Applications of high resolution mid-infrared spectroscopy for atmospheric and environmental measurements

For the past 20 years, high resolution infrared spectroscopy has served as a valuable tool to measure gas-phase concentrations of ambient gas samples. We review recent advances in atmospheric sampling using direct absorption high resolution mid-infrared spectroscopy from the perspective of light sources, detectors, and optical designs. Developments in diode, quantum cascade and interband cascade laser technology have led to thermoelectrically-cooled single-mode laser sources capable of operation between 800 \wn and 3100 \wn, with \textless10 MHz resolution and \textgreater10 mW power. Advances in detector and preamplifier technology have yielded thermoelectriocally-cooled sensors capable of room-temperature operation with extremely high detectivities. Finally, novel spectrometer optical designs have led to robust multipass absorption cells capable of \textgreater400 m effective pathlength in a compact package. In combination with accurate spectroscopic databases, these developments have afforded dramatic improvements in measurement sensitivity, accuracy, precision, and selectivity. We will present several examples of the applications of high resolution mid-IR spectrometers in real-world field measurements at sampling towers and aboard mobile platforms such as vehicles and airplanes

Using tunable infrared laser direct absorption spectroscopy for ambient hydrogen chloride detection : HCl-TILDAS

The largest inorganic, gas-phase reservoir of chlorine atoms in the atmosphere is hydrogen chloride (HCl), but challenges in quantitative sampling of this compound cause difficulties for obtaining high-quality, high-frequency measurements. In this work, tunable infrared laser direct absorption spectroscopy (TILDAS) was demonstrated to be a superior optical method for sensitive, in situ detection of HCl at the 2925.89645 cm-1 absorption line using a 3 μm inter-band cascade laser. The instrument has an effective path length of 204 m, 1 Hz precision of 7-8 pptv, and 3σ limit of detection ranging from 21 to 24 pptv. For longer averaging times, the highest precision obtained was 0.5 pptv with a 3σ limit of detection of 1.6 pptv at 2.4 min. HCl-TILDAS was also shown to have high accuracy when compared with a certified gas cylinder, yielding a linear slope within the expected 5 % tolerance of the reported cylinder concentration (slope = 0.964 ± 0.008). The use of heated inlet lines and active chemical passivation greatly improve the instrument response times to changes in HCl mixing ratios, with minimum 90 % response times ranging from 1.2 to 4.4 s depending on inlet flow rate. However, these response times lengthened at relative humidities >50 %, conditions under which HCl concentration standards were found to elicit a significantly lower response (-5.8 %). The addition of high concentrations of gas-phase nitric acid (>3.0 ppbv) were found to increase HCl signal (<10 %), likely due to acid displacement with HCl or particulate chloride adsorbed to inlet surfaces. The equilibrium model ISORROPIA suggested a potential of particulate chloride partitioning into HCl gas within the heated inlet system if allowed to thermally equilibrate, but field results did not demonstrate a clear relationship between particulate chloride and HCl signal obtained with a denuder installed on the inlet

Chemical Evolution of Atmospheric Organic Carbon over Multiple Generations of Oxidation

The evolution of atmospheric organic carbon (OC) as it undergoes oxidation has a controlling influence on concentrations of key atmospheric species, including particulate matter, ozone, and oxidants. However, the full characterization of OC over hours to days of atmospheric processing has been stymied by its extreme chemical complexity. Here we study the multigenerational oxidation of -pinene in the laboratory, characterizing products with several state-of-the-art analytical techniques. While quantification of some early-generation products remains elusive, full carbon closure is achieved (within uncertainty) by the end of the experiments. This enables new insights into the effects of oxidation on OC properties (volatility, oxidation state, and reactivity), and the atmospheric lifecycle of OC. Following an initial period characterized by functionalization reactions and particle growth, fragmentation reactions dominate, forming smaller species. After approximately one day of atmospheric aging, most carbon is sequestered in two long-lived reservoirs, volatile oxidized gases and low-volatility particulate matter

ANALYSIS OF THE HIGH RESOLUTION ELECTRONIC SPECTRUM OF 1, 2, 3, 6, 7, 8 - HEXAHYDROPYRENE

Author Institution: Department of Chemistry, University of PittsburghThe rotationally resolved electronic spectra of the two conformers of 1, 2, 3, 6, 7, 8 - hexahydropyrene (HHP) in the gas phase were obtained and fit using a rigid rotor Hamiltonian. Two electronic origins separated by $14 cm^{-1}$ are observed in the vibronically resolved spectrum, and have been assigned to the boat and chair conformers based on the analysis of their rotationally resolved spectra. The electronic transition moments of both conformers are compared to those of other substituted naphthalenes. Ground and excited state dynamics and agreement with theory are discussed

BASE PAIR ANALOGS IN THE GAS PHASEaPHASE^{a}

$^{a}$ Work supported by NSF. $^{a}$ Ming Chao, Ellory Schempp, and Robert D. Rosenstein, Acta Cryst. B31, 2922 (1975) $^{b}$ H. W. Yang and B. M. Craven, Acta Cryst. 54, 912 (1998)Author Institution: Department of Chemistry, University of PittsburghA rotationally resolved electronic spectrum of the 2-aminopyridine/2-pyridone (2AP/2PY) dimer in the gas phase has been obtained and interpreted. Using structures based on x-ray $crystallography^{ab}$, a dimer structure is proposed, and used to determine that the hydrogen bond lengths are similar to those found elsewhere, with little change upon electronic excitation. Further analyses of the transition moment reveals that, while 2PY is the primary chromophore, 2AP is also excited, indicating a single-photon, double-chromophore excitation. Possible theoretical interpretations of the excitation are discussed

INFRARED SPECTRA OF X−^-.^.H2_2O (X=F,Cl,Br) IN THE 600-800 cm−^-1^1 REGION: UNDERSTANDING THE THREE-DIMENSIONAL CONFINEMENT OF THE SHARED PROTON

Author Institution: Sterling Chemistry Laboratory, Yale University, PO Box 208107, New Haven, CT 06520Recent advances in nonlinear mixing technology have allowed us to obtain infrared spectra of the X$^-$$^.$H$_2$O (X=F,Cl,Br) complexes in the 600-1800 cm$^-$$^1$ (16.7-5.6 ${\mu}$m) region. While transitions in this regime are typically associated with the three-dimensional confinement of the shared proton, unexpected spectral activity is observed. The out-of-plane vibrational overtone of the Cl$^-$$^.$H$_2$O complex exhibits anomalous intensity, which is explained by a curvilinear motion of the proton. The F$^-$$^.$H$_2$O spectrum shows evidence for Fermi interactions between several energy levels. Analysis of the spectra in the context of the potential surface probed by the shared proton is discussed