Atmospheric and kinetic studies of OH and HO2 by the FAGE technique

Atmospheric reactivity of biogenic volatile organic compounds in a maritime pine forest during the LANDEX episode 1 field campaign

Trace gas measurements were performed during the LANDEX (the LANDes EXperiment) Episode 1 field campaign in the summer 2017, in one of the largest European maritime pine forests (> 95% Pinus pinaster) located in southwestern France. Efforts have been focused on obtaining a good speciation of 20 major biogenic volatile organic compounds (BVOCs, including pinenes, carenes, terpinenes, linalool, camphene, etc.). This was made possible by the development of a new and specific chromatographic method. In order to assess the role of BVOCs in the local gas phase chemistry budget, their reactivity with the main atmospheric oxidants (hydroxyl radicals (OH), ozone (O₃) and nitrate radicals (NO₃)) and the corresponding consumption rates were determined. When considering the OH reactivity with BVOCs, isoprene and linalool accounted for 10-47% of the OH depletion during daytime, and monoterpenes for 50-65%, whereas monoterpenes were the main contributors during the night (70-85%). Sesquiterpenes and monoterpenes were the main contributors to the ozone reactivity, especially β-caryophyllene (30-70%), with a maximum contribution during nighttime. Nighttime nitrate reactivity was predominantly due to monoterpenes (i.e. 90-95%). Five specific groups have been proposed to classify the 19 BVOCs measured in the forest, according to their reactivity with atmospheric oxidants and their concentrations. The total amount of BVOCs consumed under and above the forest canopy was evaluated for 7 BVOCs (i.e. isoprene, α-pinene, β-pinene, myrcene, limonene + cis-ocimene and Δ³-carene). The reactivity of atmospheric oxidants and BVOCs at a local level are discussed in order to highlight the compounds (BVOCs, other VOCs), the atmospheric oxidants and the main associated reactive processes observed under the canopy of a maritime pine forest.

Hydroxyl, hydroperoxyl free radicals determination methods in atmosphere and troposphere

The hydroxyl radical (•OH) has a crucial function in the oxidation and removal of many atmospheric compounds that are harmful to health. Nevertheless, high reactivity, low atmospheric abundance, determination of hydroxyl, and hydroperoxyl radical's quantity is very difficult. In the atmosphere and troposphere, hydroperoxyl radicals (HO₂) are closely demanded in the chemical oxidation of the troposphere. But advances in technology have allowed researchers to improve the determination methods on the research of free radicals through some spectroscopic techniques. So far, several methods such as laser-induced fluorescence (LIF), high-performance liquid chromatography (HPLC), and chemical ionization mass spectroscopy have been identified and mostly used in determining the quantity of hydroxyl and hydroperoxyl radicals. In this systematic review, we have advised the use of scavenger as an advance for further researchers to circumvent some of these problems caused by free radicals. The primary goal of this review is to deepen our understanding of the functions of the most critical free radical (•OH, HO₂) and also understand the currently used methods to quantify them in the atmosphere and troposphere.

Sensitive detection of HO radicals produced in an atmospheric pressure plasma using Faraday rotation cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) is a well-established, highly sensitive absorption technique whose sensitivity and selectivity for trace radical sensing can be further enhanced by measuring the polarization rotation of the intracavity light by the paramagnetic samples in the presence of a magnetic field. In this paper, we highlight the use of this Faraday rotation cavity ring-down spectroscopy (FR-CRDS) for the detection of HO₂ radicals. In particular, we use a cold atmospheric pressure plasma jet as a highly efficient source of HO₂ radicals and show that FR-CRDS in the near-infrared spectral region (1506 nm) has the potential to be a useful tool for studying radical chemistry. By simultaneously measuring ring-down times of orthogonal linearly polarized light, measurements of Faraday effect-induced rotation angles (θ) and absorption coefficients (α) are retrieved from the same data set. The Faraday rotation measurement exhibits better long-term stability and enhanced sensitivity due to its differential nature, whereby highly correlated noise between the two channels and slow drifts cancel out. The bandwidth-normalized sensitivities are α_min=2.2×10^-11 cm^-1 Hz^-1/2 and θ_min=0.62 nrad Hz^-1/2. The latter corresponds to a minimum detectable (circular) birefringence of Δn_min=5×10^-16 Hz^-1/2. Using the overlapping ^qQ₃(N = 4-9) transitions of HO₂, we estimate limits of detection of 3.1 × 10⁸ cm^-3 based on traditional (absorption) CRDS methods and 6.7 × 10⁷ cm^-3 using FR-CRDS detection, where each point of the spectrum was acquired during 2 s. In addition, Verdet constants for pertinent carrier (He, Ar) and bulk (N₂, O₂) gases were recorded in this spectral region for the first time. These show good agreement with recent measurements of air and values extrapolated from reported Verdet constants at shorter wavelengths, demonstrating the potential of FR-CRDS for measurements of very weak Faraday effects and providing a quantitative validation to the computed rotation angles.

Development of a field system for measurement of tropospheric OH radical using laser-induced fluorescence technique

A ground-based system for measuring tropospheric OH radical based on laser-induced fluorescence (AIOFM-LIF) was developed in this work. In this system, ambient air is expanded through a 0.4 mm nozzle to low pressure in a detection chamber, where OH radical is irradiated by the 308 nm laser pulse at a repetition rate of 8.5 kHz. Then, the resultant fluorescence corresponding to the A²Σ⁺(υ^'=0)←X²Π_i(ν^''=0) transition at 308 nm is detected using gated photon counting. The AIOFM-LIF system was integrated into a mobile observing platform for the field observation following the series of laboratory characterization. A portable standard OH radical source by water photolysis-ozone actinometry was established and optimized for accurate system calibration. The factors affecting the system sensitivity were quantified. It was shown that the ultimate system sensitivity is 9.9 × 10^-8 cps (molecules cm^-3)^-1 mw^-1; the minimum detection limits are (1.84 ± 0.26) × 10⁵ cm^-3 and (3.69 ± 0.52) × 10⁵ cm^-3 at night and noon, respectively; and the whole error of AIOFM-LIF system is about 16%. Then, the system was deployed in Shenzhen, China, during the "A comprehensive STudy of the Ozone foRmation Mechanism in Shenzhen" (STORM) campaign. Valid OH radical concentrations for 31 days were obtained, and the peak of the daily average concentration was 6.6 × 10⁶ cm^-3 around 12:00. And a high correlation (R² = 0.77) between OH and j(O¹D) was also observed in this field campaign. The relationship between OH concentration and NOx was attentively discussed. The deployment of AIOFM-LIF system in STORM campaign has demonstrated its capability of measuring tropospheric OH radical with high sensitivity and accuracy in a polluted environment.

A hydroxyl radical detection system using gas expansion and fast gating laser-induced fluorescence techniques

An OH radical measurement instrument based on Fluorescence Assay by Gas Expansion (FAGE) has been developed in our laboratory. Ambient air is introduced into a low-pressure fluorescence cell through a pinhole aperture and irradiated by a dye laser at a high repetition rate of 8.5kHz. The OH radical is both excited and detected at 308nm using A-X(0,0) band. To satisfy the high efficiency needs of fluorescence collection and detection, a 4-lens optical system and a self-designed gated photomultiplier (PMT) is used, and gating is actualized by switching the voltage applied on the PMT dynodes. A micro channel photomultiplier (MCP) is also prepared for fluorescence detection. Then the weak signal is accumulated by a photon counter in a specific timing. The OH radical excitation spectrum range in the wavelength of 307.82-308.2nm is detected and the excited line for OH detection is determined to be Q₁(2) line. The calibration of the FAGE system is researched by using simultaneous photolysis of H₂O and O₂. The minimum detection limit of the instrument using gated PMT is determined to be 9.4×10⁵molecules/cm³, and the sensitivity is 9.5×10^-7cps/(OH·cm^-3), with a signal-to-noise ratio of 2 and an integration time of 60sec, while OH detection limit and the detection sensitivity using MCP is calculated to be 1.6×10⁵molecules/cm³ and 2.3×10^-6cps/(OH·cm^-3). The laboratory OH radical measurement is carried out and results show that the proposed system can be used for atmospheric OH radical measurement.

Identification of the major HOx radical pathways in an indoor air environment

OH and HO₂ profiles measured in a real environment have been compared to the results of the INCA-Indoor model to improve our understanding of indoor chemistry. Significant levels of both radicals have been measured and their profiles display similar diurnal behavior, reaching peak concentrations during direct sunlight (up to 1.6×10⁶ and 4.0×10⁷  cm^-3 for OH and HO₂ , respectively). Concentrations of O₃ , NO_x , volatile organic compounds (VOCs), HONO, and photolysis frequencies were constrained to the observed values. The HO_x profiles are well simulated in terms of variation for both species (Pearson's coefficients: p_OH =0.55, p_HO2 =0.76) and concentration for OH (mean normalized bias error: MNBE_OH =-30%), HO₂ concentration being always underestimated (MNBE_HO2 =-62%). Production and loss pathways analysis confirmed HONO photolysis role as an OH precursor (here up to 50% of the production rate). HO₂ formation is linked to OH-initiated VOC oxidation. A sensitivity analysis was conducted by varying HONO, VOCs, and NO concentrations. OH, HO₂ , and formaldehyde concentrations increase with HONO concentrations; OH and formaldehyde concentrations are weakly dependent on NO, whereas HO₂ concentrations are strongly reduced with increasing NO. Increasing VOC concentrations decreases OH by consumption and enhances HO₂ and formaldehyde.

Assessment of indoor HONO formation mechanisms based on in situ measurements and modeling

The photolysis of HONO has been found to be the oxidation driver through OH formation in the indoor air measurement campaign SURFin, an extensive campaign carried out in July 2012 in a classroom in Marseille. In this study, the INCA-Indoor model is used to evaluate different HONO formation mechanisms that have been used previously in indoor air quality models. In order to avoid biases in the results due to the uncertainty in rate constants, those parameters were adjusted to fit one representative day of the SURFin campaign. Then, the mechanisms have been tested with the optimized parameters against other experiments carried out during the SURFin campaign. Based on the observations and these findings, we propose a new mechanism incorporating sorption of NO₂ onto surfaces with possible saturation of these surfaces. This mechanism is able to better reproduce the experimental profiles over a large range of conditions.

An instrument to measure fast gas phase radical kinetics at high temperatures and pressures

Fast radical reactions are central to the chemistry of planetary atmospheres and combustion systems. Laser-induced fluorescence is a highly sensitive and selective technique that can be used to monitor a number of radical species in kinetics experiments, but is typically limited to low pressure systems owing to quenching of fluorescent states at higher pressures. The design and characterisation of an instrument are reported using laser-induced fluorescence detection to monitor fast radical kinetics (up to 25 000 s(-1)) at high temperatures and pressures by sampling from a high pressure reaction region to a low pressure detection region. Kinetics have been characterised at temperatures reaching 740 K and pressures up to 2 atm, with expected maximum operational conditions of up to ∼900 K and ∼5 atm. The distance between the point of sampling from the high pressure region and the point of probing within the low pressure region is critical to the measurement of fast kinetics. The instrumentation described in this work can be applied to the measurement of kinetics relevant to atmospheric and combustion chemistry.

Experimental and modeling study of the oxidation of n-butane in a jet stirred reactor using cw-CRDS measurements

The gas-phase oxidation of n-butane has been studied in an atmospheric jet-stirred reactor (JSR) at temperatures up to 950 K. For the first time, continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared has been used, together with gas chromatography (GC), to analyze the products formed during its oxidation. In addition to the quantification of formaldehyde and water, which is always difficult by GC, cw-CRDS allowed as well the quantification of hydrogen peroxide (H2O2). A comparison of the obtained mole fraction temperature profiles with simulations using a detailed gas-phase mechanism shows a good agreement at temperatures below 750 K, but an overestimation of the overall reactivity above this temperature. Also, a strong overestimation was found for the H2O2 mole fraction at higher temperatures. In order to improve the agreement between model and experimental results, two modifications have been implemented to the model: (a) the rate constant for the decomposition of H2O2 (+M) ↔ 2OH (+M) has been updated to the value recently proposed by Troe (Combust. Flame, 2011, 158, 594-601) and (b) a temperature dependent heterogeneous destruction of H2O2 on the hot reactor walls with assumed rate parameters has been added. The improvement (a) slows down the overall reactivity at higher temperatures, but has a negligible impact on the maximal H2O2 mole fraction. Improvement (b) has also a small impact on the overall reactivity at higher temperatures, but a large effect on the maximal H2O2 mole fraction. Both modifications lead to an improved agreement between model and experiment for the oxidation of n-butane in a JSR at temperatures above 750 K.

Quantification of OH and HO2 radicals during the low-temperature oxidation of hydrocarbons by Fluorescence Assay by Gas Expansion technique

•OH and •HO2 radicals are known to be the key species in the development of ignition. A direct measurement of these radicals under low-temperature oxidation conditions (T = 550-1,000 K) has been achieved by coupling a technique named fluorescence assay by gas expansion, an experimental technique designed for the quantification of these radicals in the free atmosphere, to a jet-stirred reactor, an experimental device designed for the study of low-temperature combustion chemistry. Calibration allows conversion of relative fluorescence signals to absolute mole fractions. Such radical mole fraction profiles will serve as a benchmark for testing chemical models developed to improve the understanding of combustion processes.