Kinetics of the HO_2 + BrO reaction over the temperature range 233–348 K

The reaction BrO + HO_2 → products is the rate-limiting step in a key catalytic ozone destruction cycle in the lower stratosphere. In this study a discharge-flow reactor coupled with molecular beam mass spectrometry has been used to study the BrO + HO_2 reaction over the temperature range 233-348 K. Rate constants were measured under pseudo-first-order conditions in separate experiments with first HO_2 and then BrO in excess in an effort to identify possible complications in the reaction conditions. At 298 K, the rate constant was determined to be (1.73 ± 0.61) x 10^(-11) cm^3 molecule^(-1) s^(-1) with HO_2 in excess and (2.05 ± 0.64) x 10^(-11) cm^3 molecule^(-1) s^(-1) with BrO in excess. The combined results of the temperature-dependent experiments gave the following fit to the Arrhenius expression : k = (3.13 ± 0.33)]10^(-12) exp(536 ± 206/T) where the quoted uncertainties represent two standard deviations. The reaction mechanism is discussed in light of recent ab initio results on the thermochemistry of isomers of possible reaction intermediates

Face-To-Face With Scorching Wildfire: Potential toxicant Exposure and the Health Risks of Smoke For Wildland Firefighters at the Wildland-Urban interface

As wildfire risks have elevated due to climate change, the health risks that toxicants from fire smoke pose to wildland firefighters have been exacerbated. Recently, the International Agency for Research on Cancer (IARC) has reclassified wildland firefighters\u27 occupational exposure as carcinogenic to humans (Group 1). Wildfire smoke contributes to an increased risk of cancer and cardiovascular disease, yet wildland firefighters have inadequate respiratory protection. The economic cost of wildland fires has risen concurrently, as illustrated by the appropriation of $45 billion for wildfire management over FYs 2011-2020 by the U.S. Congress. Occupational epidemiological studies of wildland firefighters are crucial for minimizing health risks; however, they must account for the mixture of exposures in wildfire smoke. This review focuses on four aspects of wildland firefighters\u27 health risks at the wildland-urban interface: 1) economic costs and health impact, 2) respiratory protection, 3) multipollutant mixtures, and 4) proactive management of wildfires

Interaction of peroxynitric acid with solid H_2O ice

The uptake of peroxynitric acid (PNA), HO_2NO_2 or HNO_4, on solid H_2O ice at 193 K (−80°C) was studied using a fast flow‐mass spectrometric technique. An uptake coefficient of 0.15 ± 0.10 was measured, where the quoted uncertainty denotes 2 standard deviations. The uptake process did not result in the production of gas phase products. The composition of the condensed phase was investigated using programmed heating (3 K min^(−1)) of the substrate coupled with mass spectrometric detection of desorbed species. Significant quantities of HNO_4 and HNO_3 desorbed from the substrates at temperatures above 225 K and 246 K, respectively. The desorbed HNO_3, which was less than 9% of the desorbed HNO_4 and remained unchanged upon incubation of the substrate, was likely due to impurities in the HNO_4 samples rather than reaction of HNO_4 on the substrate. The onset temperatures for HNO_4 desorption increased with increasing H_2O to HNO_4 ratios, indicating that HNO_4, like HNO_3, tends to be hydrated in the presence of water. These observations suggest possible mechanisms for removal of HNO_4 or repartitioning of total odd nitrogen species in the Earth's upper troposphere and stratosphere

Experimental and Theoretical Investigation of Homogeneous Gaseous Reaction of CO₂(g) + <i>n</i>H₂O(g) + <i>n</i>NH₃(g) → Products (<i>n</i> = 1, 2)

Decreasing CO₂ emissions into the atmosphere is key for reducing global warming. To facilitate the CO₂ emission reduction efforts, our laboratory conducted experimental and theoretical investigations of the homogeneous gaseous reaction of CO₂(g) + <i>n</i>H₂O­(g) + <i>n</i>NH₃(g) → (NH₄)­HCO₃(s)/(NH₄)₂CO₃(s) (<i>n</i> = 1 and 2) using Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy and ab initio molecular orbital theory. Our FTIR-ATR experimental results indicate that (NH₄)₂CO₃(s) and (NH₄)­HCO₃(s) are formed as aerosol particulate matter when carbon dioxide reacts with ammonia and water in the gaseous phase at room temperature. Ab initio study of this chemical system suggested that the reaction may proceed through formation of NH₃·H₂O­(g), NH₃·CO₂(g), and CO₂·H₂O­(g) complexes. Subsequent complexes, NH₃·H₂O·CO₂ and (NH₃)₂·H₂O·CO₂, can be formed by adding gaseous reactants to the NH₃·H₂O­(g), NH₃·CO₂(g), and CO₂·H₂O­(g) complexes, respectively. The NH₃·H₂O·CO₂ and (NH₃)₂·H₂O·CO₂ complexes can then be rearranged to produce (NH₄)­HCO₃ and (NH₄)₂CO₃ as final products via a transition state, and the NH₃ molecule acts as a medium accepting and donating hydrogen atoms in the rearrangement process. Our computational results also reveal that the presence of an additional water molecule can reduce the activation energy of the rearrangement process. The high activation energy predicted in the present work suggests that the reaction is kinetically not favored, and our experimental observation of (NH₄)­HCO₃(s) and (NH₄)₂CO₃(s) may be attributed to the high concentrations of reactants increasing the reaction rate of the title reactions in the reactor

Kinetic and Dynamic Investigations of OH Reaction with Styrene

The kinetics of hydroxyl radical reaction with styrene has been studied at 240–340 K and a total pressure of 1–3 Torr using the relative rate/discharge flow/mass spectrometry technique. In addition, the dynamics of the reaction was also studied using the ab initio molecular orbital method. The reaction was found to be essentially pressure independent over 1–3 Torr at both 298 and 340 K. At 298 K, the average rate constant was determined, using four different reference compounds, to be <i>k</i>_styrene+OH = (5.80 ± 0.49) × 10^–11 cm³ molecule^–1 s^–1. At 240–340 K, the rate constant of this reaction was found to be negatively dependent on temperature with an Arrhenius expression determined to be <i>k</i>_styrene+OH = (1.02 ± 0.10) × 10^–11 exp[(532 ± 28)/<i>T</i>] cm³ molecule^–1 s^–1. Observation of mass spectral evidence of adduct products and their respective fragment ions suggests that the reaction proceeds with addition of the OH to the vinyl carbons of the styrene molecule. Ab initio calculations of both the addition and the abstraction pathways predict that the addition pathways are more energetically favorable because of large exothermicity and essentially barrierless transition state associated with the additions, which is consistent with the experimental observations. Using the styrene + OH rate constant determined at 277 K in the present work, the atmospheric lifetime of styrene was estimated to be 4.9 h