Collection Method for Isotopic Analysis of Gaseous Nitrous Acid

The sources and chemistry of gaseous nitrous acid (HONO) in the environment are of great interest. HONO is a major source of atmospheric hydroxyl radical (OH), which impacts air quality and climate. HONO is also a major indoor pollutant that threatens human health. However, the large uncertainty of HONO sources and chemistry hinders an accurate prediction of the OH budget. Isotopic analysis of HONO may provide a tool for tracking the sources and chemistry of HONO. In this study, a modified annular denuder system (ADS) was developed to quantitatively capture HONO for offline nitrogen and oxygen isotopic analysis (δ¹⁵N and δ¹⁸O) using the denitrifier method. The ADS method was tested using laboratory generated HONO (400 ppbv to 1 ppmv) and validated by parallel HONO collection with a standard, basic impinger (BI) method. The ADS system shows complete capture of HONO without isotopic fractionation. The uncertainty (1σ) based on repeated measurements across the entire analytical procedure is 0.6‰ for δ¹⁵N and 0.5‰ for δ¹⁸O. The ADS method was also tested in roadside collections of ambient HONO (0.4–1.3 ppbv) for isotopic analysis and was found to be robust for low concentration collections over 3 and 12 h collection times. In order to ensure ability to use this method in the laboratory and in the field, storage conditions for the collected HONO samples were tested and samples can be stored with consistent δ¹⁵N and δ¹⁸O for 60 days. This method enables future work to utilize the isotopic composition of HONO for studying HONO chemical formation pathways, as well as atmospheric sources and chemistry

Rate Constants and Kinetic Isotope Effects for Methoxy Radical Reacting with NO₂ and O₂

Relative rate studies were carried out to determine the temperature dependent rate constant ratio <i>k</i>₁/<i>k</i>_2a: CH₃O· + O₂ → HCHO + HO₂· and CH₃O· + NO₂ (+M) → CH₃ONO₂ (+M) over the temperature range 250–333 K in an environmental chamber at 700 Torr using Fourier transform infrared detection. Absolute rate constants <i>k</i>₂ were determined using laser flash photolysis/laser-induced fluorescence under the same conditions. The analogous experiments were carried out for the reactions of the perdeuterated methoxy radical (CD₃O·). Absolute rate constants <i>k</i>₂ were in excellent agreement with the recommendations of the JPL Data Evaluation panel. The combined data (i.e., <i>k</i>₁/<i>k</i>₂ and <i>k</i>₂) allow the determination of <i>k</i>₁ as 1.3_–0.5^+0.9 × 10^–14 exp[−(663 ± 144)/<i>T</i>] cm³ s^–1, corresponding to 1.4 × 10^–15 cm³ s^–1 at 298 K. The rate constant at 298 K is in excellent agreement with previous work, but the observed temperature dependence is less than was previously reported. The deuterium isotope effect, <i>k</i>_H/<i>k</i>_D, can be expressed in the Arrhenius form as <i>k</i>₁/<i>k</i>₃ = (1.7_–0.4^+0.5) exp((306 ± 70)/<i>T</i>). The deuterium isotope effect does not appear to be greatly influenced by tunneling, which is consistent with a previous theoretical work by Hu and Dibble. (Hu, H.; Dibble, T. S., <i>J. Phys. Chem. A</i> <b>2013</b>, <i>117</i>, 14230–14242.