Novel Broadband Cavity-Enhanced Absorption Spectrometer for Simultaneous Measurements of NO2 and Particulate Matter.

A novel instrument based on broadband cavity-enhanced absorption spectroscopy has been developed using a supercontinuum broadband light source, which showcases its ability in simultaneous measurements of the concentration of NO2 and the extinction of particulate matter. Side-by-side intercomparison was carried out with the reference NOx analyzer for NO2 and OPC-N2 particle counter for particulate matter, which shows a good linear correlation with r2 > 0.90. The measurement limits (1σ) of the developed instrument were experimentally determined to be 230 pptv in 40 s for NO2 and 1.24 Mm-1 for the extinction of particulate matter in 15 s. This work provides a promising method in simultaneously monitoring atmospheric gaseous compounds and particulate matter, which would further advance our understanding on gas-particle heterogeneous interactions in the context of climate change and air quality.

Rate Coefficients for the Gas-Phase Reactions of Nitrate Radicals with a Series of Furan Compounds.

The atmospheric reaction of a series of furan compounds (furan (F), 2-methylfuran (2-MF), 3-methylfuran (3-MF), 2,5-dimethylfuran (2,5-DMF), and 2,3,5-trimethylfuran (2,3,5-TMF)) with nitrate radical (NO3) has been investigated using the relative rate kinetic method in the CHamber for the Atmospheric Reactivity and the Metrology of the Environment (CHARME) simulation chamber at the laboratoire de Physico-Chimie de l'Atmosphere (LPCA) laboratory (Dunkerque, France). The experiments were performed at (294 ± 2) K atmospheric pressure and under dry conditions (relative humidity, RH < 2%) with proton transfer mass reaction-time of flight-mass spectrometer (PTR-ToF-MS) for the chemical analysis. The following rate coefficients (in units cm3 molecule-1 s-1) were determined: furan, k(F) = (1.51 ± 0.38) × 10-12, 2-methylfuran, k(2-MF) = (1.91 ± 0.32) × 10-11, 3-methylfuran, k(3-MF) = (1.49 ± 0.33) × 10-11, 2,5-dimethylfuran, k(2,5-DMF) = (5.82 ± 1.21) × 10-11, and 2,3,5-trimethylfuran, k(2,3,5-TMF) = (1.66 ± 0.69) × 10-10. The uncertainty on the measured rate coefficient (ΔkFC) includes both the uncertainty on the measurement and that on the rate coefficient of the reference molecule. To our knowledge, this work represents the first determination for the rate coefficient of the 2,3,5-TMF reaction with NO3. This work shows that the reaction between furan and methylated furan compounds with nitrate radical (NO3) is the dominant removal pathway during the night with lifetimes between 0.5 and 55 min for the studied molecules.

Absolute determination of chemical kinetic rate constants by optical tracking the reaction on the second timescale using cavity-enhanced absorption spectroscopy.

We report a new spectroscopic platform coupled to an atmospheric simulation chamber for the direct determination of chemical rate constants with high accuracy at a second time-scale resolution. These developed analytical instruments consist of an incoherent broadband cavity enhanced absorption spectrometer using a red light emitting diode (LED) emitting at ∼662 nm (LED-IBBCEAS) associated with a multipass cell direct absorption spectrometer (MPC-DAS) coupled to an external cavity quantum cascade laser (EC-QCL) operating in the mid-infrared region at approximately 8 µm (EC-QCL-MPC-DAS). Spectrometers were employed to investigate the NO3-initiated oxidation of four selected volatile organic compounds (VOCs) for the determination of the corresponding rate constants with a dynamic range of 5 orders of magnitude (from 10-11 to 10-16 cm3 molecule-1 s-1). Rate constants of (6.5 ± 0.5) × 10-15, (7.0 ± 0.4) × 10-13, and (5.8 ± 0.5) × 10-16 cm3 molecule-1 s-1 for propanal, isoprene and formaldehyde, respectively, were directly determined by fitting the measured concentration-time profiles of NO3 and VOCs (measured using a proton transfer reaction time-of-flight mass spectrometer, PTR-ToF-MS) to chemical models based on the FACSIMILE simulation software (version 4.2.50) at 760 torr and 293 ± 2 K. The obtained rate constants are in good agreement with the most recent recommendations of the IUPAC (International Union of Pure and Applied Chemistry). In addition, a rate constant of (2.60 ± 0.30) × 10-11 cm3 molecule-1 s-1 for the oxidation of 2-methoxyphenol by NO3 radicals was first determined using the absolute kinetic method. Compared to the mostly used indirect relative rate method, the rate constant uncertainty is reduced from ∼20% to ∼12%. The results demonstrated the high potential of using modern spectroscopic techniques to directly determine the chemical reaction rate constants.

Hygroscopic growth and CCN activity of secondary organic aerosol produced from dark ozonolysis of γ-terpinene.

The hygroscopic growth factor (GF) and cloud condensation nuclei (CCN) activity of secondary organic aerosol (SOA) particles produced during dark ozonolysis of γ-terpinene under different reaction conditions were investigated. The SOA particles were produced in the presence or absence of cyclohexane, an OH scavenger; 1,3,5-trimethylbenzene, an anthropogenic volatile organic compound; and (NH4)2SO4 seed particles. A hygroscopicity tandem differential mobility analyzer was used to determine the GFs of the SOA particles at RHs ≤ 93%. For some experiments, a CCN counter was used for size-resolved measurement of CCN activation at supersaturation (S) in the range of 0.1 to 1%. The single hygroscopicity parameter κ was derived from both the GF and CCN measurements. Under subsaturated conditions, all the SOA (except those in the presence of the (NH4)2SO4 seeds) showed small GF values. These GFs demonstrated that SOA mass loading affected the GF. A decrease in the SOA mass loading led to increased GF and corresponding κGFvalues. However, in a supersaturation regime, the SOA mass loading and the size of the particles did not significantly alter the CCN activity of the SOA. Our CCN measurements showed higher κCCN values (κCCN = 0.20-0.24) than those observed in most monoterpene ozonolysis studies (κCCN = 0.1-0.14). This difference may have been due to the presence of the two endocyclic double bonds in the γ-terpinene structure, which may have affected the SOA chemical composition, in contrast to monoterpenes that contain an exocyclic double bond. Our comparisons of sub- and supersaturated conditions showed a larger range of κ values than other experiments. Average κCCN/κGF ratios of ~7 and 14 were obtained in the unseeded SOA experiments at low and high SOA mass loadings, respectively. The average κCCN of 0.23 indicated that the SOA produced during ozonolysis of γ-terpinene exhibited fairly high CCN activity.

Incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS)-based strategy for direct measurement of aerosol extinction in a lidar blind zone.

In this Letter, the development of a custom-designed incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) and its application to in situ measurement of aerosol extinction near the ground surface are described in an effort to address the issue of missing data in the light detection and ranging (lidar) blind zone in the first hundreds of meters of the observation range. Combined measurements of aerosol extinction at the same location using lidar remote sensing at 355 nm and in situ IBBCEAS operating in the UV spectral region around 370 nm showed results with a good correlation (${{\rm R}^2} = {0.90}$R2=0.90) between the two measurement techniques. This Letter highlights a new strategy for near-end lidar calibration, using a ground-based compact and robust IBBCEAS located at the lidar measurement site to determine the vertical profile of the aerosol extinction coefficient with a higher accuracy.

High-accuracy and high-sensitivity spectroscopic measurement of dinitrogen pentoxide (N2O5) in an atmospheric simulation chamber using a quantum cascade laser.

A spectroscopic instrument based on a mid-infrared external cavity quantum cascade laser (EC-QCL) was developed for high-accuracy measurements of dinitrogen pentoxide (N2O5) at the ppbv-level. A specific concentration retrieval algorithm was developed to remove, from the broadband absorption spectrum of N2O5, both etalon fringes resulting from the EC-QCL intrinsic structure and spectral interference lines of H2O vapour absorption, which led to a significant improvement in measurement accuracy and detection sensitivity (by a factor of 10), compared to using a traditional algorithm for gas concentration retrieval. The developed EC-QCL-based N2O5 sensing platform was evaluated by real-time tracking N2O5 concentration in its most important nocturnal tropospheric chemical reaction of NO3 + NO2 ↔ N2O5 in an atmospheric simulation chamber. Based on an optical absorption path-length of Leff = 70 m, a minimum detection limit of 15 ppbv was achieved with a 25 s integration time and it was down to 3 ppbv in 400 s. The equilibrium rate constant Keq involved in the above chemical reaction was determined with direct concentration measurements using the developed EC-QCL sensing platform, which was in good agreement with the theoretical value deduced from a referenced empirical formula under well controlled experimental conditions. The present work demonstrates the potential and the unique advantage of the use of a modern external cavity quantum cascade laser for applications in direct quantitative measurement of broadband absorption of key molecular species involved in chemical kinetic and climate-change related tropospheric chemistry.

Infrared spectroscopy of secondary organic aerosol precursors and investigation of the hygroscopicity of SOA formed from the OH reaction with guaiacol and syringol.

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) synchrotron analyses supplemented by density functional theory (DFT) anharmonic calculations have been undertaken to study the fundamental vibrational signatures of guaiacol and syringol, two methoxyphenol compounds found at the highest concentrations in fresh wood smoke and precursors of secondary organic aerosols (SOA) affecting the radiative balance and chemistry of the atmosphere. Nitroderivatives of these two compounds have also been studied experimentally for nitroguaiacol and theoretically for nitrosyringol. All the active fundamental vibrational bands have been assigned and compared to available gas phase measurements, providing a vibrational database of the main precursors for the analysis of SOA produced by atmospheric oxidation of methoxyphenols. In addition, the SOA formed in an atmospheric simulation chamber from the OH reaction with guaiacol and syringol were analyzed using the ATR-FTIR synchrotron spectroscopy and their hygroscopic properties were also investigated. The vibrational study confirms that nitroguaiacol and nitrosyringol are the main oxidation products of methoxyphenols by OH and are key intermediates in SOA production. The hydration experiments highlight the hydrophilic and hydrophobic characters of nitrosyringol and nitroguaiacol, respectively.

Sensing atmospheric reactive species using light emitting diode by incoherent broadband cavity enhanced absorption spectroscopy.

We overview our recent progress in the developments and applications of light emitting diode-based incoherent broadband cavity enhanced absorption spectroscopy (LED-IBBCEAS) techniques for real-time optical sensing chemically reactive atmospheric species (HONO, NO3, NO2) in intensive campaigns and in atmospheric simulation chamber. New application of optical monitoring of NO3 concentration-time profile for study of the NO3-initiated oxidation process of isoprene in a smog chamber is reported.

Kinetic Study of the Gas-Phase Reactions of Nitrate Radicals with Methoxyphenol Compounds: Experimental and Theoretical Approaches.

The gas-phase reactions of five methoxyphenols (three disubstituted and two trisubstituted) with nitrate radicals were studied in an 8000 L atmospheric simulation chamber at atmospheric pressure and 294 ± 2 K. The NO3 rate constants were investigated with the relative kinetic method using PTR-ToF-MS and GC-FID to measure the concentrations of the organic compounds. The rate constants (in units of cm(3) molecule(-1) s(-1)) determined were: 2-methoxyphenol (guaiacol; 2-MP), k(2-MP) = (2.69 ± 0.57 × 10(-11); 3-methoxyphenol (3-MP), k(3-MP) = (1.15 ± 0.21) × 10(-11); 4-methoxyphenol (4-MP), k(4-MP) = (13.75 ± 7.97) × 10(-11); 2-methoxy-4-methylphenol, k(2-M-4-MeP) = (8.41 ± 5.58) × 10(-11) and 2,6-dimethoxyphenol (syringol; 2,6-DMP), k(2,6-DMP) = (15.84 ± 8.10) × 10(-11). The NO3 rate constants of the studied methoxyphenols are compared with those of other substituted aromatics, and the differences in the reactivity are construed regarding the substituents (type, number and position) on the aromatic ring. This study was also supplemented by a theoretical approach of the methoxyphenol reactions with nitrate radicals. The upper limits of the NO3 overall rate constants calculated were in the same order of magnitude than those experimentally determined. Theoretical calculations of the minimum energies of the adducts formed from the reaction of NO3 radicals with the methoxyphenols were also performed using a DFT approach (M06-2X/6-31G(d,p)). The results indicate that the NO3 addition reactions on the aromatic ring of the methoxyphenols are exothermic, with energy values ranging between -13 and -21 kcal mol(-1), depending on the environment of the carbon on which the oxygen atom of NO3 is attached. These energy values allowed identifying the most suitable carbon sites for the NO3 addition on the aromatic ring of the methoxyphenols: at the exception of the 3-MP, the NO3 ipso-addition to the hydroxyl group is one of the favored sites for all the studies compounds.

Reaction Kinetics of Catechol (1,2-Benzenediol) and Guaiacol (2-Methoxyphenol) with Ozone.

The kinetic reactions of 1,2-benzenediol (catechol) and 2-methoxyphenol (guaiacol) with ozone were studied in a simulation chamber (8 m(3)) under dark conditions. The rate coefficients were measured at 294 ± 2 K, atmospheric pressure and dry conditions (relative humidity, RH < 1%), except for 1,2-benzenediol where they were also measured as a function of relative humidity (RH = 1-80%). The concentrations of organic compounds were followed by a PTR-ToF-MS for a continuous monitoring of gas-phase species. The O3 rate coefficients were obtained using both the pseudo-first-order and relative rate methods. The values (in cm(3) molecule(-1) s(-1)) determined for catechol and guaiacol under dry conditions are (13.5 ± 1.1) × 10(-18) and (0.40 ± 0.31) × 10(-18), respectively. The rate coefficient of catechol was found to be independent of RH below 20% and above 60%, whereas for RH between 20% and 60% it decreases with increasing RH. The determined rate coefficients have been used to evaluate the atmospheric lifetime of each compound with respect to O3. To our knowledge, this study represents the first determination of the ozone rate coefficient with guaiacol and is also the first kinetic investigation for the influence of the relative humidity on the oxygenated aromatic ozonolysis.

Rate Coefficients for the Gas-Phase Reactions of Hydroxyl Radicals with a Series of Methoxylated Aromatic Compounds.

Rate coefficients for the reactions of hydroxyl radicals (OH) with a series of oxygenated aromatics (two methoxybenzene and five methoxyphenol isomers) have been obtained using the relative kinetic method in 1080 and 480 L photoreactors at the University of Wuppertal, Germany. The experiments were realized at 295 ± 2 K and 1 bar total pressure of synthetic air using in situ Fourier transform infrared spectroscopy for the chemical analysis. The following rate coefficients (in units of cm(3) molecule(-1) s(-1)) were determined: methoxybenzene (anisole), (2.08 ± 0.21) × 10(-11); 1-methoxy-2-methylbenzene, (4.56 ± 0.50) × 10(-11); 2-methoxyphenol (guaiacol), (5.40 ± 0.72) × 10(-11); 3-methoxyphenol, (6.93 ± 0.67) × 10(-11); 4-methoxyphenol, (5.66 ± 0.55) × 10(-11); 2-methoxy-4-methylphenol, (7.51 ± 0.68) × 10(-11); 2,3-dimethoxyphenol, (7.49 ± 0.81) × 10(-11); and 2,6-dimethoxyphenol (syringol), (8.10 ± 0.98) × 10(-11). The rate coefficients for the reactions of OH with 2,3-dimethoxyphenol and 1-methoxy-2-methylbenzene are first time measurements. The rate coefficients determined in this work are compared with previous determinations reported in the literature and also with the values estimated using a structure-activity relationship method. A comparison is performed between the OH rate coefficients obtained for methoxylated aromatics with those of other substituted aromatics in order to understand the influence of the type, number, and position of the different substituents on the reactivity of aromatics toward OH. In addition, a comparison is made between the OH and Cl rate coefficients for the compounds. The principal atmospheric sink of these methoxylated aromatic compounds during daytime is their reaction with OH radicals. The corresponding lifetimes for reaction with OH radicals and Cl atoms are 2-8 and 11-50 h, respectively.