Photochemistry and transport of tropospheric ozone and its precursors in urban and remote environments

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.

OBSERVATIONS AND EMISSIONS OF ENERGY-ASSOCIATED OZONE PRECURSORS IN THE MID-ATLANTIC UNITED STATES

Surface ozone is formed in the presence of NOx (NO + NO2) and volatile organic compounds (VOCs) and is hazardous to human health. A better understanding of these precursors is needed for developing effective policies to improve air quality. To evaluate the year-to-year changes in source contributions to total VOCs, Positive Matrix Factorization (PMF) was used to perform source apportionment using available hourly observations from June through August at a Photochemical Assessment Monitoring Station (PAMS) in Essex, MD for each year from 2007-2015. Results suggest that while gasoline and vehicle exhaust emissions have fallen, the contribution of natural gas sources to total VOCs has risen. To investigate this increasing natural gas influence, ethane measurements from PAMS sites in Essex, MD and Washington, D.C. were examined. Following a period of decline, daytime ethane concentrations have increased significantly after 2009. This trend appears to be linked with the rapid shale gas production in upwind, neighboring states, especially Pennsylvania and West Virginia. Back-trajectory analyses similarly show that ethane concentrations at these monitors were significantly greater if air parcels had passed through counties containing a high density of unconventional natural gas wells. In addition to VOC emissions, the compressors and engines involved with hydraulic fracturing operations also emit NOx and particulate matter (PM). The Community Multi-scale Air Quality (CMAQ) Model was used to simulate air quality for the Eastern U.S. in 2020, including emissions from shale gas operations in the Appalachian Basin. Predicted concentrations of ozone and PM show the largest decreases when these natural gas resources are hypothetically used to convert coal-fired power plants, despite the increased emissions from hydraulic fracturing operations expanded into all possible shale regions in the Appalachian Basin. While not as clean as burning natural gas, emissions of NOx from coal-fired power plants can be reduced by utilizing post-combustion controls. However, even though capital investment has already been made, these controls are not always operated at optimal rates. CMAQ simulations for the Eastern U.S. in 2018 show ozone concentrations decrease by ~5 ppb when controls on coal-fired power plants limit NOx emissions to historically best rates.

Pathways to adulthood and their precursors: Roles of gender and race

Research on the transition to adulthood dates back nearly four decades, but a growing body of research has taken a new approach by investigating multiple demographic markers in the transition to adulthood simultaneously. Using the life course perspective, this dissertation is built on the literature by first examining contemporary young adults’ pathways to adulthood from ages 18 to 30 and their differences by gender. Data for this study were drawn from the National Longitudinal Survey of Youth 1997; the final sample included 2,185 men and 2,086 women. The college-educated single workers pathway, the college-educated married working parents pathway, and the high-school-educated single parents pathway were identified in both genders. For men, the study also identified the high-school-educated single workers pathway and the high-school-educated married working parents pathway. For women, the study also identified the high-school-educated workers pathway and the high-school-educated married parents pathway. Not only did the definitions of some pathways differ by gender, but even in the pathways with the same definition, gender differences were found in the probabilities of being married, of being a parent, or of being employed full-time. Based on the pathways to adulthood identified, this research examined the family and adolescent precursors and whether race moderates the associations between family structure experiences and young adults’ pathways to adulthood. Parental education, family structure, GPA, delinquency, early sexual activity, and race/ethnicity were the family and adolescent precursors that distinguished among pathways taken by the youth. Two interactions between race and family structure/instability were identified. The positive association between growing up in a single-parent family and the odds of taking the high-school-educated single workers pathway compared to the college-educated married working parents pathway was weaker for Black males than for White males. The positive association between family instability and the odds of taking the college-educated single workers pathway compared to the college-educated married working parents pathway was weaker for Black females than for White females. This dissertation accounted for changes in the multiple statuses related to becoming an adult by following contemporary young adults for 12 years. More research on contemporary young adults’ pathways to adulthood and subgroup differences in the effects of precursors are recommended. Limitations and implications of this study are discussed.