J. Weinstein-Lloyd

Chemical and physical properties of plumes of anthropogenic pollutants transported over the North Atlantic during the North Atlantic Regional Experiment

Plumes of photochemical pollutants transported from the industrialized regions of the northeast United States and Canada were sampled over the North Atlantic Ocean at distances up to 1000 km from the coast. The plumes were found in well defined layers up to 1 km thick and were usually isolated from the surface by a low altitude inversion. Plume composition was consistent with the occurrence of extensive photochemical processing during transit from source regions as indicated by high 03 concentrations (03 maximum -150 parts per billion by volume (ppbv)), generally high fractional conversion (>85%) of NOx to its oxidation products, and high peroxide concentrations (median 3.6 ppbv; maximum 11 ppbv). These observations are in accord with processing times estimated from back trajectory analysis. CO and 03 concentrations were well correlated (r 2 = 0.64) with a slope (0.26) similar to previous measurements in photochemically aged air. Good correlations were also observed between CO and accumulation mode particle number densities (r 2 = 0.64), and CO and NOy (r 2 = 0.67). 03 was found to depend nonlinearly on the NOx oxidation product concentration. At low values of (NOy-NOx), the slope (14) was within the range of values measured previously in photochemically aged air masses, at higher concentrations the slope was much lower (4.6). The low slope at high concentrations is attributed to minimization of losses of NOx oxidation products in spatially well-defined plumes during transport. A strong linear correlation (r 2 = 0.73) was found between 03, and the concentration of radical sink species as represented by the quantity ((NOy-NOx) + 2H202).

Measurements of peroxides and related species during the 1995 summer intensive of the Southern Oxidants Study in Nashville, Tennessee

Hydroperoxide measurements are presented for 12 flights of the U.S. Department of Energy G-1 aircraft during the summer 1995 intensive of the Southern Oxidants/Middle Tennessee study. A three-channel analyzer, utilizing both peroxidase/p-hydroxy phenylacetic acid (pOHPAA) and ferrous sulfate/benzoic acid (FeBA) reagents permitted continuous measurements of hydrogen peroxide (H202), methyl hydroperoxide (CH3OOH or MHP), and hydroxymethyl hydroperoxide (HOCH2OOH or HMHP). The median concentration of total hydroperoxide was 5.2 ppbv, with median concentrations of 2.4, 1.7, and 0.97 ppbv for H202, MHP, and HMHP

Analysis of the processing of Nashville urban emissions on July 3 and July 18, 1995

This paper analyzes data obtained on July 3 and 18, 1995, during the summer 1995 Southern Oxidant Study (SOS) field campaign. In a previous paper (Nunnermacker et al., 1998) we analyzed measurements of key species that contribute to formation of O3 in the Nashville urban plume and presented a semiquantitative picture of O3 production in the plume from the point of emission to locations where no net O3 was being formed. In this paper we use a box model constrained by observed concentrations of stable species to obtain a detailed mechanistic description of the instantaneous processing of urban emissions at various times in the chemical evolution of the urban plume. Instantaneous ozone production rates and efficiencies with respect to NOx and to primary radical production are examined. At high NOx concentrations in the fresh urban plume the O3 production rate was found to be directly proportional to the hydrocarbon to NOx reactivity ratio. At lower NOx concentrations, corresponding to the mature urban plume and the background atmosphere, the O3 production rate was found to be directly proportional to the NOx concentration and independent of the hydrocarbon reactivity. NOx was found to be most efficiently used for ozone production at low NOx concentrations. In contrast, the efficiency with which the system uses primary radicals was found to be very low at low NOx concentrations and to peak at a NOx concentration of approximately 4 ppbv. A sensitivity study of the instantaneous O3 production rates to changes in NOx or hydrocarbon concentrations showed that the instantaneous 03 production rate at the center of the urban plume, when half of the urban NOx emissions had been processed, is hydrocarbon sensitive. However, 03 production becomes NOx sensitive as the plume matures.

Analysis of O3 formation during a stagnation episode in central Tennessee in summer 1995

O3 production in the Nashville urban plume during the O3 episode that occurred on July 11–July 13 1995, is examined to characterize the factors that control the ozone production rate and efficiency, and to examine the relative importance of natural and anthropogenic sources of hydrocarbons to ozone production in the urban center and outlying areas. The analysis focuses on data collected during aircraft flights on July 11 when the Nashville area was sampled more or less continuously from about 1000 to 1800 LT. The instantaneous ozone production rate P(O3) in the downtown area from late morning through midafternoon on July 11 ranged between 10 and greater than 30 ppbv/h depending on location. After 1700 local time, production rates dropped to a few ppbv/h owing to the diminished solar intensity. Instantaneous production efficiencies with respect to NOx in the downtown area ranged between 2.5 and 8, linearly depending on the ratio of the hydrocarbon to NOx, OH reactivity. Integral O3 production efficiencies corrected for NOz losses ranged between 1.5 and 4. The lowest efficiency was observed in the downtown area in the morning where NOx concentrations were high and hydrocarbon to NOx reactivity ratios were the lowest. Throughout the day, P(O3) in the downtown area was limited by the availability of hydrocarbons. Anthropogenic hydrocarbons and CO contributed about 66% of the total hydrocarbon OH reactivity in the downtown area. In the mature urban plume downwind of Nashville, P(O3) dropped to 6–9 ppbv/h at midafternoon and was controlled by the availability of NOx. Integral O3 production efficiencies in the mature urban plume ranged between 3.5 and 4. When present in large quantities (1–3 ppbv), isoprene significantly increased both the rate and efficiency of ozone production as long as the photochemical system was not strongly NOx-limited.