Measuring reactive nitrogen emissions from point sources using visible spectroscopy from aircraft.

Accurate measurements of nitrogen dioxide (NO2), a key trace gas in the formation and destruction of tropospheric ozone, are important in studies of urban pollution. Nitrogen dioxide column abundances were measured during the Texas Air Quality Study 2000 using visible absorption spectroscopy from an aircraft. The method allows for quantification of the integrated total number of nitrogen dioxide molecules in the polluted atmosphere and is hence a useful tool for measuring plumes of this key trace gas. Further, we show how such remote-sensing observations can be used to obtain information on the fluxes of nitrogen dioxide into the atmosphere with unique flexibility in terms of aircraft altitude, and the height and extent of mixing of the boundary layer. Observations of nitrogen dioxide plumes downwind of power plants were used to estimate the flux of nitrogen oxide emitted from several power plants in the Houston and Dallas metropolitan areas and in North Carolina. Measurements taken over the city of Houston were also employed to infer the total flux from the city as a whole.

Ground-based differential absorption lidar system for day or night measurements of ozone throughout the free troposphere.

The National Oceanic and Atmospheric Administration Aeronomy Laboratory's rapid tunable daylight differential absorption lidar system for monitoring ozone throughout the free troposphere is described. The system components are optimized to provide continuously and rapidly profiles of ozone, day or night, with a vertical resolution of 1 km and an absolute accuracy of +/-10% to the tropopause under clear sky conditions. Routine observations of ozone with frequent error assessments are made by scanning wavelengths between 286 and 292 nm.

Identification and correction of analog-to-digital-converter nonlinearities and their implications for differential absorption lidar measurements.

Differential absorption lidar (DIAL) is a powerful remote-sensing technique widely used to probe the spatial and temporal distribution of ozone and other gaseous atmospheric trace constituents. Although conceptually simple, the DIAL technique presents many challenging and often subtle technical difficulties that can limit its useful range and accuracy. One potentially serious source of error for many DIAL experiments is nonlinearity in the analog-to-digital converters used to capture lidar return signals. The impact of digitizer nonlinearity on DIAL measurements is examined, and a simple and inexpensive low-frequency dithering technique that significantly reduces the effects of ADC nonlinearity in DIAL and other applications in which the signal is repetitively averaged is described.

Natural vegetation as a source or sink for atmospheric ammonia: a case study.

Measurements of gaseous ammonia above a montane-subalpine forest in the Colorado mountains show that the role of the forest as a source or sink depends on the atmospheric concentrations. The canopy appeared to be an ammonia source when exposed to air containing low concentrations, but a sink when exposed to air enriched by nearby agricultural sources. The forest-averaged compensation point was 0.8 part per billion by volume at 20 degrees C. The net burden of ammonia and other nitrogen species of anthropogenic origin at this site was much less than at forest sites in the eastern United States and Europe and may provide a valuable resource for this nitrogen-limited ecosystem.