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Grid-type flight patterns at an altitude of 30 m were executed in the summer of 1991 by the Canadian Twin Otter flux research aircraft over a 15 km x 16.5 km agricultural area, as part of the San Joaquin Valley Air Quality Study/California Ozone Deposition Experiment (SJVAQS/CODE). Fast-response on board sensors for turbulence, temperature and gas concentrations permitted the spatial mapping of fluxes of momentum, sensible heat, moisture, CO$ sb2$ and ozone. Flux maps were produced in the form of GIS-interpolated 1 km averages, and in the discrete form of those coherent structures of the turbulent process, intermittent in time and space, which dominate the exchange of scalars between the ground and the atmosphere. The magnitude of surface-related mesoscale contributions to the flux was also quantified. Flux observations were compared against radiometrically observed surface temperatures and vegetation indices (NDVI), observed from aircraft and satellite (NOAA AVHRR), and surface characteristics from ground surveys. / Flux maps showed the expected correspondence between greenness, evapo(trans)ration (ET) and CO$ sb2$ exchange. Discrepancies between ozone flux maps and maps of greenness, ET or CO$ sb2$ were more pronounced than would be consistent with the hypothesis of stomatal control of ozone uptake. More insight into control mechanisms on ozone exchange is gained by an examination of the spatial coincidence between transporting structures for the various scalars (heat, moisture, CO$ sb2$ and ozone), through the Jaccard coefficient of co-location (J), which showed a lower value ($ rm0.3<J<0.6$) for coincidence in transfer between ozone and moisture than between moisture and CO$ sb2$ ($ rm0.5<J<0.8$). Analysis of J over the various land-use and crop-types in the test area, opens a door to a more differentiated understanding of the physical and physiological driving forces behind ozone uptake by soil and vegetation.

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Exposure to ground-level ozone (O₃), which forms secondarily in the atmosphere, intensifies the risk of respiratory and cardiovascular diseases. Effective mitigation strategies require understanding the spatiotemporal variability of O₃ precursors, including nitrogen oxides (NOx) and volatile organic compounds (VOCs), as well as O₃ formation photochemistry. This thesis examines the concentrations of trace gases closely related to O₃ production, specifically nitrogen dioxide (NO₂, the dominant component of NOx) and formaldehyde (HCHO, a proxy for VOC reactivity), as well as photochemical conditions. I investigate how these factors differ on high-O₃ days, change diurnally, and respond to the temporal resolution of anthropogenic emissions. The focus is on the summer of 2018 due to the availability of trace gas retrievals from the TROPOspheric Monitoring Instrument (TROPOMI) and in situ measurements from field campaigns. I first investigate New York City (NYC) and the Baltimore/Washington D.C. area, where high O₃ levels frequently occur in summer. On high-O₃ days (when the maximum daily 8-hour average (MDA8) O₃ exceeds 70 ppb), tropospheric vertical column densities (VCDTrop) of HCHO and NO₂ increase in urban centers. The HCHO/NO2 VCDTrop ratio, proposed as an indicator of local surface O₃ production sensitivity to its precursors, generally rises due to a more pronounced increase in HCHO VCDTrop. This suggests a shift toward a more NOx-sensitive O₃ production regime that could enhance the effectiveness of NOx controls on the highest O₃ days. As retrievals of tropospheric trace gases from Low Earth Orbit (LEO) satellites like TROPOMI are limited to one overpass per day (early afternoon), I then analyze spatial variability in HCHO and NO₂ concentration diurnal patterns and connect changes in column densities with surface concentrations. Diurnal HCHO patterns indicate the impact of temperature-dependent VOC emissions, while a bimodal surface NO₂ pattern reflects diurnal patterns of local anthropogenic NOx emissions and boundary layer dynamics. Column concentration peaks generally occur about four hours after surface concentration peaks (morning for NO2 and midday for HCHO), highlighting the challenge of relating column densities to health-related surface concentrations. I also explore how the temporal resolution of anthropogenic emissions influences air pollution levels and diurnal variations. Surface NOx and O3 concentrations show different spatial patterns of change when switching from daily mean to hourly varying nitric oxide emissions. In urban areas of both the western and eastern CONUS, adding hourly NO emissions increases daytime emissions, leading to O₃ decreases, indicating NOx-saturated O₃ chemistry. In the western CONUS, monthly mean surface NO₂ increases, while in the eastern CONUS, characterized by shorter NO₂ lifetimes, NO₂ decreases. These sensitivities highlight the importance of accounting for diurnal changes when inferring emissions from concentrations. This thesis advances our understanding of O₃-NOx-VOC air pollution by exploring variations in both surface and column conditions across urban-rural gradients. It integrates in situ measurements, space-based observations, and modeling techniques and assesses advanced modeling tools for future applications. These findings support the future applications of geostationary satellite retrievals for continuous trace gas observation throughout daylight hours, supplementing the once-a-day LEO satellite data used in this thesis, with implications such as aiding source attribution and targeting cost-effective control measures for O₃ mitigation.

There are two parts in this study. The first part is to test the validity of the assumption of thermodynamic equilibrium between fine particulate (PM2.5) nitrate and ammonium and gas-phase nitric acid (HNO3(g)) and ammonia (NH3(g)). A rough estimation of the characteristic time to achieve thermodynamic equilibrium is first carried out, which suggests that PM2.5 and gas-phase species are in thermodynamic equilibrium. Then equilibrium is tested by calculating the equilibrium concentrations of HNO3(g) and NH3(g) implied by the PM2.5 inorganic composition, temperature and relative humidity observed at the Atlanta Supersite 1999 using ISORROPIA model. The second part of this study is to analyze the ground-level ozone pollution precursor relationships in Hong Kong area. Characteristics of O3 precursors are explored. Trace gases NO and CO, VOCs, absorption coefficient, temperature and solar radiation are associated with the O3 formation. Specific VOC and VOC-sources that contribute most to the formation of photochemical smog are identified. The accuracy of pollutant emission inventories for Hong Kong and PRD region is also assessed. Combined with back trajectory information, dCO/dNOy is used to define whether O3 is locally or regionally occurred. An OBM is used to investigate the relative benefits of various emission-control strategies. Generally the formation of O3 throughout much of Hong Kong area is limited by VOC, in which reactive aromatics are dominant.

An analysis of air quality data from British Columbia has identified the Lower Mainland and surrounding rural areas as one of the regions in Canada where the Canadian Maximum Acceptable Air Quality Objective of 0.082 ppm ozone for one hour is frequently exceeded. Ozone at this level has the potential for affecting crops in the Fraser Valley. Field experiments were undertaken to attempt to evaluate the effect of randomly fluctuating levels of ozone on the yield of two cultivars each of Brassica oleracea L. (broccoli), Phaseolus vulgaris L. (bean), Pisum sativum L. (pea), Daucus carota L. (carrot) in 1985 and on one cultivar each of Solanum tuberosum L. (potato) and Pisum sativum L. (pea) in 1986, using a zonal air pollution system. As there is no current consensus regarding the most appropriate numerical expression of pollutant exposure to use in vegetation response studies, a comparison of various exposure terms was also undertaken. Ozone was added in various proportions to ambient levels between 0700 and 2100 hours (PDT) throughout the growing season. Three levels of ozone addition were used in 1985 and 12 in 1986. In 1985, treatments were assigned to three blocks over which ozone was released. Each block was supplied with different total amounts of supplementary ozone, a fourth block serving as an ambient air control. In 1986, ozone treatments were randomly assigned to four sub-plots on each of the three blocks over which ozone was released, with each block receiving the same total amount of supplementary ozone. Different treatments were achieved by each sub-plot being subject to different rates of release and degrees of mixing. For both years the ozone concentration distributions achieved over the season were approximately log-normal. Additional analysis of the air quality data from the ambient air plot found other types of skewed distributions such as the three parameter Weibull, three parameter gamma and Johnsons SB (four parameter log-normal) provided better descriptions of the data. The distribution providing the best fit depended on the concentration averaging time, the daily time span over which the ozone concentrations are analyzed and the selection criterion used. In 1985, field observations indicated that there were numerous plot to plot differences for disease and soil factors, which were confounded with the ozone treatments applied. Without true replication of the treatments, differentiation between the effects due to ozone and those from abiotic and biotic causes was not possible, and hence no clear conclusions concerning ozone response could be drawn. In 1986, without the confounding of ozone and plot location, significant linear reductions in yield were found for pea and pod fresh weight using the number of days on which the concentration exceeded 25 ppb, during the vegetative growing period (D25²). A significant linear reduction in fresh potato tuber weight was found using the geometric mean of all geometric mean ozone concentrations computed between 1200 and 1259-h for the season (GH12) as the exposure statistic. A significant multiple linear regression was found for pea fresh weight using the D25¹, statistic together with the number of occurrences in which the concentration exceeded 25 ppb for two, three and four consecutive hours (2C25, 3C25 and 4C25 respectively) in an episode; and for pod fresh weight using D25, 2C25 and 3C25 as independent variates. From the results presented it seems clear that ozone at the concentrations dispensed here would have a significant negative impact on crops grown in the Fraser Valley. Based on the 1986 experiment the best case estimate indicates that yield reductions of 28% could be expected for peas and potatoes at 37 ppb ozone (expressed as the season-long 7-h mean, M7). The exposure statistics used in the present study in comparison with the season-long 7 and 12 hour means provided good fits with the data. They are easily calculated from ambient air quality data and present attractive alternatives to those exposure statistics currently in use, for assessing the potential impact of ozone on crops in the Fraser Valley and for use in the setting of air quality standards. / Land and Food Systems, Faculty of / Graduate