A new gas-phase chemical mechanism for the modeling of regional atmospheric chemistry, the “Regional Atmospheric Chemistry Mechanism” (RACM) is presented. The mechanism is intended to be valid for remote to polluted conditions and from the Earth's surface through the upper troposphere. The RACM mechanism is based upon the earlier Regional Acid Deposition Model, version 2 (RADM2) mechanism [Stockwell et al., 1990] and the more detailed Euro-RADM mechanism [Stockwell and Kley, 1994]. The RACM mechanism includes rate constants and product yields from the most recent laboratory measurements, and it has been tested against environmental chamber data. A new condensed reaction mechanism is included for biogenic compounds: isoprene, α-pinene, and d-limonene. The branching ratios for alkane decay were reevaluated, and in the revised mechanism the aldehyde to ketone ratios were significantly reduced. The relatively large amounts of nitrates resulting from the reactions of unbranched alkenes with NO3 are now included, and the production of HO from the ozonolysis of alkenes has a much greater yield. The aromatic chemistry has been revised through the use of new laboratory data. The yield of cresol production from aromatics was reduced, while the reactions of HO, NO3, and O3 with unsaturated dicarbonyl species and unsaturated peroxynitrate are now included in the RACM mechanism. The peroxyacetyl nitrate chemistry and the organic peroxy radical-peroxy radical reactions were revised, and organic peroxy radical +NO3 reactions were added.

/pdf/a-new-mechanism-for-regional-atmospheric-chemistry-modeling-1xgvkhtnkg.pdf

A new mechanism for regional atmospheric chemistry modeling

The nitrate radical: Physics, chemistry, and the atmosphere

Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NOx), ozone (O3), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O3, the nitrate radical (NO3), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HOx and NOx free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a sim...

Gas-Phase Reactions of Isoprene and Its Major Oxidation Products.

. Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry–climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

/pdf/nitrate-radicals-and-biogenic-volatile-organic-compounds-48l9l9gpi9.pdf

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Detailed theoretical (Part I, this article) and experimental (Part II) investigations are presented for the mechanism of the atmospheric photooxidation of dimethyl sulfide (CH3SCH3) and dimethyl disulfide (CH3SSCH3). In this paper, comprehensive mechanisms for the atmospheric chemistry of CH3SCH3 and CH3SSCH3 are developed based on fundamental considerations of all available kinetic and mechanistic information.

Photooxidation of dimethyl sulfide and dimethyl disulfide. I: Mechanism development

The kinetics and mechanism of the reaction NO3+CH2=C(CH3)-CH=CH2→products were studied in two laboratories at 298 K in the pressure range 0.7-3 torr using the discharge-flow mass-spectrometric method. The rate constant obtained under pseudo-first-order conditions with excess of either NO3 or isoprene was: k1=(7.8±0.6)×10-13 cm3 molecule-1 s-1. The product analysis indicated that the primary addition of NO3 occurred on both π-bonds of the isprene molecule. © 1991 Kluwer Academic Publishers.

A discharge flow mass-spectrometric study of the reaction between the NO3 radical and isoprene

The reactions of NO3 with H (1), OH (2) and HO2 (3) radicals have been investigated at 298 K using a discharge flow system equipped with mass spectrometric and fluorescence detectors. The rate constants k(1) and k(2) were determined by numerical modeling of experimental [OH] profiles from reactions (1) and (2). In the reaction of NO3 with HO2 (3) the growth and decay of [OH] as well as the decay of [HO2] were measured. Comparison of these two profiles provided evidence for two different reaction paths (3a) and (3b), for which the rate constants were determined by numerical modeling. Experiments carried out in a wide range of reactant concentrations yielded in units of cm3 molecule−1 s−1 k(1) = (0.94 ± 0.2) · 10−10, k(2) = (1.22 ±0.35) · 10−11, k(3a) = (2.5 ± 0.7) · 10−12, and k(3b) = (1.9 ± 0.8) · 10−12. The results are compared with literature data.

Determination of the Rate Constants for the Gas Phase Reactions of NO3 with H, OH and HO2 Radicals at 298 K

The reaction of HNO3 with atomic fluorine (1) has been studied in a discharge-flow system at 298 K. Mass spectrometric detection with single ion counting was employed. The rate constant for the primary reaction (1) at 298 K. has been determined from the rate of HF and NO, formation under conditions [HNO3] ≫ [F]0 and the rate of HNO3-removal under conditions [F] ≫ [HNO3]0










From the patterns of growth and decay of the primary product NO3 and the secondary product FO under various ratios of the initial concentrations of the reactants, the rate constants of the following secondary reactions have been determined:










Further, the rate constants for the reactions of NO3 with CH3SH, 2-methylpropene, 1,3-butadiene and 2,3-dimethyl-2-butene at 298 K have been determined by introducing these reactants into the flow system and measuring their consumption under pseudo-first-order conditions. The values of the rate constants obtained are 7.7 ± 0.5, 3.3 ± 0.5, 1.7 ± 0.3 and 384 ± 38 in units of 10−13 cm5 s−1 respectively. A comparison is made between the values determined in this work and those reported in the literature.

A Gasphase Kinetic Investigation of the System F + HNO3 and the Determination of Absolute Rate Constants for the Reaction of the NO3 Radical with CH3SH, 2‐Methylpropene, 1,3‐Butadiene and 2,3‐Dimethyl‐2‐Butene

The kinetics of the reaction of NO3 radicals with five alkenes was investigated at room temperature in a discharge fast flow system with mass spectrometric detection. NO3 radicals were produced by hydrogen abstraction from HNO3 with F atoms. The 1:1 stoichiometry of the reactions was confirmed. Rate constants were determined by measuring the decay of NO3 and/or of alkene under pseudo-first-order conditions. The following values of rate constants are reported: trans-2-butene (3.9 ± 0.3)·10−13, cis-2-butene (3.8±0.2)·10−13, iso-butene (3.9±0.4)·10−13, 2-methyl-2-butene (8.4±0.6)·10−12, 2,3-dimethyl-2-butene (4.1±0.4)·10−11 in units of cm3 molecule−1 s−1. The results are compared with literature data.

The Determination of Rate Constants for the Reactions of Some Alkenes with the NO3Radical

The reaction (1) Cl + NO3 ClO + NO2 was studied at 298 K by observing the kinetic behaviour of NO3, Cl and ClO using discharge-flow mass-spectrometric technique. A rate constant k(1) = (2.26 ± 0.17) · 10–11 cm3 molecule−1 s−1 was obtained. The secondary processes between NO3 and ClO were also investigated and OClO was detected as a reaction product. For the rate constant of the reaction (2a) ClO + NO3 ClOO + NO2 an upper limit of k(2a) = 1 · 10−13 cm3 molecule−1 s−1 was found. In contrast to previous findings it is shown that reaction (2b) ClO + NO3 OClO + NO2 is the predominant channel with k(2b) = (4.3 ± 1.0) · 10−13 cm3 molecule−1 s−1.

An Investigation of the Reactions of NO3 Radicals with Cl and ClO

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