Michael O. Rodgers

Ozone precursor relationships in the ambient atmosphere

The concentrations of ozone, nitrogen oxides, and nonmethane hydrocarbons measured near the surface in a variety of urban, suburban, rural, and remote locations are analyzed and compared in order to elucidate the relationships between ozone, its photochemical precursors, and the sources of these precursors. While a large gradient is found among remote, rural, and urban/suburban nitrogen oxide concentrations, the total hydrocarbon reactivity in all continental locations is found to be comparable. Apportionment of the observed hydrocarbon species to mobile and stationary anthropogenic sources and biogenic sources suggests that present-day emission inventories for the United States underestimate the size of mobile emissions. The analysis also suggests a significant role for biogenic hydrocarbon emissions in many urban/suburban locations and a dominant role for these sources in rural areas of the eastern United States. As one moves from remote locations to rural locations and then from rural to urban/suburban locations, ozone and nitrogen oxide concentrations tend to increase in a consistent manner while total hydrocarbon reactivity does not.

Correlation of ozone with NOy in photochemically aged air

During the summer of 1988, measurements of photochemical trace species were made at a coordinated network of seven rural sites in the eastern United States and Canada. At six of these sites concurrent measurements of ozone and the sum of the reactive nitrogen species, NOy, were made, and at four of the sites a measure for the reaction products of the NOx oxidation was obtained. Common to all sites, ozone, in photochemically aged air during the summer, shows an increase with increasing NOy levels, from a background value of 30–40 parts per billion by volume (ppbv) at NOy mixing ratios below 1 ppbv to values between 70 to 100 ppbv at NOy levels of 10 ppbv. Ozone correlates even more closely with the products of the NOx oxidation. The correlations from the different sites agree closely at mixing ratios of the oxidation products below 5 ppbv, but systematic differences appear at higher levels. Variations in the biogenic hydrocarbon emissions may explain these differences.

Pacific Exploratory Mission in the tropical Pacific: PEM‐Tropics A, August‐September 1996

The NASA Pacific Exploratory Mission to the Pacific tropics (PEM-Tropics) is the third major field campaign of NASAs Global Tropospheric Experiment (GTE) to study the impact of human and natural processes on the chemistry of the troposphere over the Pacific basin. The first two campaigns, PEM-West A and B were conducted over the northwestern regions of the Pacific and focused on the impact of emissions from the Asian continent. The broad objectives of PEM-Tropics included improving our understanding of the oxidizing power of the tropical atmosphere as well as investigating oceanic sulfur compounds and their conversion to aerosols. Phase A of the PEM-Tropics program, conducted between August-September 1996, involved the NASA DC-8 and P-3B aircraft. Phase B of this program is scheduled for March/April 1999. During PEM-Tropics A, the flight tracks of the two aircraft extended zonally across the entire Pacific Basin and meridionally from Hawaii to south of New Zealand. Both aircraft were instrumented for airborne measurements of trace gases and aerosols and meteorological parameters. The DC-8, given its long-range and high-altitude capabilities coupled with the lidar instrument in its payload, focused on transport issues and ozone photochemistry, while the P-3B, with its sulfur-oriented instrument payload and more limited range, focused on detailed sulfur process studies. Among its accomplishments, the PEM-Tropics A field campaign has provided a unique set of atmospheric measurements in a heretofore data sparse region; demonstrated the capability of several new or improved instruments for measuring OH, H2SO4, NO, NO2, and actinic fluxes; and conducted experiments which tested our understanding of HOx and NOx photochemistry, as well as sulfur oxidation and aerosol formation processes. In addition, PEM-Tropics A documented for the first time the considerable and widespread influence of biomass burning pollution over the South Pacific, and identified the South Pacific Convergence Zone as a major barrier for atmospheric transport in the southern hemisphere.

Simulation of summertime ozone over North America

The concentrations of O3 and its precursors over North America are simulated for three summer months with a three-dimensional, continental-scale photochemical model using meteorological input from the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The model has 4°×5° grid resolution and represents non linear chemistry in urban and industrial plumes with a subgrid nested scheme. Simulated median afternoon O3 concentrations at rural U.S. sites are within 5 ppb of observations in most cases, except in the south central United States where concentrations are overpredicted by 15–20 ppb. The model captures successfully the development of regional high-O3 episodes over the northeastern United States on the back side of weak, warm, stagnant anticyclones. Simulated concentrations of CO and nonmethane hydrocarbons are generally in good agreement with observations, concentrations of NOx are underpredicted by 10–30%, and concentrations of peroxyacylnitrates (PANs) are overpredicted by a factor of 2 to 3. The overprediction of PANs is attributed to flaws in the photochemical mechanism, including excessive production from oxidation of isoprene, and may also reflect an underestimate of PANs deposition. Subgrid nonlinear chemistry as captured by the nested plumes scheme decreases the net O3 production computed in the United States boundary layer by 8% on average.

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.

Measurements of PAN, PPN, and MPAN made during the 1994 and 1995 Nashville Intensives of the Southern Oxidant Study: Implications for regional ozone production from biogenic hydrocarbons

Isoprene and a variety of other reactive hydrocarbons are released in large quantities by vegetation in forested regions and are thought to participate in the NOx-catalyzed production of ozone, a serious air quality problem in North America and Europe [National Research Council, 1991]. The determination of the fraction of O3 formed from anthropogenic NOx and biogenic hydrocarbons (BHC) is a crucial step in the formulation of effective control strategies. Peroxymethacrylic nitric anhydride (MPAN, CH2C(CH3)C(O)OONO2) is formed almost entirely from the atmospheric oxidation of isoprene in the presence of NOx and is an excellent indicator of recent ozone production from isoprene and therefore biogenic hydrocarbons. Measurements are presented here of MPAN, peroxyacetic nitric anhydride (PAN, CH3C(O)OONO2), peroxypropionic nitric anhydride (PPN, CH3CH2C(O)OONO2) and ozone from separate data sets acquired during the 1994 and 1995 Nashville intensive studies of the Southern Oxidant Study. It was found that PAN, a general product of HC-NOx photochemistry, could be well represented as a simple linear combination of contributions from BHC and anthropogenic hydrocarbon (AHC) chemistries as indicated by MPAN and PPN, respectively. The PAN:MPAN ratios found to be characteristic of BHC-dominated chemistry ranged from 6 to 10. The PAN:PPN ratios found to be characteristic of AHC-dominated chemistry ranged from 5.8 to 7.4. These BHC and AHC attributions were used to estimate the contributions of anthropogenic and biogenic hydrocarbons to regional tropospheric ozone production, and substantial BHC-O3 (50–60 ppbv) was estimated in cases where high NOx from power plants was present in areas of high BHC emission. This estimation method provides direct evidence of significant photochemical ozone production from the oxidation of biogenic hydrocarbons in the presence of NOx.

Internal consistency tests for evaluation of measurements of anthropogenic hydrocarbons in the troposphere

Measurements of tropospheric nonmethane hydrocarbons (NMHCs) made in continental North America should exhibit a common pattern determined by photochemical removal and dilution acting upon the typical North American urban emissions. We analyze 11 data sets collected in the United States in the context of this hypothesis, in most cases by analyzing the geometric mean and standard deviations of ratios of selected NMHCs. In the analysis we attribute deviations from the common pattern to plausible systematic and random experimental errors. In some cases the errors have been independently verified and the specific causes identified. Thus this common pattern provides a check for internal consistency in NMHC data sets. Specific tests are presented which should provide useful diagnostics for all data sets of anthropogenic NMHC measurements collected in the United States. Similar tests, based upon the perhaps different emission patterns of other regions, presumably could be developed. The specific tests include (1) a lower limit for ethane concentrations, (2) specific NMHCs that should be detected if any are, (3) the relatively constant mean ratios of the longer-lived NMHCs with similar atmospheric lifetimes, (4) the constant relative patterns of families of NMHCs, and (5) limits on the ambient variability of the NMHC ratios. Many experimental problems are identified in the literature and the Southern Oxidant Study data sets. The most important conclusion of this paper is that a rigorous field intercomparison of simultaneous measurements of ambient NMHCs by different techniques and researchers is of crucial importance to the field of atmospheric chemistry. The tests presented here are suggestive of errors but are not definitive; only a field intercomparison can resolve the uncertainties.

A photostationary state analysis of the NO2-NO system based on airborne observations from the subtropical/tropical North and South Atlantic

The Chemical Instrumentation Test and Evaluation 3 (CITE 3) NO-NO2 database has provided a unique opportunity to examine important aspects of tropospheric photochemistry as related to the rapid cycling between NO and NO2. Our results suggest that when quantitative testing of this photochemical system is based on airborne field data, extra precautions may need to be taken in the analysis. This was particularly true in the CITE 3 data analysis where different regional environments produced quite different results when evaluating the photochemical test ratio (NO2)Expt/(NO2)Calc, designated here as RE/RC. The quantity (NO2)Calc was evaluated using the following photostationary state expression: [NO2]Calc = (k1[O3] + k4[HO2] + k5[CH3O2] + k6[RO2]) [NO]Expt/J2. The four most prominent regional environmental data sets identified in this analysis were those labeled here as free-tropospheric northern hemisphere (FTNH), free-tropospheric tropical northern hemisphere (FTTNH), free-tropospheric southern hemisphere (FTSH), and tropical-marine boundary layer (plume) (TMBL(P)). The respective RE/RC mean and median values for these four data subsets were 1.74, 1.69; 3.00, 2.79; 1.01, 0.97; and 0.99, 0.94. Of the four data subsets listed, the two that were statistically the most robust were FTNH and FTSH; for these the respective RE/RC mean and standard deviation of the mean values were 1.74 ± 0.07 and 1.01, ± 0.04. The FTSH observations were in good agreement with theory, whereas those from the FTNH data set were in significant disagreement. An examination of the critical photochemical parameters O3, UV(zenith), NO, NO2, and non-methane hydrocarbons (NMHCs) for these two databases indicated that the most likely source of the RE/RC bias in the FTNH results was the presence of a systematic error in the observational data rather than a shortcoming in our understanding of fundamental photochemical processes. Although neither a chemical nor meteorological analyses of these data identified this error with complete certainty, they did point to the three most likely possibilities: (1) an NO2 interference from a yet unidentified NOy species; (2) the presence of unmeasured hydrocarbons, the integrated reactivity of which would be equivalent to ∼2.7 parts per billion by volume (ppbv) of toluene; or (3) some combination of points (1) and (2). Details concerning hypotheses (1) and (2) as well as possible ways to minimize these problems in future airborne missions are discussed.

Free tropospheric and boundary layer measurements of NO over the central and eastern North Pacific Ocean

Reported in this paper are the Georgia Institute of Technology NO results from the fall 1983 NASA GTE/CITE 1 Airborne Field Sampling Program. These data were predominantly collected over a geographical area defined by the eastern and central North Pacific Ocean, spanning the latitude range of 15°–42°N. These NO measurements were taken using the two-photon laser-induced fluorescence technique. The data show a general trend of increasing levels of NO from the boundary layer up to altitudes of nearly 10 km. The average midday value of No at altitudes of ≤1.8 km was 4 parts per trillion by volume (pptv), and at ∼6 km, 20 pptv, whereas that at ∼9 km was 25–35 pptv, the higher value reflecting the inclusion of NO data collected from the outflow region of two electrically active cumulonimbus clouds. The high-altitude NO data strongly suggest that at least during the time of the GTE flight operation, the major sources of NO for remote regions of the Pacific Ocean were those resulting from lightning and the downward transport of stratospheric air.

PHOTOCHEMISTRY OF OZONE FORMATION IN ATLANTA, GA-MODELS AND MEASUREMENTS*

Chemical measurements made during an air pollution event in Atlanta, GA have been compared with results from several photochemical simulations. Measurements included Os, primary reactive organic gases (ROG), aldehydes, PAN, total reactive nitrogen (NO,,) and HzOz, with vertical profiles for primary ROG. Photochemical models using two different chemical representations and a range of assumptions about winds, vertical mixing and emissions were used to simulate the event. Results show that assumptions about vertical mixing can cause a variation in simulated surface concentrations of primary hydrocarbons of a factor of two or more. A tendency to underestimate isoprene was found in comparison with measured vertical profiles. The models tend to overestimate concentrations of HCHO, H,02 and PAN in comparison with measurements. Peak 0s and concurrent NO, from helicopter measurements was used as a basis for evaluating individual model scenarios. Scenarios were developed with different Op-NO,-ROG sensitivity, but only the NO,-sensitive scenarios are consistent with measured 09, NO, and isoprene. Key word index: Ozone, nitrogen oxides, hydrocarbons, photochemical smog, hydrogen, peroxide.