Kenneth J. Olszyna

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.

O3 and NO y relationships at a rural site

Measurements of O3, NO, NO2, peroxyacetyl nitrate (PAN), HNO3, and NOy were made during a 6-week period in the summer of 1991 in Giles County, Tennessee. These data were analyzed to determine the factors controlling the relationship between O3 and NOy at this rural site. A strong association was observed between the O3 and NOx oxidation product (NOz = NOy - NOx) levels. The higher O3 levels were associated with air masses impacted by higher NOx emissions that had been photochemically processed. An analysis of the data indicates that the ultimate O3 production is about 10 molecules of O3 produced for each molecule of NOx emitted. The analysis results also suggest that O3 net production continues until about 70% of the NOx has been converted into NOz. The PAN/HNO3 ratios observed suggest that the air masses in Giles County are composed of higher volatile organic carbon/NOx ratios than the air masses observed at other rural sites in eastern North America. A comparison of the data analysis results to model simulations and smog chamber experiments suggests that most of the time, Giles County is in an NOx-limited regime for O3 production.

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.

Decadal change in carbon monoxide to nitrogen oxide ratio in U.S. vehicular emissions

[1] Accurate emission inventories and their temporal trends must be incorporated into pollutant inventories to allow for reliable modeling of the country’s past, present, and future air quality. Measured carbon monoxide (CO) and nitrogen oxide (NOx) concentrations from two urban areas show that the CO/NOx vehicular emission ratio has decreased at an average rate of 7–9% per year from 1987 to 1999. This amounts to a factor of nearly 3 over the 12 years. The current U.S. Environmental Protection Agency tabulations of estimated pollutant emission trends indicate a rate of decrease smaller by a factor of 2–3. The trend in maximum ambient CO levels in U.S. cities suggests a 5.2 ± 0.8% per year average annual decrease in CO vehicular emissions, which implies a 2–3% annual increase in NOx emissions from vehicles. Thus over the decade of the 1990s, annual U.S. CO emissions from vehicles have decreased from 65 to 38 Tg, representing approximate decreases of 6 and 3% in the annual global fuel-use CO emissions and in total global anthropogenic CO emissions, respectively. It is expected that the volatile organic compound (VOC)/NOx vehicular exhaust emission ratio has decreased similarly, implying that the character of atmospheric photochemistry in U.S. urban areas has changed significantly over the decade. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; KEYWORDS: vehicle emissions, carbon monoxide, nitrogen oxides, emission inventories, temporal change

Gas-phase, cloud and rain-water measurements of hydrogen peroxide at a high-elevation site

Abstract Gas-phase hydrogen peroxide (H 2 O 2 ) measurements were made at the summit of Whitetop Mountain (1689 m), VA, during the summer and fall of 1986. Aqueous-phase (clouds and rain) H 2 O 2 measurements were made at the same location during the spring, summer and fall of 1986. Measurements indicate a strong seasonal dependence for H 2 O 2 , with highest levels in the summer and lower concentrations in spring and fall. The mean gas-phase H 2 O 2 concentration measured during the summer study was 0.8 ppb while the fall mean was 0.15 ppb. Gas-phase concentrations were strongly correlated with O 3 , ambient temperature and dew-point and only weakly correlated with light intensity. Hydrogen peroxide exhibited a slight diurnal variation with daytime values exceeding night-time levels by 26%. Cloud-water H 2 O 2 showed no significant correlation with any of the major ions present in cloud-water. The cloud-water H 2 O 2 levels reported include the highest value (247 μM) thus far reported in the literature. The H 2 O 2 concentrations in cloud samples were usually, but not always higher than concentrations measured in rain samples. However, samples collected during a simultaneous cloud-rain event yielded higher H 2 O 2 concentrations in the rain, indicating that H 2 O 2 levels aloft exceeded those measured near the ground.

Nighttime isoprene chemistry at an urban-impacted forest site

Isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) were measured over a 4 week period in July of 1995 at a rural/forest site near Nashville, Tennessee, as part of the 1995 Southern Oxidants Study (SOS) field intensive. High nighttime isoprene mixing ratios, measured during a 3 day period of stagnant high pressure, are reported. These high nighttime isoprene events are interpreted as a result of continuing emission of isoprene into a developing shallow nocturnal boundary layer in the early evening, followed by advective transport under the inversion to the measurement site. During some evenings, there is very rapid decay of isoprene just after sunset. These events occurred when the product [O 3 ].[NO 2 ] was relatively large, consistent with loss via reaction with NO 3 . A chemical box model showed that isoprene decays were consistent with the NO 3 mechanism but only for relatively high NO x conditions. This study indicates that nighttime processing of isoprene can be important for forested regions susceptible to high-NO x transport events. We also find that this nighttime NO 3 chemistry can lead to conditions where, at least at the surface, a significant fraction of the NO y is in the form of organic nitrates that are products of the NO 3 -isoprene reaction and that the NO 3 -isoprene reaction can be the dominant NO 3 sink.

Production of hydrogen peroxide and organic peroxides in the gas phase reactions of ozone with natural alkenes

The formation of H{sub 2}O{sub 2} and organic peroxides in the reaction of O{sub 3} with trans-2-butene and naturally occurring alkenes has been studied using a 31 m{sup 3} reaction chamber. H{sub 2}O{sub 2} and organic peroxides were found to be products of the O{sub 3} reaction with trans-2-butene, isoprene, {alpha} and {beta}-pinene, and limonene. Water is necessary for the formation of H{sub 2}O{sub 2} and most of the H{sub 2}O{sub 2} is formed via a route that does not involve HO{sub 2} radicals. These results indicate that the reaction of O{sub 3} with natural alkenes may be a significant source of atmospheric H{sub 2}O{sub 2}, particularly in forest and rural areas.

Observations of isoprene chemistry and its role in ozone production at a semirural site during the 1995 Southern Oxidants Study

Isoprene and its oxidation products, methyl vinyl ketone (MVK) and methacrolein (MACR), were measured in a semirural environment that was occasionally heavily impacted by urban emissions. At this site, isoprene was the most important hydrocarbon in terms of kOH·[hydrocarbon], but the aldehydes HCHO and CH3CHO also appear to be very important. The local isoprene photochemistry appears to be occasionally enhanced in NOx-rich urban plumes that are adverted to the site over intermediate forested land. When O3 was being rapidly produced in urban plumes adverted to this forested site, isoprene was found to contribute ≈28% of the total ozone production. We observe that many of the peaks in isoprene oxidation products at this surface site arise from downward mixing of more photochemically processed air aloft, as the nocturnal inversion breaks up in the morning. We estimate that, in the daytime, typically 1–2% of the NOy at this NOx-rich site is composed of isoprene nitrates.

The correlation of temperature and rural ozone levels in southeastern U.S.A.

Abstract As a part of the southern oxidant study, a comprehensive air quality study was performed at a rural site in southern Tennessee. The observations performed between 25 July and 2 September 1991 included measurements of primary and secondary pollutants as well as meteorological measurements. Relatively high levels of NOy were observed when CO and SO2 levels were at estimated regional background indicating a significant non-combustion source. A multivariate linear regression analysis suggested that nearly 2 3 of the NOy above the estimated regional background level relates to SO2 emitting sources with the remaining 1 3 to CO emitting sources. The association between primary pollutants and temperature was found to be weak or insignificant. However, most secondary pollutants (except PAN) positively correlated with temperature. In the case of O3, a better association was found with a combination of temperature and NOy. Comparison between the observation and model simulation suggested that approximately 50% of the increase in O3 levels observed at the Giles site may relate to the intrinsic dependence of rate constants on temperature. The number of O3 molecules produced per NOy molecule present increases with temperature (between 22 and 33°C) as did the chemical air mass age ( NO x NO y ). On the other hand, the number of O3 molecules produced per molecule of NOx consumed remained nearly constant. The difference between NOz and the sum of the individual NOz species measured separately (PAN, HNO3 and nitrate aerosol) suggests the presence of an additional NOz species that increased with temperature.

Sequential oxidation products from tropospheric isoprene chemistry : MACR and MPAN at a NOx-rich forest environment in the southeastern United States

Measurements of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and peroxymethacryloyl nitrate (MPAN) were made during the Southern Oxidants Study 1995 Nashville/Middle Tennessee Ozone Study at the Youth Inc. Ranch southeast of Nashville from June 29 to July 26. These measurements were made along with those of isoprene and its oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK), other carbonyl compounds, and supporting measurements. This data set represents the first high-frequency, simultaneous measurements of MPAN and its precursor, MACR as well as PPN and its precursor propanal. The NOx sensitivity of isoprene chemistry can be studied with data from this site because large and widely fluctuating levels of NOx were experienced as a result of the proximity to the Nashville urban center. Mean mixing ratios of PAN, PPN, MPAN, and MACR were 485, 50, 30, and 290 parts per trillion by volume respectively. The mean diurnal cycle of MPAN closely tracks that of MACR and was found to be considerably sharper than the mean diurnal cycles of PAN, PPN, and O3, showing that MPAN is closely dependent on the availability of MACR. Considerable levels of MPAN and MACR appear to develop at night above a nocturnal boundary layer and are partly responsible for a commonly observed morning increase. Early morning OH reaction with MACR also produces MPAN in the morning. With the high summer temperatures at Youth Inc., the MPAN lifetime is largely determined by thermal decomposition, although OH and O3 chemistry could substantially reduce the ambient lifetime. Additional loss mechanisms for MPAN and dependence on a single source make MPAN more sensitive to photochemical activity and NOx. than is PAN. Unless quickly advected to colder, less photochemically active regions of the atmosphere, the contribution of MPAN to O3 formation through long-range transport of NOx is likely to be less significant in comparison with PAN and PPN. Its influence is more limited to local O3 production, so MPAN can be a useful indicator of local-scale, active biogenic photochemistry.