Stephen R. Springston

Ozone formation at a rural site in the southeastern United States

Trace gas measurements pertinent to understanding the transport and photochemical formation of O3 were made at a surface site in rural Georgia as part of the Southern Oxidant Study during the summer of 1991. It was found that there was a strong correlation between O3 and the oxidation products of NOx: O3(ppb) = 27 + 11.4 (NOy(ppb) − NOx(ppb)), r2 = 0.78. This fit is similar to that observed at other rural sites in eastern North America and indicates a nominal background O3 level of 27 ppb; values higher than 27 ppb are due to photochemical production in the recent past, which varied from near zero to ≈50 ppb. The origin of the O3 above background was investigated by using a free radical budget equation to calculate an in situ O3 production rate in terms of measured concentrations of NO and free radical precursors (O3, HCHO, peroxides, and other carbonyls). A comparison of observed and predicted diurnal trends in O3 indicates significant O3 production in the afternoon at a time when O3 concentration is either steady or decreasing. The afternoon near-surface layer is thereby a source region for O3 which can be exported. In situ production accounts for approximately one half of the morning increase in O3 concentration on days with high O3; the remainder is due to entrainment of dirty air aloft by the growing convective boundary layer. Additional evidence for the role of vertical transport in controlling the hour-to-hour changes in O3 is found in the diurnal cycles of SO2 and HNO3 which also have rapid increases in the morning. The day-to-day variability of O3 was investigated using a back trajectory model. NOy concentration at the measurement site could be reasonably accounted for by considering NOx emission sources located within 1-day transport distance. In as much as there is a strong correlation between O3 and NOy, the coincidence between trajectory location and NOx emission sources appears to be an important factor influencing midday O3 concentration. Hydrocarbon measurements are consistent with NOx being the limiting factor for formation of O3.

Atmospheric chemistry and distribution of formaldehyde and several multioxygenated carbonyl compounds during the 1995 Nashville/Middle Tennessee Ozone Study

Airborne measurements of formaldehyde (FA), glycolaldehyde (GA), glyoxal (GL), methylglyoxal (MG), and pyruvic acid (PD) were made on board instrumented aircraft platforms, the Department of Energy G1 and National Oceanic and Atmospheric Administration P3 (FA only), during the 1995 Nashville/Middle Tennessee Ozone Study. FA data determined on these two aircraft during three intercomparison flights agreed to within ∼10%. The mean and median (in parentheses) concentrations observed within the boundary layer ( 0.8) observed between FA and two other isoprene products, GA and MG. Further, the magnitudes of the nonzero FA intercept exhibited in these correlation plots are found to qualitatively agree with the fraction of precursors that did not concomitantly produce GA and MG. Inspection of specific flights showed direct evidence of the dominance of isoprene as a precursor for FA, appreciable contribution of FA to CO, and negligible decay of FA overnight. Because of the dominant role isoprene plays as a precursor of FA, FA could be used as a proxy of isoprene for assessing the applicability of various versions of biogenic emission inventory.

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.

Peroxy radical concentration and ozone formation rate at a rural site in the southeastern United States

As part of the Southern Oxidants Study, Brookhaven National Laboratory operated an intensive measurement site near Metter, Georgia, during parts of the summers of 1991 and 1992. Measurements were made of photochemically active trace gases and meteorological parameters relevant to determining causes for elevated ambient ozone concentration. The 1992 data set was used to calculate peroxy radical concentration and ozone formation rate based on determining the departure from the photostationary state (PSS) and based on a radical budget equation, such as applied previously to the 1991 data set. Averaged over the 28-day experimental period, we find maximum radical production occurring near noon at 2.5 ppb h−1, maximum peroxy radical concentration also occurring near noon at 80 ppt, and maximum ozone production of 8 ppb h−1 occurring near 1000 EST. Ozone photolysis accounts for 55% of radical production, HCHO and other carbonyl compounds about 40%. The radical budget and PSS methods depend in different ways on atmospheric photochemistry and a comparison between them affords a test of our understanding of the photochemical production of O3. We find that these methods agree to the extent expected based on uncertainty estimates. For the data set as a whole, the median estimate for fractional error in hourly average peroxy radical concentration determined from the radical budget method is approximately 30% and from the PSS method, 50%. Error estimates for the PSS method are highly variable, becoming infinite as peroxy radical concentration approaches zero. This behavior can be traced back to the difference form of the PSS equations. To conduct a meaningful comparison between the methods, the data set was segregated into subsets based on PSS uncertainty estimates. For the low-uncertainty subset, consisting of a third of the whole data set, we find that the ratio of peroxy radical concentration predicted from the PSS method to that predicted from the radical budget method to be 1.22±32%.

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

Measurement of O3 and related compounds over southern Nova Scotia: 1. Vertical distributions

As part of the North Atlantic Regional Experiment, the National Research Council of Canada Twin Otter aircraft was used to measure the concentration of O3 and related compounds in the atmosphere over southern Nova Scotia. Forty-eight flights were conducted, primarily over the surface sampling site in Chebogue Point, Nova Scotia, or over the adjoining Atlantic Ocean. A typical flight included one or more vertical profiles from 30 m above the surface to an altitude of 3 or 5 km. We present here O3 measurements and supporting chemical and meteorological data including NOy, CO, accumulation mode aerosol particles, winds, temperature, and dew point. Data are presented in a format which illustrates day-to-day variability and vertical structure. We find that Nova Scotia is impacted by a wide variety of air masses with varying chemical content depending on flow conditions relative to the locations of upwind emission regions. As an aid to understanding the chemical composition of the air, we characterize four types of events: (1) moist continental boundary layer air with high concentrations of O3 and other anthropogenic pollutants which is advected to Nova Scotia in relatively thin vertical layers, usually with a base altitude of several hundred meters; (2) “background” air with concentrations of anthropogenic ingredients much lower than experienced in continental pollution episodes but higher than observed in more remote regions of Canada, suggesting a dilute anthropogenic or biomass burning influence; (3) near-surface air which because of a strong temperature inversion over the Atlantic Ocean, is decoupled from air aloft, with the consequence that near-surface measurements do not give a representative view of the eastward transport of the North American plume; and (4) dry air masses with high O3 concentration in which we have to distinguish between boundary layer and upper atmosphere source regions.