Comparison of ultraviolet absorbance, chemiluminescence, and DOAS instruments for ambient ozone monitoring

Comparison of ozone measurement methods in biomass burning smoke: an evaluation under field and laboratory conditions

In recent years wildland fires in the United States have had significant impacts on local and regional air quality and negative human health outcomes. Although the primary health concerns from wildland fires come from fine particulate matter (PM2.5), large increases in ozone (O3) have been observed downwind of wildland fire plumes (DeBell et al., 2004; Bytnerowicz et al., 2010; Preisler et al., 2010; Jaffe et al., 2012; Bytnerowicz et al., 2013; Jaffe et al., 2013; Lu et al., 2016; Lindaas et al., 2017; McClure and Jaffe, 2018; Liu et al., 2018; Baylon et al., 2018; Buysse et al., 2019). Conditions generated in and around wildland fire plumes, including the presence of interfering chemical species, can make the accurate measurement of O3 concentrations using the ultraviolet (UV) photometric method challenging if not impossible. UV photometric method instruments are prone to interferences by volatile organic compounds (VOCs) that are present at high concentrations in wildland fire smoke. Four different O3 measurement methodologies were deployed in a mobile sampling platform downwind of active prescribed grassland fire lines in Kansas and Oregon and during controlled chamber burns at the United States Forest Service, Rocky Mountain Research Station Fire Sciences Laboratory in Missoula, Montana. We demonstrate that the Federal Reference Method (FRM) nitric oxide (NO) chemiluminescence monitors and Federal Equivalent Method (FEM) gas-phase (NO) chemical scrubber UV photometric O3 monitors are relatively interference-free, even in near-field combustion plumes. In contrast, FEM UV photometric O3 monitors using solid-phase catalytic scrubbers show positive artifacts that are positively correlated with carbon monoxide (CO) and total gas-phase hydrocarbon (THC), two indicator species of biomass burning. Of the two catalytic scrubber UV photometric methods evaluated, the instruments that included a Nafion® tube dryer in the sample introduction system had artifacts an order of magnitude smaller than the instrument with no humidity correction. We hypothesize that Nafion®-permeating VOCs (such as aromatic hydrocarbons) could be a significant source of interference for catalytic scrubber UV photometric O3 monitors and that the inclusion of a Nafion® tube dryer assists with the mitigation of these interferences. The chemiluminescence FRM method is highly recommended for accurate measurements of O3 in wildland fire plume studies and at regulatory ambient monitoring sites frequently impacted by wildland fire smoke.

Gas-phase reactivity of acyclic α,β-unsaturated carbonyls towards ozone

We evaluated different approaches to discuss the reactivity of α,β-unsaturated carbonyls comparative to alkene analogues. It was found that the reactivity factors x_r, defined as the relative ratio between the rate coefficient of the carbonyl and a core structure, allow a semi-quantitative estimation of substituent effects in α,β-unsaturated acids, aldehydes and esters when the carbonyl containing substituent is replaced by a hydrogen atom. By contrast, it can be shown that the reactivity of the corresponding ketones differs from the other carbonyls. A linear correlation is presented between the x_r- values and the number of carbon atoms of the alkyl group of the unsaturated esters, which can be used to predict ozonolysis rate coefficients. For this systematic analysis the following rate coefficients (in 10^-18 cm³ molecule^-1 s^-1) have been determined at 298 ± 2 K and 990 ± 15 mbar and under dry conditions using the relative rate method: k(O₃ + methyl methacrylate) = 7.0 ± 0.9, k(O₃ + methyl crotonate) = 5.5 ± 1.4, k(O₃ + methyl 3-methyl-3-butenoate) = 1.3 ± 0.3, k(O₃ + methyl tiglate) = 65 ± 11, k(O₃ + 3-penten-2-one) = 31 ± 7, k(O₃ + 3-methyl-3-penten-2-one) = 80 ± 19, k(O₃ + 4-methyl-3-penten-2-one) = 8.4 ± 0.8.

Effects of Water Removal Devices on Ambient Inorganic Air Pollutant Measurements

Water vapor is a pivotal obstacle when measuring ambient air pollutants. The effects of water vapor removal devices which are called KPASS (Key-compound PASSer) and Cooler. On the measurement of O₃, SO₂, and CO at ambient levels were investigated. Concentrations of O₃, SO₂, and CO were 100 ppb, 150 ppb, and 25 ppm, respectively. The amount of water vapor varied at different relative humidity levels of 30%, 50%, and 80% when the temperature was 25 °C and the pressure was 1 atm. Water vapor removal efficiencies and recovery rates of target gases were also determined. The KPASS showed a better performance than the Cooler device, removing 93.6% of water vapor and the Cooler removing 59.2%. In terms of recovery, the KPASS showed a better recovery of target gases than the Cooler. Consequently, it is suggested that the KPASS should be an alternative way to remove water vapor when measuring O₃, SO₂, and CO.

Ozone from fireworks: Chemical processes or measurement interference?

Fireworks have been identified as one ozone source by photolyzing NO₂ or O₂ and are believed to potentially be important for the nighttime ozone during firework events. In this study, we conducted both lab and field experiments to test two types of fireworks with low and high energy with the goal to distinguish whether the visible ozone signal during firework displays is real. The results suggest that previous understanding of the ozone formation mechanism during fireworks is misunderstood. Ultraviolet ray (UV)-based ozone monitors are interfered by aerosols and some specific VOCs. High-energy fireworks emit high concentrations of particular matters and low VOCs that the artificial ozone can be easily removed by an aerosol filter. Low-energy fireworks emit large amounts of VOCs mostly from the combustion of the cardboard from fireworks that largely interferes with the ozone monitor. Benzene and phenol might be major contributors to the artificial ozone signal. We further checked the nighttime ozone concentration in Jinan and Beijing, China, during Chinese New Year, a period with intense fireworks. A signal of 3-8ppbv ozone was detected and positively correlated to NO and SO₂, suggesting a considerable influence of these chemicals in interfering with ambient ozone monitoring.

Evaluation of low-cost electro-chemical sensors for environmental monitoring of ozone, nitrogen dioxide, and carbon monoxide

Development of an air quality monitoring network with high spatio-temporal resolution requires installation of a large number of air pollutant monitors. However, state-of-the-art monitors are costly and may not be compatible with wireless data logging systems. In this study, low-cost electro-chemical sensors manufactured by Alphasense Ltd. for detection of CO and oxidative gases (predominantly O₃ and NO₂) were evaluated. The voltages from three oxidative gas sensors and three CO sensors were recorded every 2.5 sec when exposed to controlled gas concentrations in a 0.125-m³ acrylic glass chamber. Electro-chemical sensors for detection of oxidative gases demonstrated sensitivity to both NO₂ and O₃ with similar voltages recorded when exposed to equivalent environmental concentrations of NO₂ or O₃ gases, when evaluated separately. There was a strong linear relationship between the recorded voltages and target concentrations of oxidative gases (R² > 0.98) over a wide range of concentrations. Although a strong linear relationship was also observed for CO concentrations below 12 ppm, a saturation effect was observed wherein the voltage only changes minimally for higher CO concentrations (12-50 ppm). The nonlinear behavior of the CO sensors implied their unsuitability for environments where high CO concentrations are expected. Using a manufacturer-supplied shroud, sensors were tested at 2 different flow rates (0.25 and 0.5 Lpm) to mimic field calibration of the sensors with zero air and a span gas concentration (2 ppm NO2 or 15 ppm CO). As with all electrochemical sensors, the tested devices were subject to drift with a bias up to 20% after 9 months of continuous operation. Alphasense CO sensors were found to be a proper choice for occupational and environmental CO monitoring with maximum concentration of 12 ppm, especially due to the field-ready calibration capability. Alphasense oxidative gas sensors are usable only if it is valuable to know the sum of the NO₂ and O₃ concentrations.

A method to measure the ozone penetration factor in residences under infiltration conditions: application in a multifamily apartment unit

Recent experiments have demonstrated that outdoor ozone reacts with materials inside residential building enclosures, potentially reducing indoor exposures to ozone or altering ozone reaction byproducts. However, test methods to measure ozone penetration factors in residences (P) remain limited. We developed a method to measure ozone penetration factors in residences under infiltration conditions and applied it in an unoccupied apartment unit. Twenty-four repeated measurements were made, and results were explored to (i) evaluate the accuracy and repeatability of the new procedure using multiple solution methods, (ii) compare results from 'interference-free' and conventional UV absorbance ozone monitors, and (iii) compare results against those from a previously published test method requiring artificial depressurization. The mean (±s.d.) estimate of P was 0.54 ± 0.10 across a wide range of conditions using the new method with an interference-free monitor; the conventional monitor was unable to yield meaningful results due to relatively high limits of detection. Estimates of P were not clearly influenced by any indoor or outdoor environmental conditions or changes in indoor decay rate constants. This work represents the first known measurements of ozone penetration factors in a residential building operating under natural infiltration conditions and provides a new method for widespread application in buildings.

Sensitive detection of ozone by a practical resorufin-based spectroscopic probe with extremely low background signal

Ozone (O₃) has attracted much attention because of its key role in human health and disease, and its detection is of great importance for various biochemical studies as well as environmental evaluation. Here we develop a simple and practical spectroscopic off-on O₃ probe based on resorufin and the specific reaction of but-3-enyl with O₃. The probe shows an extremely low background spectroscopic signal, but reacts with O₃ producing a distinct color and fluorescence change. The detection limit of the probe for O₃ is 5.9 nM, which corresponds to an ozone concentration of 0.056 mg m⁻³ in air in this study and is lower than the international ambient air quality standard of 0.1 mg m⁻³. More importantly, the proposed probe is worth popularizing, and its applicability has been successfully demonstrated on both the determination of O₃ in real ambient air samples and the imaging of O₃ in biological cells.

Influence of temperature changes on ambient air NOx chemiluminescence measurements

Users of automatic air pollution monitors are largely unaware of how certain parameters, like temperature, can affect readings. The present work examines the influence of temperature changes on chemiluminescence NO(x) measurements made with a Thermo Scientific 42i analyzer, a model widely used in air monitoring networks and air pollution studies. These changes are grouped into two categories according to European Standard EN 14211: (1) changes in the air surrounding the analyzers and (2) changes in the sampled air. First, the sensitivity tests described in Standard EN 14211 were performed to determine whether the analyzer performance was adapted to the requirements of the standard. The analyzer met the performance criteria of both tests; however, some differences were detected in readings with temperature changes even though the temperature compensator was on. Sample temperature changes were studied more deeply as they were the most critical (they cannot be controlled and differences of several tens of degrees can be present in a single day). Significant differences in readings were obtained when changing sample temperature; however, maximum deviations were around 3% for temperature ranges of 15°C. If other possible uncertainty contributions are controlled and temperature variations with respect to the calibration temperature are not higher than 15°C, the effect of temperature changes could be acceptable and no data correction should have to be applied.

Specific fluorogenic probes for ozone in biological and atmospheric samples

Ozone exposure is a growing global health problem, especially in urban areas. While ozone in the stratosphere protects the earth from harmful ultraviolet light, tropospheric or ground-level ozone is toxic and can damage the respiratory tract. It has recently been shown that ozone may be produced endogenously in inflammation and antibacterial responses of the immune system; however, these results have sparked controversy owing to the use of a non-specific colorimetric probe. Here we report the synthesis of fluorescent molecular probes able to unambiguously detect ozone in both biological and atmospheric samples. Unlike other ozone-detection methods, in which interference from different reactive oxygen species is often a problem, these probes are ozone specific. Such probes will prove useful for the study of ozone in environmental science and biology, and so possibly provide some insight into the role of ozone in cells.

Measurement of atmospheric ozone by cavity ring-down spectroscopy

Ozone plays a key role in both the Earth's radiative budget and photochemistry. Accurate, robust analytical techniques for measuring its atmospheric abundance are of critical importance. Cavity ring-down spectroscopy has been successfully used for sensitive and accurate measurements of many atmospheric species. However, this technique has not been used for atmospheric measurements of ozone, because the strongest ozone absorption bands occur in the ultraviolet spectral region, where Rayleigh and Mie scattering cause significant cavity losses and dielectric mirror reflectivities are limited. Here, we describe a compact instrument that measures O3 by chemical conversion to NO2 in excess NO, with subsequent detection by cavity ring-down spectroscopy. This method provides a simple, accurate, and high-precision measurement of atmospheric ozone. The instrument consists of two channels. The sum of NO2 and converted O3 (defined as Ox) is measured in the first channel, while NO2 alone is measured in the second channel. NO2 is directly detected in each channel by cavity ring-down spectroscopy with a laser diode light source at 404 nm. The limit of detection for O3 is 26 pptv (2 sigma precision) at 1 s time resolution. The accuracy of the measurement is ±2.2%, with the largest uncertainty being the effective NO2 absorption cross-section. The linear dynamic range of the instrument has been verified from the detection limit to above 200 ppbv (r2>99.99%). The observed precision on signal (2 sigma) with 41 ppbv O3 is 130 pptv in 1 s. Comparison of this instrument to UV absorbance instruments for ambient O3 concentrations shows linear agreement (r2=99.1%) with slope of 1.012±0.002.

A practical alternative to chemiluminescence-based detection of nitrogen dioxide: cavity attenuated phase shift spectroscopy

We present results obtained from a greatly improved version of a previously reported nitrogen dioxide monitor (Anal Chem. 2005, 77, 724-728) that utilizes cavity attenuated phase shift spectroscopy (CAPS). The sensor, which detects the optical absorption of nitrogen dioxide within a 20 nm bandpass centered at 440 nm, comprises a blue light emitting diode, an enclosed stainless steel measurement cell (26 cm length) incorporating a resonant optical cavity of near-confocal design and a vacuum photodiode detector. An analog heterodyne detection scheme is used to measure the phase shift in the waveform of the modulated light transmitted through the cell induced by the presence of nitrogen dioxide within the cell. The sensor, which operates at atmospheric pressure, fits into a 19 in.-rack-mounted instrumentation box, weighs 10 kg, and utilizes 70 W of electrical power with pump included. The sensor response to nitrogen dioxide (calculated as the cotangent of the phase shift) is demonstrated to be linear (r2 > 0.9999) within +/- 1 ppb over a range of 0-320 ppb (by volume). The device exhibits a detection limit (3sigma precision) of less than 60 parts per trillion (0.060 ppb) with 10 s integration, a value derived from measurements at NO2 concentration levels of both 0 and 20 ppb; the detection limit improves as the integration time is increased to several hundred seconds. The observed baseline drift is less than +/- 0.5 ppb overthe course of a month. An intercomparison of measurements of ambient NO2 concentrations over several days using this sensor with a quantum cascade laser-based infrared absorption spectrometer and a standard chemiluminescence-based NOx analyzer is presented. The data from the CAPS sensor are highly correlated (r2 > 0.99) with the other two instruments. The absolute agreement between the CAPS and each of the two other instruments is within the expected statistical noise associated with the infrared laser-based absorption spectrometer (+/- 0.3 ppb with 10 s sampling) and chemiluminescence analyzer (+/- 0.4 ppb with 60 s averaging). The major limitation concerning accuracy is a direct spectral interference with phototchemically produced 1,2-dicarbonyl species (e.g., glyoxal, methylglyoxal). However, this interference can be readily removed by shifting the detection band to a slightly longer wavelength and ensuring that the lower edge of the detection band is greater than 455 nm.