Atmospheric & Oceanic Science
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Formerly known as the Department of Meteorology.
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Item Photochemistry and transport of tropospheric ozone and its precursors in urban and remote environments(2016) Anderson, Daniel Craig; Dickerson, Russell R; Salawitch, Ross J; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tropospheric ozone (O3) adversely affects human health, reduces crop yields, and contributes to climate forcing. To limit these effects, the processes controlling O3 abundance as well as that of its precursor molecules must be fully characterized. Here, I examine three facets of O3 production, both in heavily polluted and remote environments. First, using in situ observations from the DISCOVER-AQ field campaign in the Baltimore/Washington region, I evaluate the emissions of the O3 precursors CO and NOx (NOx = NO + NO2) in the National Emissions Inventory (NEI). I find that CO/NOx emissions ratios derived from observations are 21% higher than those predicted by the NEI. Comparisons to output from the CMAQ model suggest that CO in the NEI is accurate within 15 ± 11%, while NOx emissions are overestimated by 51-70%, likely due to errors in mobile sources. These results imply that ambient ozone concentrations will respond more efficiently to NOx controls than current models suggest. I then investigate the source of high O3 and low H2O structures in the Tropical Western Pacific (TWP). A combination of in situ observations, satellite data, and models show that the high O3 results from photochemical production in biomass burning plumes from fires in tropical Southeast Asia and Central Africa; the low relative humidity results from large-scale descent in the tropics. Because these structures have frequently been attributed to mid-latitude pollution, biomass burning in the tropics likely contributes more to the radiative forcing of climate than previously believed. Finally, I evaluate the processes controlling formaldehyde (HCHO) in the TWP. Convective transport of near surface HCHO leads to a 33% increase in upper tropospheric HCHO mixing ratios; convection also likely increases upper tropospheric CH3OOH to ~230 pptv, enough to maintain background HCHO at ~75 pptv. The long-range transport of polluted air, with NO four times the convectively controlled background, intensifies the conversion of HO2 to OH, increasing OH by a factor of 1.4. Comparisons between the global chemistry model CAM-Chem and observations show that consistent underestimates of HCHO by CAM-Chem throughout the troposphere result from underestimates in both NO and acetaldehyde.Item An evaluation of a severe smog episode in the Eastern U.S. using regional modeling and satellite measurements(2011) Yegorova, Elena Andreyevna; Dickerson, Russell R; Allen, Dale J; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An ensemble of regional chemical modeling (WRF/Chem with RADM2) simulations, satellite, ozonesonde, and surface observations during July 7-11, 2007 was used to examine the horizontal and vertical signature of one of the worst smog events in the eastern U.S. in the past decade. The general features of this event -- a broad area of high pressure, weak winds and heavy pollution, terminated by the passage of a cold front -- were well simulated by the model. Average 8-hr maximum O3 has a mean (±Σ) bias of 0.59 (±11.0) ppbv and a root mean square error of 11.0 ppbv. WRF/Chem performed the best on poor air quality days, simulating correctly the spatial pattern of surface O3. Yet the model underpredicted O3 maxima by 5-7 ppbv in the Northeast and overpredicted by 8-11 ppbv in the Southeast. High O3 biases in the Southeast are explained by overpredicted temperatures in the model (>1.5°C). Sensitivity simulations with 1) accelerated O3 dry deposition velocity and 2) suppressed multiphase nitric acid formation pushed the model closer to observations. Simulated O3 vertical profiles over Beltsville, MD showed good agreement with ozonesonde measurements, but the modeled boundary layer depth was overpredicted on July 9, contributing to the low bias over this region. During this severe smog episode, space-borne TES detected high total tropospheric column ozone (TCO) over the Western Atlantic Ocean off the coast near North and South Carolina. The standard product (OMI/MLS) missed the magnitude of these local maxima, but the level-2 ozone profile (OMI) confirmed the TES observations. HYSPLIT back trajectories from these O3 maxima intersected regions of strong convection over the Southeast and Great Lakes regions. When lightning NO emissions were implemented in WRF/Chem, the high concentrations of NOx and O3 off the coast were well reproduced, showing that the exported O3 was produced by a combination of natural NO and pollutants lofted from the lower atmosphere. Lastly, WINTER MONEX O3 data from 1978 are presented for the first time here in discussion of open cell convection over Indonesia.Item Observed Relationship of Ozone air Pollution with Temperature and Emissions(American Geophysical Union (AGU), 2009) Bloomer, Bryan J. ; Stehr, Jeffrey W. ; Piety, Charles A.; Salawitch, Ross J.; Dickerson, Russell R.Higher temperatures caused by increasing greenhouse gas concentrations are predicted to exacerbate photochemical smog if precursor emissions remain constant. We perform a statistical analysis of 21 years of ozone and temperature observations across the rural eastern U.S. The climate penalty factor is defined as the slope of the ozone/temperature relationship. For two precursor emission regimes, before and after 2002, the climate penalty factor was consistent across the distribution of ozone observations. Prior to 2002, ozone increased by an average of ~3.2 ppbv/°C. After 2002, power plant NOx emissions were reduced by 43%, ozone levels fell ~10%, and the climate penalty factor dropped to ~2.2 ppbv/°C. NOx controls are effective for reducing photochemical smog and might lessen the severity of projected climate change penalties. Air quality models should be evaluated against these observations, and the climate penalty factor metric may be useful for evaluating the response of ozone to climate change.Item The Sensitivity of Modeled Ozone to the Temporal Distribution of Point, Area, and Mobile Emissions in the Eastern US(Elsevier, 2009) Castellanos, Patricia; Ehrman, Sheryl H.; Stehr, Jeffrey W.; Dickerson, Russell R.Ozone remains one of the most recalcitrant air pollution problems in the US. Hourly emissions fields used in air quality models (AQMs) generally show less temporal variability than corresponding measurements. In order to understand how the daily cycle of estimated emissions affects modeled ozone, we analyzed the effects of altering all anthropogenic emissions’ temporal distributions by source group 2 on 2002 summer-long simulations of ozone using the Community Multi-Scale Air Quality Model (CMAQ) v4.5 and the carbon bond IV (CBIV) chemical mechanism with a 12 km grid. We find that when mobile source emissions were made constant over the course of a day, 8-hour maximum ozone predictions changed by ±7 parts per billion by volume (ppbv) in urban areas on days when ozone concentrations greater than 80 ppbv were simulated in the base case. Increasing the temporal variation of point sources resulted in ozone changes of +6 and –6 ppbv, but only for small areas near sources. Changing the daily cycle of mobile source emissions produces substantial changes in simulated ozone, especially in urban areas at night; implications for abatement strategy are discussed.Item Air Pollution Response to Changing Weather and Power Plant Emissions in the Eastern United States(2008-11-20) Bloomer, Bryan Jaye; Dickerson, Russell R; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Air pollution in the eastern United States causes human sickness and death as well as damage to crops and materials. NOX emission reduction is observed to improve air quality. Effectively reducing pollution in the future requires understanding the connections between smog, precursor emissions, weather, and climate change. Numerical models predict global warming will exacerbate smog over the next 50 years. My analysis of 21 years of CASTNET observations quantifies a climate change penalty. I calculate, for data collected prior to 2002, a climate penalty factor of ~3.3 ppb O3/°C across the power plant dominated receptor regions in the rural, eastern U.S. Recent reductions in NOX emissions decreased the climate penalty factor to ~2.2 ppb O3/°C. Prior to 1995, power plant emissions of CO2, SO2, and NOX were estimated with fuel sampling and analysis methods. Currently, emissions are measured with continuous monitoring equipment (CEMS) installed directly in stacks. My comparison of the two methods show CO2 and SO2 emissions are ~5% lower when inferred from fuel sampling; greater differences are found for NOX emissions. CEMS are the method of choice for emission inventories and commodity trading and should be the standard against which other methods are evaluated for global greenhouse gas trading policies. I used CEMS data and applied chemistry transport modeling to evaluate improvements in air quality observed by aircraft during the North American electrical blackout of 2003. An air quality model produced substantial reductions in O3, but not as much as observed. The study highlights weaknesses in the model as commonly used for evaluating a single day event and suggests areas for further investigation. A new analysis and visualization method quantifies local-daily to hemispheric-seasonal scale relationships between weather and air pollution, confirming improved air quality despite increasing temperatures across the eastern U.S. Climate penalty factors indicate amplified smog formation in areas of the world with rising temperatures and increasing emissions. Tools developed in this dissertation provide data for model evaluation and methods for establishing air quality standards with an adequate margin of safety for cleaning the air and protecting the public's health in a world with changing climate.Item Classification of Northern Hemisphere Stratospheric Ozone and Water Vapor Profiles by Meteorological Regime: Validation, Climatology, and Trends(2007-01-19) Follette, Melanie Beth; Hudson, Robert D.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The presence of stratospheric ozone is essential for the survival of life on the Earth's surface. The decrease in the column content of ozone over mid-latitudes from 1979-1991 has previously been attributed to destruction by anthropogenic halogens, and changes in the general circulation. The research presented here shows that a poleward movement of the subtropical and polar upper troposphere fronts is responsible for 35% of this observed decrease. In Hudson et al. (2003) we showed that the Northern Hemisphere total ozone field could be separated into meteorological regimes, bounded by the subtropical and polar upper troposphere fronts. These regimes were characterized by relatively constant total ozone, tropopause height, and ozonepause height. Negative trends in total ozone within each regime were found for the time period January 1979-May 1991. These trends corresponded to a statistically significant increase in the relative area of the tropical regime, and decrease in the relative area of the polar regime, indicating a net poleward movement of the subtropical and polar fronts over this time period. This poleward frontal movement was responsible for ~35% of the negative zonal trend in total ozone over this time period and latitude range, the remaining 65% being the result of total ozone changes within the meteorological regimes. Ozone and water vapor profiles from 1997-2004, from the HALOE and SAGE II satellite-based instruments, were classified by regime. Each regime was characterized by a distinct ozonepause and hygropause height, and profile shape below ~25km, over a wide latitude range (25°-60°N). Therefore, previously reported zonal trends in the lower stratosphere and upper troposphere are a combination of both tropospheric and stratospheric air. Trends within each regime were calculated for both ozone and water vapor from 1997-2004 and from October 1984-May 1991. The relationship between the observed zonal vertical trends and the trends within each regime were consistent with the idea of meteorological regimes and reinforce the major conclusion of this work. A true understanding of zonal trends in either the column or in the lower stratosphere involves understanding both changes within each regime and changes in the relative weighting of each regime over time.