Peroxypropionyl Nitrate Research Articles

Infrared absorption cross-sections for peroxyacyl nitrates (nPANs)

Absorption spectra of peroxyacyl nitrates (nPANs) at room temperature (298 ± 3 K) have been measured in the mid-infrared range (700–2050 cm −1) using an environmental chamber/FTIR absorption system. nPANs are generated in situ as main products in the NO 3 radical reactions with aldehydes. Both peak absorption cross-sections and integrated band intensities for the principal absorption bands in the peroxypropionyl nitrate (PPN), peroxy- n-butyryl nitrate (PnBN), peroxy- n-valeryl nitrate (PnVN), peroxyacryloyl nitrate (APAN) and peroxycrotonyl nitrate (CPAN) spectra have been derived from quantitative laboratory measurements. Results are compared with literature data when available and in those cases, a good agreement is observed.

Quantum chemical study of the atmospheric C 2H 5C(O)OONO 2 (PPN) molecule and of the C 2H 5C(O)OO and C 2H 5C(O)O radicals

Ab initio and density functional calculations have been performed on atmospheric peroxypropionyl nitrate, C 2H 5C(O)OONO 2 (PPN) molecule and its radical decomposition products C 2H 5C(O)OO and C 2H 5C(O)O. Potential barriers for the internal rotations have been computed at the B3LYP/6-311++G(d,p) level of theory. Geometries, harmonic vibrational frequencies and thermochemical properties of stable rotational conformers and transition states have been calculated at the B3LYP/6-311++G(3df,3pd) and G3MP2B3 levels. For PPN, the results shown a structure in which the atoms nearest to the O O bond are in approximately perpendicular planes, τ(COON) = 85.9°. The standard enthalpies of formation at 298 K have been calculated using isodesmic reactions at the G3MP2//B3LYP/6-311++G(3df,3pd) level of theory. The resulting values for PPN, C 2H 5C(O)OO, and C 2H 5C(O)O are −66.5, −43.2, and −46.6 kcal mol −1, respectively. Dissociation enthalpies of 31.5 and 37.5 kcal mol −1 have been predicted for the C 2H 5C(O)OO–NO 2 and C 2H 5C(O)O–ONO 2 bonds from the above enthalpies of formation. For comparative purposes, results derived from similar calculations are also reported for the peroxyacetyl nitrate, CH 3C(O)OONO 2 (PAN) and for the CH 3C(O)OO and CH 3C(O)O radicals.

Eddy covariance fluxes of peroxyacetyl nitrates (PANs) and NOy to a coniferous forest

We employed a fast response thermal dissociation‐chemical ionization mass spectrometer (TD‐CIMS) system to measure eddy covariance fluxes of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN) and peroxymethacryloyl nitrate (MPAN). Fluxes were measured for eight consecutive days in July 2003 at a Loblolly pine forest in North Carolina along with eddy covariance NOy fluxes. Covariances between PAN concentration and vertical wind velocity indicated consistent deposition fluxes that ranged up to approximately −14 ng N m−2 s−1. The average daytime flux peaked at −6.0 ng N m−2 s−1 and accounted for ∼20% of the daytime NOy flux. Calculations suggest minimum daytime surface resistances for PAN in the range of 70–130 s m−1. It was estimated that approximately half of daytime uptake was through plant stomates. Average PAN deposition velocities, Vd(PAN), showed a daytime maximum of ∼10.0 mm s−1; however, deposition did not cease during nighttime periods. Vd(PAN) was highly variable at night and increased when canopy elements were wet from either precipitation or dew formation. Diel patterns of deposition velocity of MPAN and PPN were similar to that of PAN. These results suggest that deposition of PAN, at least to coniferous forest canopies, is much faster than predicted with current deposition algorithms. Although deposition of PAN is unlikely to compete with thermal dissociation during warm summer periods, it will likely play an important role in removing PAN from the atmosphere in colder regions or during winter. The fate of PAN at the surface and within the plants remains unknown, but may present a previously ignored source of nitrogen to ecosystems.

Gas‐phase chemical characteristics of Asian emission plumes observed during ITCT 2K2 over the eastern North Pacific Ocean

The gas‐phase chemical characteristics of emission plumes transported from Asia across the Pacific Ocean observed during the Intercontinental Transport and Chemical Transformation experiment in 2002 (ITCT 2K2) are described. Plumes measured in the troposphere from an aircraft were separated from the background air in data analysis using 1‐s measurements of carbon monoxide (CO), total reactive nitrogen (NOy), and other gas‐phase species along with back trajectory analysis. On the basis of these measurements, Asian transport plumes with CO mixing ratios greater than 150 ppbv were observed on seven flights. Correlations between 1‐s observations of CO, ozone (O3), and NOy are used to characterize the plumes. The NOy/CO ratios were similar in each plume and significantly lower than those derived from estimated Asian emission ratios, indicating substantial removal of soluble NOy species during transport. Observations of nitric oxide (NO), nitrogen dioxide (NO2), nitric acid (HNO3), peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and alkyl nitrates are used with the NOy measurements to further distinguish the transport plumes by their NOy partitioning. NOy was primarily in the form of PAN in plumes that were transported in cold high‐latitude and high‐altitude regions, whereas in plumes transported in warmer, lower latitude and altitude regions, NOy was mainly HNO3. Additional gas‐phase species enhanced in these plumes include sulfuric acid, methanol, acetone, propane, and ethane. The O3/CO ratio varied among the plumes and was affected by the mixing of anthropogenic and stratospheric influences. The complexity of this mixing prevents the determination of the relative contribution of anthropogenic and stratospheric influences to the observed O3 levels.

Measurements of the sum of HO<sub>2</sub>NO<sub>2</sub> and CH<sub>3</sub>O<sub>2</sub>NO<sub>2</sub> in the remote troposphere

Abstract. The chemistry of peroxynitric acid (HO2NO2) and methyl peroxynitrate (CH3O2NO2)is predicted to be particularly important in the upper troposphere where temperatures are frequently low enough that these compounds do not rapidly decompose. At temperatures below 240K, we calculate that about 20% of NOy in the mid- and high-latitude upper troposphere is HO2NO2. Under these conditions, the reaction of OH with HO2NO2 is estimated to account for as much as one third of the permanent loss of hydrogen radicals. During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign, we used thermal dissociation laser-induced fluorescence (TD-LIF) to measure the sum of peroxynitrates (PNs HO2NO2+CH3O2NO2+PAN+PPN+...) aboard the NCAR C-130 research aircraft. We infer the sum of HO2NO2 and CH3O2NO2 as the difference between PN measurements and gas chromatographic measurements of the two major peroxy acyl nitrates, peroxy acetyl nitrate (PAN) and peroxy propionyl nitrate (PPN). Comparison with NOy and other nitrogen oxide measurements confirms the importance of HO2NO2 and CH3O2NO2 to the reactive nitrogen budget and shows that current thinking about the chemistry of these species is approximately correct. During the spring high latitude conditions sampled during the TOPSE experiment, the model predictions of the contribution of (HO2NO2+CH3O2NO2) to NOy are highly temperature dependent: on average 30% of NOy at 230K, 15% of NOy at 240K, and 5% of NOy above 250K. The temperature dependence of the inferred concentrations corroborates the contribution of overtone photolysis to the photochemistry of peroxynitric acid. A model that includes IR photolysis (J=1x10-5s-1) agreed with the observed sum of HO2NO2+CH3O2NO2 to better than 35% below 240K where the concentration of these species is largest.

Photochemical Studies of CH3C(O)OONO2 (PAN) and CH3CH2C(O)OONO2 (PPN): NO3 Quantum Yields

The quantum yields for the production of NO3 in the gas-phase UV photolysis of peroxyacetyl nitrate (PAN, CH3C(O)OONO2) and peroxypropionyl nitrate (PPN, CH3CH2C(O)OONO2) at 248 and 308 nm were measured. We report room-temperature quantum yield values of 0.19 ± 0.04 and 0.41 ± 0.10 at 248 and 308 nm, respectively, for PAN and 0.22 ± 0.04 and 0.39 ± 0.04 at 248 and 308 nm, respectively, for PPN. UV absorption cross sections for PPN between 200 and 340 nm over the temperature range 296−253 K are also reported. The atmospheric implications of these results are discussed.

Ambient PAN and PPN in southern California from 1960 to the SCOS97-NARSTO

Abstract We examine long-term trends in ambient levels of peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) in southern California from the first measurements made in 1960 to those made in 1997 as part of SCOS97-NARSTO. We analyze peak concentrations, 24-h averages and monthly averages. Although the data record is incomplete with respect to frequency and duration of measurements and is limited with respect to seasonal and spatial variations, peak concentrations have decreased from ca. 60–70 ppb to ca. 5–10 ppb for PAN and from ca. 5–6 to 1 ppb or less for PPN. Twenty-four-hour averages and monthly averages also show downward trends in ambient PAN and PPN. The PPN/PAN concentration ratios range from 0.10 to 0.28 and the limited data record give no indication of a long-term temporal trend. The downward trend in peak PAN concentrations is similar to that of peak ozone concentrations during the same 37-year period and is likely to result from increasingly stringent controls on emissions of oxides of nitrogen and of volatile organic compounds that are precursors to ozone, PAN, PPN and other photochemical oxidants.

Peroxyacetyl nitrate and peroxypropionyl nitrate in Porto Alegre, Brazil

For 41 days between 25 May 1996 and 27 March 1997, peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) have been measured by electron capture gas chromatography at Santa Rita near Porto Alegre, RS, Brazil, where light-duty vehicles used either ethanol or a gasoline–MTBE blend. Daily maximum concentrations ranged from 0.19 to 6.67 ppb for PAN and 0.06 to 0.72 ppb for PPN. Linear regression of maximum PPN vs. maximum PAN yielded a slope of 0.105±0.004 ( R 2=0.974). Diurnal variations of ambient PAN often followed those of ozone with respect to time of day but not with respect to amplitude. This was reflected in the large relative standard deviations associated with the study-averaged PAN/ozone concentration ratio, 0.037±0.105 (ppb/ppb, n=789) and the maximum PAN/maximum ozone concentration ratio, 0.028±0.015 (ppb/ppb, range 0.005–0.078, n=41). On several days PAN accounted for large fractions of the total ambient NO x in the late morning and afternoon hours, e.g., PAN/NO x ⩽0.58 and PAN/(NO x –NO) ⩽0.76 on 27 March 1997. The amount of PAN lost by thermal decomposition (TPAN) was comparable in magnitude to that present in ambient air. The ratios TPAN/(PAN+TPAN) were up to 0.53, 0.67 and 0.64 during the warm afternoons of 25, 26 and 27 March 1997, respectively. The highest calculated value of TPAN was 5.6 ppb on 27 March 1997. On that day the 24 h-averaged value of TPAN (1.01 ppb) was nearly the same as that of PAN (1.09 ppb). Using computer kinetic modeling (SAPRC 97 chemical mechanism) and sensitivity analysis of VOC incremental reactivity, we ranked VOC present in Porto Alegre ambient air for their importance as precursors to PAN and to PPN. Using as input data the averages of VOC concentrations measured in downtown Porto Alegre during the ca. 1 yr period March 1996–April 1997, we calculated that the most important precursors to PAN and PPN were the SAPRC 97 model species ARO2 (which includes the aromatics xylenes, trimethylbenzenes, ethyltoluenes, etc.), which accounted for ca. 17% of the total PAN and total PPN formation potentials. Overall, the results indicate a major role for aromatics and alkenes and a minor role for acetaldehyde and ethanol as precursors to peroxyacyl nitrates in the Porto Alegre urban area.

Seasonal variations of PAN, PPN, and O3 at the upper Midwest PROPHET site

Continuous measurements of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and ozone (O3) were made from June 1997 through August 1999 at a ground site in the upper Midwest as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET). Relatively clean air originating in Canada is transported to the site from the northwest, while more polluted air originating in the metropolitan regions of the Midwest is transported mainly from the southwest and the southeast. Monthly averages of PAN are a minimum in summer /early winter (July‐January) with mixing ratios ranging from 125 to 259 parts per trillion by volume (pptv), while maximum monthly average PAN levels are observed in February through June with mixing ratios of 325–390 pptv. Monthly average ozone mixing ratios ranged from 42 to 46 ppbv in the spring/summer months and 23–32 ppbv in the autumn/early winter months. Correlation among species were examined on a monthly basis. There is a dramatic variation in the PPN/PAN slope from summer to winter with the general trend a maximum slope in the winter ranging from 0.20 to 0.23 and a minimum slope during the summer months ranging from 0.09 to 0.13. The degree of association, r2, is strong in all months, ranging from 0.76 to 0.96. A photochemical box model was used to determine if the seasonal variation could be attributed solely to the summertime production of PAN from isoprene. Box model results indicate that the seasonal variation in the PPN/PAN slope when produced from anthropogenic precursors can be explained by seasonal changes in temperature and photolysis rates, and the production of PAN from isoprene in the summer only serves to increase the seasonal difference in slope.

Fast‐turnaround alkyl nitrate measurements during the PROPHET 1998 summer intensive

An automated gas chromatographic system for making fast‐turnaround measurements of alkyl nitrates was developed and tested. Every 30 min, samples were automatically acquired on a 1/16‐inch OD Tenax trap made from fused silica‐lined stainless steel tubing and injected by thermal desorption into the gas chromatograph. The system exhibited good chromatographic resolution, minimal breakthrough, and on‐trap stability for C3‐C5 nitrates. The detection limit of the instrument was <0.1 parts per trillion by volume for C3‐C5 nitrates with an overall uncertainty of 30% for isopropyl nitrate and 50% for all other quantified compounds. Overall uncertainty of the method was limited by calibration precision rather than by sampling method or chromatography. The system was deployed in the field as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport intensive held at the University of Michigan Biological Station in northern Michigan during the summer of 1998 from July 15 to August 22. Ambient results show a significant dynamic range with absolute levels affected by air mass history and pollution sources. The sum of measured C3‐C5 alkyl nitrates obtained during this intensive ranged from 3.45 to 65.8 pptv, with a mean of 18.6 pptv and contributed 1% on average to total NOy, with a range of 0.5% to 3%. Differences in the contribution to NOy are found when the data are filtered according to air mass origin. A positive relationship between n‐propyl nitrate and peroxypropionyl nitrate was observed over the course of the intensive.

Peroxyacetyl nitrate and peroxypropionyl nitrate during SCOS 97-NARSTO.

Peroxyacyl nitrates [RC(O)OONO2] play an important role in urban air quality and tropospheric chemistry. They also receive attention as mutagens, phytotoxins, and possible air quality indicators of changes in vehicle fuel composition. Ambient concentrations of PAN (R = CH3) and PPN (R = C2H5) have been measured during summer 1997 at two southern California locations, Azusa (July 14-October 16) and Simi Valley (June 18-October 16). The highest concentrations were 4.8 ppb for PAN and 0.72 ppb for PPN in Azusa and 3.0 ppb for PAN and 0.28 ppb for PPN in Simi Valley. Ambient levels of PAN and PPN during summer 1997 were lower than those measured in the last three studies carried out in southern California in the summers of 1990, 1991, and 1993. Average PPN/PAN concentration ratios were about the same in Azusa (0.142+/-0.025, n = 132) and in Simi Valley (0.135+/-0.028, n = 138). The PPN/PAN ratio measured in Azusa was the same as that measured at that location in 1993 prior to the introduction in 1996 of California Phase 2 reformulated gasoline. Diurnal variations of PAN and PPN generally followed those of ozone with respect to time of day but not with respect to amplitude. The PAN/ozone ratio was lower in Simi Valley than in Azusa, and daytime minima were recorded at both locations. The amount of PAN lost by thermal decomposition accounted for large fractions of the amount of PAN formed (measured + decomposed) during daytime hours at both locations. The amount of PAN lost by thermal decomposition was higher in Azusa and was up to ca. 8.5 ppb, i.e., 4-5 times more than that measured, when afternoon temperatures were ca. 40 degrees C.

Analysis of C 2–C 4 peroxyacyl nitrates and C 1–C 5 alkyl nitrates with a non-polar capillary column

Analysis of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate, peroxy- n-butyryl nitrate, and peroxyisobutyryl nitrate (PiBN) with a non-polar capillary column indicated PAN not to coelute with PiBN. Isopropyl nitrate coeluted with PAN, n-butyl nitrate with PiBN and ethyl nitrate with chloroform, at oven temperature 30°C. In addition methyl nitrate, n-propyl nitrate, 2-butyl nitrate, 2-methyl-2-butyl nitrate, 3-pentyl nitrate and 2-pentyl nitrate were studied. Their response factors in the electron-capture detector were also determined. Tetrachloroethylene, carbon tetrachloride and chloroform whose vapors occur in the atmosphere were also examined.

FTIR spectroscopic study of the OH-induced oxidation of two linear acetates: ethyl and n-propyl acetates

OH-induced oxidation mechanisms of ethyl and n-propyl acetates have been investigated at room temperature (298 ± 5 K) and atmospheric pressure by photolysing CH3ONO/acetate/NO mixtures with FTIR spectroscopy as analytical device. The main oxidation products and their yields were as follows: from ethyl acetate, acetic acid (0.75 ± 0.13), acetoxyacetaldehyde (0.15 ± 0.05), acetic anhydride (0.02 ± 0.01), formic acetic anhydride (0.02 ± 0.01) and peroxyacetyl nitrate (PAN); from n-propyl acetate, acetoxyacetaldehyde (0.22 ± 0.06), formic acetic anhydride (0.28 ± 0.03), acetic acid (0.15 ± 0.02), acetaldehyde (0.35 ± 0.10), peroxypropionyl nitrate (PPN) and probably acetoxypropionaldehyde (0.30 ± 0.10). From these data, oxidation schemes of these two acetates were elucidated. This study reveals in particular the specific reactivity of acetates by confirming the novel α-ester rearrangement proposed recently by Tuazon et al. (J. Phys. Chem. A, 1998, 102, 2316) and then by showing that oxygenated alkoxyl radicals may not follow the same rules of reactivity as other alkoxyl radicals. This last observation shows the necessity for further experiments to understand the influence of the oxygenated function on alkoxyl reactivity.

Simple cryoconcentration technique for the determination of peroxyacyl and alkyl nitrates in the atmosphere

A gaseous mixture of methyl nitrate, n-propyl nitrate, peroxyacetyl nitrate and peroxypropionyl nitrate was successfully cryoconcentrated using a gas sampling valve with its loop filled with glass beads placed inside the GC oven which was cooled at –50°C. The technique offered linearity over a ten fold concentration variation and sample volume range of 1–280 ml. The detection limit for 150 ml sample varied from 0.02–10 pptv depending on the compound.

Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic

A large number of oxygenated organic chemicals (peroxyacyl nitrates, alkyl nitrates, acetone, formaldehyde, methanol, methylhydroperoxide, acetic acid and formic acid) were measured during the 1997 Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) airborne field campaign over the Atlantic. In this paper, we present a first picture of the distribution of these oxygenated organic chemicals (Ox‐organic) in the troposphere and the lower stratosphere, and assess their source and sink relationships. In both the troposphere and the lower stratosphere, the total atmospheric abundance of these oxygenated species (ΣOx‐organic) nearly equals that of total nonmethane hydrocarbons (ΣNMHC), which have been traditionally measured. A sizable fraction of the reactive nitrogen (10–30%) is present in its oxygenated organic form. The organic reactive nitrogen fraction is dominated by peroxyacetyl nitrate (PAN), with alkyl nitrates and peroxypropionyl nitrate (PPN) accounting for <5% of total NOy. Comparison of observations with the predictions of the Harvard three‐dimensional global model suggests that in many key areas (e.g., formaldehyde and peroxides) substantial differences between measurements and theory are present and must be resolved. In the case of CH3OH, there appears to be a large mismatch between atmospheric concentrations and estimated sources, indicating the presence of major unknown removal processes. Instrument intercomparisons as well as disagreements between observations and model predictions are used to identify needed improvements in key areas. The atmospheric chemistry and sources of this group of chemicals is poorly understood even though their fate is intricately linked with upper tropospheric NOx and HOx cycles.

Measurements of peroxyacyl nitrates (PANS) in Mexico City: implications for megacity air quality impacts on regional scales

Peroxyacyl nitrates (PANs) were measured using gas chromatography with electron capture detection (GC/ECD) in north central Mexico City during February–March of 1997. Peroxyacetyl nitrate (PAN) was observed to exceed 30 ppb during five days of the study, with peroxypropionyl nitrate (PPN) and peroxybutryl nitrate (PBN) reaching 6 and 1 ppb maximum, respectively. Levels of total PANs typically exceeded 10 ppb during the period of measurement and showed a very strong diurnal variation with PANs maximum during the early afternoon and falling to less than 0.1 ppb during the evening hours. These levels of PANs are the highest reported values in North America (and the world) for an urban center, since levels of approximately 30 ppb were reported during the late 1970s in the Los Angeles area (South Coast Air Basin, Tuazon et al., 1978). Hydrocarbon measurements indicate that the levels of olefins, specifically butenes are significant in Mexico City. A time series taken of source indicator hydrocarbons taken before and during a Mexican National Holiday with reduced automobile traffic clearly show that mobile sources of butenes are as important as liquefied petroleum gas. Observations of 10–40 ppb C methyl- t-butyl ether (MTBE) are consistent with MTBE/gasoline fuel usage as a source of isobutene and formaldehyde. Both these reactive species can lead to increased oxidant and PAN formation. The strong diurnal profiles of PANs are consistent with regional clearing of the Mexico City air basin on a daily basis. Estimates are given using a simple box model calculation for a number of key primary and secondary pollutant emissions from this megacity on an annual basis. These calculations indicate that megacities can be important sources of both primary and secondary pollutants, and that PANs produced in megacity environments are likely to contribute strongly to regional scale ozone and aerosol productions during long range transport.

A new measurement technique of peroxyacetyl nitrate at parts per trillion by volume levels: Gas chromatography/negative ion chemical ionization mass spectrometry

A new technique for measuring peroxyacetyl nitrate (PAN) at parts per trillion by volume (pptv) levels has been developed using gas chromatography with negative ion chemical ionization mass spectrometry (GC/NICI MS). Four fragment ions, CH3C(O)O− (mass‐to‐charge ratios (m/z) = 59), NO−3 (m/z = 62), CH3C(O)OO− (m/z = 75), and NO−2 (m/z = 46), were identified by NICI mass spectra. NO−3 (m/z = 62) was used in selected ion monitoring (SIM) mode among these fragment ions, since it gave the best signal‐to‐noise (S/N) ratio, and a potential interference from isopropyl nitrate (IPN) was excluded. Optimization of mass spectrometry and use of capillary gas chromatography enabled us to detect PAN at a retention time of 2.6±0.1 min in ambient air mixing ratio range. The present detection limit is 15±4 pptv (S/N = 3), which is comparable to those of previously published GC/ECD methods, with good linearity at pptv levels. The instrumental accuracy and precision are estimated to be ±20% and ±15%, respectively. Other PAN analogs (i.e., peroxypropionyl nitrate and peroxymethacryloyl nitrate) were also measured within 10 min. Exploratory ambient air measurements were made at our laboratory in a suburban area of Tokyo, Japan. These results demonstrated the performance of GC/NICI MS for ambient air measurements.

Formation of Nitrogenous Compounds in the Photooxidation of n-Butane under Atmospheric Conditions

The photooxidation of n-butane under atmospheric conditions in the presence of NOx resulted in the formation of the following nitrogenous products: peroxy acetyl nitrate 23, sec-butyl nitrate 16, n-butyl nitrate 1.3, ethyl nitrate 1.3, peroxy n-butyryl nitrate 1.3, and peroxy propionyl nitrate 0.5% of the initially added odd nitrogen. In addition, an electron capturing compound eluting at the retention time of sec-propyl nitrate was also observed accounting for 5% of initial NOx. Butan-2-one was the major product of conversion of n-butane with a yield of 37%. From product ratios it is evident that the formation of sec-butyl nitrate is favored over that of n-butyl nitrate by a factor of 2.1. The rate of reaction of sec-butoxy radicals with oxygen is equal to their decomposition rate.

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.

Airborne peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) measurements during tract 1992

Airborne PAN and PPN measurements with high spatial resolution were made during the TRACT 1992 intensive campaign. Relationships between PAN, PPN, ozone and reactive nitrogen species are discussed. PPN mixing ratios are found to be typically 12% of the PAN mixing ratios. PAN was always tightly correlated with CO, and in most cases with ozone, in agreement with a simple photochemical box model which was run with input parameters representative of the flight situations. The observed PPN/PAN ratios were lower than predicted by the box model. This may be due to an overestimation of 2-pentene emissions which were used as input to the model, and/or to the omission of isoprene and other biogenic hydrocarbons. The observed PAN/NO y ratios in photochemically aged air masses exceeded both model predictions and previous observations. This inconsistency is most likely due to HNO 3 losses in the aircraft sampling line.