The atmospheric chemistry of organic nitrates

Thermal decomposition of peroxy acetyl nitrate CH3C(O)OONO2

The thermal decomposition of peroxy acetyl nitrate (PAN) is investigated by low pressure flash thermolysis of PAN highly diluted in noble gases and subsequent isolation of the products in noble gas matrices at low temperatures and by density functional computations. The IR spectroscopically observed formation of CH3C(O)OO and H2CCO (ketene) besides NO2, CO2, and HOO implies a unimolecular decay pathway for the thermal decomposition of PAN. The major decomposition reaction of PAN is bond fission of the O-N single bond yielding the peroxy radical. The O-O bond fission pathway is a minor route. In the latter case the primary reaction products undergo secondary reactions whose products are spectroscopically identified. No evidence for rearrangement processes as the formation of methyl nitrate is observed. A detailed mapping of the reaction pathways for primary and secondary reactions using quantum chemical calculations is in good agreement with the experiment and predicts homolytic O-N and O-O bond fissions within the PAN molecule as the lowest energetic primary processes. In addition, the first IR spectroscopic characterization of two rotameric forms for the radical CH3C(O)OO is given.

Atmospheric chemistry of nonanal

During the Southern Oxidants Study 1999 field campaign at Dickson, TN, we conducted measurements of the n-aldehydes propanal, pentanal, hexanal, heptanal, octanal, and nonanal. Propanal and nonanal tended to have the largest concentrations, with afternoon maxima of approximately 0.3 ppb. These aldehydes typically represented a significant fraction of the VOC reactivity defined as k(OH)[VOC]. However, this information is misleading with regard to the impact of these aldehydes on ozone formation, as their oxidation can represent a significant NOx sink. Motivated by the relatively large nonanal concentrations, we conducted a laboratory study of the products of the nonanal + OH reaction. The OH + nonanal reaction rate constant was determined via the relative rate technique and found to be 3.6 (+/- 0.7) x 10(-11) cm3 molecule(-1) s(-1). Under conditions of high [NO2]/[NO], we determined that 50 +/- 6% of OH-nonanal reaction occurs via abstraction of the aldehydic H-atom through measurement of the peroxynonanyl nitrate yield. We also studied the production of organic nitrates from OH reaction with nonanal in the presence of NO. As expected, a major product (20% at large [NO]/[NO2]) of this reaction was 1-nitrooxy octane. We calculate that the branching ratio for 1-nitrooxy octane formation from peroxyoctyl radicals is 0.40 +/- 0.05. On the basis of these measurements, we find that for more than 50% of the time OH reacts with nonanal (for midday summer conditions) an organic nitrate or PAN compound is formed, making this important atmospheric aldehyde an effective NOx sink.

Vibrational analysis of isopropyl nitrate and isobutyl nitrate

Raman and infrared spectra of isopropyl nitrate and isobutyl nitrate are reported. These spectra are used in combination with computational studies employing density functional theory at the B3-LYP/6-31G* level to assign the vibrational transitions to their corresponding normal coordinates. Similar to other alkyl nitrates, the frequency of the NO2 symmetric stretch remains relatively unchanged while the asymmetric stretch shifts to lower frequency with increasing alpha-carbon substitution. The mode assignments involving the photochemically relevant -ONO2 chromophore agree well with those from previous infrared work. Raman depolarization ratios are also presented, and provide evidence that the condensed phase, ground-state molecular structure of isobutyl nitrate is of Cs symmetry. In contrast, the minimum energy structure of isopropyl nitrate is predicted to contain a pronounced twist around the C-O bond relative to the Cs-symmetry structure that lies 2.6 kcal/mol higher in energy. Infrared intensities of isopropyl nitrate are consistent with the twisted geometry, demonstrating that this conformer is favored in solution.

Levels and pattern of volatile organic nitrates and halocarbons in the air at Neumayer Station (70 degrees S), Antarctic

Levels and patterns of C1-C4/C9 organic nitrates were measured for the first time in Antarctica. The sampling was done by adsorptive enrichment on Tenax TA followed by thermodesorption cold-trap high resolution capillary gas chromatography with electron capture detection. 2-70 1 air on-column have been analyzed this way. C1-C9 alkyl mononitrates, C2-C4 alkyl dinitrates, C2-C4 hydroxy alkyl nitrates, and halocarbons could be identified in air samples collected near the German Neumayer Research Station, Antarctica, in February 1999. Volatile biogenic and anthropogenic halocarbons were used to assess the origin of the air parcels analyzed. The average concentration measured for sigmaC2-C6 alkyl nitrates was in the range of 9.2 +/- 1.8 ppt(v), while the sum of the mixing ratios of six C2-C4 hydroxy alkyl nitrates was in the range of 1.1 +/- 0.2 ppt(v). Moreover, C2-C4 alkyl dinitrates were found at levels near the detection limit of 0.1-0.5 ppt(v). The concentrations of organic nitrates found in Antarctic air represent ultimate baseline levels due to chemical and physical loss processes during long-range transport in the air. The South Atlantic and the Antarctic Ocean as a general secondary source for organic nitrates in terms of an air/sea exchange equilibrium has to be evaluated yet, but it seems logical. Our results confirm the common assumption that there are no biogenic marine sources of C2-C9 organonitrates. We have found a level of > 80 ppt(v) for methyl nitrate. This level if it can be confirmed in a systematic survey requires a strong biogenic source of methyl nitrate in the Antarctic Ocean.

Direct evidence for a marine source of C1 and C2 alkyl nitrates

Alkyl nitrates are a significant component of the "odd nitrogen" reservoir and play an important role in regulating tropospheric ozone levels in remote marine regions. Measurements of methyl and ethyl nitrate in seawater and air samples along two Atlantic Ocean transects provide the first direct evidence for an oceanic source of these compounds. Equatorial surface waters were highly supersaturated (up to 800%) in both species, with the waters in the temperate regions generally being closer to equilibrium. A simple box model calculation suggests that the equatorial source could be a major component of the local atmospheric alkyl nitrate budget.

Determination of aldehydes and ketones in air samples using cryotrapping sampling

A cryotrapping sampling technique using glass traps cooled in liquid nitrogen for monitoring carbonyl compounds in air has been developed. Sampling was followed by derivatization by addition of acidified 2,4-dinitrophenyl hydrazine (DNPH) solution to the traps and an aliquot of the sample was analysed with a high performance liquid chromatograph system (HPLC), equipped with a diode array detector. The procedure was optimised concerning derivatization conditions and analytical parameters on formaldehyde, acetaldehyde, propanal, acetone, butanal and benzaldehyde. The technique was applied in monitoring their concentration in the urban atmosphere in Ljubljana.

Analysis of C2–C4 peroxyacyl nitrates and C1–C5 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 degrees 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.

Proton affinity of peroxyacetyl nitrate. A computational study of topical proton affinities

The structure and energetics of the peroxyacetyl nitrate conformers syn- and anti-PAN and several cations formed by PAN protonation were investigated by a combination of density functional theory and ab initio calculations. syn-PAN is the more stable conformer that is predicted to predominate in gas-phase equilibria. The acetyl carbonyl oxygen was found to be the most basic site in PAN, the oxygen atoms of the peroxide and NO(2) groups being less basic. The 298 K proton affinity of syn-PAN was calculated as 759-763 kJ mol(-1) by effective QCISD(T)/6-311 + G(3df,2p) and 771-773 kJ mol(-1) by B3-MP2/6-311 + G(3df,2p). The calculated values are 25-39 kJ mol(-1) lower than the previous estimate by Srinivasan et al. (Rapid Commun. Mass Spectrom. 1998; 12: 328) that was based on competitive dissociations of proton-bound dimers (the kinetic method). The calculated threshold dissociation energies predicted the formation of CH(3)CO(+) + syn - HOONO(2) and CH(3)COOOH + NO(2)(+) to be the most favorable fragmentations of protonated PAN that required 83 and 89 kJ mol(-1) at the respective thermochemical thresholds at 298 K. The previously observed dissociation to CH(3)COOH + NO(3)(+) was calculated by effective QCISD(T)/6-311 + G(3df,2p) to require 320 kJ mol(-1). The disagreement between the experimental data and calculated energetics is discussed.

Importance of handling organic atmospheric pollutants for assessing air quality

Volatile organic compounds (VOCs) are the main precursors of tropospheric ozone production, playing an important role in photochemical pollution of the atmosphere and, consequently, in the degradation of air quality. Air quality photochemical models require a specific VOC profile for each different main source, based on the most important group compounds. Chromatographic techniques have been used to identify and measure VOC in Portugal. These measured values were used to simulate photochemical pollution, and modelling results were compared with those from another simulation using VOC class distributions from the literature. Ozone concentration values estimated via both simulations indicate the importance of using VOC data from Portugal instead of those obtained in different conditions.

Alkyl nitrates as achiral and chiral solute probes in gas chromatography. Novel properties of a beta-cyclodextrin derivative and characterization of its enantioselective forces

Achiral and chiral interactions of alkyl nitrates (R-O-NO2) with heptakis-(3-O-acetyl-2,6-di-O-pentyl)-beta-cyclodextrin (LIPODEX-D) in the gas phase were investigated chromatographically. Two major outcomes can be summarized. First, LIPODEX-D shows very fast temperature-dependent variations of the selectivity up to changes in the order of elution for C1-C5 alkyl nitrates. These changes in selectivity reveal that LIPODEX-D possesses different shape selectivities for small alkyl nitrates at different temperatures (40-80 degrees C), i.e. with increasing isothermal separation temperature extended (chain-like) alkyl nitrates have increased retention relative to bulky alkyl nitrates. The observations are highly reproducible and might indicate conformational changes of the cyclodextrin, however, an ultimate proof would require further spectroscopic investigation. Secondly, the chiral separations of systematically varied sets of C4-C11 alkyl nitrates allowed the thermodynamic characterization of enantiodiscriminating interactions. Quantitative evidence is provided showing that the presence of an ethyl group at the asymmetric carbon atom of an n-alkyl nitrate gives a strong enthalpic contribution to the resulting enantioselectivity. The Gibbs free energy differences -deltaR,S(deltaG) decrease systematically three to six times if the ethyl group is either shortened or enlarged by only a -CH2-increment. The results are based on two separate thermodynamic approaches, i.e. the determination of thermodynamic quantities (-deltaR,S(deltaG), -deltaR,S(deltaH), -deltaR,S(deltaS), Tiso) and a theoretical concept of enthalpy-entropy compensation. The data from our laboratory experiments also indicate that van der Waals interactions are responsible for chiral discrimination.

C3-C14-alkyl nitrates in remote South Atlantic air

Alkyl nitrates with chain lengths up to fourteen carbons have been detected in the remote troposphere of the South Atlantic Ocean. The concentrations of numerous iso- and n-alkyl nitrates in the range of C3-C12 have been measured. Tenax-GC analysis was applied to determine short chain alkyl nitrates (< C6), while the novel detection of long chain iso- and n-alkyl nitrates was achieved with a high volume sampling technique using silica gel. C3-C12-alkyl nitrates show the lowest concentrations in the trade wind region (3.5 pptv). Higher concentrations in the west wind belt (11.1 pptv) reveal the influence of the South American continent as the source for the alkyl nitrates after long range transport. The concentrations of alkyl nitrates increase in proximity to South America. Measurements of long chain alkyl nitrates in rural air near the city of Ulm (Germany) are given for comparison. In addition, pattern analysis of long chain alkyl nitrates reveal this complex mixture of new compounds to be useful as trace indicators to distinguish continental and marine air masses. Despite the lower photochemical formation yields of primary n-alkyl nitrates compared to secondary n-alkyl nitrates, the primary n-alkyl nitrates are relatively increased in marine air. The reason for this finding cannot be explained so far, but the correlation of secondary/primary n-alkyl nitrates shows a significant differentiation of air samples with continental or marine character.