Browsing by Author "Thames, Alexander B."
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Item Constraining remote oxidation capacity with ATom observations(Copernicus Publications, 2020-07-03) Travis, Katherine R.; Heald, Colette L.; Allen, Hannah M.; Apel, Eric C.; Arnold, Stephen R.; Blake, Donald R.; Brune, William H.; Chen, Xin; Commane, Róisín; Crounse, John D.; Daube, Bruce C.; Diskin, Glenn S.; Elkins, James W.; Evans, Mathew J.; Hall, Samuel R.; Hintsa, Eric J.; Hornbrook, Rebecca S.; Kasibhatla, Prasad S.; Kim, Michelle J.; Luo, Gan; McKain, Kathryn; Millet, Dylan B.; Moore, Fred L.; Peischl, Jeffrey; Ryerson, Thomas B.; Sherwen, Tomás; Thames, Alexander B.; Ullmann, Kirk; Wang, Xuan; Wennberg, Paul O.; Wolfe, Glenn; Yu, FangqunThe global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half of the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations from the first two deployments of the NASA ATom aircraft campaign during July–August 2016 and January–February 2017 to evaluate the oxidation capacity over the remote oceans and its representation by the GEOS-Chem chemical transport model. The model successfully simulates the magnitude and vertical profile of remote OH within the measurement uncertainties. Comparisons against the drivers of OH production (water vapor, ozone, and NOy concentrations, ozone photolysis frequencies) also show minimal bias, with the exception of wintertime NOy. The severe model overestimate of NOy during this period may indicate insufficient wet scavenging and/or missing loss on sea-salt aerosols. Large uncertainties in these processes require further study to improve simulated NOy partitioning and removal in the troposphere, but preliminary tests suggest that their overall impact could marginally reduce the model bias in tropospheric OH. During the ATom-1 deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and this is not captured by the sum of its measured components (cOHRobs) or by the model (cOHRmod). This enhancement could suggest missing reactive VOCs but cannot be explained by a comprehensive simulation of both biotic and abiotic ocean sources of VOCs. Additional sources of VOC reactivity in this region are difficult to reconcile with the full suite of ATom measurement constraints. The model generally reproduces the magnitude and seasonality of cOHRobs but underestimates the contribution of oxygenated VOCs, mainly acetaldehyde, which is severely underestimated throughout the troposphere despite its calculated lifetime of less than a day. Missing model acetaldehyde in previous studies was attributed to measurement uncertainties that have been largely resolved. Observations of peroxyacetic acid (PAA) provide new support for remote levels of acetaldehyde. The underestimate in both model acetaldehyde and PAA is present throughout the year in both hemispheres and peaks during Northern Hemisphere summer. The addition of ocean sources of VOCs in the model increases cOHRmod by 3 % to 9 % and improves model–measurement agreement for acetaldehyde, particularly in winter, but cannot resolve the model summertime bias. Doing so would require 100 Tg yr⁻¹ of a long-lived unknown precursor throughout the year with significant additional emissions in the Northern Hemisphere summer. Improving the model bias for remote acetaldehyde and PAA is unlikely to fully resolve previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.Item Global airborne sampling reveals a previously unobserved dimethyl sulfide oxidation mechanism in the marine atmosphere(PNAS, 2020-02-18) Veres, Patrick R.; Neuman, J. Andrew; Bertman, Timothy H.; Assaf, Emmanuel; Wolfe, Glenn M.; Williamson, Christina J.; Weinzierl, Bernadett; Tilmes, Simone; Thompson, Chelsea R.; Thames, Alexander B.; Schroder, Jason C.; Saiz‐Lopez, Alfonso; Rollins, Andrew W.; Roberts, James M.; Price, Derek; Peischl, Jeff; Nault, Benjamin A.; Møller, Kristian H.; Miller, David O.; Meinardi, Simone; Li, Qinyi; Lamarque, Jean-François; Kupc, Agnieszka; Kjaergaard, Henrik G.; Kinnison, Douglas; Jimenez, Jose L.; Jernigan, Christopher M.; Hornbrook, Rebecca S.; Hills, Alan; Dollner, Maximilian; Day, Douglas A.; Cuevas, Carlos A.; Campuzano-Jost, Pedro; Burkholder, James; Bui, T. Paul; Brune, William H.; Brown, Steven S.; Brock, Charles A.; Bourgeois, Ilann; Blake, Donald R.; Apel, Eric C.; Ryerson, Thomas B.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, HOOCH₂SCHO), 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.Item Missing OH reactivity in the global marine boundary layer(Copernicus Publications, 2020-04-02) Thames, Alexander B.; Brune, William H.; Miller, David O.; Allen, Hannah M.; Apel, Eric C.; Blake, Donald R.; Bui, T. Paul; Commane, Roisin; Crounse, John D.; Daube, Bruce C.; Diskin, Glenn S.; DiGangi, Joshua P.; Elkins, James W.; Hall, Samuel R.; Hanisco, Thomas F.; Hannun, Reem; Hintsa, Eric; Hornbrook, Rebecca S.; Kim, Michelle J.; McKain, Kathryn; Moore, Fred L.; Nicely, Julie M.; Peischl, Jeffrey; Ryerson, Thomas B.; St. Clair, Jason; Sweeney, Colm; Teng, Alex; Thompson, Chelsea R.; Ullmann, Kirk; Wennberg, Paul O.; Wolfe, GlennThe hydroxyl radical (OH) reacts with thousands of chemical species in the atmosphere, initiating their removal and the chemical reaction sequences that produce ozone, secondary aerosols, and gas-phase acids. OH reactivity, which is the inverse of OH lifetime, influences the OH abundance and the ability of OH to cleanse the atmosphere. The NASA Atmospheric Tomography (ATom) campaign used instruments on the NASA DC-8 aircraft to measure OH reactivity and more than 100 trace chemical species. ATom presented a unique opportunity to test the completeness of the OH reactivity calculated from the chemical species measurements by comparing it to the measured OH reactivity over two oceans across four seasons. Although the calculated OH reactivity was below the limit of detection for the ATom instrument used to measure OH reactivity throughout much of the free troposphere, the instrument was able to measure the OH reactivity in and just above the marine boundary layer. The mean measured value of OH reactivity in the marine boundary layer across all latitudes and all ATom deployments was 1.9 s⁻¹, which is 0.5 s⁻¹ larger than the mean calculated OH reactivity. The missing OH reactivity, the difference between the measured and calculated OH reactivity, varied between 0 and 3.5 s⁻¹, with the highest values over the Northern Hemisphere Pacific Ocean. Correlations of missing OH reactivity with formaldehyde, dimethyl sulfide, butanal, and sea surface temperature suggest the presence of unmeasured or unknown volatile organic compounds or oxygenated volatile organic compounds associated with ocean emissions.