A COMPARISON OF AIRBORNE AND SURFACE TRACE GAS MEASUREMENTS DURING THE SOUTHERN OXIDANTS STUDY (SOS)

The NOAA Twin Otter conducted more than a dozen overflights of ground- level air quality monitoring stations during the 1995 Southern Oxidant s Study (SOS) Nashville/Middle Tennessee Ozone Project Field Intensive . Surface and aircraft observations of ozone and ozone precursors were examined to identify systematic sampling errors, and to assess the de gree to which surface measurements may be considered representative of the larger planetary boundary layer (PBL). Overall agreement between surface and aircraft trace gas measurements was excellent in the well developed mixed layer, especially in rural-regional background air and under stagnant conditions, where surface concentrations change only s lowly. On July 2, surface level measurements were representative of th e larger mixed layer over distances as far as 70 km in background air, and 30 km in the weakly advected urban plume. Vertical variations in trace gas concentrations were often minimal in the well-mixed PBL, and measurements at the surface always agreed well with aircraft observat ions up to 460 m above ground level. Under conditions of rapidly varyi ng surface concentrations (e.g., during episodes of power plant plume fumigation and early morning boundary layer development), agreement be tween surface and aloft is dependent upon the spatial (aircraft) and t emporal (ground) averaging intervals used in the comparison. Under the se conditions, surface sites may be representative of the PBL only to within a few kilometers. Under clear skies in the well-mixed PBL, regr ession of aircraft trace gas data collected within 5 km of the ground sites against 15-20 min average surface concentrations centered on the times of the overflights yielded the following relationships: [O-3](a ircraft) = ([O-3](surface) x 0.9374) + 4.86 ppbv (r(2)=0.9642); [CO](a ircraft) = ([CO](surface) x 0.8914) + 16.4 ppbv (r(2)=0.9673); [SO2](a ircraft) = ([SO2](surface) x 0.9414) - 0.069 ppbv (r(2)=0.9945). Altho ugh O-3, CO, and SO2 measurements at the surface and aloft generally a greed well, isolated examples of unexplained measurement discrepancies emerged, illustrating the need for side-by-side instrument comparison s. NOY measurements agreed poorly between surface and aircraft: [NOY]( aircraft) = ([NOY](surface) X 0.9184) + 4.56 ppbv (r(2) = 0.9188).