Halogen atoms and oxides are highly reactive and can profoundly affect atmospheric composition. Chlorine atoms can decrease the lifetimes of gaseous elemental mercury and hydrocarbons such as the greenhouse gas methane. Chlorine atoms also influence cycles that catalytically destroy or produce tropospheric ozone, a greenhouse gas potentially toxic to plant and animal life. Conversion of inorganic chloride into gaseous chlorine atom precursors within the troposphere is generally considered a coastal or marine air phenomenon. Here we report mid-continental observations of the chlorine atom precursor nitryl chloride at a distance of 1,400 km from the nearest coastline. We observe persistent and significant nitryl chloride production relative to the consumption of its nitrogen oxide precursors. Comparison of these findings to model predictions based on aerosol and precipitation composition data from long-term monitoring networks suggests nitryl chloride production in the contiguous USA alone is at a level similar to previous global estimates for coastal and marine regions. We also suggest that a significant fraction of tropospheric chlorine atoms may arise directly from anthropogenic pollutants.

Recent advances in threshold photoelectron photoion coincidence (TPEPICO) make possible the analysis of several parallel and sequential dissociations of energy selected ions. The use of velocity focusing optics for the simultaneous collection of threshold and energetic electrons not only improves the resolution, but also permits subtraction of coincidences associated with "hot" electrons, thereby yielding TPEPICO data with no contamination from "hot" electrons. The data analysis takes into account the thermal energy distribution of the sample and uses statistical theory rate constants and energy partitioning in dissociation reactions to model the time of flight distributions and the breakdown diagram. Examples include CH2BrCl and P(C2H5)3. Of particular interest is the ability to extract error limits for rate constants and dissociation energies.

Abstract. We report a new method for the simultaneous in situ detection of nitryl chloride (ClNO 2 ) and dinitrogen pentoxide (N 2 O 5 ) using chemical ionization mass spectrometry (CIMS). The technique relies on the formation and detection of iodide ion-molecule clusters, I(ClNO 2 ) − and I(N 2 O 5 ) − . The novel N 2 O 5 detection scheme is direct. It does not suffer from high and variable chemical interferences, which are associated with the typical method of nitrate anion detection. We address the role of water vapor, CDC electric field strength, and instrument zero determinations, which influence the overall sensitivity and detection limit of this method. For both species, the method demonstrates high sensitivity (>1 Hz/pptv), precision (∼10% for 100 pptv in 1 s), and accuracy (∼20%), the latter ultimately determined by the nitrogen dioxide (NO 2 ) cylinder calibration standard and characterization of inlet effects. For the typically low background signals (<10 Hz) and high selectivity, we estimate signal-tonoise (S/N) ratios of 2 for 1 pptv in 60 s averages, but uncertainty associated with the instrumental zero currently leads to an ultimate detection limit of ∼5 pptv for both species. We validate our approach for the simultaneous in situ measurement of ClNO 2 and N 2 O 5 while on board the R/V Knorr as part of the ICEALOT 2008 Field Campaign.

The Uintah Basin in northeastern Utah, a region of intense oil and gas extraction, experienced ozone (O₃) concentrations above levels harmful to human health for multiple days during the winters of 2009–2010 and 2010–2011. These wintertime O₃ pollution episodes occur during cold, stable periods when the ground is snow-covered, and have been linked to emissions from the oil and gas extraction process. The Uintah Basin Winter Ozone Study (UBWOS) was a field intensive in early 2012, whose goal was to address current uncertainties in the chemical and physical processes that drive wintertime O₃ production in regions of oil and gas development. Although elevated O₃ concentrations were not observed during the winter of 2011–2012, the comprehensive set of observations tests our understanding of O₃ photochemistry in this unusual emissions environment. A box model, constrained to the observations and using the near-explicit Master Chemical Mechanism (MCM) v3.2 chemistry scheme, has been used to investigate the sensitivities of O₃ production during UBWOS 2012. Simulations identify the O₃ production photochemistry to be highly radical limited (with a radical production rate significantly smaller than the NO_x emission rate). Production of OH from O₃ photolysis (through reaction of O(¹D) with water vapor) contributed only 170 pptv day⁻¹, 8% of the total primary radical source on average (primary radicals being those produced from non-radical precursors). Other radical sources, including the photolysis of formaldehyde (HCHO, 52%), nitrous acid (HONO, 26%), and nitryl chloride (ClNO₂, 13%) were larger. O₃ production was also found to be highly sensitive to aromatic volatile organic compound (VOC) concentrations, due to radical amplification reactions in the oxidation scheme of these species. Radical production was shown to be small in comparison to the emissions of nitrogen oxides (NO_x), such that NO_x acted as the primary radical sink. Consequently, the system was highly VOC sensitive, despite the much larger mixing ratio of total non-methane hydrocarbons (230 ppbv (2080 ppbC), 6 week average) relative to NO_x (5.6 ppbv average). However, the importance of radical sources which are themselves derived from NO_x emissions and chemistry, such as ClNO₂ and HONO, make the response of the system to changes in NO_x emissions uncertain. Model simulations attempting to reproduce conditions expected during snow-covered cold-pool conditions show a significant increase in O₃ production, although calculated concentrations do not achieve the highest seen during the 2010–2011 O₃ pollution events in the Uintah Basin. These box model simulations provide useful insight into the chemistry controlling winter O₃ production in regions of oil and gas extraction

The dissociative photoionization studies have been performed for a set of dihalomethane CH(2)XY (X,Y = Cl, Br, and I) molecules employing the threshold photoelectron photoion coincidence (TPEPICO) technique. Accurate dissociation onsets for the first and second dissociation limits have been recorded in the 10-13 eV energy range, and ionization potentials have been measured for these compounds. By using our experimental dissociation onsets and the known heat of formation of CH(2)Cl(2) molecule, it has been possible to derive the 0 and 298 K heats of formation of all six neutral dihalomethanes as well as their ionic fragments, CH(2)Cl(+), CH(2)Br(+), and CH(2)I(+), to a precision better than 3 kJ/mol. These new measurements serve to fill the lack of reliable experimental thermochemical information on these molecules, correct the old literature values by up to 19 kJ/mol, and reduce their uncertainties. From our thermochemical results it has also been possible to derive a consistent set of bond dissociation energies for the dihalomethanes.

The dissociative photoionization onsets for the production of CH 3 CO + + CH 3 • from acetone and CH 3 CO + + CH 3 CO • from butanedione have been measured by threshold photoelectron photoion coincidence (TPEPICO) in which time-of-flight (TOF) mass spectra are obtained as a function of the ion internal energy. The use of velocity focusing for threshold electrons and the subtraction of "hot" electron coincidences from the TPEPICO spectra allow the 0 K dissociation onset to be measured with a precision of 1 kJ/mol. The experimental onset for CH 3 • loss from CH 3 COCH 3 was measured to be 10.563 ( 0.010 eV and the onset for CH 3 CO • loss from CH 3 COCOCH 3 was found to be 10.090 ( 0.006 eV. A 298 K heat of formation of the CH 3 CO + of 659.4 ( 1.1 kJ/mol is obtained by combining the measured dissociation onset with the well-established heats of formation of acetone and the methyl radical. A 298 K heat of formation of the CH 3 CO • radical of -9.8 ( 1.8 kJ/mol is obtained by combining the measured dissociation onset with the well-known heat of formation of butanedione and the measured heat of formation of CH 3 CO + . The acetone and butanedione ionization energies were measured to be 9.708 ( 0.004 and 9.21 ( 0.05 eV, respectively.

Alkylamines (RCH(2)NH(2), R = H, CH(3), C(2)H(5), C(3)H(7), i-C(3)H(7)) have been investigated by dissociative photoionization by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). The 0 K dissociation limits (9.754 +/- 0.008, 9.721 +/- 0.008, 9.702 +/- 0.012, and 9.668 +/- 0.012 eV for R = CH(3), C(2)H(5), C(3)H(7), i-C(3)H(7), respectively) have been determined by preparing energy-selected ions and collecting the fractional abundances of parent and daughter ions. All alkylamine cations produce the methylenimmonium ion, CH(2)NH(2)+, and the corresponding alkyl free radical. Two isodesmic reaction networks have also been constructed. The first one consists of the alkylamine parent molecules, and the other of the alkyl radical photofragments. Reaction heats within the isodesmic networks have been calculated at the CBS-APNO and W1U levels of theory. The two networks are connected by the TPEPICO dissociation energies. The heats of formation of the amines and the alkyl free radicals are then obtained by a modified least-squares fit to minimize the discrepancy between the TPEPICO and the ab initio values. The analysis of the fit reveals that the previous experimental heats of formation are largely accurate, but certain revisions are suggested. Thus, Delta(f)Ho(298K)[CH(3)NH(2)(g)] = -21.8 +/- 1.5 kJ mol-1, Delta(f)Ho(298K)[C(2)H(5)NH(2)(g)] = -50.1 +/- 1.5 kJ mol(-1), Delta(f)Ho(298K)[C(3)H(7)NH(2)(g)] = -70.8 +/- 1.5 kJ mol(-1), Delta(f)Ho(298K)[C(3)H(7)*] = 101.3 +/- 1 kJ mol(-1), and Delta(f)Ho(298K)[i-C(3)H(7)*] = 88.5 +/- 1 kJ mol(-1). The TPEPICO and the ab initio results for butylamine do not agree within 1 kJ mol-1; therefore, no new heat of formation is proposed for butylamine. It is nevertheless indicated that the previous experimental heats of formation of methylamine, propylamine, butylamine, and isobutylamine may have been systematically underestimated. On the other hand, the error in the ethyl radical heat of formation is found to be overestimated and can be decreased to +/- 1 kJ mol(-1); thus, Delta(f)Ho(298K)[C(2)H(5).] = 120.7 +/- 1 kJ mol(-1). On the basis of the data analysis, the heat of formation of the methylenimmonium ion is confirmed to be Delta(f)Ho(298K)[CH(2)NH(2)+] = 750.3 +/- 1 kJ mol(-1).

Energy selected trimethyl phosphine ions were prepared by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. This ion dissociates via H, CH(3), and CH(4) loss, the latter two involving hydrogen transfer steps. The ion time-of-flight distribution and the breakdown diagram are analyzed in terms of the statistical RRKM theory, which includes tunneling. Ab initio and DFT calculations provide the vibrational frequencies required for the RRKM modeling. CH(3) loss could produce both the P(CH(3))(2)(+) by a simple bond dissociation step, and the more stable HP(CH(2))CH(3)(+) ion by a hydrogen transfer step. Quantum chemical calculations are extensively used to uncover the reaction scheme, and they strongly suggest that the latter product is exclusively formed via an isomerization step in the energy range of the experiment. The data analysis, which includes modeling with the trimethyl phosphine thermal energy distribution, provides accurate onset energies for both H (E(0K) = 1024.1 +/- 3.5 kJ/mol) and CH(3) (E(0K) = 1024.8 +/- 3.5 kJ/mol) loss reactions. From this analysis, we conclude that the Delta(f)H(298K) degrees [HP(CH(2))(CH(3))(+)] = 783 +/- 8 kJ/mol and Delta(f)H(298K) degrees [P(CH(2))(CH(3))(2)(+)] = 711 +/- 8 kJ/mol.