Peroxyacyl Nitrates Research Articles

Kinetics and products of the OH radical-initiated reaction of 3-methyl-2-butenal

Kinetics and products of the gas-phase reaction of OH radicals with 3-methyl-2-butenal [(CH3)2C=CHCHO] have been investigated at room temperature and atmospheric pressure of air. Using a relative rate method with methacrolein as the reference compound, a rate constant for the reaction of OH radicals with 3-methyl-2-butenal of (6.21 +/- 0.18) x 10(-11) cm3 molecule(-1) s(-1) at 296 +/- 2 K was measured, where the indicated error does not include the uncertainty in the rate constant for the methacrolein reference compound. Products of this reaction were investigated using in situ Fourier transform infrared (FT-IR) spectroscopy and solid phase microextraction (SPME) fibers coated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine for on-fiber derivatization of carbonyl compounds, with subsequent thermal desorption and analysis by gas chromatography. The products observed and the molar formation yields were: glyoxal, 40 +/- 3%; acetone, 74 +/- 6%; 2-hydroxy-2-methylpropanal, 4.6 +/- 0.7%; CO2, 39% initially, decreasing to 30% at greater extents of reaction; peroxyacyl nitrate(s) [RC(O)OONO2], 5-8%, increasing with the extent of reaction and with the sum of the CO2 and RC(O)OONO2 yields being 38 +/- 6%; and organic nitrates [RONO2], 8.5 +/- 2.3%. The formation of these products is readily explained by a reaction mechanism based on those previously formulated for the corresponding reactions of the alpha,beta-unsaturated aldehydes acrolein, crotonaldehyde and methacrolein. Based on the mechanism proposed, at room temperature H-atom abstraction from the CHO group accounts for 40 +/- 6% of the overall reaction, and OH radical addition to the carbon atoms of the C=C bond accounts for 53 +/- 4% of the overall reaction. Hence 93 +/- 8% of the reaction products and pathways are accounted for.

Systematic reduction of complex tropospheric chemical mechanisms, Part II: Lumping using a time-scale based approach

Abstract. This paper presents a formal method of species lumping that can be applied automatically to intermediate compounds within detailed and complex tropospheric chemical reaction schemes. The method is based on grouping species with reference to their chemical lifetimes and reactivity structures. A method for determining the forward and reverse transformations between individual and lumped compounds is developed. Preliminary application to the Leeds Master Chemical Mechanism (MCMv2.0) has led to the removal of 734 species and 1777 reactions from the scheme, with minimal degradation of accuracy across a wide range of test trajectories relevant to polluted tropospheric conditions. The lumped groups are seen to relate to groups of peroxy acyl nitrates, nitrates, carbonates, oxepins, substituted phenols, oxeacids and peracids with similar lifetimes and reaction rates with OH. In combination with other reduction techniques, such as sensitivity analysis and the application of the quasi-steady state approximation (QSSA), a reduced mechanism has been developed that contains 35% of the number of species and 40% of the number of reactions compared to the full mechanism. This has led to a speed up of a factor of 8 in terms of computer calculation time within box model simulations.

Analysis of the atmospheric distribution, sources, and sinks of oxygenated volatile organic chemicals based on measurements over the Pacific during TRACE‐P

Airborne measurements of a large number of oxygenated volatile organic chemicals (OVOC) were carried out in the Pacific troposphere (0.1–12 km) in winter/spring of 2001 (24 February to 10 April). Specifically, these measurements included acetone (CH3COCH3), methylethyl ketone (CH3COC2H5, MEK), methanol (CH3OH), ethanol (C2H5OH), acetaldehyde (CH3CHO), propionaldehyde (C2H5CHO), peroxyacylnitrates (PANs) (CnH2n+1COO2NO2), and organic nitrates (CnH2n+1ONO2). Complementary measurements of formaldehyde (HCHO), methyl hydroperoxide (CH3OOH), and selected tracers were also available. OVOC were abundant in the clean troposphere and were greatly enhanced in the outflow regions from Asia. Background mixing ratios were typically highest in the lower troposphere and declined toward the upper troposphere and the lowermost stratosphere. Their total abundance (ΣOVOC) was nearly twice that of nonmethane hydrocarbons (ΣC2‐C8 NMHC). Throughout the troposphere, the OH reactivity of OVOC is comparable to that of methane and far exceeds that of NMHC. A comparison of these data with western Pacific observations collected some 7 years earlier (February–March 1994) did not reveal significant differences. Mixing ratios of OVOC were strongly correlated with each other as well as with tracers of fossil and biomass/biofuel combustion. Analysis of the relative enhancement of selected OVOC with respect to CH3Cl and CO in 12 plumes originating from fires and sampled in the free troposphere (3–11 km) is used to assess their primary and secondary emissions from biomass combustion. The composition of these plumes also indicates a large shift of reactive nitrogen into the PAN reservoir thereby limiting ozone formation. A three‐dimensional global model that uses state of the art chemistry and source information is used to compare measured and simulated mixing ratios of selected OVOC. While there is reasonable agreement in many cases, measured aldehyde concentrations are significantly larger than predicted. At their observed levels, acetaldehyde mixing ratios are shown to be an important source of HCHO (and HOx) and PAN in the troposphere. On the basis of presently known chemistry, measured mixing ratios of aldehydes and PANs are mutually incompatible. We provide rough estimates of the global sources of several OVOC and conclude that collectively these are extremely large (150–500 Tg C yr−1) but remain poorly quantified.

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.

A Review of Instrumentation and Measurement Techniques for Ground-Based and Airborne Field Studies of Gas-Phase Tropospheric Chemistry

ABSTRACT The development, applications and intercomparisons of instrumentation and measurement techniques for ground-based and airborne field studies of gas-phase tropospheric chemistry are reviewed. Filter radiometry, chemical actinometry and scanning spectroradiometry for j-NO2 and j-(O1D) are described. Detection of OH using L-POAS/DOAS, MOAS, LIF/FAGE and CIMS is discussed. Observations of NO3 using DOAS, MIESR and CRDS, and of HO2 and RO2 with CA, CIMS/IMR-MS, MIESR, and LIF/FAGE are also reviewed. GC-FID, GC-ECD and GC-MS analyses of NMHCs and alkyl, peroxyacyl and bi-functional organic nitrates are described, together with applications of CIMS/PTR-MS, DOAS and LIF. CO measurements utilising GFC, GC-HgO/UV, VUV-RF and TDLAS are presented. Measurements of O3 using UV photometry, chemiluminescence, electrochemistry, LIDAR/DIAL and DOAS are discussed. Chemiluminescence and LIF detection methods for NO with photochemical and thermal convertors for NO2 and NOy are also discussed, as are TDLAS, MIESR, DOAS, CRDS and other approaches for NO2. HONO measurements using DOAS, CIMS, CRDS, TDLAS, denuder systems, chemiluminescence and on-line analyses of NO2 − are described. For HONO2, filter packs, denuder systems, chemiluminescence methods, mist chambers, TDLAS, CIMS and LIF are presented. Direct and indirect fluorimetric, chromatographic and spectroscopic detection techniques are discussed for CH2O and higher carbonyls. Observations of H2O2 and ROOH utilising colorimetry, chemiluminescence, fluorescence, HPLC and TDLAS are also described.

Validation of proton transfer reaction‐mass spectrometry (PTR‐MS) measurements of gas‐phase organic compounds in the atmosphere during the New England Air Quality Study (NEAQS) in 2002

Organic compounds were measured by proton transfer reaction‐mass spectrometry (PTR‐MS) on board the National Oceanic and Atmospheric Administration's research ship Ronald H. Brown during the New England Air Quality Study (NEAQS) in July and August of 2002. PTR‐MS has the potential to measure many important organic species with a fast time response, but its validity has not been proven sufficiently. The results obtained by PTR‐MS during NEAQS were compared with those from (oxygenated) hydrocarbon measurements by gas chromatography/mass spectrometry (GC‐MS), peroxyacyl nitrate measurements by gas chromatography/electron capture detection, and carboxylic acid measurements by mist chamber/ion chromatography. The PTR‐MS and GC‐MS data for methanol, acetonitrile, acetone, isoprene, benzene, and toluene agreed within the measurement uncertainties. The comparison for C8 aromatics and acetaldehyde was less quantitative due to calibration inaccuracies. In addition, PTR‐MS measured the sum of methyl vinyl ketone and methacrolein at 71 amu, the sum of C9 aromatics at 121 amu, and the sum of monoterpenes at 81 and 137 amu. The PTR‐MS signal at 61 amu was found to correlate well with data for acetic acid. The signal at 73 amu correlated reasonably well with methyl ethyl ketone data, but the quantitative disagreement suggested interference from other species, possibly methyl glyoxal. The signal at 77 amu correlated well with data for peroxyacetyl nitrate, and the sensitivity inferred from the field data agreed within 30% with the results from laboratory calibrations. Finally, the signal at 105 amu was attributed to styrene and peroxy isobutyryl nitrate. These results prove that many important organic species can be measured accurately and with a fast response time by PTR‐MS.

Passive Samplers for NOx Monitoring: A Critical Review

Nitrogen oxide is an important gaseous air pollutant. It plays a major role in atmospheric chemistry, particularly in the formation of secondary air pollutants such as ozone, peroxyacyl nitrate (PAN), nitrate aerosols, and contributes to environmental acidification. A comprehensive assessment of NO2 levels in the atmosphere is required for developing effective strategies for control of air pollution and improvement of air quality. Consequently, spatial NO2 monitoring has become an important aspect of air quality assessment. Present NO2 monitoring networks are mostly confined to urban areas, having a limited number of monitoring sites. Wide spatial NO2 monitoring in India is constrained because of the limited availability of the monitoring equipment, such as continuous chemiluminesence monitors, high volume and handy samplers, which are expensive and need to be imported. Moreover, they require elaborate infrastructure, scientific personnel, technical support and uninterrupted power supply. In an effort to overcome the above shortcomings, passive samplers have been successfully used for large scale monitoring of NO2 in the ambient environment.

Determination of PAN, PPN, P nBN and selected pentyl nitrates in Athens, Greece

Quasi-continuous measurements of PAN, PPN, P nBN and the alkyl nitrates—2-methyl-2-butyl nitrate, 3-pentyl nitrate and 2-pentyl nitrate were carried out in Athens using a simple cryoconcentration technique. The maximum mixing ratios measured were 6.6, 1.0 and 0.07 ppbv for PAN, PPN and P nBN, respectively, for the peroxyacyl nitrates, and 0.3, 0.09 and 0.03 ppb for 2-methyl-2-butyl nitrate, 2-pentyl nitrate and 3-pentyl nitrate, respectively. Mean ratios of PPN/PAN mixing ratios were 0.102 and of P nBN/PAN 0.012. 2PN/3PN mean ratios were 1.8 near the theoretical value of 1.6. All maximum values of measured nitrogenous compounds were associated with maximum mixing ratios of ozone and NO x and occurred when southwestern winds prevailed in association with a temperature inversion.

A new peroxycarboxylic nitric anhydride identified in the atmosphere: CH2=CHC(O)OONO2 (APAN)

We detected a peroxyacyl nitrate (PAN)‐type compound, peroxyacryloyl nitrate (CH2=CHC(O)OONO2, APAN) which has not previously been reported in the atmosphere. Identification of APAN was made from negative ion chemical ionization (NICI) mass spectra of a cryogenically concentrated ambient air sample by comparison with a synthesized standard. The thermally fragile nature of the compound, and the retention time of the synthesized standard provide additional evidence for the existence of APAN in ambient air. Observations made at a suburban site in Tokyo, and at Rishiri Island, a remote site in northern Japan, suggest the ubiquitous presence of APAN in the global atmosphere. While its mixing ratio was usually low (<1% of PAN), high‐APAN episodes (∼4% of PAN) were observed at Rishiri Island. APAN can be formed from acrolein, which is directly emitted from vehicle exhaust, and may be produced by oxidation of 1,3‐butadiene and aromatics. Further measurements of APAN along with acrolein and other NMHCs are required to understand its distribution, behavior, and sources in the atmosphere.

Products of the Chlorine-Atom- and Hydroxyl-Radical-Initiated Oxidation of CH3CN

FTIR smog chamber techniques were used in the first investigation of the products following oxidation of CH3C⋮N initiated by Cl atom or OH radicals in air at 296 ± 2 K. It is shown that reaction of Cl atoms with CH3C⋮N gives •CH2C⋮N radicals in essentially 100% yield. These radicals add O2 to form the peroxy radical N⋮CCH2O2•. Self-reaction of the peroxy radicals gives the alkoxy radical N⋮CCH2O• which reacts exclusively with O2 to give HC(O)C⋮N (formyl cyanide). The reaction of Cl atoms with HC(O)C⋮N proceeds with a rate constant of (7.0 ± 1.0) × 10-13 cm3 molecule-1 s-1 via H-abstraction to give •C(O)C⋮N radicals which add O2 to give N⋮CC(O)OO• radicals. The subsequent chemistry of these radicals leads to the production of HC⋮N and CO2 or the acyl peroxy nitrate N⋮CC(O)OONO2, the IR spectrum of which is reported here. The reaction of OH radicals with CH3C⋮N in 700 Torr of air gives HC(O)C⋮N in (40 ± 20)% yield implying that up to half of the OH + CH3C⋮N reaction could proceed via H-atom abstraction, with the remaining fraction proceeding via addition to the −C⋮N group. These results are discussed in terms of the atmospheric oxidation mechanism of CH3C⋮N.

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.

Global impact of fossil fuel combustion on atmospheric NOx

Fossil fuel combustion is the largest global source of NOx to the troposphere. This source is concentrated in polluted continental boundary layers, and the extent to which it impacts tropospheric chemistry on a global scale is uncertain. We use a global three‐dimensional model of tropospheric chemistry and transport to study the impact of fossil fuel combustion on the global distribution of NOx during nothern hemisphere summer. In the model, we tag fossil fuel NOx and its reservoir NOy species in order to determine the relative contribution of fossil fuel combustion to NOx concentrations in different regions of the world. Our model includes a detailed representation of NOx‐O3‐nonmethane hydrocarbon (NMHC) chemistry, which is necessary to properly simulate the export of reactive nitrogen, including organic nitrates such as peroxyacyl nitrates (PANs), from the continental boundary layer. We find that fossil fuel combustion accounts for over 40% of NOx. concentrations in the lower and middle troposphere throughout the extratropical northern hemisphere. PANs are shown to provide an important mechanism for transporting NOx from source regions to the remote troposphere, accounting for over 80% of the fossil fuel NOx in the lower troposphere over most of the ocean. Sources in the United States are found to contribute about half of the fossil fuel NOx over the North Atlantic Ocean. Emissions from China, which are expected to increase rapidly in the coming decades, currently account for about half of the fossil fuel NOx over the western North Pacific Ocean; the influence of these emissions extends into the tropics. Because of this tropical influence, emissions from China have more potential than emissions in the United States to perturb the global oxidizing power of the atmosphere.

Aircraft Measurements of Nitrogen Dioxide and Peroxyacyl Nitrates Using Luminol Chemiluminescence with Fast Capillary Gas Chromatography

Fast capillary gas chromatography with luminol detection has been used to make airborne measurements of nitrogen dioxide (NO2) and peroxyacetyl nitrate (PAN). The analysis system allows for the simultaneous measurement of NO2 and peroxyacyl nitrates (PANs) with time resolution of less than 1 min, an improvement of a factor of 4−5 over previously reported methods using electron capture detection. Data presented were taken near Pasco, Washington, in August 1997, during a test flight onboard the U.S. Department of Energy G-1 aircraft. We report measurements of NO2 in the boundary layer in a paper mill plume and a plume from a grass fire, in addition to analyses for free tropospheric NO2 and PAN. Ratios of PAN/NO2 were observed to increase with altitude (decreasing temperature) and to reach values of 2−4 above the boundary layer, consistent with the thermal equilibrium of the peroxyacetyl radical and NO2 with PAN. Estimates for the peroxyacetyl radical in the continental free troposphere, calculated from this equilibrium, were found to be in the range of 104−105 molecules per cubic centimeter. These results demonstrate the application of this approach for airborne measurements of NO2 and PAN in a wide range of field study scenarios.

Seasonal budgets of reactive nitrogen species and ozone over the United States, and export fluxes to the global atmosphere

A three‐dimensional, continental‐scale photochemical model is used to investigate seasonal budgets of O3 and NOy species (including NOx and its oxidation products) in the boundary layer over the United States and to estimate the export of these species from the U.S. boundary layer to the global atmosphere. Model results are evaluated with year‐round observations for O3, CO, and NOy species at nonurban sites. A seasonal transition from NOx to hydrocarbon‐limited conditions for O3 production over the eastern United States is found to take place in the fall, with the reverse transition taking place in the spring. The mean NOx/NOy molar ratio in the U.S. boundary layer in the model ranges from 0.2 in summer to 0.6 in winter, in accord with observations, and reflecting largely the seasonal variation in the chemical lifetime of NOx. Formation of hydroxy organic nitrates during oxidation of isoprene, followed by decomposition of these nitrates to HNO3, is estimated to account for 30% of the chemical sink of NOx in the U.S. boundary layer in summer. Model results indicate that peroxyacylnitrates (PANs) are most abundant in the U.S. boundary layer in spring (25% of total NOy.), reflecting a combination of active photochemistry and low temperatures. About 20% of the NOx emitted from fossil fuel combustion in the United States in the model is exported out of the U.S. boundary layer as NOx or PANs (15% in summer, 25% in winter). This export responds less than proportionally to changes in NOx emissions in summer, but more than proportionally in winter. The annual mean export of NOx and PANs from the U.S. boundary layer is estimated to be 1.4 Tg N yr−1, representing an important source of NOx on the scale of the northern hemisphere troposphere. The eventual O3 production in the global troposphere due to the exported NOx and PANs is estimated to be twice as large, on an annual basis, as the direct export of O3 pollution from the U.S. boundary layer. Fossil fuel combustion in the United States is estimated to account for about 10% of the total source of O3 in the northern hemisphere troposphere on an annual basis.

Capillary gas chromatographic analysis of nitrogen dioxide and pans with luminol chemiluminescent detection

Peroxyacyl nitrates (PANs) and nitrogen dioxide are important atmospheric air pollutants in the troposphere. These atmospheric nitrogen species are strongly coupled chemically by a clearly temperature-dependent equilibrium in the troposphere. A chemical method that can measure both nitrogen dioxide and PANs rapidly and with sub-part-per-billion detection is described that is based upon a modified luminoldetection system coupled to a capillary gas chromatographic column by using helium as a carrier. The system can readily separate and detect nitrogen dioxide, peroxyacetyl nitrate, peroxyproprionyl nitrate, and peroxybutyrl nitrate with detection limits in the low tens of parts per trillion with total analysis time of less than 1 min. Calibration of PAN by thermal decomposition to nitrogen dioxide is demonstrated with PAN detection sensitivities approximately 75% of the sensitivities observed for NO 2 luminol detection by using helium as a carrier gas. The advantages of this method for simultaneous measurement of nitrogen dioxide and PANs over ozone chemiluminescent detection and electron capture detection are discussed, as well as potential applications of this method for heterogeneous surface chemistry studies of PANs and nitrogen dioxide and for tropospheric measurements.

Potential reactions among indoor pollutants

Reactions among indoor pollutants can produce products that, otherwise, might not be present in an indoor environment. To be relevant in an indoor setting, a chemical reaction must occur within a time interval shorter than or comparable to the residence time for a packet of indoor air. At typical air exchange rates, the reactions that meet this criterion include those of ozone with nitric oxide, nitrogen dioxide, and selected unsaturated hydrocarbons; thermal decomposition of peroxyacyl nitrates; numerous free radical reactions; and selected heterogeneous processes. Stable products include aldehydes, ketones, carboxylic acids and various organic nitrates. These reactions also generate free radicals, starting with the nitrate radical, Criegree biradicals, and peroxyacyl radicals, and leading to the hydroxyl, alkyl, alkylperoxy, hydroperoxy, and alkoxy radicals. Such radicals can react with other indoor species yielding additional aldehydes, ketones, carboxylic acids, dinitrates and peroxyacyl nitrates. Some of the potential products are known or suspected to be irritating (e.g. methacrolein, nonanoic acid, 1,2-propanediol dinitrate, peroxybenzoyl nitrate, and radical anions of the type [Cl… NO 2] −) However, some of these same products are difficult to detect using the sampling and analysis techniques currently applied to indoor air.

A Numerical Model for Chemical and Meteorological Processes in the Atmospheric Boundary Layer. Part I: A Model Description and a One-Dimensional Parameter Study

A numerical flow model is presented for the atmospheric boundary layer, including dispersion and chemical transformations of air pollutants. The model is a three-dimensional time-dependent one for the mesoscale based on the conservation equations for mass, heat, motion, water, and chemical species. The present version is hydrostatic, with a turbulence closure of second order. Only gas-phase chemistry is included and the chemical reaction scheme used is a modified condensed version of the carbon bond mechanism. A parameter study with a one-dimensional version of the model system is performed for the meteorological conditions of a typical summer day. Simulations with constant and diurnally varied deposition velocity are compared. The conclusion is that the difference in the surface concentration is minor. The difference in the maximum concentration between a simulation with a diurnally varied deposition velocity and a simulation with constant value equal to the maximum of the diurnally varied deposition velocity is about 1%. Other parameters affecting the results studied here are the temperature, the influence of pressure and water vapor on the rate constants, and the actinic flux. The resulting concentrations of ozone, peroxyacyl nitrate, and nitric acid are presented. The timing and the magnitude of peak concentrations of these species are sensitive to alterations in the parameters. When comparing the various simulations with a control run, the largest discrepancies are seen in the first simulation day, for the cases with higher albedo, and when deposition is included.

Atmospheric Chemistry of 1-Octene, 1-Decene, and Cyclohexene: Gas-Phase Carbonyl and Peroxyacyl Nitrate Products

Atmospheric reactions of 1-octene, 1-decene, and cyclohexene have been studied in laboratory experiments involving alkene−ozone mixtures in the dark and alkene−NO and aldehyde−NO mixtures in sunlight. Major carbonyl products of the alkene−ozone reaction (with sufficient cyclohexane added to scavenge OH) were heptanal from 1-octene, nonanal from 1-decene, formaldehyde from both, and pentanal from cyclohexene. For 1-octene and 1-decene, carbonyl formation yields were consistent with the simple mechanism: RCH = CH_2 + O_3 → 0.5(HCHO + RCHOO) + 0.5(RCHO + H_2COO). Other carbonyls were formed and accounted for ca. 0.10−0.20 of the subsequent reactions of the RCHOO biradical. Pentanal accounted for ca. 0.16 of the cyclohexene−O_3 reaction. Sunlight irradiation of alkene−NO and aldehyde−NO mixtures leads to carbonyls, alkyl nitrates, and peroxyacyl nitrates. Major carbonyl products of the OH−alkene reaction were heptanal from 1-octene, nonanal from 1-decene, and formaldehyde from both. Experimental data indicated that addition is an important pathway of the overall OH−1-alkene reaction, decomposition is important for the β-hydroxyalkoxy radicals that form following OH addition, and reaction with O2 is of some importance for the alkoxy radicals that form in several pathways in the alkene−NO (sunlight), aldehyde−NO (sunlight), and peroxyacyl nitrate−NO (dark) systems. The extent of isomerization of alkoxy and β-hydroxyalkoxy radicals could not be assessed. Several peroxyacyl nitrates [RC(O)OONO_2] were formed including those with R = n-C_4H_9 from cyclohexene, R = n-C_5H_(11) from hexanal, and R = n-C_6H_(13) from heptanal and from 1-octene. The thermal decomposition rates of n-C_5H_(11)C(O)OONO_2 and n-C_6H_(13)C(O)OONO_2, which were synthesized in the liquid phase and were characterized in a number of tests using electron capture gas chromatography, were (in units of 10^(-4) s^(-1)) 0.72−2.02 (T = 291−299 K) and 0.61−1.19 (T = 291−295 K), respectively, at p = 1 atm of air. Comparison with data for lower molecular weight homologues including PAN (R = CH_3) suggests that the thermal stability of peroxyacyl nitrates may increase with the size of the n-alkyl substituent. The atmospheric persistence of the atmospheric oxidation products of 1-octene, 1-decene, and cyclohexene is briefly discussed.

Henry's law and hydrolysis-rate constants for peroxyacyl nitrates (PANs) using a homogeneous gas-phase source

The solubilities and hydrolysis rates of PAN (peroxyacetyl nitrate) and its homologues PPN (peroxypropionyl nitrate), PnBN (peroxy-n-butyl nitrate), PiBN (peroxy-isobutyl nitrate) and MPAN (peroxymethacryloyl nitrate) in liquid water have been studied at 20 °C. Temperature dependencies were measured for PAN and PPN. The solubilities of peroxyacyl nitrates decrease smoothly with increasing carbon-chain length fromH (293 K)=4.1 M atm−1 (PAN) toH (293 K)=1.0 M atm−1 (PiBN). Hydrolysis-rate constants, which cover the range fromk h (293 K)=(2.4−7.4)×10−4 s−1, do not show a systematic chain-length dependency. Solubilities of PAN and PPN in solutions which mimic the composition and ionic strength of sea water are 15% and 20% lower than in pure water. The hydrolysis rate constants are not affected.