TRACE GAS CONCENTRATIONS AND METEOROLOGY IN RURAL VIRGINIA .2. REACTIVE NITROGEN-COMPOUNDS

In this second of a two-part paper on the air chemistry of the Big Meadows site in Shenandoah National Park, Virginia, we present measurements of reactive nitrogen compounds that play important roles in atmospheric photochemistry and acidity, and as nutrients in sod and water. These compounds am, however, poorly understood in terms of spatial and temporal variability. Measurements of nitric oxide, NO, and total reactive nitrogen, NOy, were made for over a year during 1988 and 1989 at a mountain site that we feel is representative of the regional tropospheric composition. We also measured total nitrate (HNO3 + NO3-) for three weeks during this period. For the whole year, the mean. standard deviation, (and median) NOy and daytime NO mixing ratios were 5.1 +/- 3.1 (4.3) ppbv and 0.59 +/- 0.94 (0.23) ppbv. respectively. A histogram for NOy appears to conform to a lognormal distribution, but fails a chi2 test of significance at the 0.05 level. The speciation of NOy indicates considerable photochemical aging, except in winter Over the year during daylight, the median ratio NO/NOy was 0.06. Total nitrate (measured in August and September) accounted for 39% of the total NOy. Total reactive nitrogen shows no appreciable diurnal cycle, but NO, NOy, and the ratio NO/NOy show a summer minimum. For winter, spring, summer, and fall, the mean (and median) NOy mixing ratios were: 5.7 +/- 3.3 (5.0), 5.4 +/- 3.2 (4.4), 4.0 +/- 2.1 (3.5), and 5.2 +/- 3.2 (4.4) ppbv, respectively. Similarly, seasonally averaged daytime NO values were: 0.95 +/- 1.07 (0.61), 0.66 +/- 0.93 (0.28), 0.18 +/- 0.29 (0.13), and 0.70 +/- 1.10 (0.30) ppbv. Wet deposition of nitrate shows the opposite seasonality we estimate the lifetime of NOy with respect to wet deposition to be 2.7 days in summer and 5.8 days in winter. Ozone and NOy were positively correlated in the summer and negatively correlated in the winter. Carbon monoxide and NOy were positively correlated in all seasons. The slope of a scatter plot of CO vs. NOy, 4.85, provides a lower limit to the molar emissions ratio of CO and NOx. Emissions inventories give a ratio of 4.15.