Tropospheric ozone is a major atmospheric pollutant and a greenhouse gas. Nevertheless, many processes influencing its spatio-temporal distribution are still poorly understood, mainly due to the lack of adequate observations. One such region is the Middle East, where ozone measurements are scant. In this study, the GEOS-Chem chemical transport model is used to interpret newly available tropospheric ozone data from the Tropospheric Emission Spectrometer (TES) satellite instrument. TES reveals elevated ozone in the mid-troposphere (500-300 hPa) over the Middle East in summer 2005. This study demonstrates that the Arabian anticyclone in the mid-troposphere over the Middle East plays a critical role in facilitating the buildup of ozone. Additionally, the South Asian High in the upper troposphere helps transport ozone from the Asian monsoon region. Transport from Asia and local production are predominantly responsible for the ozone buildup, each contributing 30-35% to the ozone abundance in the region. Ozone transported from the boundary layer accounts for about 25% of local production. TES retrievals of water vapour and deuterated water are used for the first time to provide an independent assessment of the ozone transport pathways. Using a GEOS-Chem simulation from 1987 to 2006, it is found that this ozone buildup fluctuates interannually by about ±7% (or ±6 ppbv). The major contributors, ozone transported from Asia and ozone produced locally, vary by ±30% (±7 ppbv) and ±15% (±3 ppbv), respectively. The variations of Asian and local sources are related to the strengths of the South Asian High and the Arabian anticyclone, respectively. It is found that in years when the Asian influence is weaker in the region, transport from other areas, such as North America, is enhanced. This tradeoff between transport from Asia and other regions is found to be linked to the position and strength of the subtropical westerly jet over central Asia. These results suggest that climate-related changes in the general circulation of the atmosphere will have implications for the transport of pollution into the Middle East. Such changes in pollution in the region could have feedbacks on the climate through changes in the radiative forcing associated with tropospheric ozone.

Tropospheric ozone is a major atmospheric pollutant and a greenhouse gas. Nevertheless, many processes influencing its spatio-temporal distribution are still poorly understood, mainly due to the lack of adequate observations. One such region is the Middle East, where ozone measurements are scant. In this study, the GEOS-Chem chemical transport model is used to interpret newly available tropospheric ozone data from the Tropospheric Emission Spectrometer (TES) satellite instrument. TES reveals elevated ozone in the mid-troposphere (500-300 hPa) over the Middle East in summer 2005. This study demonstrates that the Arabian anticyclone in the mid-troposphere over the Middle East plays a critical role in facilitating the buildup of ozone. Additionally, the South Asian High in the upper troposphere helps transport ozone from the Asian monsoon region. Transport from Asia and local production are predominantly responsible for the ozone buildup, each contributing 30-35% to the ozone abundance in the region. Ozone transported from the boundary layer accounts for about 25% of local production. TES retrievals of water vapour and deuterated water are used for the first time to provide an independent assessment of the ozone transport pathways. Using a GEOS-Chem simulation from 1987 to 2006, it is found that this ozone buildup fluctuates interannually by about ±7% (or ±6 ppbv). The major contributors, ozone transported from Asia and ozone produced locally, vary by ±30% (±7 ppbv) and ±15% (±3 ppbv), respectively. The variations of Asian and local sources are related to the strengths of the South Asian High and the Arabian anticyclone, respectively. It is found that in years when the Asian influence is weaker in the region, transport from other areas, such as North America, is enhanced. This tradeoff between transport from Asia and other regions is found to be linked to the position and strength of the subtropical westerly jet over central Asia. These results suggest that climate-related changes in the general circulation of the atmosphere will have implications for the transport of pollution into the Middle East. Such changes in pollution in the region could have feedbacks on the climate through changes in the radiative forcing associated with tropospheric ozone.

Observations show that the interannual variability of the Northern Hemisphere (NH) extratropical winter stratosphere is strongly correlated with the quasi-biennial oscillation (QBO) of tropical stratospheric winds, particularly during early winter. Most current general circulation models (GCMs) do not exhibit a QBO and therefore do not represent this important mode of tropical-extratropical interaction. In this study we examine the QBO-extratropical correlation using a 150-year GCM simulation in which a QBO occurs. Since no external forcings or interannual variations in sea surface temperatures are imposed, the modelled tropical-extratropical interactions represent an internal mode of atmospheric variability. The QBO itself is spontaneously forced by a combination of resolved and parameterized waves. The effects of this QBO on the climatological mean state and its interannual variability are considered, both by comparison with a control simulation (also 150 years in length, but with no QBO) and by compositing winters according to the phase of the QBO. Careful attention is given to the definition of QBO phase. Comparisons of the model results with observations (reanalysis data) are also made. QBO-induced changes in the climatological state of the model are found to have high statistical significance above the tropopause. In the extratropical winter stratosphere, these mean-state changes arise predominantly from the influence of the QBO on the propagation and dissipation of planetary-scale waves. This behaviour is shown to depend on the seasonal cycle, which argues for the usefulness of considering tropical-extratropical interactions in a GCM context. QBO influence on the interannual variability of the extratropical winter stratosphere is also seasonal, and the tropical-extratropical interaction is sensitive to the phase alignment of the QBO with respect to the annual cycle. This phase alignment is strongly affected by the seasonality of QBO phase transitions, which - due to the QBO being spontaneously generated, rather than having an imposed period - is somewhat realistic in the model. This leads to fluctuations in the strength of the modelled tropical-extratropical interaction occurring on a decadal timescale as an internal mode of atmospheric variability.

Observations show that the interannual variability of the Northern Hemisphere (NH) extratropical winter stratosphere is strongly correlated with the quasi-biennial oscillation (QBO) of tropical stratospheric winds, particularly during early winter. Most current general circulation models (GCMs) do not exhibit a QBO and therefore do not represent this important mode of tropical-extratropical interaction. In this study we examine the QBO-extratropical correlation using a 150-year GCM simulation in which a QBO occurs. Since no external forcings or interannual variations in sea surface temperatures are imposed, the modelled tropical-extratropical interactions represent an internal mode of atmospheric variability. The QBO itself is spontaneously forced by a combination of resolved and parameterized waves. The effects of this QBO on the climatological mean state and its interannual variability are considered, both by comparison with a control simulation (also 150 years in length, but with no QBO) and by compositing winters according to the phase of the QBO. Careful attention is given to the definition of QBO phase. Comparisons of the model results with observations (reanalysis data) are also made. QBO-induced changes in the climatological state of the model are found to have high statistical significance above the tropopause. In the extratropical winter stratosphere, these mean-state changes arise predominantly from the influence of the QBO on the propagation and dissipation of planetary-scale waves. This behaviour is shown to depend on the seasonal cycle, which argues for the usefulness of considering tropical-extratropical interactions in a GCM context. QBO influence on the interannual variability of the extratropical winter stratosphere is also seasonal, and the tropical-extratropical interaction is sensitive to the phase alignment of the QBO with respect to the annual cycle. This phase alignment is strongly affected by the seasonality of QBO phase transitions, which - due to the QBO being spontaneously generated, rather than having an imposed period - is somewhat realistic in the model. This leads to fluctuations in the strength of the modelled tropical-extratropical interaction occurring on a decadal timescale as an internal mode of atmospheric variability.

The motivation for this thesis was to study chemical and dynamical processes in the Arctic stratosphere, using data from two ground-based spectrometers (GBSs). The GBSs took atmospheric trace gas measurements at the Polar Environment Atmospheric Research Laboratory (PEARL), which is located at Eureka, Nunavut, Canada (80.05°N, 86.42°W) and operated by the Canadian Network for the Detection of Atmospheric Change. The University of Toronto GBS took measurements at Eureka on a campaign basis from 1999 2011. The PEARL GBS was installed permanently at Eureka in 2006 and has taken measurements during the sunlit part of the year since then. GBS and other ground based ozone and NO2 column measurements were compared with Atmospheric Chemistry Experiment (ACE) and Optical Spectrograph and Infra Red Imaging System (OSIRIS) satellite measurements above Eureka. Ozone from all instruments agreed within 9.2%, while NO2 from most instruments, including the GBS, agreed to within 20%. On 1 August 2008, a solar eclipse of 98% totality passed over Eureka. GBS NO2 increased to 1.84 times normal levels. This agrees with a ratio of 1.91 that was calculated using a photochemical model, adjusted for reduced sunlight during the eclipse. In spring/winter 2011, up to 47% (250 DU) ozone loss was calculated using GBS and modeled passive ozone. This was the largest ozone loss in the 11 year GBS measurement record. GBS OClO was elevated, indicating chlorine activation and NO2 was low, suggesting denitrification. GBS, satellite, and chemical transport model data were used to investigate the 2011 vortex breakup. NOx transport led to middle stratosphere ozone loss within an anticyclone. Furthermore, isolated, or “frozen-in”, vortex and lower-latitude airmasses were observed following the vortex breakup. Stratospheric BrO was retrieved from spring 2008 GBS zenith sky measurements, using an optimal estimation technique. GBS BrO was compared with OSIRIS and Ozone Monitoring Instrument (OMI) satellite data. Discrepancies are partly attributed to bromine explosions in the boundary layer. New off axis GBS measurements taken in spring 2010 are sensitive to boundary layer bromine. The combination of GBS stratospheric and tropospheric BrO measurements will be useful for future estimates of the Arctic bromine budget.

Atmospheric carbon monoxide (CO) is a product of incomplete combustion and a byproduct of the oxidation of hydrocarbons. It plays a key role in controlling the oxidative capacity of the atmosphere since it is the main sink for the hydroxyl radical (OH), the primary tropospheric oxidant. As a result of its lifetime, CO is a useful tracer of long-range transport in models. However, estimates of the regional sources of CO are uncertain. Inverse modeling has become a widely used approach for better quantifying the sources, but a fundamental assumption in these inversions, which is typically not valid, is that the observations and models are unbiased. In this thesis, the GEOS-Chem model and observations of CO from the Measurement Of Pollution In The Troposphere (MOPITT) instrument are employed to study the impact of systematic model errors on inversion analyses of CO. The impact of the treatment of biogenic non-methane volatile organic compounds (NMVOCs), aggregation errors, and discrepancies in the meteorological fields and OH distribution on the CO source estimates are examined. The influence of vertical transport errors on the source estimates is assessed using newly available MOPITT version 5 (V5) retrievals in a comparative inversion analysis employing surface level, profile, and column data. To quantify the potential impact of discrepancies in long-range transport on the source estimates, a high-resolution, regional inversion over North America, with optimized lateral boundary conditions, was conducted and compared with the results of a global inversion. The influence of the spatial-temporal distribution of the observations on the source estimates was also assessed through a comparison of the inversion analyses of MOPITT data and aircraft data from the Intercontinental Transport Experiment – North America, Phase A (INTEX-A) aircraft campaign. The results presented in the thesis provide a more comprehensive understanding of the potential impact of system model errors on inversion analyses of CO. This work also represents the first inverse modeling analysis of the MOPITT v5 retrievals. The results demonstrate the potential utility of these new data for characterizing vertical transport errors in models and they reveal that the new data can provide reliable constraints in regional CO source estimates.

The motivation for this thesis was to study chemical and dynamical processes in the Arctic stratosphere, using data from two ground-based spectrometers (GBSs). The GBSs took atmospheric trace gas measurements at the Polar Environment Atmospheric Research Laboratory (PEARL), which is located at Eureka, Nunavut, Canada (80.05°N, 86.42°W) and operated by the Canadian Network for the Detection of Atmospheric Change. The University of Toronto GBS took measurements at Eureka on a campaign basis from 1999 2011. The PEARL GBS was installed permanently at Eureka in 2006 and has taken measurements during the sunlit part of the year since then. GBS and other ground based ozone and NO2 column measurements were compared with Atmospheric Chemistry Experiment (ACE) and Optical Spectrograph and Infra Red Imaging System (OSIRIS) satellite measurements above Eureka. Ozone from all instruments agreed within 9.2%, while NO2 from most instruments, including the GBS, agreed to within 20%. On 1 August 2008, a solar eclipse of 98% totality passed over Eureka. GBS NO2 increased to 1.84 times normal levels. This agrees with a ratio of 1.91 that was calculated using a photochemical model, adjusted for reduced sunlight during the eclipse. In spring/winter 2011, up to 47% (250 DU) ozone loss was calculated using GBS and modeled passive ozone. This was the largest ozone loss in the 11 year GBS measurement record. GBS OClO was elevated, indicating chlorine activation and NO2 was low, suggesting denitrification. GBS, satellite, and chemical transport model data were used to investigate the 2011 vortex breakup. NOx transport led to middle stratosphere ozone loss within an anticyclone. Furthermore, isolated, or “frozen-in”, vortex and lower-latitude airmasses were observed following the vortex breakup. Stratospheric BrO was retrieved from spring 2008 GBS zenith sky measurements, using an optimal estimation technique. GBS BrO was compared with OSIRIS and Ozone Monitoring Instrument (OMI) satellite data. Discrepancies are partly attributed to bromine explosions in the boundary layer. New off axis GBS measurements taken in spring 2010 are sensitive to boundary layer bromine. The combination of GBS stratospheric and tropospheric BrO measurements will be useful for future estimates of the Arctic bromine budget.

Atmospheric carbon monoxide (CO) is a product of incomplete combustion and a byproduct of the oxidation of hydrocarbons. It plays a key role in controlling the oxidative capacity of the atmosphere since it is the main sink for the hydroxyl radical (OH), the primary tropospheric oxidant. As a result of its lifetime, CO is a useful tracer of long-range transport in models. However, estimates of the regional sources of CO are uncertain. Inverse modeling has become a widely used approach for better quantifying the sources, but a fundamental assumption in these inversions, which is typically not valid, is that the observations and models are unbiased. In this thesis, the GEOS-Chem model and observations of CO from the Measurement Of Pollution In The Troposphere (MOPITT) instrument are employed to study the impact of systematic model errors on inversion analyses of CO. The impact of the treatment of biogenic non-methane volatile organic compounds (NMVOCs), aggregation errors, and discrepancies in the meteorological fields and OH distribution on the CO source estimates are examined. The influence of vertical transport errors on the source estimates is assessed using newly available MOPITT version 5 (V5) retrievals in a comparative inversion analysis employing surface level, profile, and column data. To quantify the potential impact of discrepancies in long-range transport on the source estimates, a high-resolution, regional inversion over North America, with optimized lateral boundary conditions, was conducted and compared with the results of a global inversion. The influence of the spatial-temporal distribution of the observations on the source estimates was also assessed through a comparison of the inversion analyses of MOPITT data and aircraft data from the Intercontinental Transport Experiment – North America, Phase A (INTEX-A) aircraft campaign. The results presented in the thesis provide a more comprehensive understanding of the potential impact of system model errors on inversion analyses of CO. This work also represents the first inverse modeling analysis of the MOPITT v5 retrievals. The results demonstrate the potential utility of these new data for characterizing vertical transport errors in models and they reveal that the new data can provide reliable constraints in regional CO source estimates.

A ground-based, zenith-sky, UV-visible triple grating spectrometer was installed at the Polar Environment Atmospheric Research Laboratory (PEARL) in the Canadian High Arctic during polar springtime from 2004 to 2007 as part of the Canadian Arctic ACE (Atmospheric Chemistry Experiment) Validation Campaigns. From the solar spectra, ozone, NO2, and BrO vertical column densities (VCDs) have been retrieved using the DOAS (Differential Optical Absorption Spectroscopy) technique. This spectrometer, the UT-GBS (University of Toronto Ground-Based Spectrometer), was also deployed as part of the fourth Middle Atmosphere Nitrogen TRend Assessment (MANTRA) campaign in Vanscoy, Saskatchewan in August and September 2004. A near-identical spectrometer, the PEARL-GBS, was permanently installed at PEARL in August 2006 as part of the refurbishment of the laboratory by CANDAC (Canadian Network for the Detection of Atmospheric Change). Since then, the instrument has been making continuous measurements, with the exception of during polar night. Vertical columns of ozone and NO2 can be retrieved year-round. During the 2007 sunrise campaign, differential slant column densities (DSCDs) of OClO and VCDs of BrO were also retrieved. Ozone and NO2 DSCDs and VCDs from the UT-GBS were compared to the DSCDs and VCDs from three other UV-visible, ground-based, grating spectrometers that also participated in the MANTRA and Eureka campaigns. Two methods developed by the UV-visible Working Group of the NDACC (Network for the Detection of Atmospheric Composition Change) were followed. During MANTRA, the instruments were found to partially meet the NDACC standards. The comparisons from Eureka were an improvement on the MANTRA comparisons, and also partially met the NDACC standards. In 2007, the columns from the UT-GBS and PEARL-GBS were compared, and were found to agree within the NDACC standards for both species. Ozone and NO2 VCDs from the ground-based instruments were also compared to integrated partial columns from the ACE-FTS (ACE-Fourier Transform Spectrometer) and ACE-MAESTRO (ACE-Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) on board the ACE satellite. ACE-FTS partial columns were found to agree with the ground-based total columns, while the ACE-MAESTRO partial columns were found to be smaller than expected for ozone and larger than expected for NO2.

The measurement of trace gases through remote sounding techniques has led to a better understanding of the processes controlling the structure and variability of the stratosphere. Differences between measurements over space and time are due to atmospheric variability and instrument errors: thus, comparison of measurements can be used to test our knowledge of both. Comparisons of measurements over long time periods are used to identify trends. Balloon-borne infrared emission radiometer instruments have been used to make measurements of midlatitude stratospheric HNO3 spanning a period of twelve years. The timing of the measurements is notable, since they occur before and well after the eruption of Mt. Pinatubo, which significantly perturbed HNO3 levels, complicating prior trend analyses. No significant differences are found between the HNO3 retrievals, although large measurement uncertainties preclude any conclusion concerning trends. Comparisons of measurements that are closely spaced in space and time are useful for satellite validation, where one aims to reduce the effect of atmospheric variability on the estimation of systematic and random errors. A novel technique for the estimation of systematic error, which differentiates between additive and multiplicative bias, is introduced. In a comparison of measurements by the ACE-FTS and Aura MLS instruments, significant multiplicative biases are identified and described. In order to validate the reported random errors (RREs) of measurements, satellite validation studies often focus on measurements in the tropical stratosphere, where variability is weak. The scatter in tropical measurements can then be used as an upper limit on instrument precision. In an analysis of tropical measurements by the ACE-FTS, scatter is found to be roughly consistent with the RREs for H2O and CO. The scatter in measurements of O3, HNO3, and N2O, while larger than the reported random errors, is roughly consistent with the variability simulated in the Canadian Middle Atmosphere Model. This work implies that the random error of the ACE-FTS measurements is smaller than the weak natural variability of the tropical stratosphere.