Temperature and Pressure Studies of the Reactions of CH3O2, HO2, and 1,2-C4H9O2 with NO2

Impact of temperature-dependent non-PAN peroxynitrate formation, RO2NO2, on nighttime atmospheric chemistry

The formation of peroxynitrates (RO2NO2) from the reaction of peroxy radicals (RO2) and nitrogen dioxide (NO2) and their subsequent redissociation are typically not included in chemical mechanisms. This is often done to save computational time as the assumption is that the equilibrium is strongly towards the RO2 + NO2 reaction for most conditions. Exceptions are the reactions of the methyl peroxy radical due to its abundance in the atmosphere and of acyl-RO2 radicals due to the long lifetime of peroxyacyl nitrates RO2NO2 (PANs). In this study, the nighttime oxidation of cis-2-butene and trans-2-hexene in the presence of NO2 is investigated in the atmospheric simulation chamber SAPHIR, Forschungszentrum Jülich, Germany, at atmospherically-relevant conditions at different temperatures (≈276 K, ≈293 K, ≈305 K). Measured concentrations of peroxy and hydroperoxy radicals as well as other trace gases (ozone, NO2, volatile organic compounds) are compared to state-of-the-art zero-dimensional box model calculations. Good model-measurement agreement can only be achieved when reversible RO2 + NO2 reactions are included for all RO2 species using literature values available from the latest SAR by [Jenkin et al., Atmos. Chem. Phys., 2019, 19, 7691]. The good agreement observed gives confidence that the SAR, derived originally for aliphatic RO2, can be applied to a large range of substituted RO2 radicals, simplifying generalised implementation in chemical models. RO2NO2 concentrations from non-acyl RO2 radicals of up to 2 × 10 cm-3 are predicted at 276 K, impacting effectively the kinetics of RO2 radicals. Under these conditions, peroxy radicals are slowly regenerated downwind of the pollution source and may be lost in the atmosphere through deposition of RO2NO2. Based on this study, 60% of RO2 radicals would be stored as RO2NO2 at a temperature of 10 °C and in the presence of a few ppbv of NO2. The fraction increases further at colder temperatures and/or higher NO2 mixing ratios. This does not only affect the expected concentrations of RO2 radicals but, as the peroxynitrates can react with OH radicals or photolyse, they could comprise a net sink for RO2 radicals as well as increase the production of NOx (= NO + NO2) in different locations depending on their lifetime. Omitting this chemistry from the kinetic model can lead to misinterpreted product formation and may prevent reconciling observations and model predictions.

A novel photochemical conversion technique for reliable calibration of peroxyacetyl nitrate (PAN) analyzers

Photochemical synthesis of peroxyacetyl nitrate (PAN) through irradiating air mixtures of NO and acetone is prevailingly adopted for calibrating PAN analyzers, but few users of PAN analyzers provide evidence to certify the calibration reliability. Here we report a nonnegligible variation (up to ~50 %) of PAN synthesized in the calibration unit of a commercial PAN analyzer, whereas PAN synthesized in the two custom-made reactors could achieve stable values with variations of <2.5 %. Compared with a straight quartz tube flow reactor (SQTFR), PAN synthesized by a coiled quartz tube flow reactor (CQTFR) could achieve more stable (relative standard deviation: <0.66 % versus 2.49 %) and larger (PAN_CQTFR/PAN_SQTFR: 1.04-1.10) values. The residence time and reaction temperature of photochemical mixtures in CQTFR were found to be the key factors affecting PAN synthesis, with their optimal values of 30-60 s and 30-35 °C for achieving the highest PAN levels. The photochemical conversion efficiencies of NO to PAN in CQTFR under the optimal conditions were successfully measured to be 98.5 ± 0.5 % based on the alkaline-absorption method. Therefore, CQTFR is suggested to be adopted for calibrating PAN analyzers to reduce calibration uncertainties.

HO2 + NO2: Kinetics, Thermochemistry, and Evidence for a Bimolecular Product Channel

A master equation (ME) analysis of available experimental data has been carried out on the reaction HO₂ + NO₂ + M ⇋ HO₂NO₂ + M (1a)/(-1a). The analysis, based on the ME code MESMER, uses both the association and dissociation kinetic data from the literature, and provides improved thermochemistry on reaction 1a. Our preferred model assigns two low-frequency vibrations of HO₂NO₂ as hindered rotors and couples these to the external rotations. This model gives Δ_rH°₀(1a) = -93.9 ± 1.0 kJ mol^-1, which implies that Δ_fH°₀ HO₂NO₂ = -42.0 ± 1.0 kJ mol^-1 (uncertainties are 2σ). A significant contributor to the uncertainty derives from modeling the interaction between the internal and external rotors. Using this improved kinetics for reaction 1a/-1a, data at elevated temperatures, 353-423 K, which show no evidence of the expected equilibration, have been reanalyzed, indicating that an additional reaction is occurring that masks the equilibration. Based on a published ab initio study, this additional channel is assigned to the bimolecular reaction HO₂ + NO₂ → H-NO₂ + O₂ (1b); H-NO₂ is nitryl hydride and has not previously been directly observed in experiments. The output of the master equation analysis has been parametrized and Troe expressions are provided for an improved description of k_1a(p,T) and k_-1a(p,T).

Short-term association of NO2 with hospital visits for chronic kidney disease and effect modification by temperature in Hefei, China: A time series study

BACKGROUND

A large body of evidence has linked air pollution and temperature with chronic kidney disease (CKD) prevalence and hospitalizations. However, most studies have focused on the influence of heat stress on CKD prevalence, and the potential effect modification of temperature on the association between air pollution and CKD has not been well-investigated. In this study, we examined the associations of the whole temperature spectrum and air pollution with CKD-related hospital visits and explored whether temperature modifies the short-term association of air pollution with CKD-related hospital visits.

METHODS AND FINDINGS

We collected 40 276 CKD-related hospital visits from the first Affiliated Hospital of Anhui Medical University and Anhui Provincial Hospital in Hefei, China, during 2015-2019. A two-stage time-series design was conducted to investigate the associations of air pollution and daily mean temperature with CKD-related hospital visits. First, we estimated the associations between air pollution and CKD-related hospital visits as well as temperature and CKD-related hospital visits. Second, we analyzed the associations of air pollution with CKD hospital visits at different temperatures. We found that NO₂ exposure and low temperature were associated with an increased risk of CKD-related hospital visits. Low temperature enhanced the association between NO₂ exposure and CKD-related hospital visits, with an increase of 4.30% (95% CI: 2.47-5.92%) per 10 μg/m³ increment in NO₂ at low temperature. Effect modification of the association between NO₂ and the risk of CKD-related hospital visits was stronger at low temperature across the whole population.

CONCLUSIONS

Our findings indicate that low temperature-related chronic kidney damage should be of immediate public health concern. Impact of NO₂ exposure on the risk of CKD-related hospital visits may increase under the low temperature, which suggests the need for NO₂ exposure mitigation strategies in the context of climate change and an enhanced understanding of the mechanisms underlying the temperature variance of air pollution effect to help reduce the magnitude of the CKD burden on the healthcare systems.

Global Master Equation Analysis of Rate Data for the Reaction C2H4 + H ⇄ C2H5: ΔfH0⊖C2H5

While forward and reverse rate constants are frequently used to determine enthalpies of reaction and formation, this process is more difficult for pressure-dependent association/dissociation reactions, especially since the forward and reverse reactions are usually studied at very different temperatures. The problems can be overcome by using a data-fitting procedure based on a master equation model. This approach has been applied to existing experimental pressure-dependent forward and reverse rate coefficients for the reaction C₂H₄ + H ⇄ C₂H₅ (k₁, k_-1) using the MESMER code to determine Δ_fH₀^⊖C₂H₅ from the enthalpy of the reaction. New measurements of k₁, k_-1 were included in analysis. They are based on laser flash photolysis with direct observation of H atom time profiles by vacuum ultraviolet laser-induced fluorescence under conditions where the approach to equilibrium could be observed. Measurements were made over the temperature range 798-828 K and with [He] from 2.33 to 7.21 × 10¹⁸ molecule cm^-3. These data were then combined with a wide range of existing experimental data with helium as the bath gas (112 measurements of k₁ and k_-1, covering the temperature range 285-1094 K, and [He] = 7.1 × 10¹⁵-1.9 × 10¹⁹ molecule cm^-3) and fitted using the master equation solver MESMER. The required vibrational frequencies and rotational constants of the system were obtained from ab initio calculations, and the activation threshold for association (ΔE_thresh), enthalpy of reaction (Δ_rH₀^⊖), imaginary frequency (υ_imag), and helium energy-transfer parameters (⟨ΔE⟩_d,298(T/298)ⁿ) were optimized. The resulting parameters (errors are 2σ) are ΔE_thresh = 11.43 ± 0.34 kJ mol^-1, Δ_rH₀^⊖ = -145.34 ± 0.60 kJ mol^-1, υ_imag = 730 ± 130 cm^-1, ⟨ΔE⟩_d,298 = 54.2 ± 7.6 cm^-1, and n = 1.17 ± 0.12. A value of Δ_fH_298.15^⊖(C₂H₅) = 120.49 ± 0.57 kJ mol^-1 is obtained by combining Δ_rH₀^⊖ with standard enthalpies of formation for H and C₂H₄ and making the appropriate temperature corrections. The dependence of these parameters on how the internal rotor and CH₂ inversion modes are treated has been explored. The experimental data for other bath gases have been analyzed, and data sets compatible with the potential energy surface parameters determined above have been identified. The parameters are virtually identical but with slightly smaller error limits. Parameterization of k₁, k_-1 using the Troe formalization has been used to investigate competition between ethyl decomposition and reaction with oxygen under combustion conditions.

Effect of changes in season and temperature on cardiovascular mortality associated with nitrogen dioxide air pollution in Shenzhen, China

BACKGROUND

The intricate association of mortality risk with ambient air pollution and temperature is of growing concern. Little is known regarding effect of changes in season and temperature on daily cardiovascular mortality associated with air pollutant nitrogen dioxide (NO₂).

OBJECTIVES

Our study aimed to assess the effect of NO₂ on cardiovascular mortality modified by season and daily air temperature in the effect, and further to identify the population highly susceptible to cardiovascular mortality associated with NO₂ and air temperature.

METHODS

We collected daily cause-specific death data, weather conditions, and air pollutant concentrations in Shenzhen from 2013 to 2017. Distributed-lag linear models were employed to analyze the effect of season on the NO₂-associated mortality. Furthermore, generalized additive models were combined with stratification parametric analysis to estimate the interaction effect of NO₂ with air temperature on cardiovascular mortality.

RESULTS

In the cold season, the percentage increase in daily mortality for every 10 μg/m³ increment in NO₂ concentration over lags of 0-2 days was 4.45% (95% CI: 2.71-6.21%). However, no statistically significant effect of NO₂ was observed in the warm season. Compared with high-temperature days (>median temperature), a 3.51% increase in mortality (95% CI: 2.04-5.01%) over low-temperature days (≤median temperature) for the same increase in NO₂ was significant. Air temperature modified the effect of NO₂ on daily mortality by 4.08% (95% CI: 2.28-5.91%) for the elderly (age ≥ 65 years) on low-temperature days vs. -0.82% (95% CI: -3.88-2.34%) on high-temperature days, and 3.38% (95% CI: 1.50-5.29%) for males on low-temperature days vs. -0.73% (95% CI: -3.83-2.47%) on high air temperature days.

CONCLUSIONS

The cold season and low temperatures could significantly enhance the effect of NO₂ on cardiovascular mortality. The elderly and males suffering from cardiovascular disease should take precautions against low temperature and NO₂ air pollution.

An Experimental and Master Equation Study of the Kinetics of OH/OD + SO2: The Limiting High-Pressure Rate Coefficients

The kinetics of the reaction OH/OD + SO₂ were studied using a laser flash photolysis/laser-induced fluorescence technique. Evidence for two-photon photolysis of SO₂ at 248 nm is presented and quantified, and which appears to have been evident to some extent in most previous photolysis studies, potentially leading to values for the rate coefficient k₁ that are too large. The kinetics of the reaction OH(v = 0) + SO₂ (T = 295 K, p = 25-300 torr) were measured under conditions where SO₂ photolysis was taken into account. These results, together with literature data, were modeled using a master equation analysis. This analysis highlighted problems with the literature data: the rate coefficients derived from flash photolysis data were generally too high and from the flow tube data too low. Our best estimate of the high-pressure limiting rate coefficient k₁^∞ was obtained from selected data and gives a value of (7.8 ± 2.2) × 10^-13 cm³ molecule^-1 s^-1, which is lower than that recommended in the literature. A parametrized form of k₁([N₂],T) is provided. The OD(v = 0) + SO₂ (T = 295 K, p = 25-300 torr) data are reported for the first time, and master equation analysis reinforces our assignment of k₁^∞.