Observed distributions of nitrogen oxides in the remote free troposphere from the Nasa Global Tropospheric Experiment Programs

Reactive oxides of nitrogen are critical catalysts that can limit the formation rates of tropospheric oxidants. As a result, they control the efficiency with which the troposphere cleanses itself of the many natural and artificial compounds that would otherwise reach toxic levels. They play a pivotal role in the troposphere's capacity to generate O3 via photochemical processes. Even at concentrations of a few parts per trillion by volume, as are typically seen in the remote tropical Pacific and Arctic/Antarctic regions, reactive nitrogen oxides lead to ozone formation at rates estimated to be several times larger than the influx of ozone from the stratosphere. Reflecting its multilevel importance in our evolving chemical environment, significant effort has been expended toward defining the factors that control the global distribution of reactive nitrogen oxides. Developing this understanding, however, has not been a simple task. The chemical reactivities that are inherently associated with these compounds allow them to undergo transformations on timescales that range from minutes to months. In addition, a wide variety of sources exist for this family. Some of these are near the Earth's surface (e.g., by-products of emissions from fossil fuel combustion, biomass burning, and nitrification/denitrification of soils), whereas others are within the troposphere itself (i.e., lightning, emissions from subsonic aircraft, stratospheric intrusions, and the oxidation of reduced nitrogen compounds). As a result, large temporal and regional variations can be expected and in fact are found from this array of sources. The variations are dependent on factors ranging from the effects of soil moisture on microbial activity to the convective potential of cumulus clouds. Recent airborne field measurement campaigns have made significant progress toward unraveling many of the NOx distribution controlling factors, particularly as they relate to the remote troposphere. This review concentrates on the progress that has been made during the last 15 years. The major focus is on compounds within the reactive nitrogen oxide family, together with associated chemically coupled species. The measurements reported are predominately those from airborne platforms that have been part of NASA's Global Tropospheric Experiment (GTE) program. However, comparison is also made with other prominent airborne studies including NASA's Airborne Arctic Stratospheric Experiment (AASE) I and II as well as the non-U.S. programs Stratospheric Ozone Experiment (STRATOZ) II and III and International Stratospheric Chemistry (INSTAC) 1. Major findings from these programs as reflected in this review can be summarized as follows: (1) Throughout much of the remote troposphere, NOx levels (inferred from NO observations) are at sufficient levels to significantly impact on the photochemistry of this region. This was found to be particularly true in regard to the O3 production. Thus average column production of O3 in the remote troposphere is estimated to be several times the average stratospheric intrusion flux of O3. (2) A persistent characteristic of observed NO is that mixing ratios in the remote upper troposphere are generally enhanced by a factor of 3 or more compared with middle and lower altitudes. This typically results in net O3 production in the upper troposphere and net loss at the lowest tropospheric altitudes. (3) The largest source of NOx in the upper troposphere appears to be from lightning. However, convection of surface sources such as fossil fuel combustion and biomass burning were also found to be important. In addition, recycling of NOx through other odd nitrogen species may contribute significantly to maintaining the level of NOx, particularly in the upper troposphere, but much uncertainty remains concerning the mechanisms responsible for this. (4) Substantial uncertainties also still exist in the NOx production rate from lightning and from oxidation of NH3.

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