Observations and tropospheric photochemistry in central North Carolina

The southeastern U.S. environment exhibits some of the highest average O 3 concentrations in the nation, especially during the summertime. Reactive nitrogen compounds and carbon monoxide were monitored for 2 weeks in rural North Carolina in August 1991 as part of the Southern Oxidants Study. The daytime (0600–1900 EST) mean NO concentration was ∼200 parts per trillion (ppt), and nonzero abundances, probably due to soil emissions, were observed at night (mean NO ∼ 60 ppt). Total reactive nitrogen compounds (NO y ) ranged between 1.5 and 12 parts per billion (minimum and maximum hourly observed averages). Back trajectories suggested the potentially important impact of power plants northwest of the site. The mean contribution of nitrogen oxides (NO x ) to NO y during our experiment was 26% (±5% at ±1σ level of significance), with the diurnal variation of all trace gases strongly affected by the evolution of the planetary boundary layer. The good agreement between the observed O 3 production and the results of a simple physical/chemical scheme based on deviations from the NO/O 3 /NO 2 photostationary state provides confidence in the reliability of a photolytic NO 2 converter. The same scheme indicated the enormously misleading conclusions that may result from the use of an inappropriate technique for monitoring NO 2 .