This thesis explores the sources, transport, and fate of reactive
nitrogen in the atmosphere. Nitrogen oxides (NOx) play a major
role in the chemistry of the troposphere by influencing the main
oxidants of the atmosphere, ozone and hydroxyl radical (OH),
and impact the global biosphere through formation of nitric acid
(HNO3), a significant contributor to acid rain.

The goal of both the modeling and measurement studies
presented here is to assess the controls on NOx emissions,
distribution and chemistry. A new biomass burning source,
including six different types of biomass, is used as input into the
National Oceanic and Atmospheric Administration’s Geophysical
Fluid Dynamics Laboratory three-dimensional global chemical
transport model (GFDL GCTM) to quantify the contribution from
biomass burning emissions to the global distribution of nitrogen
oxides (NOx), carbon monoxide (CO), and tropospheric ozone.
The GFDL GCTM is also used to address the episodic nature of
pollution transported across the Pacific Ocean from Asia.
Comparison with observations from the west coast of the United
States reveals that the model realistically simulates the
magnitude and frequency of short-term fluctuations. The impact
of Asian emissions on NOx and CO, and indirectly on ozone
production is investigated in present and future emissions
scenarios.

Stable isotope measurements of HNO3 (or nitrate) offers a tool
for the study of the sources and chemistry contributing to HNO3
deposition. Measurement of the 15N/14N and 18O/16O isotope
ratios of nitrate in rain collected on the island of Bermuda and in
snow from Summit, Greenland, reveals seasonal variations in
both 15N/14N and 18O/16O. Results from both studies
demonstrate that the 18O/16O of nitrate is dependent upon
NOx oxidation processes. The higher 15N/14N of nitrate in
Bermuda rain during the warm season, and Greenland snow
during spring and summer, suggest a change in the NOx
sources that contribute to nitrate deposition. The interpretation
of 15N/14N and 18O/16O fluctuations in terms of the sources
and chemistry of HNO3 is consistent with global and regional
model predictions. The isotopic composition of nitrate
represents a new tool for the study of changes in NOx sources
and chemistry relative to changes in climate, and results from a
Greenland ice core show significant changes in the isotopic
composition of nitrate between the Holocene and last glacial
period. This has implications for future studies of the connection
between the biosphere, atmospheric composition and climate
change.