L. Newman

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.

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.

Measurements of the chemical composition of stratiform clouds

Abstract Measurements are reported of the chemical composition of the liquid water and interstitial air in warm (> 0°C), non-precipitating stratus and strato-cumulus clouds at various locations in the eastern United States. Inorganic ionic composition of the cloud water was generally dominated by H + , NH 4 + , NO 3 − and SO 4 2− , similar to the composition of precipitation in this region of the U.S. Concentrations of the corresponding interstitial aerosol species and gaseous HNO 3 were invariably low in comparison to concentrations of the respective ionic species in cloudwater. In contrast, the concentration of NO x (i.e. NO + NO 2 + organic nitrates) was invariably comparable to or in excess of that of cloudwater nitrate. Sulfur dioxide was found at varying concentrations relative to cloudwater sulfate. In many cases, the SO 2 concentration was quite low ( 4 2− (> 1 ppb equivalent gas-phase concentration), suggesting large fractional conversion and incorporation into cloudwater. In other cases, in which dilute SO 2 plumes (pso, > 5 ppb) were observed in the cloud interstitial air, the gaseous SO 2 concentration substantially exceeded the cloudwater sulfate concentration. Concentrations of H 2 O 2 in cloudwater were found to exhibit strong inverse correlation with interstitial SO 2 . Appreciable concentrations of SO 2 in cloud interstitial air and H 2 O 2 in cloudwater were only rarely observed to coexist, for the most part only one or the other being present above the limit of detection. These observations are consistent with aqueous-phase oxidation of SO 2 by H 2 O 2 , as has been inferred previously on the basis of laboratory kinetic studies, and with the hypothesis that depending on relative concentrations, either of these species can be a limiting reagent for in-cloud SO 2 oxidation. The uptake of NO x as cloudwater nitrate is less complete than the uptake of SO 2 as sulfate, and evidence for the occurrence of similar in-cloud processes causing the conversion of NO or NO 2 to cloudwater nitrate has not been found.

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.

Determination of atmospheric nitrate and nitric acid employing a diffusion denuder with a filter pack

A diffusion denuder assembly for sampling and analyzing atmospheric gaseous and particulate nitrate without significant artifactual error was constructed and evaluated. The system consists of parallel filter packs, one to determine total inorganic nitrate and the other, preceded by a diffusion denuder to remove nitric acid, to measure particulate nitrate. Nitric acid is determined by difference. Ambient sampling data for total nitrate (sum of nitric acid and particulate nitrate) were statistically identical by either high volume or low volume sequential filter packs. Differences between nitric acid and nitrate data by filter pack and diffusion denuder methods were attributed principally to known sources of error in the former-adsorption of HNO3 on collected particles and evaporation of nitric acid from collected ammonium nitrate particles. Negative correlation of particulate nitrate and strong acid levels was observed. During long sampling times adsorption of HNO3 on collected particles is apparently the dominant source of error eliminated by use of the diffusion denuder method; for short sampling times, evaporative loss of NH4NO3 is dominant. Losses of nitrate particles by impaction or ammonium nitrate by evaporation in the denuder appear to be negligibly small for particles ⩽ 3pm.

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.

Methods for determination of low molecular weight carbonyl compounds in the atmosphere: A review

Abstract Determination of carbonyl compounds in the ambient atmosphere is receiving increasing attention because of the critical role these compounds play in tropospheric organic chemistry. Currently, field measurements are very limited mainly because of the analytical challenges posed by trace concentrations and interferences arising from atmospheric co-pollutants. We review here the methods used for determination of carbonyl compounds, from an atmospheric chemistry perspective, emphasizing the principles, advantages and limitations. Since a large number of varied types of methods have been used specifically for determination of formaldehyde, it is considered separately from other carbonyls. It is clear that despite more than a decade of work, many problems related to sampling, interferences and artifacts have not been resolved. Because of the increasing demand for time-series measurements in field studies, an automated method for continuous sampling and analysis of carbonyls is very much required. Important issues to be considered in the development of a suitable field method and potential approaches are discussed.

Determination of total inorganic nitrate utilizing collection of nitric acid on NaCl—impregnated filters

Abstract Cellulose filters impregnated with NaCl will collect nitric acid vapor from ambient atmospheres at efficiencies of ~95% when placed downstream from pretreated quartz particle filters in a high-volume sampler. No adsorption of NO2 on the NaCl filters with retention as nitrate was observed. Conversion of NO2 to nitrate on quartz (either retained as nitrate or released as artifact nitric acid) was negligibly low with the removal of the less than 1% of the incident NO2. At high humidities adsorption of HNO3 on the quartz prefilter can become significant Quartz filters which were preloaded with ambient particulate matter lost nitrate when exposed to aerosol H2SO4. Sulfur dioxide may be collected on potassium-carbonate-impregnated cellulose filters placed downstream from quartz and NaCl papers without any loss on the preceding filters. Ammonium nitrate on quartz filters is partially volatilized during ambient air sampling.

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).