Photochemical evolution of the molecular composition of dissolved organic carbon and dissolved brown carbon from wood smoldering.

Recently, extreme wildfires occur frequently around the world and emit substantial brown carbon (BrC) into the atmosphere, whereas the molecular compositions and photochemical evolution of BrC remain poorly understood. In this work, primary smoke aerosols were generated from wood smoldering, and secondary smoke aerosols were formed by the OH radical photooxidation in an oxidation flow reactor, where both primary and secondary smoke samples were collected on filters. After solvent extraction of filter samples, the molecular composition of dissolved organic carbon (DOC) was determined by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). The molecular composition of dissolved BrC was obtained based on the constraints of DOC formulae. The proportion of dissolved BrC fractions accounted for approximately 1/3-1/2 molecular formulae of DOC. The molecular characteristics of dissolved BrC showed higher levels of carbon oxidation state, double bond equivalents, and modified aromaticity index than those of DOC, indicating that dissolved BrC fractions were a class of organic structures with relatively higher oxidation state, unsaturated and aromatic degree in DOC fractions. The comparative analysis suggested that aliphatic and olefinic structures dominated DOC fractions (contributing to 70.1%-76.9%), while olefinic, aromatic, and condensed aromatic structures dominated dissolved BrC fractions (contributing to 97.5%-99.9%). It is worth noting that dissolved BrC fractions only contained carboxylic-rich alicyclic molecules (CRAMs)-like structures, unsaturated hydrocarbons, aromatic structures, and highly oxygenated compounds. CRAMs-like structures were the most abundant species in both DOC and dissolved BrC fractions. Nevertheless, the specific molecular characteristics for DOC and dissolved BrC fractions varied with subgroups after aging. The results highlight the similarities and differences in the molecular compositions and characteristics of DOC and dissolved BrC fractions with aging. This work will provide insights into understanding the molecular composition of DOC and dissolved BrC in smoke.

Optimizing a twin-chamber system for direct ozone production rate measurement.

High Ozone Production Rate (OPR) leads to O3 pollution episodes and adverse human health outcomes. OPR observation (Obs-OPR) and OPR modelling (Mod-OPR) have been obtained from observed and modelled peroxy radicals and nitrogen oxides. However, discrepancies between them remind of an imperfect understanding of O3 photochemistry. Direct measurement of OPR (Mea-OPR) by a twin-chamber system emerges. Herein, we optimized Mea-OPR design, i.e., minimizing the chamber surface area to volume ratio (S/V) to 9.8 m-1 from 18 m-1 and the dark uptake coefficient of O3 to 9.9 × 10-9 from 7.1 × 10-8 in the literature. In addition, control experiments further revealed and quantified a photo-enhanced O3 uptake, and therefore recommended an essential correction of Mea-OPR. We finally characterized a measurement uncertainty of ±38% and a detection limit of 3.2 ppbv h-1 (3SD), which suggested that Mea-OPR would be sensitive enough to measure OPR in urban or suburban environments. Further application of this system in urban Beijing during the Beijing 2022 Olympic Winter Games recorded a noontime OPR of 7.3 (±3.3, 1SD) ppbv h-1. These observational results added up to our confidence in future field application of Mea-OPR, to facilitate pollution control policy evaluation and to shed light on O3 photochemistry puzzle.

A broadband picometer resolution visible CCD spectrometer based on virtually imaged phased array technology.

Coincidental realization of broadband spectral coverage and high resolution in one spectrometer system has always been a challenge. Here, we report the development of a high-resolution visible CCD spectrometer based on the virtually imaged phase array (VIPA) technique. By using a thin glass plate and a reflective grating, a two-dimensional cross-dispersion was realized. A broadband coverage of ∼14.94 THz and a high resolution of ∼1 GHz at 632.996 nm were achieved with a simple structure. The effects of the surface quality of VIPA etalon, the pixel size of the CCD camera, the pinhole size of the input beam, and the focal length of the imaging lens on the resolution of the spectrometer and the transverse spot size on the detector plane were considered. A comparison between the experimental results by changing the imaging lens and the theoretical calculation results proved a better simulation of these two parameters, which is a helpful contribution to the design and construction of a VIPA spectrometer. The developed spectrometer will provide a useful tool for the study of high-resolution spectroscopy and for simultaneous multi-species trace detection.

Peroxy radical chemistry during ozone photochemical pollution season at a suburban site in the boundary of Jiangsu-Anhui-Shandong-Henan region, China.

Ambient peroxy radical (RO2⁎ = HO2 + RO2) concentrations were measured at a suburban site in a major prefecture-level city (Huaibei) in the boundary of Jiangsu-Anhui-Shandong-Henan region, which is the connecting belt of air pollution in the Beijing-Tianjin-Hebei region and the Yangtze River Delta. Measurements were carried out during the period of September to October 2021 to elucidate the formation mechanism of O3 pollution. The observed maximum concentration of peroxy radicals was 73.8 pptv. A zero-dimensional box model (Framework for 0-Dimensional Atmospheric Modeling, F0AM) based on Master Chemical Mechanism (MCM3.3.1) was used to predict radical concentrations for comparison with observations. The model reproduced the daily variation of peroxy radicals well, but discrepancies still appear in the morning hours. As in previous field campaigns, systematic discrepancies between modelled and measured RO2⁎ concentrations are observed in the morning for NO mixing ratios higher than 1 ppbv. Between 6:00 and 9:00 am, the model significantly underpredicts RO2⁎ by a mean factor of 7.2. This underprediction can be explained by a missing RO2⁎ source of 1.2 ppbv h-1 which originated from the photochemical conversion of an alkene-like chemical species. From the model results it shows that the main sources of ROx (= OH + HO2 + RO2) are the photolysis of oxygenated volatile organic compounds (OVOCs, 33 %), O3 and HONO (25 %), and HCHO (24 %). And the major sinks of ROx transitioned from a predominant reaction of radicals with NOx in the morning to a predominant peroxy self- and cross-reaction in the late afternoon. The introduction of an alkene-like species increased RO2 radical concentration and resulted in 14 % increase in net daily integrated ozone production, indicating the possible significance of the mechanism of alkene-like species oxidation to peroxy radicals. This study provides important information for subsequent ozone pollution control policies in Jiangsu-Anhui-Shandong-Henan region.

High band-width mid-infrared frequency-modulated Faraday rotation spectrometer for time resolved measurement of the OH radical.

We present a novel mid-infrared frequency-modulated Faraday rotation spectrometer (FM-FRS) for highly sensitive and high bandwidth detection of OH radicals in a photolysis reactor. High frequency modulation (up to 150 MHz) of the probe laser using an electro-optical modulator (EOM) was used to produce a modulation sideband on the laser output. An axial magnetic field was applied to the multi-pass Herriott cell, causing the linearly polarized light to undergo Faraday rotation. OH radicals were generated in the cell by photolyzing a mixture of ozone (O3) and water (H2O) with a UV laser pulse. The detection limit of OH reaches 6.8 × 108 molecule/cm3 (1σ, 0.2 ms) after 3 and falling to 8.0 × 107 molecule/cm3 after 100 event integrations. Relying on HITRAN absorption cross section and line shape data, this corresponds to minimum detectable fractional absorption (Amin) of 1.9 × 10-5 and 2.2 × 10-6, respectively. A higher signal-to-noise ratio and better long-term stability was achieved than with conventional FMS because the approach was immune to interference from diamagnetic species and residual amplitude modulation noise. To our knowledge, this work reports the first detection of OH in a photolysis reactor by FM-FRS in the mid-infrared region, a technique that will provide a new and alternative spectroscopic approach for the kinetic study of OH and other intermediate radicals.

Unmanned-aerial-vehicle-borne cavity enhanced albedometer: a powerful tool for simultaneous in-situ measurement of aerosol light scattering and absorption vertical profiles.

Vertical profiles of aerosol light scattering (bscat), absorption (babs), as well as the single scattering albedo (SSA, ω), play an important role in the effects of aerosols on climate, air quality, and local photochemistry. High-precision in-situ measurements of the vertical profiles of these properties are challenging and therefore uncommon. We report here the development of a portable cavity-enhanced albedometer operating at λ = 532 nm for use aboard an unmanned aerial vehicle (UAV). Multi-optical parameters, bscat, babs, extinction coefficient bext, and ω, can be measured simultaneously in the same sample volume. The achieved detection precisions in laboratory were 0.38, 0.21, and 0.43 Mm-1 for bext, bscat, and babs, respectively, for a 1 s data acquisition time. The albedometer was installed on an hexacopter UAV and simultaneous in-situ measurements of the vertical distributions of bext, bscat, babs, and ω were realized for the first time. Here we report a representative vertical profile up to a maximum height of 702 m with a vertical resolution of better than 2 m. The UAV platform and the albedometer demonstrate good performance and will be a valuable and powerful tool for atmospheric boundary layer research.

Broadband spectrum characteristics and radiative effects of primary brown carbon from wood pyrolysis.

Brown carbon (BrC), known as light-absorbing organic aerosol in the near-ultraviolet (UV) and short visible region, plays a significant role in the global and regional climate change. A detailed understanding of the spectral optical properties of BrC is beneficial for reducing the uncertainty in radiative forcing calculation. In this work, the spectral properties of primary BrC were investigated by using a four-wavelength broadband cavity-enhanced albedometer with central wavelengths at 365, 405, 532 and 660 nm. The BrC samples were generated by the pyrolysis of three types of wood. During the pyrolysis process, the measured average single scattering albedo (SSA) at 365 nm was about 0.66 to 0.86, where the average absorption Ångström exponent (AAE) was between 5.8 and 7.8, and the average extinction Ångström exponent (EAE) was within 2.1 to 3.5. The full spectral measurement of SSA (300-700 nm) was realized by an optical retrieval method and the retrieved SSA spectrum was directly applied to evaluate aerosol direct radiative forcing (DRF) efficiency. The DRF efficiency over ground of various primary BrC emissions increased from 5.3 % to 68 % as compared to the non-absorbing organic aerosol assumption. A decrease of about 35 % in SSA would cause the DRF efficiency over ground to change from cooling effect to warming effect (from -0.33 W/m2 to +0.15 W/m2) in the near-UV band (365-405 nm). The DRF efficiency over ground of strongly absorptive primary BrC (lower SSA) contributed 66 % more than weakly absorptive primary BrC (higher SSA). These findings proved the importance of broadband spectral properties of BrC, which are substantial for radiative forcing evaluation of BrC and should be considered in global climate models.

[Characteristics of O3 Production in the Western Suburb of Hefei in Summer Based on the Observation of Total Peroxy Radical].

In August 2020, the observations of total peroxy radical concentrations were carried out in the western suburb site of Hefei using a peroxy radical chemical amplifier (PERCA) instrument. The ozone production and its sensitivity were characterized with the measured O3 and its precursors. The results showed that the daily variation in total peroxy radical concentrations exhibited an obvious convex tend, with the highest value at approximately 12:00; the average peak peroxy radical concentration was 43.8×10-12; and the concentrations of the peroxy radical and ozone were driven by strong solar radiation and high temperature. The photochemical ozone production rate could be determined with peroxy radical and NO concentration. The average ozone peak production rate in summer was 10.6×10-9 h-1, which was more sensitive to NO concentration. Based on the ratio of the radical loss rate due to reactions with NOx to the radical loss rate (Ln/Q), the characteristics of O3 production in the western suburb of Hefei in summer were analyzed. The results showed that O3 production sensitivity varied greatly during the day. The summer O3 production regime shifted from the VOC-sensitive chemistry in the early morning to NOx-sensitive chemistry in the afternoon, and this regime transition typically occurred in the morning.

Validating HONO as an Intermediate Tracer of the External Cycling of Reactive Nitrogen in the Background Atmosphere.

In the urban atmosphere, nitrogen oxide (NOx═NO + NO2)-related reactions dominate the formation of nitrous acid (HONO). Here, we validated an external cycling route of HONO and NOx, i.e., formation of HONO resulting from precursors other than NOx, in the background atmosphere. A chemical budget closure experiment of HONO and NOx was conducted at a background site on the Tibetan Plateau and provided direct evidence of the external cycling. An external daytime HONO source of 100 pptv h-1 was determined. Both soil emissions and photolysis of nitrate on ambient surfaces constituted likely candidate mechanisms characterizing this external source. The external source dominated the chemical production of NOx with HONO as an intermediate tracer. The OH production was doubled as a result of the external cycling. A high HONO/NOx ratio (0.31 ± 0.06) during the daytime was deduced as a sufficient condition for the external cycling. Literature review suggested the prevalence of high HONO/NOx ratios in various background environments, e.g., polar regions, pristine mountains, and forests. Our analysis validates the prevalence of external cycling in general background atmosphere and highlights the promotional role of external cycling regarding the atmospheric oxidative capacity.

Portable cavity ring-down spectrometer for an HO2 radical measurement: instrument's performance and potential improvement using a narrow linewidth laser.

We report the development of a portable cavity ring-down spectrometer (CRDS) for direct and absolute measurement of HO2 radical concentration using a distributed feedback (DFB) diode laser operating at 1506 nm. The spectrometer has a compact design with all optics in a 1000 × 400 × 140 mm3 box. At a pressure of 100 mbar and a ring-down time (τ0) of 136 µs, the detection limit of the CRDS spectrometer was ∼ 7.3 × 107 molecule/cm3 (1σ, 10s). The corresponding detection sensitivity was 1.5 × 10-11 cm-1, which was close to the state-of-the-art performance. By replacing the DFB diode laser with a narrow linewidth erbium-doped fiber (EDF) laser, the amplitude fluctuation caused by the laser phase noise was reduced and the cavity mode injection efficiency was improved. The sensitivity was improved to 3.9 × 10-12 cm-1 with a short data-acquisition time of 0.2 s. Compared with the DFB laser, the improvement was nearly an order of magnitude. The use of the narrow linewidth laser is attractive. The instrument can achieve very high sensitivity without the need for a complex locking technique, ensuring simple and ease of use in future field applications.

Fast and Efficient Atmospheric NO2 Collection for Isotopic Analysis by a 3D-Printed Denuder System.

Being major species of atmospheric reactive nitrogen, nitrogen oxides (NOx = NO + NO2) have important implications for ozone and OH radical formation in addition to nitrogen cycles. Stable nitrogen isotopes (δ15N) of NOx have been sought to track NOx emissions and NOx chemical reactivities in the atmosphere. The current atmospheric NOx collection methods for isotopic analysis, however, largely suffer from unverified collection efficiency and/or low collection speed (<10 L/min). The latter makes it difficult to study δ15N(NOx) in pristine regions with low NOx concentrations. Here, we present a three-dimensional (3D)-printed honeycomb denuder (3DP-HCD) system, which can effectively collect atmospheric NO2 (a major part of NOx) under a variety of laboratory and field conditions. With a coating solution consisting of 10% potassium hydroxide (KOH) and 25% guaiacol in methanol, the denuder system can collect NO2 with nearly 100% efficiency at flow rates of up to 70 L/min, which is 7 times higher than that of the existing method and allows high-resolution (e.g., diurnal or finer resolution) NO2 collection even in pristine sites. Besides, the δ15N of NO2 collected by the 3DP-HCD system shows good reproducibility and consistency with the previously tested method. Preliminary results of online NO oxidation by a chrome trioxide (CrO3) oxidizer for simultaneous NO and NO2 collection are also presented and discussed.

Effect of photooxidation on size distribution, light absorption, and molecular compositions of smoke particles from rice straw combustion.

Organic aerosol (OA) emitted from biomass burning (BB) impacts air quality and global radiation balance. However, the comprehensive characterization of OA remains poorly understood because of the complex evolutionary behavior of OA in atmospheric processes. In this work, smoke particles were generated from rice straw combustion. The effect of OH radicals photooxidation on size distribution, light absorption, and molecular compositions of smoke particles was systematically investigated. The results showed that the median diameters of smoke particles increased by a factor of approximately 1.2 after photooxidation. In the particle compositions, although both non-polar fractions (n-hexane-soluble organic carbon, HSOC) and polar fractions (water-soluble organic carbon, WSOC) underwent photobleaching after aging, the photobleaching properties of HSOC (1.87-2.19) was always higher than that of WSOC (1.52-1.33). Besides, the light-absorbing properties of HSOC were higher than that of WSOC, showing a factor of approximately 1.75 times for mass absorption efficiency at 365 nm (MAE365). Consequently, the simple forcing efficiency (SFE) caused by absorption showed that HSOC has higher radiation effects than WSOC. After photooxidation, the concentration of 16 PAHs in HSOC fractions significantly decreased by 15.3%-72.5%. In WSOC fractions, the content of CHO, CHONS, and CHOS compounds decreased slightly, while the content of CHON compounds increased. Meantime, the variations in molecular properties supported the decrease in light absorption of WSOC fractions. These results reveal the aging behavior of smoke particles, then stress the importance of non-polar organic fractions in particles, providing new insights into understanding the atmospheric pollution caused by BB smoke particles.

Optical-feedback cavity-enhanced absorption spectroscopy for OH radical detection at 2.8†µm using a DFB diode laser.

We report the development of an optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) instrument for OH detection at 2.8 µm using a DFB diode laser. Two different approaches, symmetry analysis and wavelength modulation, were performed to achieve laser frequency locking to the cavity mode. Compared with the symmetry analysis method, the wavelength modulation method continuously locked the laser frequency to the cavity mode and eliminated decoupling the laser from the cavity mode. A detection sensitivity of 1.7×10-9 cm-1 was achieved in a 25 s sampling time and was about 3 times better than that of the symmetry analysis method. The corresponding OH detection limit was ∼ 2×108 molecule/cm3. Further improvement can be achieved by using higher reflectivity mirrors and other high-sensitivity approaches, such as frequency modulation spectroscopy and Faraday rotation spectroscopy.

Amplitude-Modulated Cavity-Enhanced Absorption Spectroscopy with Phase-Sensitive Detection: A New Approach Applied to the Fast and Sensitive Detection of NO2.

Accurate and sensitive measurements of NO2 play an extremely important role in atmospheric studies. Increasingly, studies require NO2 measurements with parts per trillion by volume (pptv-level) detection limits. Other desirable instrument attributes include ease of use, long-term stability, and low maintenance. In this work, we report the development of an amplitude-modulated multimode-diode-laser-based cavity-enhanced absorption spectroscopy (AM-CEAS) system operating at 406 nm that uses phase-sensitive detection for extremely sensitive NO2 detection. The laser was TTL-modulated at 35 kHz. The mirror reflectivity was determined to be 99.985% based on the ring-down time measurement. The cavity base length was 47.5 cm, giving an effective absorption pathlength of ∼3.26 km. AM-CEAS achieved a 1σ detection precision of 35 pptv in a 1 s data acquisition time (4.98 × 10-10 cm-1), over 4 times lower than that attained using a ring-down approach and the same optical system. The AM-CEAS precision improved to 8 pptv over a data acquisition time of 30 s (1.14 × 10-10 cm-1). The AM-CEAS method with the multimode diode laser integrates the advantages of high light injection efficiency like on-axis alignment cavity ring-down spectroscopy, low cavity-mode noise like off-axis alignment CEAS, and narrow-bandwidth high-sensitivity weak signal detection of modulation spectroscopy, providing a powerful, straightforward, and general method for ultrasensitive absorption and extinction measurements.

Light absorption properties and molecular compositions of water-soluble and methanol-soluble organic carbon emitted from wood pyrolysis and combustion.

Organic carbon (OC) emitted from biomass burning (BB) plays an important role in the global radiation budget. In this work, primary OC emitted from wood pyrolysis and combustion under nitrogen (N2) and air conditions in a tube furnace was investigated. The absorption spectra, chemical functional groups, and molecular compositions of OC were analyzed using UV-Vis-NIR spectrophotometer, Fourier transform infrared spectroscopy (FTIR), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), respectively. The light absorption properties showed that the mass absorption efficiency at 365 nm (MAE365) of methanol-soluble OC (MSOC) is 3.1-3.8 times higher than that of water-soluble OC (WSOC). Moreover, the MAE365 values derived from the N2 pyrolysis atmosphere are higher than that from the air atmosphere for both MSOC and WSOC. These results indicated that OC extracted by methanol has higher light absorption, especially for the OC emitted from the N2 pyrolysis atmosphere. Although the FTIR spectra showed identical functional groups for the OC from the air and N2 conditions, molecular compositions from the FT-ICR MS analysis presented significant differences. The molecular weight (MW), double bonds equivalent (DBE), DBE/C, and modified aromaticity index (AImod) of extracted OC showed higher values in MSOC than those in WSOC, and higher values under the N2 atmosphere than those under the air atmosphere. In addition, MAE365 showed positive correlations with MW (r = 0.94), DBE (r = 0.88), DBE/C (r = 0.96), and AImod (r = 0.97), whereas negative correlations with H/C (r = -0.97), O/C (r = -0.90), N/C (r = -0.88), and S/C (r = -0.93). These results indicated that molecules with larger MW and a high level of unsaturation and aromaticity present higher light absorption, while molecules with high elemental ratios of H/C, O/C, N/C, and S/C are adverse to light absorption.

Light Absorption Properties of Organic Aerosol from Wood Pyrolysis: Measurement Method Comparison and Radiative Implications.

Growing evidence indicates that organic aerosol (OA) is a significant absorber of solar radiation. Such absorptive OA is known as "brown carbon" (BrC). However, a formal analytical method for BrC is currently lacking although several methods have been applied to determine its absorption properties. Reported imaginary refractive index (kOA) values from various combustion sources span 2 orders of magnitude. Measurement methods are an important factor affecting this kOA variation. In this work, isolated OA from wood pyrolysis was used to compare four methods to determine absorbing properties of OA. The generated aerosol was lognormally distributed, spherical, and nearly pure organic matter. Optical closure was considered as the reference method. kOA calculated from the extract bulk light absorbance measurement was comparable to that determined by optical closure. kOA and mass absorption cross section obtained by online and offline filter-based transmission measurements were similar, but 3.5 to 5.0 times greater than those determined by optical closure. Absorption Ångström Exponents determined by the four methods were comparable and ranged from 6.1 to 6.8. A clear-sky radiative transfer model implied that using the optical parameters derived from different methods in the full climate model could produce different radiative impacts of primary OA emissions.

Time-Resolved Laser-Flash Photolysis Faraday Rotation Spectrometer: A New Tool for Total OH Reactivity Measurement and Free Radical Kinetics Research.

The total OH reactivity (kOH') is an important parameter for quantitative assessment of the atmospheric oxidation capacity. Although laboratory measurement of kOH' has been achieved 20 years ago, the instruments required are often costly and complex. Long-term atmospheric observations remain challenging and elusive. In this work, a novel instrument combining laser-flash photolysis with a mid-infrared Faraday rotation spectrometer (LFP-FRS) has been developed for the measurement of kOH' and for studying gas phase free radical kinetics. The reactor is composed of a Herriott-type optical multipass cell, and OH radicals were generated by flash photolysis of ozone with a 266 nm pulsed Nd:YAG laser. The decay of the OH signal was directly measured with a time-resolved FRS spectrometer at 2.8 µm. The overlapping path length between the pump beam and probe beam was 25 m. High performance was achieved by subtracting the signals before and after flash photolysis to eliminate interferences caused by H2O absorption and background drift. The optimum precisions (1σ) of OH concentration and kOH' measurement were 4 × 106 molecules cm-3 and 0.09 s-1 over data acquisition times of 56 and 112 s, respectively. The performance of the system was evaluated by the reaction of OH with CO and NO. The measured rate coefficients (kOH+CO and kOH+NO) were in good agreement with values reported in the literature. The developed LFP-FRS provides a new, high precision, and highly selective tool for atmospheric chemistry research of OH radicals and other transient paramagnetic free radicals such as HO2 radicals.

Improved spherical mirror multipass-cell-based interband cascade laser spectrometer for detecting ambient formaldehyde at parts per trillion by volume levels.

We report the development of an improved spherical mirror multipass-cell-based interband cascade laser (ICL) spectrometer for ambient formaldehyde (HCHO) detection. The multipass cell consists of two easily manufactured spherical mirrors that are low cost, and have a simple structure, large mirror area utilization, and dense spot pattern. Optical interference caused by the multipath cell was largely reduced, resulting in good sensitivity. Using wavelength modulation spectroscopy (WMS), a detection precision (${1} \sigma $1σ) of 51 pptv in 10 s was achieved with an absorption pathlength of 96 m, which compared favorably with the performance of other state-of-the-art instruments. The precision can be further improved by using a long absorption pathlength configuration and by removing fringe-like optical noise caused by the collimation lens. Ambient application of the developed spectrometer was demonstrated.

Simultaneous detection of atmospheric CO and CH4 based on TDLAS using a single 2.3 µm DFB laser.

A laser-based spectrometer with a physical size of 60× 30 ×25 cm3 has been developed to continuously monitor CO and CH4 in atmosphere based on tunable diode laser absorption spectroscopy (TDLAS). Two neighboring lines of CO and CH4 around 2.3 µm were selected as candidates for simultaneous measurement by a single diode distributed feedback (DFB) laser. A special Herriott-type multipass absorption cell, with a 72 m optical path length, was designed and used to enhance the absorption signals of sample gases. Normalized wavelength modulation spectroscopy was applied to improve the sensitivity and robustness of the spectrometer and it was implemented on a home-made electronic system based on field programmable gate array (FPGA). Meanwhile, the electronic system controlled the temperature and current of DFB laser with the precision of 0.01 °C and 40 ppm. The 2nd-harmonic signals normalized by the corresponding 1st-harmonic signals for both CO and CH4 are of high linear response to their concentrations in the range of 0.046-4.6 ppm and 0.487-48.7 ppm, respectively. According to the Allan variance, respective minimum detection limits for CO and CH4 are 0.73 ppb and 36 ppb at 122 s and 137 s. As an application example, the spectrometer has been validated through real-time and in-situ measurement of atmospheric CO and CH4 for 48 h.

Improved Chemical Amplification Instrument by Using a Nafion Dryer as an Amplification Reactor for Quantifying Atmospheric Peroxy Radicals under Ambient Conditions.

In our previous work ( Yang et al. Anal. Chem. 2018 , 90 ( 5 ), 3307 - 3312 ), we reported that a large diameter Nafion dryer can be used in the PERCA (PEroxy Radical Chemical Amplification) technique to minimize the impact of water vapor on the chain length (CL). By using a Nafion dryer, the sample was first dried to a low relative humidity (RH) and was then drawn into a FEP (fluorinated ethylene propylene) flow reactor tube to start the amplification cycles for peroxy radical measurement. This method provides a promising and simple method to minimize the sensitivity of CL to water vapor. However, there is a trade-off between inlet radical losses and moisture removal efficiency under high ambient RH conditions. In this paper, we report a further improvement by removing the inlet Nafion dryer and using it directly as an amplifier. The sample was drawn into the Nafion dryer directly without preconditioning to reduce inlet losses; the amplification and HO2 reformation cycles were started in the Nafion dryer. The CL value at 3% RHinlet was about 150. A linear relationship between CL and RHinlet up to 87% RH was observed. Low loss of the CL under high RHinlet (about 10% reduction at 87% RH) was achieved. As a result, the modified system can largely remove the uncertainty of the CL arising from water vapor under most ambient conditions.