity of literature-based group-contribution methods for estimating saturation vapor concentrations. While there is evidence for equilibrium partitioning being achieved on the chamber residence timescale (5.2 h) for some individual components, significant errors in group-contribution methods are revealed. In addition, >30% of the SOA mass, detected as low-molecular-weight semivolatile compounds, cannot be reconciled withmore » equilibrium partitioning. These compounds desorb from the FIGAERO at unexpectedly high temperatures given their molecular composition, which is indicative of thermal decomposition of effectively lower-volatility components such as larger molecular weight oligomers.« les

Whole-genome sequencing of <em>Oryza brachyantha</em> reveals mechanisms underlying <em>Oryza</em> genome evolution

The wild species of the genus Oryza contain a largely untapped reservoir of agronomically important genes for rice improvement. Here we report the 261-Mb de novo assembled genome sequence of Oryza brachyantha. Low activity of long-terminal repeat retrotransposons and massive internal deletions of ancient long-terminal repeat elements lead to the compact genome of Oryza brachyantha. We model 32,038 protein-coding genes in the Oryza brachyantha genome, of which only 70% are located in collinear positions in comparison with the rice genome. Analysing breakpoints of non-collinear genes suggests that double-strand break repair through non-homologous end joining has an important role in gene movement and erosion of collinearity in the Oryza genomes. Transition of euchromatin to heterochromatin in the rice genome is accompanied by segmental and tandem duplications, further expanded by transposable element insertions. The high-quality reference genome sequence of Oryza brachyantha provides an important resource for functional and evolutionary studies in the genus Oryza

Insights into salt tolerance from the genome of Thellungiella salsuginea

Thellungiella salsuginea, a close relative of Arabidopsis, represents an extremophile model for abiotic stress tolerance studies. We present the draft sequence of the T. salsuginea genome, assembled based on ∼134-fold coverage to seven chromosomes with a coding capacity of at least 28,457 genes. This genome provides resources and evidence about the nature of defense mechanisms constituting the genetic basis underlying plant abiotic stress tolerance. Comparative genomics and experimental analyses identified genes related to cation transport, abscisic acid signaling, and wax production prominent in T. salsuginea as possible contributors to its success in stressful environments

Highly functionalized organic nitrates in the southeast United States : Contribution to secondary organic aerosol and reactive nitrogen budgets

Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene-and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gas-particle equilibrium and (ii) have a short particle-phase lifetime (similar to 2-4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment.Peer reviewe

Chamber-based insights into the factors controlling epoxydiol (IEPOX) secondary organic aerosol (SOA) yield, composition, and volatility

We present measurements utilizing the Filter Inlet for Gases and Aerosols (FIGAERO) applied to chamber measurements of isoprene-derived epoxydiol (IEPOX) reactive uptake to aqueous acidic particles and associated secondary organic aerosol (SOA) formation. Similar to recent field observations with the same instrument, we detect two molecular components desorbing from the IEPOX SOA in high abundance: C5H12O4 and C5H10O3. The thermal desorption signal of the former, presumably 2-methyltetrols, exhibits two distinct maxima, suggesting it arises from at least two different SOA components with significantly different effective volatilities. Isothermal evaporation experiments illustrate that the most abundant component giving rise to C5H12O4 is semi-volatile, undergoing nearly complete evaporation within 1 h while the second, less volatile component remains unperturbed and even increases in abundance. We thus confirm, using controlled laboratory studies, recent analyses of ambient SOA measurements showing that IEPOX SOA is of very low volatility and commonly measured IEPOX SOA tracers such as C5H12O4 and C5H10O3, presumably 2-methyltetrols and C5-alkene triols or 3-MeTHF-3,4-diols, result predominantly from thermal decomposition in the FIGAERO-CIMS. We infer that other measurement techniques using thermal desorption or prolonged heating for analysis of SOA components may also lead to reported 2-methyltetrols and C5-alkene triols or 3-MeTHF-3,4-diol structures. We further show that IEPOX SOA volatility continues to evolve via acidity-enhanced accretion chemistry on the timescale of hours, potentially involving both 2-methyltetrols and organosulfates.Peer reviewe

Chemical and optical properties of organic aerosols in the atmosphere over continental US: formation, partitioning, and light absorption

The chemical and optical properties of particulate organic compounds remain unclear, which leaves large uncertainties in the estimation of global radiative transfer balance. Gas and find particle (PM2.5) phase formic acid concentrations were measured with online instrumentation during separate one-month studies in the summer of 2010 in Los Angeles (LA), CA, and Atlanta, GA, and the gas-particle partitioning behavior was investigated and compared with that of water-soluble organic compounds (WSOC). The diurnal profiles clearly indicated that the photochemistry production serves as a strong source for the formation of organics, while the correlation between the gas and particle phase suggested that another partitioning route, the aqueous reactions, is also very important. Later, the optical properties of light-absorbing organic compounds were examined. Little is known about the optical importance of light absorbing particulate organic compounds (brown carbon), especially its extent and absorption relative to black carbon throughout the tropospheric column. Mie theory was applied to size-resolved spectrophotometric absorption measurements of methanol and water-extracts from cascade impactor substrates collected at three surface sites around Atlanta, GA, including both urban and rural. These results were applied to similar measurements of brown carbon in extracts from aircraft bulk filter samples collected over central USA. At the surface sites predicted light absorption by brown carbon relative to total absorption (brown carbon plus pure black carbon) was about 10% and 30% at 350 nm, versus 1 and 11% at 450 nm, for water and methanol extracts, respectively. The relative contribution of brown carbon was greater in the free troposphere and significantly increased with altitude. Although this approach has limitations, it demonstrates the ubiquity and significant potential contribution of brown carbon.Ph.D