Browsing by Author "Oman, Luke D."
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Item Enhancing Long-Term Trend Simulation of Global Tropospheric OH and Its Drivers from 2005-2019: A Synergistic Integration of Model Simulations and Satellite Observations(EGU, 2024-02-29) Souri, Amir H.; Duncan, Bryan N.; Strode, Sarah A.; Anderson, Daniel; Manyin, Michael E.; Liu, Junhua; Oman, Luke D.; Zhang, Zhen; Weir, BradThe tropospheric hydroxyl radical (TOH) is a key player in regulating oxidation of various compounds in Earth’s atmosphere. Despite its pivotal role, the spatiotemporal distributions of OH are poorly constrained. Past modeling studies suggest that the main drivers of OH, including NO₂, tropospheric ozone (TO₃), and H₂O(v), have increased TOH globally. However, these findings often offer a global average and may not include more recent changes in diverse compounds emitted on various spatiotemporal scales. Here, we aim to deepen our understanding of global TOH trends for more recent years (2005-2019) at 1x1 degrees. To achieve this, we use satellite observations of HCHO and NO₂ to constrain simulated TOH using a technique based on a Bayesian data fusion method, alongside an interpretable machine learning module named ECCOH, which is integrated into NASA’s GEOS global model. This innovative module helps efficiently predict the convoluted response of TOH to its drivers/proxies. Aura Ozone Monitoring Instrument (OMI) NO₂ observations suggest that the simulation has high biases over biomass burning activities in Africa and Eastern Europe, resulting in overestimation of up to 20 % in TOH, regionally. OMI HCHO primarily impacts oceans where TOH linearly correlates with this proxy. Five key parameters including TO₃, H₂O(v), NO₂, HCHO, and stratospheric ozone can collectively explain 65 % of variance in TOH trends. The overall trend of TOH influenced by NO₂ remains positive, but it varies greatly because of the differences in the signs of anthropogenic emissions. Over oceans, TOH trends are primarily positive in the northern hemisphere, resulting from the upward trends in HCHO, TO₃, and H₂O(v). Using the present framework, we can tap the power of satellites to quickly gain a deeper understanding of simulated TOH trends and biases.Item Evaluation of NASA's high-resolution global composition simulations: Understanding a pollution event in the Chesapeake Bay during the summer 2017 OWLETS campaign(Elsevier, 2020-02-01) Dacic, Natasha; Sullivan, John T.; Knowland, K. Emma; Wolfe, Glenn M.; Oman, Luke D.; Berkoff, Timothy A.; Gronoff, Guillaume P.Recirculation of pollutants due to a bay breeze effect is a key meteorological mechanism impacting air quality near urban coastal areas, but regional and global chemical transport models have historically struggled to capture this phenomenon. We present a case study of a high ozone (O3) episode observed over the Chesapeake Bay during the NASA Ozone Water-Land Environmental Transition Study (OWLETS) in summer 2017. OWLETS included a complementary suite of ground-based and airborne observations, with which we characterize the meteorological and chemical context of this event and develop a framework to evaluate model performance. Two publicly-available NASA global high-resolution coupled chemistry-meteorology models (CCMMs) are investigated: GEOS-CF and MERRA2-GMI. The GEOS-CF R2 value for comparisons between the NASA Sherpa C-23 aircraft measurements to the GEOS-CF resulted in good agreement (R2: 0.67) on July 19th and fair agreement (R2: 0.55) for July 20th. Compared to surface observations, we find the GEOS-CF product with a 25 × 25 km2 grid box, at an hourly (R2: 0.62 to 0.87) and 15-min (R2: 0.64 to 0.87) interval for six regional sites outperforms the hourly nominally 50 × 50 km2 gridded MERRA2-GMI (R2: 0.53 to 0.76) for four of the six sites, suggesting it is better capable of simulating complex chemical and meteorological features associated with ozone transport within the Chesapeake Bay airshed. When the GEOS-CF product was compared to the TOLNet LiDAR observations at both NASA Langley Research Center (LaRC) and the Chesapeake Bay Bridge Tunnel (CBBT), the median differences at LaRC were −6 to 8% and at CBBT were ±7% between 400 and 2000 m ASL. This indicates that, for this case study, the GEOS-CF is able to simulate surface level ozone diurnal cycles and vertical ozone profiles at small scales between the surface level and 2000 m ASL. Evaluating global chemical model simulations at sub-regional scales will help air quality scientists understand the complex processes occurring at small spatial and temporal scales within complex surface terrain changes, simulating nighttime chemistry and deposition, and the potential to use global chemical transport simulations in support of regional and sub-regional field campaigns.Item Evaluation of Version 3 Total and Tropospheric Ozone Columns From Earth Polychromatic Imaging Camera on Deep Space Climate Observatory for Studying Regional Scale Ozone Variations(Frontiers, 2021-09-08) Kramarova, Natalya A.; Ziemke, Jerald R.; Huang, Liang-Kang; Herman, Jay; Wargan, Krzysztof; Seftor, Colin J.; Labow, Gordon J.; Oman, Luke D.Discrete wavelength radiance measurements from the Deep Space Climate Observatory (DSCOVR) Earth Polychromatic Imaging Camera (EPIC) allows derivation of global synoptic maps of total and tropospheric ozone columns every hour during Northern Hemisphere (NH) Summer or 2 hours during Northern Hemisphere winter. In this study, we present version 3 retrieval of Earth Polychromatic Imaging Camera ozone that covers the period from June 2015 to the present with improved geolocation, calibration, and algorithmic updates. The accuracy of total and tropospheric ozone measurements from EPIC have been evaluated using correlative satellite and ground-based total and tropospheric ozone measurements at time scales from daily averages to monthly means. The comparisons show good agreement with increased differences at high latitudes. The agreement improves if we only accept retrievals derived from the EPIC 317 nm triplet and limit solar zenith and satellite looking angles to 70°. With such filtering in place, the comparisons of EPIC total column ozone retrievals with correlative satellite and groundbased data show mean differences within ±5-7 Dobson Units (or 1.5–2.5%). The biases with other satellite instruments tend to be mostly negative in the Southern Hemisphere while there are no clear latitudinal patterns in ground-based comparisons. Evaluation of the EPIC ozone time series at different ground-based stations with the correlative groundbased and satellite instruments and ozonesondes demonstrated good consistency in capturing ozone variations at daily, weekly and monthly scales with a persistently high correlation (r² > 0.9) for total and tropospheric columns. We examined EPIC tropospheric ozone columns by comparing with ozonesondes at 12 stations and found that differences in tropospheric column ozone are within ±2.5 DU (or ∼±10%) after removing a constant 3 DU offset at all stations between EPIC and sondes. The analysis of the time series of zonally averaged EPIC tropospheric ozone revealed a statistically significant drop of ∼2–4 DU (∼5–10%) over the entire NH in spring and summer of 2020. This drop in tropospheric ozone is partially related to the unprecedented Arctic stratospheric ozone losses in winter-spring 2019/2020 and reductions in ozone precursor pollutants due to the COVID-19 pandemic.Item Formaldehyde in the Tropical Western Pacific: Chemical Sources and Sinks, Convective Transport, and Representation in CAM‐Chem and the CCMI Models(AGU Pubication, 2017-10-06) Anderson, Daniel C.; Nicely, Julie M.; Wolfe, Glenn M.; Hanisco, Thomas F.; Salawitch, Ross J.; Canty, Timothy P.; Dickerson, Russell R.; Apel, Eric C.; Baidar, Sunil; Bannan, Thomas J.; Blake, Nicola J.; Chen, Dexian; Dix, Barbara; Fernandez, Rafael P.; Hall, Samuel R.; Hornbrook, Rebecca S.; Huey, L. Gregory; Josse, Beatrice; Jöckel, Patrick; Kinnison, Douglas E.; Koenig, Theodore K.; Breton, Michael Le; Marécal, Virginie; Morgenstern, Olaf; Oman, Luke D.; Pan, Laura L.; Percival, Carl; Plummer, David; Revell, Laura E.; Rozanov, Eugene; Saiz‐Lopez, Alfonso; Stenke, Andrea; Sudo, Kengo; Tilmes, Simone; Ullmann, Kirk; Volkamer, Rainer; Weinheimer, Andrew J.; Zeng, GuangFormaldehyde (HCHO) directly affects the atmospheric oxidative capacity through its effects on HOx. In remote marine environments, such as the tropical western Pacific (TWP), it is particularly important to understand the processes controlling the abundance of HCHO because model output from these regions is used to correct satellite retrievals of HCHO. Here we have used observations from the Convective Transport of Active Species in the Tropics (CONTRAST) field campaign, conducted during January and February 2014, to evaluate our understanding of the processes controlling the distribution of HCHO in the TWP as well as its representation in chemical transport/climate models. Observed HCHO mixing ratios varied from ~500 parts per trillion by volume (pptv) near the surface to ~75 pptv in the upper troposphere. Recent convective transport of near surface HCHO and its precursors, acetaldehyde and possibly methyl hydroperoxide, increased upper tropospheric HCHO mixing ratios by ~33% (22 pptv); this air contained roughly 60% less NO than more aged air. Output from the CAM‐Chem chemistry transport model (2014 meteorology) as well as nine chemistry climate models from the Chemistry‐Climate Model Initiative (free‐running meteorology) are found to uniformly underestimate HCHO columns derived from in situ observations by between 4 and 50%. This underestimate of HCHO likely results from a near factor of two underestimate of NO in most models, which strongly suggests errors in NOx emissions inventories and/or in the model chemical mechanisms. Likewise, the lack of oceanic acetaldehyde emissions and potential errors in the model acetaldehyde chemistry lead to additional underestimates in modeled HCHO of up to 75 pptv (~15%) in the lower troposphere.Item Investigation of observed dust trends over the Middle East region in NASA GEOS Earth system model simulations(EGU, 2023-08-21) Rocha-Lima, Adriana; Colarco, Peter R.; Darmenov, Anton S.; Nowottnick, Edward P.; da Silva, Arlindo M.; Oman, Luke D.Satellite observations and ground-based measurements have indicated a high variability in the Aerosol Optical Depth (AOD) in the Middle East region in recent decades. In the period that extends from 2003 to 2012, observations show a positive trend of 0.01–0.04 AOD per year or a total increase of 0.1–0.4 per decade. This study aimed to investigate if the observed trend was also captured by the NASA Goddard Earth Observing System (GEOS) Earth system model. To this end, we examined changes in the simulated dust emissions and dust AOD during this period. To understand the factors driving the increase of AOD in this region we also examined meteorological and surface parameters important for dust emissions, such as wind fields and soil moisture. Two GEOS model simulations were used in this study: the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) Reanalysis (with meteorological and aerosol AOD data assimilated) and MERRA-2 GMI Replay (with meteorology constrained by MERRA-2 Reanalysis, but without aerosol assimilation). We did not find notable changes in the modeled 10-meter wind speed and soil moisture. However, analysis of MODIS Normalized Difference Vegetation Index (NDVI) data, did show an average decrease of 8 % per year in the region encompassing Syria and Iraq, which prompted us to quantify the effects of vegetation on dust emissions and AOD in the Middle East region. This was done by performing a sensitivity experiment in which we enhanced dust emissions in grid cells where NDVI decreased. The simulation results supported our hypothesis that the loss of vegetation cover and the associated increase of dust emissions over Syria and Iraq can partially explain the increase of AOD downwind. The model simulations indicated dust emissions need to be tenfold larger in those grid cells in order to reproduce the observed AOD and trend in the model.Item A machine learning examination of hydroxyl radical differences among model simulations for CCMI-1(EGU Publications, 2020-02-05) Nicely, Julie M.; Duncan, Bryan N.; Hanisco, Thomas F.; Wolfe, Glenn M.; Salawitch, Ross J.; Deushi, Makoto; Haslerud, Amund S.; Jöckel, Patrick; Josse, Béatrice; Kinnison, Douglas E.; Klekociuk, Andrew; Manyin, Michael E.; Marécal, Virginie; Morgenstern, Olaf; Murray, Lee T.; Myhre, Gunnar; Oman, Luke D.; Pitari, Giovanni; Pozzer, Andrea; Quaglia, Ilaria; Revell, Laura E.; Rozanov, Eugene; Stenke, Andrea; Stone, Kane; Strahan, Susan; Tilmes, Simone; Tost, Holger; Westervelt, Daniel M.; Zeng, GuangThe hydroxyl radical (OH) plays critical roles within the troposphere, such as determining the lifetime of methane (CH₄), yet is challenging to model due to its fast cycling and dependence on a multitude of sources and sinks. As a result, the reasons for variations in OH and the resulting methane lifetime (τCH₄), both between models and in time, are difficult to diagnose. We apply a neural network (NN) approach to address this issue within a group of models that participated in the Chemistry-Climate Model Initiative (CCMI). Analysis of the historical specified dynamics simulations performed for CCMI indicates that the primary drivers of τCH₄ differences among 10 models are the flux of UV light to the troposphere (indicated by the photolysis frequency JO¹D), the mixing ratio of tropospheric ozone (O₃), the abundance of nitrogen oxides (NOₓ≡NO+NO₂), and details of the various chemical mechanisms that drive OH. Water vapour, carbon monoxide (CO), the ratio of NO:NOₓ, and formaldehyde (HCHO) explain moderate differences in τCH₄, while isoprene, methane, the photolysis frequency of NO₂ by visible light (JNO₂), overhead ozone column, and temperature account for little to no model variation in τCH₄. We also apply the NNs to analysis of temporal trends in OH from 1980 to 2015. All models that participated in the specified dynamics historical simulation for CCMI demonstrate a decline in τCH₄ during the analysed timeframe. The significant contributors to this trend, in order of importance, are tropospheric O₃, JO¹D, NOₓ, and H₂O, with CO also causing substantial interannual variability in OH burden. Finally, the identified trends in τCH₄ are compared to calculated trends in the tropospheric mean OH concentration from previous work, based on analysis of observations. The comparison reveals a robust result for the effect of rising water vapour on OH and τCH₄, imparting an increasing and decreasing trend of about 0.5 % decade⁻¹, respectively. The responses due to NOₓ, ozone column, and temperature are also in reasonably good agreement between the two studies.Item Mapping hydroxyl variability throughout the global remote troposphere via synthesis of airborne and satellite formaldehyde observations(PNAS, 2019-05-20) Wolfe, Glenn M.; Nicely, Julie M.; St. Clair, Jason; Hanisco, Thomas F.; Liao, Jin; Oman, Luke D.; Brune, William B.; Miller, David; Thames, Alexander; Abad, Gonzalo González; Ryerson, Thomas B.; Thompson, Chelsea R.; Peischl, Jeff; McKain, Kathryn; Sweeney, Colm; Wennberg, Paul O.; Kim, Michelle; Crounse, John D.; Hall, Samuel R.; Ullmann, Kirk; Diskin, Glenn; Bui, Paul; Chang, Cecilia; Dean-Day, JonathanThe hydroxyl radical (OH) fuels tropospheric ozone production and governs the lifetime of methane and many other gases. Existing methods to quantify global OH are limited to annual and global-to-hemispheric averages. Finer resolution is essential for isolating model deficiencies and building process-level understanding. In situ observations from the Atmospheric Tomography (ATom) mission demonstrate that remote tropospheric OH is tightly coupled to the production and loss of formaldehyde (HCHO), a major hydrocarbon oxidation product. Synthesis of this relationship with satellite-based HCHO retrievals and model-derived HCHO loss frequencies yields a map of total-column OH abundance throughout the remote troposphere (up to 70% of tropospheric mass) over the first two ATom missions (August 2016 and February 2017). This dataset offers unique insights on near-global oxidizing capacity. OH exhibits significant seasonality within individual hemispheres, but the domain mean concentration is nearly identical for both seasons (1.03 ± 0.25 × 10⁶ cm⁻³), and the biseasonal average North/South Hemisphere ratio is 0.89 ± 0.06, consistent with a balance of OH sources and sinks across the remote troposphere. Regional phenomena are also highlighted, such as a 10-fold OH depression in the Tropical West Pacific and enhancements in the East Pacific and South Atlantic. This method is complementary to budget-based global OH constraints and can help elucidate the spatial and temporal variability of OH production and methane lossItem Response of the Upper-Level Monsoon Anticyclones and Ozone to Abrupt CO₂ Changes(AGU, 2021-10-05) Tweedy, Olga V.; Oman, Luke D.; Waugh, Darryn W.; Schoeberl, Mark R.; Douglass, Anne R.; Li, FengThe summer monsoon anticyclones are the dominant climatological features of the Northern Hemispheric (NH) summertime circulation in the upper troposphere and lower stratosphere (UTLS). However, the response of these anticyclones to the increased levels of E CO₂ remains highly uncertain, as does the impact on the distribution of UTLS ozone and other tracers. This study examines the response of the NH summertime monsoon anticyclones and UTLS ozone to the abrupt increase in E CO₂ forcing using output from a suite of coupled ocean–atmosphere general circulation model simulations. These models show an equatorward shift of the Asian summer monsoon anticyclone, a weakening of the North American summer monsoon anticyclone, and a stronger westerly flow penetrating deep into the tropics above the Pacific Ocean and North America. We use additional idealized experiments from atmosphere-only general circulation models with prescribed SSTs and sea ice concentration to isolate the direct atmospheric radiative effects from the indirect effect of SST warming on the UTLS monsoon anticyclones. Comparison between atmosphere-only and coupled ocean–atmosphere experiments shows that SST warming is the principal mechanism producing UTLS monsoonal circulation changes. The 4*CO₂ experiments result in a significant reduction up to 40%–50% of the UTLS ozone in the northern tropics, which could have an impact on radiative balance near the surface.Item Seasonal ventilation of the stratosphere: Robust diagnostics from one-way flux distributions(AGU, 2013-12-10) Orbe, Clara; Holzer, Mark; Polvani, Lorenzo M.; Waugh, Darryn W.; Li, Feng; Oman, Luke D.; Newman, Paul A.We present an analysis of the seasonally varying ventilation of the stratosphere using one-way flux distributions. Robust transport diagnostics are computed using GEOSCCM subject to fixed present-day climate forcings. From the one-way flux, we determine the mass of the stratosphere that is in transit since entry through the tropical tropopause to its exit back into the troposphere, partitioned according to stratospheric residence time and exit location. The seasonalities of all diagnostics are quantified with respect to the month of year (a) when air enters the stratosphere, (b) when the mass of the stratosphere is partitioned, and (c) when air exits back into the troposphere. We find that the return flux, within 3 months since entry, depends strongly on when entry occurred: (34±10)% more of the air entering the stratosphere in July leaves poleward of 45°N compared to air that enters in January. The month of year when the air mass is partitioned is also found to be important: The stratosphere contains about six times more air of tropical origin during late summer and early fall that will leave poleward of 45° within 6 months since entering the stratosphere compared to during late winter to late spring. When the entire mass of the air that entered the stratosphere at the tropics regardless of its residence time is considered, we find that (51±1)% and (39±2)% will leave poleward of 10° in the Nothern Hemisphere (NH) and Southern Hemisphere (SH), respectively.Item Stratospheric Impacts of Continuing CFC-11 Emissions Simulated in a Chemistry-Climate Model(AGU, 2021-04-09) Fleming, Eric L.; Liang, Qing; Oman, Luke D.; Newman, Paul A.; Li, Feng; Hurwitz, Margaret M.Trichlorofluoromethane (CFC-11, CFCl3) is a major anthropogenic ozone-depleting substance and greenhouse gas, and its production and consumption are controlled under the Montreal Protocol. However, recent studies show that CFC-11 emissions increased during 2014–2017 relative to 2008–2012. In this study, we use a chemistry-climate model to investigate the stratospheric impacts of potential CFC-11 emissions continuing into the future. As a sensitivity test, we use a high CFC-11 scenario in which the inferred 2013–2016 average emissions of 72.5 Gg/yr is sustained to year 2100. This increases equivalent effective stratospheric chlorine by 15% in 2100, relative to the WMO (2018) baseline scenario in which future emissions decay with a bank release rate of 6.4%/year. Consistent with recent studies, the resulting ozone response has a linear dependence on the accumulated CFC-11 emissions, yielding global and Antarctic spring total ozone sensitivity per 1,000 Gg of −0.37 and −3.9 DU, respectively, averaged over 2017–2100. The deepened ozone hole reduces UV heating, causing a colder Antarctic lower stratosphere in spring/early summer. Through thermal wind balance, this accelerates the circumpolar jet which in turn alters planetary and gravity wave propagation through the Southern Hemisphere stratosphere, and modifies the Brewer-Dobson circulation. Age of air in the high scenario is slightly younger than the baseline in the lower stratosphere globally during 2090–2099, with a maximum change of −0.1 years. Coupled atmosphere-ocean model simulations show that the resulting greenhouse gas impact of CFC-11 is small and not statistically significant throughout the troposphere and stratosphere.