Nitrogen is most abundant in the atmosphere in its molecular form, N2, which comprises 78% of Earth's atmosphere. Although virtually inert and of no direct consequence to tropospheric chemistry, this vast reservoir of atmospheric nitrogen enables the existence of trace levels of nitrogen oxides that play a number of critical roles in the chemistry of the atmosphere. Nitrogen oxides, commonly referred to as NOx, are defined by the sum of the chemical species NO and N02. These two atmospheric constituents are grouped for convenience due to their fast photochemical cycling, which brings them into equilibrium generally within a few minutes. The greater family of reactive nitrogen, conventionally denoted by the term NO,,, consists of NOx as well as a suite of other compounds including N03, N205, HN03, HONO, H02N02, peroxyacetylnitrate (PAN), and a wide array of other organic nitrogen-containing species. These compounds play important roles in the removal of reactive nitrogen from the atmosphere as well as the transport of reactive nitrogen from source regions to remote areas.

Tropospheric chemical cycles involving NOx are of fundamental importance to understanding several key atmospheric issues. For instance, the tropospheric ozone abundance is largely regulated by catalytic photochemical cycles involving NOx, CO, and hydrocarbons that produce ozone (see Chapter 3). NO, often represents the rate-limiting precursor for ozone production, especially throughout the remote atmosphere. This is due to its short lifetime relative to other precursors. On a regional scale, the role of NOx in creating high concentrations of ozone detrimental to human health is a major air quality issue in many urban areas. On a global scale, the impact

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of NOr on ozone represents an important factor in determining the oxidizing capacity of the atmosphere. NO, further impacts atmospheric oxidation rates by regulating OH concentrations, especially at high altitudes and latitudes. Since the primary mechanism for removing many pollutant gases from the atmosphere is reaction with OH, NO, is important to the atmosphere's ability to cleanse itself. This in turn relates to the issue of climate change regarding removal of greenhouse gases such as CH4. Another important link to the issue of climate change involves the impact of NO, on ozone production in the upper troposphere where it is most effective as a greenhouse gas.

The tropospheric distribution of NO, is complicated by a combination of diverse sources. Natural as well as anthropogenic sources exist both at the surface (e.g., soil emissions, biomass burning, and fossil fuel combustion) and in the free troposphere (e.g., lightning, aircraft, and stratosphere-troposphere exchange). Regeneration of NOr through chemical recycling of various NOv species represents a secondary source of NO, in the troposphere. Also, the atmospheric lifetime of NO, ranges from hours to days depending predominantly on altitude. As a result, NO, mixing ratios vary from a few parts per trillion in some remote regions to several parts per billion in highly polluted conditions. Given the high variability of NO, and its importance to several key atmospheric issues, the global NO, distribution represents a pivotal subject in efforts to fully understand the current state of our atmosphere as well as its future evolution.

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