Explore the vast range of titles available.

The detection and attribution of high background ozone (O3) events in the southwestern US is challenging but relevant to the effective implementation of the lowered National Ambient Air Quality Standard (NAAQS; 70 ppbv). Here we leverage intensive field measurements from the Fires, Asian, and Stratospheric Transport−Las Vegas Ozone Study (FAST-LVOS) in May–June 2017, alongside high-resolution simulations with two global models (GFDL-AM4 and GEOS-Chem), to study the sources of O3 during high-O3 events. We show possible stratospheric influence on 4 out of the 10 events with daily maximum 8 h average (MDA8) surface O3 above 65 ppbv in the greater Las Vegas region. While O3 produced from regional anthropogenic emissions dominates pollution events in the Las Vegas Valley, stratospheric intrusions can mix with regional pollution to push surface O3 above 70 ppbv. GFDL-AM4 captures the key characteristics of deep stratospheric intrusions consistent with ozonesondes, lidar profiles, and co-located measurements of O3, CO, and water vapor at Angel Peak, whereas GEOS-Chem has difficulty simulating the observed features and underestimates observed O3 by ∼20 ppbv at the surface. On days when observed MDA8 O3 exceeds 65 ppbv and the AM4 stratospheric ozone tracer shows 20–40 ppbv enhancements, GEOS-Chem simulates ∼15 ppbv lower US background O3 than GFDL-AM4. The two models also differ substantially during a wildfire event, with GEOS-Chem estimating ∼15 ppbv greater O3, in better agreement with lidar observations. At the surface, the two models bracket the observed MDA8 O3 values during the wildfire event. Both models capture the large-scale transport of Asian pollution, but neither resolves some fine-scale pollution plumes, as evidenced by aerosol backscatter, aircraft, and satellite measurements. US background O3 estimates from the two models differ by 5 ppbv on average (greater in GFDL-AM4) and up to 15 ppbv episodically. Uncertainties remain in the quantitative attribution of each event. Nevertheless, our multi-model approach tied closely to observational analysis yields some process insights, suggesting that elevated background O3 may pose challenges to achieving a potentially lower NAAQS level (e.g., 65 ppbv) in the southwestern US.

Wildfires are increasing in size across the western US, leading to increases in human smoke exposure and associated negative health impacts. The impact of biomass burning (BB) smoke, including wildfires, on regional air quality depends on emissions, transport, and chemistry, including oxidation of emitted BB volatile organic compounds (BBVOCs) by the hydroxyl radical (OH), nitrate radical (NO3), and ozone (O3). During the daytime, when light penetrates the plumes, BBVOCs are oxidized mainly by O3 and OH. In contrast, at night or in optically dense plumes, BBVOCs are oxidized mainly by O3 and NO3. This work focuses on the transition between daytime and nighttime oxidation, which has significant implications for the formation of secondary pollutants and loss of nitrogen oxides (NOx=NO+NO2) and has been understudied. We present wildfire plume observations made during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality), a field campaign involving multiple aircraft, ground, satellite, and mobile platforms that took place in the United States in the summer of 2019 to study both wildfire and agricultural burning emissions and atmospheric chemistry. We use observations from two research aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailed chemical box model, including updated phenolic mechanisms, to analyze smoke sampled during midday, sunset, and nighttime. Aircraft observations suggest a range of NO3 production rates (0.1–1.5 ppbv h−1) in plumes transported during both midday and after dark. Modeled initial instantaneous reactivity toward BBVOCs for NO3, OH, and O3 is 80.1 %, 87.7 %, and 99.6 %, respectively. Initial NO3 reactivity is 10–104 times greater than typical values in forested or urban environments, and reactions with BBVOCs account for >97 % of NO3 loss in sunlit plumes (jNO2 up to 4×10-3s-1), while conventional photochemical NO3 loss through reaction with NO and photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH and O3 (11 %–43 %, 54 %–88 % for alkenes; 18 %–55 %, 39 %–76 %, for furans, respectively), but phenolic oxidation is split between NO3, O3, and OH (26 %–52 %, 22 %–43 %, 16 %–33 %, respectively). Nitrate radical oxidation accounts for 26 %–52 % of phenolic chemical loss in sunset plumes and in an optically thick plume. Nitrocatechol yields varied between 33 % and 45 %, and NO3 chemistry in BB plumes emitted late in the day is responsible for 72 %–92 % (84 % in an optically thick midday plume) of nitrocatechol formation and controls nitrophenolic formation overall. As a result, overnight nitrophenolic formation pathways account for 56 %±2 % of NOx loss by sunrise the following day. In all but one overnight plume we modeled, there was remaining NOx (13 %–57 %) and BBVOCs (8 %–72 %) at sunrise.

Ice-nucleating particles catalyze ice formation in clouds, affecting climate through radiative forcing from aerosol–cloud interactions. Aviation directly emits particles into the upper troposphere where ice formation conditions are favorable. Previous studies have used proxies of aviation soot to estimate their ice nucleation activity; however, investigations with commercial aircraft soot from modern in-use aircraft engines have not been quantified. In this work, we sample aviation soot particles at ground level from different commercial aircraft engines to test their ice nucleation ability at temperatures ≤228 K as a function of engine thrust and soot particle size. Additionally, soot particles were catalytically stripped to reveal the impact of mixing state on their ice nucleation ability. Particle physical and chemical properties were further characterized and related to the ice nucleation properties. The results show that aviation soot nucleates ice at or above relative humidity conditions required for homogeneous freezing of solution droplets (RHhom). We attribute this to a mesopore paucity inhibiting pore condensation and the sulfur content which suppresses freezing. Only large soot aggregates (400 nm) emitted under 30 %–100 % thrust conditions for a subset of engines (2 out of 10) nucleate ice via pore condensation and freezing. For those specific engines, the presence of hydrophilic chemical groups facilitates the nucleation. Aviation soot emitted at thrust ≥ 100 % (sea level thrust) nucleates ice at or above RHhom. Overall, our results suggest that aviation soot will not contribute to natural cirrus formation and can be used in models to update impacts of soot–cirrus clouds.

The sum of all reactive nitrogen species (NOy) includes NOx (NO2 + NO) and all of its oxidized forms, and the accurate detection of NOy is critical to understanding atmospheric nitrogen chemistry. Thermal dissociation (TD) inlets, which convert NOy to NO2 followed by NO2 detection, are frequently used in conjunction with techniques such as laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS) to measure total NOy when set at > 600 °C or speciated NOy when set at intermediate temperatures. We report the conversion efficiency of known amounts of several representative NOy species to NO2 in our TD-CRDS instrument, under a variety of experimental conditions. We find that the conversion efficiency of HNO3 is highly sensitive to the flow rate and the residence time through the TD inlet as well as the presence of other species that may be present during ambient sampling, such as ozone (O3). Conversion of HNO3 at 400 °C, nominally the set point used to selectively convert organic nitrates, can range from 2 to 6 % and may represent an interference in measurement of organic nitrates under some conditions. The conversion efficiency is strongly dependent on the operating characteristics of individual quartz ovens and should be well calibrated prior to use in field sampling. We demonstrate quantitative conversion of both gas-phase N2O5 and particulate ammonium nitrate in the TD inlet at 650 °C, which is the temperature normally used for conversion of HNO3. N2O5 has two thermal dissociation steps, one at low temperature representing dissociation to NO2 and NO3 and one at high temperature representing dissociation of NO3, which produces exclusively NO2 and not NO. We also find a significant interference from partial conversion (5–10 %) of NH3 to NO at 650 °C in the presence of representative (50 ppbv) levels of O3 in dry zero air. Although this interference appears to be suppressed when sampling ambient air, we nevertheless recommend regular characterization of this interference using standard additions of NH3 to TD instruments that convert reactive nitrogen to NO or NO2.

The Fires, Asian, and Stratospheric Transport–Las Vegas Ozone Study (FAST-LVOS) was conducted in May and June of 2017 to study the transport of ozone (O3) to Clark County, Nevada, a marginal non-attainment area in the southwestern United States (SWUS). This 6-week (20 May–30 June 2017) field campaign used lidar, ozonesonde, aircraft, and in situ measurements in conjunction with a variety of models to characterize the distribution of O3 and related species above southern Nevada and neighboring California and to probe the influence of stratospheric intrusions and wildfires as well as local, regional, and Asian pollution on surface O3 concentrations in the Las Vegas Valley (≈ 900 m above sea level, a.s.l.). In this paper, we describe the FAST-LVOS campaign and present case studies illustrating the influence of different transport processes on background O3 in Clark County and southern Nevada. The companion paper by Zhang et al. (2020) describes the use of the AM4 and GEOS-Chem global models to simulate the measurements and estimate the impacts of transported O3 on surface air quality across the greater southwestern US and Intermountain West. The FAST-LVOS measurements found elevated O3 layers above Las Vegas on more than 75 % (35 of 45) of the sample days and show that entrainment of these layers contributed to mean 8 h average regional background O3 concentrations of 50–55 parts per billion by volume (ppbv), or about 85–95 µg m−3. These high background concentrations constitute 70 %–80 % of the current US National Ambient Air Quality Standard (NAAQS) of 70 ppbv (≈ 120 µg m−3 at 900 m a.s.l.) for the daily maximum 8 h average (MDA8) and will make attainment of the more stringent standards of 60 or 65 ppbv currently being considered extremely difficult in the interior SWUS.

Solid fuel (SF) combustions, including coal and biomass, are important sources of pollutants in the particle and gas phase and therefore have significant implications for air quality, climate, and human health. In this study, we systematically examined gas-phase emissions, using the Vocus proton-transfer-reaction time-of-flight (PTR-TOF) mass spectrometer, from a variety of solid fuels, including beech logs, spruce/pine logs, spruce/pine branches and needles, straw, cow dung, and coal. The average emission factors (EFs) for organic vapors ranged from 4.8 to 74.2 g kg−1, depending on the combustion phases and solid fuel types. Despite slight differences in modified combustion efficiency (MCE) for some experiments, increasing EFs for organic vapors were observed with lower MCE. The relative contribution of different classes showed large similarities between the combustion phases in beech logs stove burning, relative to the large change in EFs observed. The CxHyOz family is the most abundant group of the organic vapor emitted from all SF combustion. However, among these SF combustions, a greater contribution of nitrogen-containing species and CxHy families (related to polycyclic aromatic hydrocarbons) is observed in the organic vapors from cow dung burning and coal burning, respectively. Intermediate-volatility organic compounds (IVOCs) constituted a significant fraction of emissions in solid fuel combustion, ranging from 12.6 % to 39.3 %. This was particularly notable in the combustion of spruce/pine branches and needles (39.3 %) and coal (31.1 %). Using the Mann–Whitney U test on the studied fuels, we identified specific potential new markers for these fuels based on the Vocus measurements. The product from pyrolysis of coniferyl-type lignin and the extract of cedar pine needle were identified as markers in the open burning of spruce/pine branches and needles (e.g., C10H14O2, C11H14O2, C10H10O2). The product (C9H12O) from the pyrolysis of beech lignin was identified as the potential new marker for beech log stove burning. Many series of nitrogen-containing homologues (e.g., C10H11–21NO, C12H11–21N, C11H11–23NO, and C15H15–31N) and nitrogen-containing species (e.g., acetonitrile, acrylonitrile, propanenitrile, methylpentanenitrile) were specifically identified in cow dung burning emissions. Polycyclic aromatic hydrocarbons (PAHs) with 9–12 carbons were identified with significantly higher abundance from coal burning compared to emissions from other studied fuels. The composition of these organic vapors reflects the burned solid fuel types and can help constrain emissions of solid fuel burning in regional models.

Abstract Heterogeneous chemical cycles of pyrogenic nitrogen and halides influence tropospheric ozone and affect the stratosphere during extreme Pyrocumulonimbus (PyroCB) events. We report field‐derived N 2 O 5 uptake coefficients, γ (N 2 O 5 ), and ClNO 2 yields, φ (ClNO 2 ), from two aircraft campaigns observing fresh smoke in the lower and mid troposphere and processed/aged smoke in the upper troposphere and lower stratosphere (UTLS). Derived φ (ClNO 2 ) varied across the full 0–1 range but was typically <0.5 and smallest in a PyroCB (<0.05). Derived γ (N 2 O 5 ) was low in agricultural smoke (0.2–3.6 × 10 −3 ), extremely low in mid‐tropospheric wildfire smoke (0.1 × 10 −3 ), but larger in PyroCB processed smoke (0.7–5.0 × 10 −3 ). Aged biomass burning aerosol in the UTLS had a higher γ (N 2 O 5 ) of 17 × 10 −3 that increased with sulfate and liquid water, but that was 1–2 orders of magnitude lower than values for aqueous sulfuric aerosol used in stratospheric models.

Engineered switchgrass and poplar are better feedstocks for biofuel synthesis and yield more biomass in multi-year field trials. Cell walls in crops and trees have been engineered for production of biofuels and commodity chemicals, but engineered varieties often fail multi-year field trials and are not commercialized. We engineered reduced expression of a pectin biosynthesis gene ( Galacturonosyltransferase 4 , GAUT4 ) in switchgrass and poplar, and find that this improves biomass yields and sugar release from biomass processing. Both traits were maintained in a 3-year field trial of GAUT4 -knockdown switchgrass, with up to sevenfold increased saccharification and ethanol production and sixfold increased biomass yield compared with control plants. We show that GAUT4 is an α-1,4-galacturonosyltransferase that synthesizes homogalacturonan (HG). Downregulation of GAUT4 reduces HG and rhamnogalacturonan II (RGII), reduces wall calcium and boron, and increases extractability of cell wall sugars. Decreased recalcitrance in biomass processing and increased growth are likely due to reduced HG and RGII cross-linking in the cell wall.

Cell walls in crops and trees have been engineered for production of biofuels and commodity chemicals, but engineered varieties often fail multi-year field trials and are not commercialized. In this paper, we engineered reduced expression of a pectin biosynthesis gene (Galacturonosyltransferase 4, GAUT4) in switchgrass and poplar, and find that this improves biomass yields and sugar release from biomass processing. Both traits were maintained in a 3-year field trial of GAUT4-knockdown switchgrass, with up to sevenfold increased saccharification and ethanol production and sixfold increased biomass yield compared with control plants. We show that GAUT4 is an α-1,4-galacturonosyltransferase that synthesizes homogalacturonan (HG). Downregulation of GAUT4 reduces HG and rhamnogalacturonan II (RGII), reduces wall calcium and boron, and increases extractability of cell wall sugars. Finally, decreased recalcitrance in biomass processing and increased growth are likely due to reduced HG and RGII cross-linking in the cell wall.