L. J. Nunnermacker

Dependence of ozone production on NO and hydrocarbons in the troposphere

An expression for the production rate of 03, P(O 3), is derived based on a radical budget equation applicable to low and high NOx conditions. Differentiation of this equation with respect to NO or hydrocarbons (HC) gives an approximate analytic formula in which the relative sensitivity of P(O3) to changes in NO or HC depends only on the fraction of radicals which are removed by reactions with NOx. This formula is tested by comparison with results from a photochemical calculation driven by trace gas observations from the 1995 Southern Oxidants Study (SOS) campaign in Nashville, Tennessee.

Intercomparison of ground-based NOy measurement techniques

An informal intercomparison of NOy measurement techniques was conducted from June 13 to July 22, 1994, at a site in Hendersonville, Tennessee, near Nashville. The intercomparison involved five research institutions: Brookhaven National Laboratory, Environmental Science and Engineering, Georgia Institute of Technology, NOAA/Aeronomy Laboratory, and Tennessee Valley Authority. The NOy measurement techniques relied on the reduction of NOy species to NO followed by detection of NO using O3-chemiluminescence. The NOy methods used either the Au-catalyzed conversion of NOy to NO in the presence of CO or H2 or the reduction of NOy to NO on a heated molybdenum oxide surface. Other measurements included O3, NOx, PAN and other organic peroxycarboxylic nitric anhydrides, HNO3 and particulate nitrate, and meteorological parameters. The intercomparison consisted of six weeks of ambient air sampling with instruments and inlet systems normally used by the groups for field measurements. In addition, periodic challenges to the instruments (spike tests) were conducted with known levels of NO, NO2, NPN, HNO3 and NH3. The NOy levels were typically large and highly variable, ranging from 2 ppbv to about 100 ppbv, and for much of the time was composed mostly of NOx from nearby sources. The spike tests results and ambient air results were consistent only when NOx was a substantial fraction of NOy. Inconsistency with ambient air data and the other spike test results is largely attributed to imprecision in the spike results due to the high and variable NOy background. For the ambient air data, a high degree of correlation was found with the different data sets. Of the seven NOy instrument/converters deployed at the site, two (one Au and one Mo) showed evidence of some loss of conversion efficiency. This occurred when the more oxidized NOy species (e.g., HNO3) were in relatively high abundance, as shown by analysis of one period of intense photochemical activity. For five of the instruments, no significant differences were found in the effectiveness of NOy conversion at these levels of NOy with either Au or Mo converters. Within the estimated uncertainty limits there was agreement between the sum of the separately measured NOy species and the NOy measured by the five of the seven techniques. These results indicate that NOy can be measured reliably in urban and suburban environments with existing instrumentation.

Ozone production in the New York City urban plume

In the summer of 1996 the Department of Energy G-1 aircraft was deployed in the New York City metropolitan area as part of the North American Research Strategy for Tropospheric Ozone-Northeast effort to determine the causes of elevated O3 levels in the northeastern United States. Measurements of O3, O3 precursors, and other photochemically active trace gases were made upwind and downwind of New York City with the objective of characterizing the O3 formation process and its dependence on ambient levels of NOx and volatile organic compounds (VOCs). Four flights are discussed in detail. On two of these flights, winds were from the W-SW, which is the typical direction for an O3 episode. On the other two flights, winds were from the NW, which puts a cleaner area upwind of the city. The data presented include plume and background values of O3, CO, NOx, and NOy concentration and VOC reactivity. On the W-SW flow days O3 reached 110 ppb. According to surface observations the G-1 intercepted the plume close to the region where maximum O3 occurred. At this point the ratio NOx/NOy was 20–30%, indicating an aged plume. Plume values of CO/NOy agree to within 20% with emission estimates from the core of the New York City metropolitan area. Steady state photochemical calculations were performed using observed or estimated trace gas concentrations as constraints. According to these calculations the local rate of O3 production P(O3) in all four plumes is VOC sensitive, sometimes strongly so. The local sensitivity calculations show that a specified fractional decrease in VOC concentration yields a similar magnitude fractional decrease in P(O3). Imposing a decrease in NOx, however, causes P(O3) to increase. The question of primary interest from a regulatory point of view is the sensitivity of O3 concentration to changes in emissions of NOx and VOCs. A qualitative argument is given that suggests that the total O3 formed in the plume, which depends on the entire time evolution of the plume, is also VOC sensitive. Indicator ratios O3/NOz and H2O2/NOz mainly support the conclusion that plume O3 is VOC sensitive.

Ozone production efficiency in an urban area

Ozone production efficiency can be defined as the number of molecules of oxidant (O 3 + NO 2 ) produced photochemically when a molecule of NO X (NO + NO 2 ) is oxidized. It conveys information about the conditions under which O 3 is formed and is an important parameter to consider when evaluating impacts from NO x emission sources. We present calculational and observational results on ozone production efficiency based on measurements made from aircraft flights in the Phoenix metropolitan area in May and June of 1998. Constrained steady state box model calculations are used to relate a ratio of O 3 production rate to NO x consumption rate (i.e., P(0 3 )/P(NO z )) to a VOC to NO 2 ratio of OH reactivity. Lagrangian calculations show how this ratio generally increases with time due to oxidation chemistry and plume dilution. City to city differences in ozone production efficiency can be attributed to corresponding differences in VOC to NO 2 reactivity ratio which in turn reflect emission patterns. Ozone production efficiencies derived from aircraft measurements in 20 plumes show a dependence on NO x concentration similar to that calculated for P(0 3 )/P(NO z ). Calculations are based on data from a single location but are believed to be applicable to a wide range of plumes from different areas.

NO y lifetimes and O3 production efficiencies in urban and power plant plumes: Analysis of field data

In an effort to describe and characterize power plant plumes in the Nashville region, emissions from a small power plant (Gallatin) and a large power plant (Paradise) were examined using data obtained on the Department of Energy G-1 airborne sampling platform. Observations made on July 3, 7, 15, 17, and 18, 1995, were compiled, and a kinetic analysis of the chemical evolution of the power plant plumes was performed. Analysis of the power plant plume data revealed a very active photochemistry, as had been determined previously for the urban plume. Ozone production efficiencies (OPE), defined as the number of molecules of O3 formed per NOx molecule consumed, were found to be 3 for Gallatin and 2 for Paradise. Lifetimes for NOx (2.8 and 4.2 hours) and NOy (7.0 and 7.7 hours) were determined for Gallatin and Paradise, respectively. These NOx and NOy lifetimes imply rapid loss of NOz (NOz is assumed to be primarily HNO3). Lifetimes for NOz are calculated to be 3 and 2.5 hours for Gallatin and Paradise, respectively. A sensitivity analysis indicates that the Gallatin NOz lifetime could be as long as 5 hours, bringing it into agreement with the value determined for the Nashville urban plume. It is unlikely that the Paradise NOz lifetime is as long as 4 hours. If NOz loss is attributed to dry deposition, a 3 hour lifetime implies a deposition velocity greater than 10 cm s−1, which is much faster than expected based on accepted theory. Possible reasons for this discrepancy are discussed.

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.