Products and mechanism of the reaction of NO3 with selected acyclic monoalkenes

The gas-phase reactions of NO₃ with 2-methyl-2-butene, isobutene,trans-butene, 1-butene and propene, were investigated in a flow-tube at room temperature. Experiments were performed in the pressure range 1–1000 mbar in synthetic air as well as at a total pressure of 800 mbar with varying concentrations of oxygen in nitrogen.The main products found were oxiranes, nitroxy-carbonyl compounds (ketonitrates) and ketones or aldehydes. The product distribution was a function of pressure. In each case, in synthetic air, the oxirane yield increased with decreasing total pressure up to a value of about 100% at pressures less than 1 mbar.It was concluded that oxirane is a product of the excited adduct radical formed in the electrophilic addition of NO₃ to the double bond. Experiments with very low partial pressures of oxygen showed that the quenched adduct radicals also produce the corresponding oxirane.Under tropospheric conditions (1000 mbar synthetic air) the following yields of the corresponding oxiranes were found: 2-methyl-2-butene 9%, isobutene 7%,trans-butene 12%, 1-butene 18%, propene 28%. In the case oftrans-butene the total oxirane yield consists of 72%trans- and 28%cis-isomer.Dedicated to Professor Wolfgang Rolle on the occasion of his 60th birthday.     

Evaluation of alkene degradation in the detailed tropospheric chemistry mechanism, MCM v3, using environmental chamber data

The representation of alkene degradation in version 3 of the Master Chemical Mechanism (MCM v3) has been evaluated, using environmental chamber data on the photo-oxidation of ethene, propene, 1-butene and 1-hexene in the presence of NO_x, from up to five chambers at the Statewide Air Pollution Research Center (SAPRC) at the University of California. As part of this evaluation, it was necessary to include a representation of the reactions of the alkenes with O(³P), which are significant under chamber conditions but generally insignificant under atmospheric conditions. The simulations for the ethene and propene systems, in particular, were found to be sensitive to the branching ratios assigned to molecular and free radical forming pathways of the O(³P) reactions, with the extent of radical formation required for proper fitting of the model to the chamber data being substantially lower than the reported consensus. With this constraint, the MCM v3 mechanisms for ethene and propene generally performed well. The sensitivity of the simulations to the parameters applied to a series of other radical sources and sink reactions (radical formation from the alkene ozonolysis reactions and product carbonyl photolysis; radical removal from the reaction of OH with NO₂ and β-hydroxynitrate formation) were also considered, and the implications of these results are discussed. Evaluation of the MCM v3 1-butene and 1-hexene degradation mechanisms, using a more limited dataset from only one chamber, was found to be inconclusive. The results of sensitivity studies demonstrate that it is impossible to reconcile the simulated and observed formation of ozone in these systems for ranges of parameter values which can currently be justified on the basis of the literature. As a result of this work, gaps and uncertainties in the kinetic, mechanistic and chamber database are identified and discussed, in relation to both tropospheric chemistry and chemistry important under chamber conditions which may compromise the evaluation procedure, and recommendations are made for future experimental studies. Throughout the study, the performance of the MCM v3 chemistry was also simultaneously compared with that of the corresponding chemistry in the SAPRC-99 mechanism, which was developed and optimized in conjunction with the chamber datasets.     

Characterization of C2−C4 NMHCs distributions at a high altitude tropical site in India

Air samples were collected covering a full diurnal cycle during each month of the year 2002 at a mountaintop of Mt. Abu (24.6^∘ N, 72.7^∘ E, 1680 amsl). These samples were analyzed for C₂−C₄ NMHCs using a gas chromatograph (GC) equipped with flame ionization detector (FID). The seasonally averaged diurnal distributions of these NMHCs do not show significant variations in the summer season. While sharp peaks in the diurnal variation of some species during evening hours are additional features apart from higher levels in all NMHCs in the winter season. The seasonal variations in relatively long lived species (e.g. ethane, propane and acetylene) are observed to be more pronounced compared to those in reactive species (e.g. ethene, propene and butanes). The seasonal changes in transport patterns seem to be more dominant factor at this site for the observed variations in NMHCs than changes in OH radical concentration. The annual mean mixing ratios of ethane, ethene, propane, propene, i-butane, acetylene, and n-butane are 1.22 ± 0.58, 0.34 ± 0.24, 0.46 ± 0.20, 0.17 ± 0.14, 0.21 ± 0.18, 0.41 ± 0.43, and 0.31 ± 0.35 ppbv, respectively. Only few pairs of NMHCs are observed to show good correlations, mainly due to transport of air masses with different degree of photochemical processing. A comparison of this measurement with data reported for other remote sites of the globe indicates lower levels of light NMHCs in the tropical sites. The annual mean mixing ratios of various C₂−C₄ NMHCs at Mt. Abu are lower by factors ranging between 3 to 9 compared to a nearest urban site of Ahmedabad. The annual mean propylene (propene) equivalent concentrations of about 1.12 and 8.62 ppbC were calculated for Mt. Abu and Ahmedabad, respectively.     

Intercomparison of NO,NO2 and HNO3 measurements with photochemical theory

The simultaneous measurements of NO, NO₂ and HNO_A mixing‐ratio profiles carried out on the Stratoprobe balloon flight of 22 July 1974 have been simulated with a time‐dependent model using the measured temperature and ozone profiles. The calculated ratios of NO/NO₂, HNO₃/NO₂ using currently accepted photochemistry are consistent with the measured ratios within the experimental errors of the measurements. The measured NO₂/NO ratio is almost a factor of two smaller than predicted, although the discrepancy is still within the experimental errors. A remarkable proportionality in the NO₂ and O₃ profiles has been noted and is unexplained. A time‐dependent simulation has been employed to convert the measurements into diurnally‐averaged profiles suitable for intercomparison with two‐dimensional stratospheric models and a comparison with constituent profiles from Prinn et al. (1975) is carried out as an example. The NO_V mixing ratio, formed from the sum of the NO, NO₂ and HNO₂ measurements is similar to the NO_V mixing ratio from several one‐ and two‐dimensional models used to predict the effects of SST's on the ozone layer. The odd nitrogen mixing ratio is roughly constant from 20 to 35 km at 11 ppbv.     

Seasonal variation of gaseous HNO3 and NH3 at a Tropical Savannah site

Gaseous nitric acid and ammonia were sampled with annular denuders at a forest savannah site from April to December 1987. The analysis of the extract was made spectrophotometrically and by a selective electrode for NO₃ ^– and NH₄ ⁺, respectively. Higher concentrations were observed during the vegetation burning period at the end of the dry season. In the studied savannah area, large soil emissions of NO occur during the rainy season, although very low concentrations of HNO₃ (0.035 ppb) and also of particulate NO₃ ^– (0.43 g m^-3) were observed; it is likely that NO_x are lost by fast vertical transport to the upper troposphere. During the nonburning period, the average concentration of NH₃ was 2.7 ppb, which is much lower than values given in the literature for the tropical America atmosphere. The concentrations of HNO₃ and NH₃ were always below the values needed to produce ammonium nitrate aerosols.     

Processing of oxidised nitrogen compounds by passage through winter-time orographic cloud

Four case studies are described, from a three-site field experiment in October/November 1991 using the Great Dun Fell flow-through reactor hill cap cloud in rural Northern England. Measurements of total odd-nitrogen nitrogen oxides (NO _y ) made on either side of the hill, before and after the air flowed through the cloud, showed that 10 to 50% of the NO _y , called NO _z , was neither NO nor NO₂. This NO _z failed to exhibit a diurnal variation and was often higher after passage through cloud than before. No evidence of conversion of NO _z to NO₃ ^- in cloud was found. A simple box model of gas-phase chemistry in air before it reached the cloud, including scavenging of NO₃ and N₂O₅ by aerosol of surface area proportional to the NO₂ mixing ratio, shows that NO₃ and N₂O₅ may build up in the boundary layer by night only if stable stratification insulates the air from emissions of NO. This may explain the lack of evidence for N₂O₅ forming NO₃ ^- in cloud under well-mixed conditions in 1991, in contrast with observations under stably stratified conditions during previous experiments when evidence of N₂O₅ was found. Inside the cloud, some variations in the calculated total atmospheric loading of HNO₂ and the cloud liquid water content were related to each other. Also, indications of conversion of NO _x to NO _z were found. To explain these observations, scavenging of NO _x and HNO₂ by cloud droplets and/or aqueous-phase oxidation of NO₂ ^- by nitrate radicals are considered. When cloud acidity was being produced by aqueous-phase oxidation of NO _x or SO₂, NO₃ ^- which had entered the cloud as aerosol particles was liberated as HNO₃ vapour. When no aqueous-phase production of acidity was occurring, the reverse, conversion of scavenged HNO₃ to particulate NO₃ ^-, was observed.     

Verification of the Contribution of Vehicular Traffic to the Total NMVOC Emissions in Germany and the Importance of the NO3 Chemistry in the City Air

In 1997 and 1998 several field campaigns for monitoring non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NO_x) were carried out in a road traffic tunnel and in the city center of Wuppertal, Germany. C₂–C₁₀ aliphatic and aromatic hydrocarbons were monitored using a compact GC instrument. DOAS White and long path systems were used to measure aromatic hydrocarbons and oxygenated aromatic compounds. A formaldehyde monitor was used to measure formaldehyde. Chemiluminescence NO analysers with NO₂ converter were used for measuring NO and NO₂. The high mixing ratios of the NMVOCs observed in the road traffic tunnel, especially 2.9 ppbv phenol, 1.5ppbv para-cresol and 4.4 ppbv benzaldehyde, in comparison with themeasured background concentration clearly indicate that these compounds were directly emitted from road traffic. Para-Cresol was for the first timeselectively detected as primary pollutant from traffic. From the measured data a NMVOC profile of the tunnel air and the city air, normalised to benzene (ppbC/ppbC), was derived. For most compounds the observed city air NMVOC profile is almost identical with that obtained in the traffic tunnel. Since benzene originates mainly from road traffic emission, the comparison of the normalised emission ratios indicate that the road traffic emissions in Wuppertal have still the largest impact on the city air composition, which is in contrast to the German emission inventory. In both NMVOC profiles, aromatic compounds have remarkably large contributions of more than 40 ppbC%. In addtion, total NMVOC/NO_x ratios from 0.6 up to 3.0ppbC/ppb in the traffic tunnel air and 3.4± 0.5 in the city air of Wuppertal were obtained. From the observed para-cresol/toluene and ortho-cresol/toluene ratios in the city air, evidence was found thatalso during daytime NO₃ radical reactions play an important role in urban air.     

A one-dimensional photochemical model of the troposphere with planetary boundary-layer parameterization

The results from a one-dimensional photochemical model of the troposphere representative of summertime conditions at Northern Hemisphere mid-latitudes are presented. A parameterization of mixing processes within the planetary boundary layer (PBL) has been incorporated into the model for both the daytime convective PBL and the formation of the nocturnal PBL. One result of the parameterized PBL is that the concentrations of some trace species in the free troposphere are 20–30% higher than when mixing processes are described by a vertical eddy diffusion coefficient which is held constant with respect to height and time.The calculations indicate that the lifetime of the oxides of nitrogen (NO _x =NO+NO₂) against photochemical conversion to nitric acid (HNO₃) during summertime conditions is on the order of 6 h. This lifetime is short enough to deplete most of the NO _x in the PBL, resulting in the finding that other reactive nitrogen species (HNO₃ and peroxyacetyl nitrate) are more abundant than NO _x throughout the free troposphere, even though NO _x is the most abundant reactive nitrogen species at the surface. The effects of the inclusion of anthropogenic nonmethane hydrocarbon (NMHC) chemistry are also discussed. The inclusion of NMHC chemistry has a pronounced effect on the photochemistry of tropospheric oxone and increases thein situ column production by more than 30%.     

大气中由NOx形成的HNO3分布的估计

本文根据大气中NO_2形成HNO_3的主要化学反应求解带有化学反应的扩散方程而得到HNO_3生成量的空间分布。HNO_3气体形成后边扩散边被降水清洗带至地面。我们又求解了HNO_3气体的扩散方程,考虑降水清洗从而得到HNO_3被降水带至地面的湿沉降量,得到水平面上HNO_3在24h内的分布,可用来大致估计HNO_3污染程度及范围。     

Uptake of the NO <Subscript>3</Subscript> Radical on Aqueous Surfaces

Using a single drop experiment, the uptake of NO₃ radicals on aqueous solutions of the dye Alizarin Red S and NaCl was measured at 293 K. Uptake coefficients in the range (1.7–3.1) ⋅ 10^{− 3} were measured on Alizarin Red S solutions. The uptake coefficients measured on NaCl solutions were in the range of (1.1–2.0) ⋅ 10⁻³ depending on the salt concentration. Both experiments lead to a consistent result for the mass accommodation coefficient of α_NO3 = (4.2_{− 1.7}^+2.2)⋅ 10⁻³. The product H(D_l k_Cl−^II)^0.5 for the NO₃ radical was determined to be (1.9 ± 0.2) M atm^{− 1} cm s^−0.5 M^−0.5 s^−0.5 by fitting the uptake data for the NaCl solutions to the so-called resistance model. The yield of the chemical NO₃ radical source was characterized using UV-VIS and FT-IR spectroscopy. The amount of gas-phase NO₃ radicals measured at elevated humidities was less than expected. Instead, a rise of the gas-phase HNO₃ concentration was found indicating a conversion of gas-phase NO₃ radicals to gas-phase HNO₃ on the moist reactor walls.     

Laboratory measurements of the <Superscript>12</Superscript>C/<Superscript>13</Superscript>C kinetic isotope effects in the gas-phase reactions of unsaturated hydrocarbons with Cl atoms at 298 ± 3 K

The carbon kinetic isotope effects (KIEs) in the reactions of several unsaturated hydrocarbons with chlorine atoms were measured at room temperature and ambient pressure using gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). All measured KIEs, defined as the ratio of the rate constants for the unlabeled and labeled hydrocarbon reaction k ₁₂/k ₁₃, are greater than unity or normal KIEs. The KIEs, reported in per mil according to ^Cl ɛ = (k ₁₂/k ₁₃−1) × 1000‰ with the number of experimental determinations in parenthesis, are as follows: ethene, 5.65 ± 0.34 (1); propene, 5.56 ± 0.18 (2); 1-butene, 5.93 ± 1.16 (1); 1-pentene, 4.86 ± 0.63 (1); cyclopentene, 3.75 ± 0.14 (1); toluene, 2.89 ± 0.31 (2); ethylbenzene, 2.17 ± 0.17 (2); o-xylene, 1.85 ± 0.54 (2). To our knowledge, these are the first reported KIE measurements for reactions of unsaturated NMHC with Cl atoms. Relative rate constants were determined concurrently to the KIE measurements. For the reactions of cyclopentene and ethylbenzene with Cl atoms, no rate constant has been reported in refereed literature. Our measured rate constants are: cyclopentene (7.32 ± 0.88) relative to propene (2.68 ± 0.32); ethylbenzene (1.15 ± 0.04) relative to o-xylene (1.35 ± 0.21), all × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The KIEs in reactions of aromatic hydrocarbons with Cl atoms are similar to previously reported KIEs in Cl-reactions of alkanes with the same numbers of carbon atoms. Unlike the KIEs for previously studied gas-phase hydrocarbon reactions, the KIEs for alkene–Cl reactions do not exhibit a simple inverse dependence on carbon number. This can be explained by competing contributions of normal and inverse isotope effects of individual steps in the reaction mechanism. Implications for the symmetries of the transition state structures in these reactions and the potential relevance of Cl-atom reactions on stable carbon isotope ratios of atmospheric NMHC are discussed.     

A Chamber Investigation of Nitric Acid-Soot Aerosol Chemistry at 298 K

Long-pathlength infrared absorption spectroscopy wasused to investigate nitric acid-soot aerosol chemistryat 298 K and 0.5% relative humidity. Experimentswere performed by introducing nitric acid vapor(P_{HNO 3} 3 Pa, P_total 40 kPa) intoateflon-coated chamber and initiating acquisition ofinfrared spectra at 3 minute time intervals. After 36minutes of data collection, soot powder was rapidlyexpanded into nitric acid contained in the chamber togenerate a soot-HNO₃ aerosol. Infrared spectracollected before, and after, soot introduction to thechamber were used to characterize chamber wallreaction processes and soot aerosol chemistry,respectively. Three soot types were investigated(Degussa FW2, Cabot Monarch 1000, and crystallinegraphite), each yielding similar chemistry. Upon sootintroduction to the chamber both HNO₃ uptake andNO₂ production occurred, with the molar ratio ofHNO₃ uptake to NO₂ production varying from1.2 to 2.9 for the three soot types studied. Unreacted HNO₃ was present at the conclusion ofeach of the aerosol experiments, indicating incompleteconversion of HNO₃ into NO₂. Thisobservation suggested that `active' sites at the sootsurface responsible for the reduction of HNO₃ arenot regenerated (i.e., formed) in the reactionprocess. In essence, a titration occurred betweenthese active sites and HNO₃. The NO₂concentrations produced, the soot mass concentrationsused, and the BET measured specific surface area ofthe powders allowed computation of the surface densityof active sites of 4.0 × 10^-18 m²/active site(describing all three powders studied). This is thefirst reported measurement of surface density ofactive sites for nitric acid chemistry on soot. Sinceatmospheric heterogeneous reactions that exhibitsurface deactivation may, in principle, affect tracegas concentration, we perform an assessment in thisregard.     

Laboratory spectroscopic studies of atmospherically important radicals using fourier transform spectroscopy

This paper describes laboratory experiments designed to obtain the infrared spectra of some atmospherically important radical species and related compounds. A Fourier transform spectrometer was used that was capable of yielding resolutions as great as 0.0024 cm^-1, and optical paths of up to 512 m were employed. The objective of the experiments was to obtain the spectra for subsequent application to remote sounding measurements in the atmosphere.Radicals were generated by a variety of chemical reactions involving atoms or other highly reactive precursors. Spectra of the ₃ band of NO₃, at ca. 1500 cm^-1, were obtained with up to 0.005 cm^-1 resolution using the reaction between NO₂ and O₃ to produce the radical. The most satisfactory source of ClO was found to be the reaction between Cl and O₃, and the (1-0) vibration-rotation band in the region 829–880 cm^-1 was recorded at a resolution of 0.02 cm^-1. We were unable to observe infrared absorption of HO₂ with any of the radical sources that we tested. High-resolution survey spectra were obtained of compounds used as reactants, or formed as side-products in the radical-generating processes. These compounds included N₂O₅, HNO₃, ClONO₂, FNO₂, Cl₂O, HO₂NO₂, and probably FO₂.The ability to monitor concentrations of the NO₃ radical in the visible region of the spectrum as well as the concentrations of reactants and other products in the infrared region allowed us to undertake a study of the time-dependent interactions occurring when NO₂ reacts with O₃. The results indicate the importance of heterogeneous processes, especially when traces of water are present, and lend credence to suggestions that heterogeneous mechanisms in the NO₃–N₂O₅–H₂O system might be a viable source of HNO₃ in the atmosphere.     

Flow reactor and triple quadrupole mass spectrometer investigations of negative ion reactions involving nitric acid: Implications for atmospheric HNO3 detection by chemical ionization mass spectrometry

Atmospheric nitric acid measurements by ACIMS (Active Chemical Ionization Mass Spectrometry) are based on ion-molecule reactions of CO₃ ^-(H₂O) _n and NO₃ ^-(H₂O) _n with HNO₃. We have studied these reactions in the laboratory using a flow tube apparatus with mass spectrometric detection of reactant and product ions. Both product ion distributions and rate coefficients were measured. All reactions were investigated in an N₂-buffer (1–3 hPa) at room temperature. The reaction rate coefficients of OH^-, O₂ ^-, O₃ ^-, CO₄ ^-, CO₃ ^-, CO₃ ^-H₂O, NO₃ ^-, and NO₃ ^-H₂O were measured relative to the known rate k=3.0×10^-9 cm³ s^-1 for the reaction of O^- with HNO₃. The main product ion of the reaction of CO₃ ^-H₂O with HNO₃ was found to be (CO₃HNO₃)^- supporting a previous suggestion made on the basis of balloon-borne ACIMS measurements. For the reaction of bare CO₃ ^- with HNO₃ three product ions were observed, namely NO₃ ^-, (NO₃OH)^-, and (CO₃HNO₃)^-. The reaction rate coefficients for CO₃ ^-H₂O (1.7×10^-9 cm³ s^-1) and NO₃ ^-H₂O (1.6×10^-9 cm³ s^-1) were found to be close to the collision rate. The measured k values for bare CO₃ ^- (1.3×10^-9 cm³ s^-1) and NO₃ ^- (0.7×10^-9 cm³ s^-1) are somewhat smaller. The collisional dissociations of CO₃ ^-(H₂O) _n , NO₃ ^-(H₂O) _n (n=1, 2), (CO₃HNO₃)^- and (NO₃HNO₃)^-, occasionally influencing ACIMS measurements, were also studied. Fragment ion distributions were measured using a triple quadrupole mass spectrometer. The results showed that previous stratospheric nitric acid measurements were unimpaired from collisional dissociation processes whereas these processes played a major role during previous tropospheric measurements leading to an underestimation of nitric acid concentrations. Previous ACIMS HNO₃ detection was also affected by the conversion of CO₃ ^-(H₂O) _n to NO₃ ^-(H₂O) _n due to ion source-produced neutral radicals. A novel ACIMS ion source was developed in order to avoid these problems and to improve the ACIMS method.     

Multiphase chemistry and acidity of clouds at Kleiner Feldberg

The chemistry of cloud multiphase systems was studied within the Kleiner Feldberg Cloud Experiment 1990. The clouds encountered during this experimental campaign could be divided into two categories according to the origin of air masses in which the clouds formed. From the chemical point of view, clouds passing the sampling site during the first period of the campaign (26 October-4 November) were characterized by lower pollutant loading and higher pH, as compared to clouds during the final period of the experimental campaign (10–13 November). The study of multiphase partitioning of the main chemical constituents of the cloud systems and of atmospheric acidity within the multiphase systems themselves (gas + interstitial aerosol + liquid droplets) are presented in this paper. A general lack of gaseous NH₃ was found in these cloud systems, which caused a lack of buffer capacity toward acid addition. Evidence supports the hypothesis that the higher acidity of the cloud systems during this final period of the campaign was due to input of HNO₃. Our measurements, however, could not determine whether the observed input was due to scavenging of gaseous HNO₃ from the air feeding into the cloud, or to heterogeneous HNO₃ formation via NO₂ oxidation by O₃ to NO₃ and N₂O₅. Sulfate in cloud droplets mainly originated from aerosol SO ₄ ^2– scavenging, since S(IV) to S(VI) liquid phase conversion was inhibited due to both lack of H₂O₂ and low pH of cloud droplets, which made O₃ and metal catalyzed S(IV) oxidation inefficient.     

Measurements of ozone and its precursor nitrogen dioxide and crop yield losses due to cumulative ozone exposures over 40 ppb (AOT40) in rural coastal southern India

Measurements of ground level ozone (O₃), nitrogen dioxide (NO₂) and meteorological parameters (air temperature, relative humidity and wind speed and direction) has been made for 3 years from March 2007 to February 2010 at Nagercoil (8.2°N, 77.5°E, 23 m above sea level), an equatorial rural coastal site of southern India. The monthly average of daytime maximum of O₃ concentrations ranged from 28 to 50 parts per billion (ppb) with an annual average of 19.8 ppb. Similarly, monthly average of NO₂ concentration ranged from 3.4 ppb to 7.7 ppb with an annual average of 5.3 ppb. The monthly variation of meteorological parameters shows the little changes being a coastal site. The estimated summer crops yield losses by 1.1–15.6 % from present O₃ concentration level associated with AOT40 index 3.1–5 ppm h.     

Carboxylic Acids: Seasonal Variation and Relation to Chemical and Meteorological Parameters

Formic and acetic acid measured as daily averages in 1993–1994show equal and highly correlated concentrations up to 3 ppb in the summer(May–August). In the winter (October–March) the formicacid/acetic acid ratio was 0.6 and the formic acid concentrations wereusually below 1 ppb. In winter the carboxylic acids correlate withO_x, NO_y, SO₂ and particulatesulphur. The main sources are suggested to be ozonolysis of anthropogenicalkenes and reactions between peroxyacetyl radicals and RO₂radicals. In spring–summer the carboxylic acids correlate withO₃, O_x, HNO₃, PAN,NO_y, SO₂, particulate sulphur and temperature.In addition to the sources of the winter a contribution from ozonolysis ofbiogenic alkenes is likely. Quite similar formic acid/acetic acid ratios forall wind directions suggest that the source(s) are atmospheric oxidationprocesses distributed over large areas. The highest concentrations occurringfor winds from east to south and the correlation with e.g., particulatesulphur indicate chemical production in polluted air masses during longrange transport.     

The partitioning of nitrogen oxides in the lower Arctic troposphere during spring 1988

Surface observations of several nitrogen oxides in the Canadian high Arctic during the period March-April 1988 are reported. These include data on NO₂, the inorganic nitrates HNO₃ and particulate nitrate, and the organic nitrates PAN and C₃–C₇ alkyl-nitrates. It is found that the organic nitrates make up 70–80% of the sum of the measured nitrogen oxides. Based on concurrently measured sulphur oxides, the period of observation was divided into two halves with the first half representing less polluted, more aged air than the second. The preponderance of the organic nitrates was less in the first period than the second. In contrast, there was little difference in the inorganic nitrates and NO₂ concentrations. The dominant inorganic nitrate shifted from particulate nitrate in the first period towards gaseous HNO₃ in the second. No correlation between the nitrates (inorganic or organic) and O₃ was observed; although some indication of a positive correlation between NO₂ and O₃ has been reported earlier (Bottenheimet al., 1990). Possible explanations for these observations are proposed. A survey of other potential nitrogen oxides that may be present in the Arctic air but not measured in these experiments suggests that the nitrogen oxides not measured here constitute a minor fraction of the total reactive nitrogen (NO _y ).Paper submitted to the 7th International Symposium of the Commission for Atmospheric Chemistry and Global Pollution on the Chemistry of the Global Atmosphere held in Chamrousse, France, from 5 to 11 September 1990.     

Formation and occurrence of organic hydroperoxides in the troposphere: Laboratory and field observations

The formation and occurrence of hydroperoxides in the troposphere have been studied by laboratory experiments and by preliminary field measurements. Nine alkenes were reacted individually with ozone in a reaction chamber in the presence of excess water, and the amounts of hydrogen peroxide and of nine organic hydroperoxides produced in the gas and aerosol phases and deposited on the chamber walls determined by HPLC. The reactions of ethene, propene, 1-butene and isoprene gave hydroxymethyl hydroperoxide as the major product with no hydrogen peroxide observed. In the case of - and -pinene, 2-carene and limonene the major product was hydrogen peroxide. Cis-2-butene produced hydrogen peroxide and methyl hydroperoxide. Preliminary measurements of hydrogen peroxide and five organic hydroperoxides in ambient air were made at Niwot Ridge, Colorado from 24 July–4 August 1989. The gas-phase species were preconcentrated by cryotrapping with subsequent HPLC separation. The gas-phase concentrations of H₂O₂ ranged from 0.5–2 ppbv with the lowest concentrations being measured at night and the highest under conditions of strong photochemical activity. The maximum concentrations of hydroxymethyl hydroperoxide approximated those of H₂O₂. Methyl hydroperoxide concentrations ranged from <50 to 800 pptv and three other organic hydroperoxides were detected at concentrations below 200 pptv. High volume aerosol samples yielded H₂O₂ and methyl hydroperoxide concentrations <10 ng m^-3 while H₂O₂ and six organic species were detected in rainwater at concentrations in the range <0.01–50 M.     

FTIR studies of reactions between the nitrate radical and haloethenes

Products and mechanisms for the gas-phase reactions of NO₃ radicals with CH₂=CHCl, CH₂=CCl₂, CHCl=CCl₂,cis-CHCl=CHCl andtrans-CHCl=CHCl in air have been studied. The experiments were carried out at 295±2 K and 740±5 Torr in a 480-L Teflon-coated reaction chamber and at 295±2 K and 760±5 Torr in a 250-L stainless steel reactor. NO₃ was generated by the thermal dissociation of N₂O₅. Experiments with¹⁵NO₃ and CD₂CDCl have also been performed. The initially formed nitrate peroxynitrates decay into carbonyl compounds, nitrates, HCl and ClNO₂. In adidtion, there are indications of nitrooxy acid chlorides being produced. The reactions with CH₂=CCl₂ and CHCl=CCl₂ are more complex due to release of chlorine atoms which eventually lead to formation of chloroacid chlorides.A general reaction mechanism is proposed and the observed concentration-time profiles of reactants and products are simulated for each compound. The rate constants for the initial step of NO₃ addition to the chloroethenes are determined as: (2.6±0.5, 9.4±0.9, 2.0±0.4 and 1.4±0.4) × 10^–16 cm³ molecule^–1 s^–1 for CH₂=CHCl, CH₂=CCl₂, CHCl=CCl₂ andcis-CHCl=CHCl, respectively.