Long-path Absorption Spectroscopy Research Articles

Budget of nitrous acid and its impacts on atmospheric oxidative capacity at an urban site in the central Yangtze River Delta region of China

Abstract In this study, we used a wet chemistry based long-path absorption spectroscopy method to measure HONO in Changzhou, in the central Yangtze River Delta region (YRD) of China, from April 3–24, 2017. During the observation period, the average HONO mixing ratio was 1.55 ± 1.21 ppbv. In addition, the average OH formation rates of the photolysis of HONO, O3, HCHO and H2O2 along with ozonolysis of alkenes were 7.84 × 106, 2.02 × 107, 7.41 × 105, 3.79 × 105 and 1.51 × 106 molecules cm−3 s−1, respectively. At nighttime, the average conversion rate from NO2 to HONO was determined to be ~0.018 h−1. In this work, the primary emission rate of HONO was determined by the ratios of HONO to nitrogen oxides (NOx = NO + NO2) within freshly emitted plumes (NO/NOx > 0.85) and a value of ~0.69% was obtained. The sources of HONO were further investigated through a box model utilizing the Master Chemical Mechanism. The simulation results show that primary emissions contributed only ~12.3% of the total HONO budget during daytime but a substantial portion (31.4%) at night. Comparing to heterogeneous HONO sources, the gas-phase NO + OH reaction was the less important HONO source, with a contribution of 14.2% at night and 28.7% during the daytime. Heterogeneous reactions of NO2 on various surfaces (mostly the ground surface) were responsible for most of the observed HONO, i.e., 34.7% during the day and 54.4% at night. Overall, nighttime HONO can be reasonably explained by aforementioned mechanisms. However, daytime HONO cannot be fully accounted for without the consideration of nitrate photolysis, which contributed ~24.1% of the daytime HONO according to the model simulation. Our findings highlight that photolysis of particulate nitrate could be an important source of daytime HONO, providing a channel to cycle NO and OH radical back into the photochemical system and further enhancing the atmospheric oxidative capacity within the air shed of a typical industrial zone of China.

Thermal effects of an ICL-based mid-infrared CH4 sensor within a wide atmospheric temperature range

The thermal effects of an interband cascade laser (ICL) based mid-infrared methane (CH4) sensor that uses long-path absorption spectroscopy were studied. The sensor performance in the laboratory at a constant temperature of ∼25 °C was measured for 5 h and its Allan deviation was ∼2 ppbv with a 1 s averaging time. A LabVIEW-based simulation program was developed to study thermal effects on infrared absorption and a temperature compensation technique was developed to minimize these effects. An environmental test chamber was employed to investigate the thermal effects that occur in the sensor system with variation of the test chamber temperature between 10 and 30 °C. The thermal response of the sensor in a laboratory setting was observed using a 2.1 ppm CH4 standard gas sample. Indoor/outdoor CH4 measurements were conducted to evaluate the sensor performance within a wide atmospheric temperature range.

Rotational Spectroscopy of Isotopologues of Silicon Monoxide, SiO, and Spectroscopic Parameters from a Combined Fit of Rotational and Rovibrational Data

Pure rotational transitions of silicon monoxide, involving the main ((28)Si(16)O) as well as several rare isotopic species, were observed in their ground vibrational states by employing long-path absorption spectroscopy between 86 and 825 GHz (1 ≤ J" ≤ 18). Fourier transform microwave spectroscopy was used to study the J" = 0 transition frequencies in the ground and several vibrationally excited states. The vibrational excitation of the newly studied isotopologues extend to between υ = 9 and 29 for (28)Si(17)O and (30)Si(16)O, respectively. Data were extended for some previously investigated species up to υ = 51 for the main isotopologue. The high spectral resolution allowed us to resolve the hyperfine structure in (28)Si(17)O caused by the nuclear electric quadrupole and magnetic dipole moments of (17)O for the first time, and to resolve the much smaller nuclear spin-rotation splitting for isotopic species containing (29)Si. These data were combined with previous rotational and rovibrational (infrared) data to determine an improved set of spectroscopic parameters of SiO in one global fit which takes the breakdown of the Born-Oppenheimer approximation into account. Highly accurate rotational transition frequencies for this important astronomical molecule can now be predicted well into the terahertz region with this parameter set. In addition, a more complete comparison among physical properties of group 14/16 diatomics is possible.

Rotational spectroscopy, dipole moment and 14N nuclear hyperfine structure of iso-propyl cyanide

Rotational transitions of iso-propyl cyanide, (CH 3) 2CHCN, also known as iso-butyronitrile, were recorded using long-path absorption spectroscopy in selected regions between 37 and 600 GHz. Further measurements were carried out between 6 and 20 GHz employing Fourier transform microwave (FTMW) spectroscopy on a pulsed molecular supersonic jet. The observed transitions reach J and K a quantum numbers of 103 and 59, respectively, and yield accurate rotational constants as well as distortion parameters up to eighth order. The 14N nuclear hyperfine splitting was resolved in particular by FTMW spectroscopy yielding spin–rotation parameters as well as very accurate quadrupole coupling terms. In addition, Stark effect measurements were carried out in the microwave region to obtain a largely revised c-dipole moment component and to improve the a-component. The hyperfine coupling and dipole moment values are compared with values for related molecules both from experiment and from quantum chemical calculations.

LABORATORY DETECTION OF THIOCYANIC ACID HSCN

The rotational spectrum of thiocyanic acid HSCN, a highly polar isomer of the well-known astronomical molecule isothiocyanic acid HNCS, has been measured in two radio bands: in the centimeter-wave band by Fourier transform microwave spectroscopy in a molecular beam, and in the millimeter-wave band by long-path absorption spectroscopy in a low-pressure glow discharge. Twelve spectroscopic constants were derived from more than 60 a-type rotational transitions between 11 and 346 GHz with J up to 30 and Ka ≤ 6, including seven centimeter-wave transitions with resolved hyperfine structure. With these constants the rotational spectrum in the Ka = 0 and Ka = 1 ladders—those most likely to be observed in space—can now be calculated up to 400 GHz with formal uncertainties of less than 0.2 km s–1 in equivalent radial velocity. Thiocyanic acid was recently identified in Sgr B2 by Halfen et al. following the laboratory measurements, and there is possible evidence for it in cold dark clouds, with the implication that HSCN may be detectable in many galactic sources.

Broadband Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) – applicability and corrections

Abstract. Atmospheric trace gas measurements by cavity assisted long-path absorption spectroscopy are an emerging technology. An interesting approach is the combination of CEAS with broadband light sources, the broadband CEAS (BB-CEAS). BB-CEAS lends itself to the application of the DOAS technique to analyse the derived absorption spectra. While the DOAS approach has enormous advantages in terms of sensitivity and specificity of the measurement, an important implication is the reduction of the light path by the trace gas absorption, since cavity losses due to absorption by gases reduce the quality (Q) of the cavity. In fact, at wavelength, where the quality of the BB-CEAS cavity is dominated by the trace gas absorption (especially at very high mirror reflectivity), the average light path will vary nearly inversely with the trace gas concentration and the strength of the band will become only weakly dependent on the trace gas concentration c in the cavity, (the differential optical density being proportional to the logarithm of the trace gas concentration). Only in the limiting case where the mirror reflectivity determines Q at all wavelength, the strength of the band as seen by the CE-DOAS instrument becomes directly proportional to the concentration c. We investigate these relationships in detail and present methods to correct for the cases between the two above extremes, which are of course the important ones in practice.

Rotational spectra and hyperfine structure of isotopic species of deuterated cyanoacetylene, DC 3N

Cyanoacetylene enriched in deuterium was used to record pure rotational transitions of DC 3N and its rare 13C and 15N isotopic species by employing Fourier transform microwave (FTMW) spectroscopy on a supersonic-jet expansion at centimeter wavelengths (8.2–25.4 GHz) and by using long-path absorption spectroscopy at millimeter and submillimeter wavelengths (82–900 GHz). In addition, submillimeter wave measurements (304–897 GHz) have been performed for DC 3N in its v 7 = 1 lowest excited vibrational state. Hyperfine structure caused by the 14N and D nuclei has been resolved in the FTMW spectra. Quantum-chemical calculations have been performed on the hyperfine structure parameters at the coupled-cluster level (CCSD(T)) with up to core-polarized quintuple zeta basis-sets and were found to be in excellent agreement with experimental data as far as these were available. Vibrational corrections on the quadrupole coupling parameters have been evaluated at the MP2 level employing a quadruple zeta basis-set. The present and previous data were fit simultaneously to obtain accurate predictions for the identification of these species in space.

Rotational spectroscopy of the isotopic species of silicon monosulfide, SiS

Pure rotational transitions of silicon monosulfide ((28)Si(32)S) and its rare isotopic species have been observed in their ground as well as vibrationally excited states by employing Fourier transform microwave (FTMW) spectroscopy of a supersonic molecular beam at centimetre wavelengths (13-37 GHz) and by using long-path absorption spectroscopy at millimetre and submillimetre wavelengths (127-925 GHz). The latter measurements include 91 transition frequencies for (28)Si(32)S, (28)Si(33)S, (28)Si(34)S, (29)Si(32)S and (30)Si(32)S in upsilon = 0, as well as 5 lines for (28)Si(32)S in upsilon = 1, with rotational quantum numbers J''< or = 52. The centimetre-wave measurements include more than 300 newly recorded lines. Together with previous data they result in almost 600 transitions (J'' = 0 and 1) from all twelve possible isotopic species, including (29)Si(36)S and (30)Si(36)S, which have fractional abundances of about 7 x 10(-6) and 4.5 x 10(-6), respectively. Rotational transitions were observed from upsilon = 0 for the least abundant isotopic species to as high as upsilon = 51 for the main species. Owing to the high spectral resolution of the FTMW spectrometer, hyperfine structure from the nuclear electric quadrupole moment of (33)S was resolved for species containing this isotope, as was much smaller nuclear spin-rotation splitting for isotopic species involving (29)Si. By combining the measurements here with previously published microwave and infrared data in one global fit, an improved set of spectroscopic parameters for SiS has been derived which include several terms describing the breakdown of the Born-Oppenheimer approximation. With this parameter set, highly accurate rotational frequencies for this important astronomical molecule can now be predicted well into the terahertz region.

Long-path monitoring of NO_2 with a 635 nm diode laser using frequency-modulation spectroscopy

In situ monitoring of traffic-generated nitrogen dioxide (NO2) emissions using long-path absorption spectroscopy is reported. High-sensitivity detection of NO2 is achieved by employing two-tone frequency-modulation spectroscopy at a visible absorption band using a tunable high-power diode laser operated around 635 nm. A real-time absorption spectrometer is accomplished by repetitively applying a rectangular current pulse to the diode-laser operating current, allowing detection of isolated NO2 absorption lines. A detection limit of 10 microg/m3 for NO2 at atmospheric pressure using a 160 m absorption path is demonstrated. Continuous monitoring of NO2 over a road intersection at peak traffic is performed.

In-situ Measurements of Tropospheric Hydroxyl Radicals by Folded Long-Path Laser Absorption During the Field Campaign POPCORN

Absolutely calibrated in-situ measurements of tropospheric hydroxyl radicals, formaldehyde, sulfur dioxide, and naphthalene (C10H8) were performed by long-path laser absorption spectroscopy during the field campaign POPCORN. The absorption light path was folded into an open optical multiple reflection cell with a mirror separation of 38.5 m. Using a light path length of 1848 m and an integration time of 200 s, the average 1σ-detection limits of OH, HCHO, SO2 and C10H8 during POPCORN were 8.7 · 105 cm−3, 8.3 · 109 cm−3, 2.4 · 109 cm−3, 1.5 · 108 cm−3, respectively. In total, 392 identifications of OH in air spectra were made in a rural environment between August 5 and August 23, 1994. We present and discuss OH absorption spectra and diurnal OH concentration profiles of three days which are representative for measurements under different pollution conditions during POPCORN. The observed maximum and median OH radical concentrations are 1.3 · 107 OH/cm3 and 4.0 · 106 OH/cm3, respectively. The measured diurnal variation of the OH concentration shows a good correlation with the primary formation reaction of OH radicals which is the photolysis of ambient ozone. Deviations from this correlation in the morning and evening hours, when the OH concentration is higher than expected from the ozone photolysis, demonstrate the importance of other photochemical HOx production pathways during POPCORN.

Tropospheric OH concentration measurements by laser long-path absorption spectroscopy

Tropospheric OH radicals have been detected by broadband laser long-path absorption spectroscopy. This technique has been improved by application of a low-noise multichannel spectrometric detector to record OH absorption spectra. The simulataneous observation of at least three rotational lines ( Q 1 (2), Q 21 (2), R 2 (2) or Q 1 (3), Q 21 (3), P 1 (1)) of the OH ( A 2 Σ + ← X 2 Π) (0,0) vibrational band at 308 nm and the spectra evaluation by fitting OH reference spectra for proper atmospheric conditions allows a specific OH concentration determination. Furthermore, the broad spectral range of the detector system facilitates the identification and elimination of interfering absorption structures, which are produced by atmospheric trace gases like SO 2, CH 2O, CS 2 or naphthalene in polluted air. Diurnal OH concentration profiles have been observed during various field campaigns showing different temporal shapes. These are in qualitative agreement with the primary net production process of OH from ozone u.v.-photolysis and the primary net destruction of hydroxyl radicals by the reaction with NO 2 at high NO x levels.

Visible and near‐ultraviolet spectroscopy at McMurdo Station, Antarctica: 5. Observations of the diurnal variations of BrO and OClO

Observations of the diurnal variations of OClO and BrO during austral spring, 1987, using long‐path visible and near‐ultraviolet absorption spectroscopy are presented and compared to simplified model calculations. It is shown that care must be taken to compare model calculations and measurements along the line of sight of the instrument. Evening twilight observations of OClO are found to be broadly consistent with current photochemical schemes, assuming ClO and BrO levels near 50 mbar of about 0.5 parts per billion by volume (ppbv) and 7 parts per trillion by volume (pptv), respectively, throughout the observing period from late August to mid‐October. Nighttime observations of OClO obtained using the Moon as a light source display evidence for growth after sunset in late August, but not in late September. Further, the observed morning twilight OClO abundances are in agreement with model calculations in late August, but they generally fall below calculations in late September and October. Observations of BrO in mid‐September show far greater evening than morning twilight abundances. It is shown that the diurnal variations of BrO and OClO in mid‐September and October can be explained by formation of the BrONO2 reservoir species at night, although other reservoir species with comparably long lifetimes could also explain the observations. If formation of BrONO2 is the correct explanation for these data, the observations suggest that NO2 levels in the Antarctic lower stratosphere are of the order of a few pptv or less in late August, a few tens of pptv in mid‐September, and a few hundred pptv in October.

Measurement of tropospheric OH concentrations by laser long-path absorption spectroscopy

Measurement of tropospheric OH concentrations by laser long-path absorption spectroscopy

Measurement of tropospheric OH concentrations by laser long-path absorption spectroscopy

Tropospheric hydroxyl (OH) radical concentrations were measured in summer 1987 in Jiilich by broadband laser long-path absorption (laser-LPA) spectroscopy observing the Q 1 (2), Q 2 1 (2).Q 1 (3).Q 2 1 (3). and P 1 (1) rotational absorption lines around 308 nm. The measurement technique was improved using a multichannel spectrometric detector to record the OH absorption spectra. Now a much wider spectral interval can be observed at high resolution; thus, registration and correction of overlapping absorption features of interfering trace gases is improved. Simultaneous to the OH measurements, the concentrations of nitrogen dioxide (NO 2 ), formaldehyde (HCHO), and sulfur dioxide (SO 2 ) were monitored by a second LPA spectrometer. Ozone (O 3 aerosol concentrations, meteorological parameters, and absolutely calibrated photolysis frequencies of O 3 and NO 2 were also recorded. Carbon monoxide (CO), methane (CH 4 ), and light hydrocarbons were determined by gas chromatographic analysis. OH concentrations ranged up to 6.8.10 6 molecules/cm 3 . At one day in July, an 'asymmetric' diurnal profile of the OH concentration was observed, which could be qualitatively explained by a strong decrease in the NO 2 concentration during the morning hours. DOI: 10.1111/j.1600-0889.1988.tb00115.x