Quantifying Sources of Nitrogen Oxides in Remote Environments: From Biosphere-Atmosphere Exchange to Renoxification

Nitrogen oxides (NOx = NO + NO2) are an important constituent to the global atmosphere due to their strong connection to ozone (O3) and hydroxyl radical (OH). Anthropogenic NOx emissions have been regulated worldwide, leading to a decreasing trend over the past 15 years in North America and Europe, which makes biogenic sources and recycling pathways more important, especially in remote regions that are less influenced by human activities.A two-channel chemiluminescence instrument with high precision and time resolution was used to quantify NOx in the field and the laboratory. In the field, direct measurements of biosphere-atmosphere exchange of NOx were made above a temperate broadleaf forest using the eddy covariance flux method. Near-zero NOx fluxes suggest that the soil NO emissions barely escape the canopy, and an air mass back trajectory analysis indicates that the morning NOx maximum is mainly associated with long-range transport of pollution. Flux divergence observed at this site is a consequence of fast chemical processes and different light intensity above and below the canopy. In the laboratory, NOx and other products from the photolysis of suspended inorganic particulate nitrate were measured in chamber experiments as a function of wavelength, relative humidity, and presence of photosensitizers. The photolysis rate constants of particulate nitrate measured are within an order of magnitude of that of gas-phase HNO3. This finding is in contrast with several studies showing a larger enhancement factor (EF) of renoxification of particle nitrate relative to HNO3. Different values of EF were applied to monitoring data from Environment and Climate Change Canada, to estimate NOx production from the photolysis of particulate nitrate and HNO3. Results were then compared with soil NOx emissions from a global model, showing that renoxification could potentially compete with biogenic NOx in remote environments and needs further investigation.Ph.D