Preliminary Results from SOS --Do Not Cite!!

The NOAA Twin Otter, a small, twin engine turboprop aircraft, was equipped with a variety of chemistry (NO, NOx, NOy, O₃, CO, SO₂, and NMHCs) and turbulent flux (sensible and latent heat, momentum, CO₂, and O₃) measurement systems during SOS. The Twin Otter was deployed for a total of 75 hours on 18 flights in the Nashville region, from June 21 - July 19, 1995. The majority of the flights were conducted within a 50 km radius of downtown Nashville, and were designed to measure turbulent fluxes and flux divergence in the mixed layer. Flight altitudes typically ranged from approximately 300 feet above ground level (AGL) to 5000 feet above mean sea level (MSL), with occasional excursions into the lower free troposphere (up to 8500 feet MSL). Other flights were designed to study the chemistry of the advected urban plume and to investigate the breakup of the nocturnal inversion.

Map of NOAA Twin Otter flight on July 2, 1995. Flight was designed to investigate the degree of correlation between surface and aircraft trace gas measurements, and the Twin Otter overflew Level II chemistry sites at Dickson (NW of Nashville) and Youth, Inc. (S of the city), and the Level III chemistry station at New Hendersonville (NE of the city). Flight altitude was typically 1500 feet MSL, with profiles over the ground stations from 500 feet AGL to 7000-8000 feet MSL. Times denote turns or major course corrections. Winds on this day were light (<2 m/s) out of the N-NW. Preliminary data from this flight are presented below. Map courtesy of the Tennessee Valley Authority.

Map of NOAA Twin Otter flight on July 3, 1995. Flight was designed to investigate surface fluxes, flux divergence, and atmospheric chemistry in the advected urban plume. Flight altitudes ranged from 300 feet AGL to 7500 feet MSL. Times denote turns or major course corrections. Winds on this day were brisk (2-5 m/s) from the SW. Level flight legs of the Twin Otter intercepted the urban plume approximately 2 hours downwind of Nashville. Preliminary data are presented below. Map courtesy of the Tennessee Valley Authority.

Preliminary Results

• On July 2-3, the Nashville plume was embedded in a large region of uniformly polluted air, with high background concentrations of CO, O₃, and NOy. NOx/NOy ratios of approximately 10% suggest photochemically aged air. After injection of fresher emissions in the urban plume, NOx/NOy ratios of only 10-20% were observed a few hours downwind of Nashville. These observations suggest very rapid photochemical processing in the urban plume.
• Photochemical ozone generation inferred in the summertime Nashville area was rapid; ozone enhancements of 30-40 ppbv were observed within 30 km of the urban center. Given estimated plume travel times of 2-3 hours, ozone formation rates of approximately 10-15 ppbv/hr may be inferred.

• In the generally stagnant, well-mixed boundary layer, concentrations of CO, SO₂, and O₃ measured at the ground agreed very well with concentrations measured aloft, through the lower PBL. NOy discrepancies remain; Twin Otter always measured higher NOy. Some of this discrepancy may be due to the deposition of nitric acid (HNO₃) near the surface.

Vertical profiles of ozone (above, left), and SO₂ (above, right) on July 2, 1995, during overflights of SOS ground monitoring stations near Nashville. Blue trace denotes data collected during overflight of Dickson ground station; green corresponds to data collected over Youth, Inc. ground site; and red denotes data collected over New Hendersonville Level III station. Colored triangles denote average concentrations monitored at the ground during the time of the overflight. Ground level data courtesy of B. Hartsell, ESE, Inc. (Dickson); K. Olszyna, Tennessee Valley Authority (Youth, Inc.); M.O. Rodgers, GIT (New Hendersonville)

• On July 3, a variety of aromatic and alkene hydrocarbons contributed to the overall NMHC distribution, expressed on a part per billion carbon (ppbC) basis (above right). On a reactivity-weighted scale, e.g., calculated as propylene equivalent concentrations (again in units of ppbC), isoprene dominated the NMHC distribution (below, right) in the lower PBL. In background air outside of the urban plume, isoprene dominance is even greater. Data courtesy of M.O. Rodgers, GIT.

• O₃/NOy ratios suggest VOC limitation in the heart of the urban plume, with NOx limitation near the edges and in background air. These findings are preliminary and somewhat uncertain, however, given the lack of additional measurements of other key indicator species, from which NOx or VOC limitation might be inferred.

Scatter plot of ozone (left ordinate) and ozone/NOy ratio (right ordinate) as a function of NOy concentrations as measured during repeated cross-wind transects of the Nashville urban plume on July 3, 1995. An ozone/NOy ratio of 6.5 is estimated to characterize the transition between NOx and VOC limited chemistry, based upon the results of Sillman, S., J. Geophys. Res., 14,175-14,188, 1995.

• [CO]/[NOy] ratios measured downwind suggest molar CO/NO emission ratios of about 20. However, the artificial enhancement of CO/NOy ratios in the plume due to deposition of NOy species, while unlikely given the freshness of the emissions (ca 2 hours), cannot be ruled out. Similarly, a bias toward artificially low ratios due to the sampling of power plant emission plumes (with low CO and high NOy) is theoretically possible. However, concurrent measurements of SO₂, a compound which is emitted in abundance in power plant plumes, and air trajectories suggest that the Twin Otter encountered no such point source emissions this day.

For additional information, please contact:

• Winston Luke (Winston.Luke@noaa.gov; 301 713-0971)
• Bob McMillen (bob.mcmillen@noaa.gov; 423 576-1249)