Near-UV photolysis cross sections of CH_3OOH and HOCH_2OOH determined via action spectroscopy

Knowledge of molecular photolysis cross sections is important for determining atmospheric lifetimes and fates of many species. A method and laser apparatus for measurement of these cross sections in the near-ultraviolet (UV) region is described. The technique is based on action spectroscopy, where the yield of a photodissociation product (in this case OH) is measured as a function of excitation energy. For compounds yielding OH, this method can be used to measure near-UV photodissociation cross section as low as 10−23 cm2 molecule−1. The method is applied to determine the photodissociation cross sections for methyl hydroperoxide (CH3OOH; MHP) and hydroxymethyl hydroperoxide (HOCH2OOH; HMHP) in the 305–365 nm wavelength range. The measured cross sections are in good agreement with previous measurements of absorption cross sections

Vibrational overtone initiated unimolecular dissociation of HOCH_2OOH and HOCD_2OOH: Evidence for mode selective behavior

The vibrational overtone induced unimolecular dissociation of HMHP (HOCH2OOH) and HMHP-d2 (HOCD2OOH) into OH and HOCH2O (HOCD2O) fragments is investigated in the region of the 4nuOH and 5nuOH bands. The unimolecular dissociation rates in the threshold region, corresponding to the 4nuOH band, exhibit measurable differences associated with excitation of the OH stretch of the alcohol versus the peroxide functional group, with the higher energy alcohol OH stretching state exhibiting a slower dissociation rate compared to the lower energy peroxide OH stretch in both HMHP and HMHP-d2. Predictions using the Rice–Ramsperger–Kassel–Marcus theory give rates that are in reasonably good agreement with the measured dissociation rate for the alcohol OH stretch but considerably differ from the measured rates for the peroxide OH stretch in both isotopomers. The present results are interpreted as suggesting that the extent of intramolecular vibrational energy redistribution (IVR) is different for the two OH stretching states associated with the two functional groups in HMHP, with IVR being substantially less complete for the peroxide OH stretch. Analysis of the OH fragment product state distributions in conjunction with phase-space theory simulation gives a D0 value of 38±0.7 kcal/mole for breaking the peroxide bond in HMHP

Near-IR photodissociation of peroxy acetyl nitrate

Measurements of the C-H overtone transition strengths combined with estimates of the photodissociation cross sections for these transitions suggest that near-IR photodissociation of peroxy acetyl nitrate (PAN) is less significant (Jnear−IR ~3×10^−8 s^−1 at noon) in the lower atmosphere than competing sinks resulting from unimolecular decomposition and ultraviolet photolysis. This is in contrast to the photochemical behavior of a related peroxy nitrate, pernitric acid (PNA), that undergoes rapid near-IR photolysis in the atmosphere with Jnear−IR ~10^−5 s^−1 at noon (Roehl et al., 2002). This difference is attributed to the larger binding energy and larger number of vibrational degrees of 10 freedom in PAN, which make 4[Greek nu]CH the lowest overtone excitation with a high photodissociation yield (as opposed to 2[Greek nu]OH in PNA)

Cis-cis and trans-perp HOONO: Action spectroscopy and isomerization kinetics

The weakly bound HOONO product of the OH + NO_2 + M reaction is studied using the vibrational predissociation that follows excitation of the first OH overtone (2nu1). We observe formation of both cis-cis and trans-perp conformers of HOONO. The trans-perp HOONO 2nu1 band is observed under thermal (223–238 K) conditions at 6971 cm^(–1). We assign the previously published (warmer temperature) HOONO spectrum to the 2nu1 band at 6365 cm^(–1) and 2nu1-containing combination bands of the cis-cis conformer of HOONO. The band shape of the trans-perp HOONO spectrum is in excellent agreement with the predicted rotational contour based on previous experimental and theoretical results, but the apparent origin of the cis-cis HOONO spectrum at 6365 cm^(–1) is featureless and significantly broader, suggesting more rapid intramolecular vibrational redistribution or predissociation in the latter isomer. The thermally less stable trans-perp HOONO isomerizes rapidly to cis-cis HOONO with an experimentally determined lifetime of 39 ms at 233 K at 13 hPa (in a buffer gas of predominantly Ar). The temperature dependence of the trans-perp HOONO lifetime in the range 223–238 K yields an isomerization barrier of 33±12 kJ/mol. New ab initio calculations of the structure and vibrational mode frequencies of the transition state perp-perp HOONO are performed using the coupled cluster singles and doubles with perturbative triples [CCSD(T)] model, using a correlation consistent polarized triple zeta basis set (cc-pVTZ). The energetics of cis-cis, trans-perp, and perp-perp HOONO are also calculated at this level [CCSD(T)/cc-pVTZ] and with a quadruple zeta basis set using the structure determined at the triple zeta basis set [CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ]. These calculations predict that the anti form of perp-perp HOONO has an energy of DeltaE0 = 42.4 kJ/mol above trans-perp HOONO, corresponding to an activation enthalpy of DeltaH298[double-dagger]0 = 41.1 kJ/mol. These results are in good agreement with statistical simulations based on a model developed by Golden, Barker, and Lohr. The simulated isomerization rates match the observed decay rates when modeled with a trans-perp to cis-cis HOONO isomerization barrier of 40.8 kJ/mol and a strong collision model. The quantum yield of cis-cis HOONO dissociation to OH and NO2 is also calculated as a function of photon excitation energy in the range 3500–7500 cm^(–1), assuming D0 = 83 kJ/mol. The quantum yield is predicted to vary from 0.15 to 1 over the observed spectrum at 298 K, leading to band intensities in the action spectrum that are highly temperature dependent; however, the observed relative band strengths in the cis-cis HOONO spectrum do not change substantially with temperature over the range 193–273 K. Semiempirical calculations of the oscillator strengths for 2nu1(cis-cis HOONO) and 2nu1(trans-perp HOONO) are performed using (1) a one-dimensional anharmonic model and (2) a Morse oscillator model for the OH stretch, and ab initio dipole moment functions calculated using Becke, Lee, Yang, and Parr density functional theory (B3LYP), Møller-Plesset pertubation theory truncated at the second and third order (MP2 and MP3), and quadratic configuration interaction theory using single and double excitations (QCISD). The QCISD level calculated ratio of 2nu1 oscillator strengths of trans-perp to cis-cis HOONO is 3.7:1. The observed intensities indicate that the concentration of trans-perp HOONO early in the OH + NO2 reaction is significantly greater than predicted by a Boltzmann distribution, consistent with statistical predictions of high initial yields of trans-perp HOONO from the OH + NO_2 + M reaction. In the atmosphere, trans-perp HOONO will isomerize nearly instantaneously to cis-cis HOONO. Loss of HOONO via photodissociation in the near-IR limits the lifetime of cis-cis HOONO during daylight to less than 45 h, other loss mechanisms will reduce the lifetime further

Quantifying the loss of processed natural gas within California's South Coast Air Basin using long-term measurements of ethane and methane

Abstract. Methane emissions inventories for Southern California's South Coast Air Basin (SoCAB) have underestimated emissions from atmospheric measurements. To provide insight into the sources of the discrepancy, we analyze records of atmospheric trace gas total column abundances in the SoCAB starting in the late 1980s to produce annual estimates of the ethane emissions from 1989 to 2015 and methane emissions from 2007 to 2015. The first decade of measurements shows a rapid decline in ethane emissions coincident with decreasing natural gas and crude oil production in the basin. Between 2010 and 2015, however, ethane emissions have grown gradually from about 13 ± 5 to about 23 ± 3 Gg yr−1, despite the steady production of natural gas and oil over that time period. The methane emissions record begins with 1 year of measurements in 2007 and continuous measurements from 2011 to 2016 and shows little trend over time, with an average emission rate of 413 ± 86 Gg yr−1. Since 2012, ethane to methane ratios in the natural gas withdrawn from a storage facility within the SoCAB have been increasing by 0.62 ± 0.05 % yr−1, consistent with the ratios measured in the delivered gas. Our atmospheric measurements also show an increase in these ratios but with a slope of 0.36 ± 0.08 % yr−1, or 58 ± 13 % of the slope calculated from the withdrawn gas. From this, we infer that more than half of the excess methane in the SoCAB between 2012 and 2015 is attributable to losses from the natural gas infrastructure

OH-Stretch Vibrational Spectroscopy of Hydroxymethyl Hydroperoxide

We report measurement and analysis of the photodissociation spectrum of hydroxymethyl hydroperoxide (HOCH_2OOH) and its partially deuterated analogue, HOCD_2OOH, in the OH-stretching region. Spectra are obtained by Fourier transform infrared spectroscopy in the 1ν_(OH) and 2ν_(OH) regions, and by laser induced fluorescence detection of the OH fragment produced from dissociation of HOCH_2OOH initiated by excitation of the 4ν_(OH) and 5ν_(OH) overtone regions (action spectroscopy). A one-dimensional local-mode model of each OH chromophore is used with ab initio calculated OH-stretching potential energy and dipole moment curves at the coupled-cluster level of theory. Major features in the observed absorption and photodissociation spectra are explained by our local-mode model. In the 4ν_(OH) region, explanation of the photodissocation spectrum requires a nonuniform quantum yield, which is estimated by assuming statistical energy distribution in the excited state. Based on the estimated dissociation threshold, overtone photodissociation is not expected to significantly influence the atmospheric lifetime of hydroxymethyl hydroperoxide