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.

The number and size distributions of particles between 10 and 400 nm were measured in Toronto and rural Egbert during May 2007 to May 2008 to compare nucleation and growth at paired urban and rural locations. Particle formation and growth were observed in Egbert more frequently than in Toronto, and simultaneous events occurred on 34 out of 368 days. In contrast, formation and growth rates were both higher in Toronto. Further, a linear regression analysis suggested that compounds contributing to nucleation and growth processes were different in Toronto and Egbert. Vehicular emissions seemed to suppress particle formation in downtown Toronto. Nucleation also appeared to be suppressed by long-range transported pollutants originating from industrial regions in southwestern Ontario and northern Ohio in the United States. A Nucleation Indicator (NI) was developed by combining relevant parameters, and it was found to provide a reasonable measure of the probability of nucleation events occurring.

The number and size distributions of particles between 10 and 400 nm were measured in Toronto and rural Egbert during May 2007 to May 2008 to compare nucleation and growth at paired urban and rural locations. Particle formation and growth were observed in Egbert more frequently than in Toronto, and simultaneous events occurred on 34 out of 368 days. In contrast, formation and growth rates were both higher in Toronto. Further, a linear regression analysis suggested that compounds contributing to nucleation and growth processes were different in Toronto and Egbert. Vehicular emissions seemed to suppress particle formation in downtown Toronto. Nucleation also appeared to be suppressed by long-range transported pollutants originating from industrial regions in southwestern Ontario and northern Ohio in the United States. A Nucleation Indicator (NI) was developed by combining relevant parameters, and it was found to provide a reasonable measure of the probability of nucleation events occurring.

Accurate measurements of the trace nitrogenous atmospheric species amines (NR3) and nitrous acid (HONO) are essential to understanding their chemistry and potential influence on new particle formation and oxidation capacity in the atmospheric boundary-layer, respectively. Ambient Ion Monitor – Ion Chromatography (AIM-IC) methods developed in this work have provided quantitative online observations of atmospheric amines in the gaseous and condensed phases with detection limits of pptv and ng m-3 at hourly time resolution. Size-resolved particle observations demonstrated maximum amine mass loadings in 320 – 560 nm particles, and an increase in importance relative to ammonium for the smallest particles measured (56 – 180 nm). In particular, the size-resolved samples analysed in this work indicate that bulk aerosol measurements may not be appropriate for modelling the atmospheric processes that govern the incorporation of amines and ammonia in to atmospheric particles. Measurements of HONO made during the two intensive field campaigns (NACHTT, CalNex) and a lab study provided a new perspective on the interactions of this trace compound with ground surfaces. Integrated atmospheric column measurements of HONO and NO2 during NACHTT provided clear evidence that the ground surface dominates HONO production and loss at night. Simultaneous measurements of the gas and particle phases made by the AIM-IC system during CalNex demonstrated the potential for reactive uptake of HONO on mineral dust/soil as a nocturnal sink. Similarly, the potential for nitrite salts to react with strong acids, displacing HONO during the day was suggested by this dataset. Lab study results showed that HONO is taken up irreversibly on carbonate salts and real soil extracts. Relative humidity-dependent reactive uptake coefficients were derived. Subsequent release of HONO by displacement reactions with HNO3 and HCl was also confirmed. Together, these field and lab studies have produced a new picture of HONO surface interactions by providing i) a more explicit description of a nocturnal sink of HONO that could act as a surface reservoir and ii) a new mechanism for daytime HONO formation that does not require NO2.

Accurate measurements of the trace nitrogenous atmospheric species amines (NR3) and nitrous acid (HONO) are essential to understanding their chemistry and potential influence on new particle formation and oxidation capacity in the atmospheric boundary-layer, respectively. Ambient Ion Monitor – Ion Chromatography (AIM-IC) methods developed in this work have provided quantitative online observations of atmospheric amines in the gaseous and condensed phases with detection limits of pptv and ng m-3 at hourly time resolution. Size-resolved particle observations demonstrated maximum amine mass loadings in 320 – 560 nm particles, and an increase in importance relative to ammonium for the smallest particles measured (56 – 180 nm). In particular, the size-resolved samples analysed in this work indicate that bulk aerosol measurements may not be appropriate for modelling the atmospheric processes that govern the incorporation of amines and ammonia in to atmospheric particles. Measurements of HONO made during the two intensive field campaigns (NACHTT, CalNex) and a lab study provided a new perspective on the interactions of this trace compound with ground surfaces. Integrated atmospheric column measurements of HONO and NO2 during NACHTT provided clear evidence that the ground surface dominates HONO production and loss at night. Simultaneous measurements of the gas and particle phases made by the AIM-IC system during CalNex demonstrated the potential for reactive uptake of HONO on mineral dust/soil as a nocturnal sink. Similarly, the potential for nitrite salts to react with strong acids, displacing HONO during the day was suggested by this dataset. Lab study results showed that HONO is taken up irreversibly on carbonate salts and real soil extracts. Relative humidity-dependent reactive uptake coefficients were derived. Subsequent release of HONO by displacement reactions with HNO3 and HCl was also confirmed. Together, these field and lab studies have produced a new picture of HONO surface interactions by providing i) a more explicit description of a nocturnal sink of HONO that could act as a surface reservoir and ii) a new mechanism for daytime HONO formation that does not require NO2.

It has been commonly recognized that a series of global glaciation events occurred during the late Neoproterozoic Era (800 - 540 million years ago (Ma)). However, the extent of these glaciations continues to be hotly debated, namely whether the whole Earth was ice covered (ie. a “hard snowball”) or only the continents were fully ice covered but the oceans were not (“slushball/soft snowball”). Through a combination of climate modeling and carbon cycle modeling, I have investigated the plausibility of the “soft snowball” Earth hypothesis. It is demonstrated that the flow of land ice is critical to the formation of a “soft snowball”, such that low latitude land ice must be generated through ice transported from high latitudes. In order for a climate state of this kind to be realizable, continental fragments at low latitude must be well connected to those at high latitude, and the high latitude continents must be sufficiently extensive that a large ice sheet may initiate and subsequently flow to low latitude. It is found that these constraints are satisfied by the most accurate available continental reconstruction for both the initial Sturtian glaciation of the late Neoproterozoic and the subsequent Marinoan event. It is furthermore proposed that the alternative “hard snowball” hypothesis would have been prevented by a negative feedback due to the enhanced remineralization of dissolved organic carbon (DOC) in the ocean due to increased oxygen solubility in seawater at lower temperature. This process would release CO2 to the atmosphere, thus counteracting the initial climate cooling. I have also carried out detailed simulations in which an explicit model of the carbon cycle is coupled to the ice-sheet coupled climate model to investigate this feedback quantitatively. It is found that the remineralization of the DOC does indeed provide a strong negative feedback that counteracts climate cooling. The action of this feedback not only prevents the descent of the climate into a hard snowball state, but also enables the model to re-produce the δ13C carbon isotopic anomalies observed to accompany Neoproterozoic glacial events. The resistance of this carbon cycle coupled climate system to descent into a “hard snowball” state is further tested against stochastic perturbations, and shown to be robust in the presence of such influence.

It has been commonly recognized that a series of global glaciation events occurred during the late Neoproterozoic Era (800 - 540 million years ago (Ma)). However, the extent of these glaciations continues to be hotly debated, namely whether the whole Earth was ice covered (ie. a “hard snowball”) or only the continents were fully ice covered but the oceans were not (“slushball/soft snowball”). Through a combination of climate modeling and carbon cycle modeling, I have investigated the plausibility of the “soft snowball” Earth hypothesis. It is demonstrated that the flow of land ice is critical to the formation of a “soft snowball”, such that low latitude land ice must be generated through ice transported from high latitudes. In order for a climate state of this kind to be realizable, continental fragments at low latitude must be well connected to those at high latitude, and the high latitude continents must be sufficiently extensive that a large ice sheet may initiate and subsequently flow to low latitude. It is found that these constraints are satisfied by the most accurate available continental reconstruction for both the initial Sturtian glaciation of the late Neoproterozoic and the subsequent Marinoan event. It is furthermore proposed that the alternative “hard snowball” hypothesis would have been prevented by a negative feedback due to the enhanced remineralization of dissolved organic carbon (DOC) in the ocean due to increased oxygen solubility in seawater at lower temperature. This process would release CO2 to the atmosphere, thus counteracting the initial climate cooling. I have also carried out detailed simulations in which an explicit model of the carbon cycle is coupled to the ice-sheet coupled climate model to investigate this feedback quantitatively. It is found that the remineralization of the DOC does indeed provide a strong negative feedback that counteracts climate cooling. The action of this feedback not only prevents the descent of the climate into a hard snowball state, but also enables the model to re-produce the δ13C carbon isotopic anomalies observed to accompany Neoproterozoic glacial events. The resistance of this carbon cycle coupled climate system to descent into a “hard snowball” state is further tested against stochastic perturbations, and shown to be robust in the presence of such influence.

A new portable flow tube technique, the Toronto Photo-Oxidation Tube v2.0 was developed and characterized to explore its potential to control and monitor the OH-initiated formation and chemical aging of secondary organic aerosol (SOA) in-situ. The first study investigated the different operational parameters of this flow tube technique. TPOT v2.0 can generate oxidizing conditions equivalent to ambient OH exposures of 2.3 – 10.8 days. The transmission efficiency of a model organic aerosol indicated negligible losses in the oxidation tube. Differences in the residence time distribution curves measured for a gas and model organic aerosol showed that particles were subjected to approximately half of the OH exposure compared to gases. The second study examined the capacity of the TPOT technique to generate secondary aerosols due to OH oxidation. High aerosol yield was observed for H2SO4 particles, whereas a low aerosol yield was observed for α-pinene SOA.

A new portable flow tube technique, the Toronto Photo-Oxidation Tube v2.0 was developed and characterized to explore its potential to control and monitor the OH-initiated formation and chemical aging of secondary organic aerosol (SOA) in-situ. The first study investigated the different operational parameters of this flow tube technique. TPOT v2.0 can generate oxidizing conditions equivalent to ambient OH exposures of 2.3 – 10.8 days. The transmission efficiency of a model organic aerosol indicated negligible losses in the oxidation tube. Differences in the residence time distribution curves measured for a gas and model organic aerosol showed that particles were subjected to approximately half of the OH exposure compared to gases. The second study examined the capacity of the TPOT technique to generate secondary aerosols due to OH oxidation. High aerosol yield was observed for H2SO4 particles, whereas a low aerosol yield was observed for α-pinene SOA.