In urban areas, emissions of nitrogen oxide radicals (NOx ≡ NO + NO2) to the atmosphere from anthropogenic activities such as fossil fuel combustion contribute to poor air quality through the production of ozone and particulate matter. Soils are also a significant global source of NOx, but at downind forest environments the deposition of transported reactive nitrogen can be much more important than local emissions. Data from a government monitoring network in the Toronto area from 2000-2007 was used to explore the impact of long-term trends in NO2 and other ozone precursors on local ozone levels. Non-linear chemistry and the influence of meteorology explained why reductions in precursor levels during this period did not lead to significant improvements in ozone. Data from this network was also used to investigate the ability of a satellite-borne spectrometer to represent spatial patterns of ground-level NO2 in the same region. Selection biases, resulting from the need to discard satellite data on cloudy days, were shown to affect locations differently and were most severe at a receptor site. The sum of all reactive nitrogen oxides including NOx is known as NOy. A custom-built instrument for high precision and time resolution measurements of reactive nitrogen oxides was tested under various lab and field conditions, and used in field work where direct biosphere-atmosphere exchange of NOy was measured by eddy covariance above two comparable North American mixed forests (Haliburton Forest Wildlife Reserve and the University of Michigan Biological Station). While these forests were found to be small net sources of NOx, they were subject to elevated rates of NOy deposition overall, driven by the transport of polluted air from upwind source regions. Wet deposition measurements were used to show that dry deposition contributed a significant fraction of total deposition during the observation periods.

In urban areas, emissions of nitrogen oxide radicals (NOx ≡ NO + NO2) to the atmosphere from anthropogenic activities such as fossil fuel combustion contribute to poor air quality through the production of ozone and particulate matter. Soils are also a significant global source of NOx, but at downind forest environments the deposition of transported reactive nitrogen can be much more important than local emissions. Data from a government monitoring network in the Toronto area from 2000-2007 was used to explore the impact of long-term trends in NO2 and other ozone precursors on local ozone levels. Non-linear chemistry and the influence of meteorology explained why reductions in precursor levels during this period did not lead to significant improvements in ozone. Data from this network was also used to investigate the ability of a satellite-borne spectrometer to represent spatial patterns of ground-level NO2 in the same region. Selection biases, resulting from the need to discard satellite data on cloudy days, were shown to affect locations differently and were most severe at a receptor site. The sum of all reactive nitrogen oxides including NOx is known as NOy. A custom-built instrument for high precision and time resolution measurements of reactive nitrogen oxides was tested under various lab and field conditions, and used in field work where direct biosphere-atmosphere exchange of NOy was measured by eddy covariance above two comparable North American mixed forests (Haliburton Forest Wildlife Reserve and the University of Michigan Biological Station). While these forests were found to be small net sources of NOx, they were subject to elevated rates of NOy deposition overall, driven by the transport of polluted air from upwind source regions. Wet deposition measurements were used to show that dry deposition contributed a significant fraction of total deposition during the observation periods.