Abstract. 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 ] 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, a-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 NO 3 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 03 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 + NO 3 reactions were added. IntroductionThe gas-phase chemical mechanism is one of the most important components of an atmospheric chemistry model. These models require high-quality gas-phase chemical mechanisms to calculate the concentrations of atmospheric chemical species. The concentrations of ozone and other air pollutants are determined by the emissions of nitrogen oxides and reactive organic species, gas-and aqueous-phase chemical reaction rates, deposition, and meteorological conditions. There are several important mechanisms which are widely used for modeling the chemistry of the troposphere including the mechanism of Lurmann et al. chemistry on a regional scale, we have named it the "Regional Atmospheric Chemistry Mechanism" (RACM). The RACM mechanism was created to be capable of simulating the troposphere from the Earth's surface through the upper troposphere and to be valid for simulating remote to polluted urban conditions.The mechanism includes 17 stable inorganic species, 4 inorganic intermediates, 32 stable organic species (4 of these are primarily of biogenic origin), and 24 organic intermediates (Table 1). The RACM mechanism includes 237 reactions ( Table 2). The mechanism and its use are described in the subsequent text. The text is divided into section 2 on the inorganic chemistry and section 3 on the organic chemistry. The organic chemistry section includes a description of the aggregation procedures for emissions and the development of the chemistry for alkanes, carbonyls, alkenes, aromatics; the decomposition...

Peroxynitrates are thermally unstable intermediates (at ambient temperatures) in the atmospheric degradation of hydrocarbons. In this work, thermal lifetimes of nine peroxynitrates have been measured as a function of temperature and, for two of them, also, as a function of total pressure. In the presence of excess NO, relative concentrations of the peroxynitrates were followed in a 420 I reaction chamber as a function of time by means of longpath IR absorption using a Fourier transform spectrometer. Original data on the unimolecular decomposition rate constants are presented for the peroxynitrates RO 2 NO 2 with R ϭ C 6 H 11 , CH 3 C(O)CH 2 , C 6 H 5 CH 2 , CH 2 I, CH 3 C(O)OC(H)CH 3 , C 6 H 5 OCH 2 , (CH 3 ) 2 NC(O), C 6 H 5 OC(O), and C 2 H 5 C(O). Thermal lifetimes at room temperature and atmospheric pressure are very short (in the order of seconds) for substituted methyl peroxynitrates (i.e., RЈCH 2 O 2 NO 2 ) but rather long for substituted formyl peroxynitrates (i.e., RЉC(O)O 2 NO 2 ). Kinetic data from this and previous work from our laboratory are used to derive structure-stability relationships which allow an estimate of the thermal lifetimes of peroxynitrates from readily available 13 C n.m.r. shift data.

No abstract

No abstract

The reactions of organic peroxy radicals with NO, HO2, organic peroxy radicals, and NO3 along with the formation and decomposition of peroxyacetyl nitrate are all important for the modeling of atmospheric chemistry. Recent laboratory measurements of the rate constants for peroxy radical-peroxy radical reactions show that there are large differences between these rate constants. We present methods of estimating organic peroxy radical self-reaction rate constants from an empirical expression. These selfreaction rate constants may be used to estimate the rates of peroxy radical cross reactions. There are also new data available on the rate constants for peroxyacetyl nitrate formation, decomposition, the reaction of acetyl peroxy radicals and other organic peroxy radicals with NO, and the reactions of NO3 with organic peroxy radicals. To estimate the importance of these reactions, these new data along with revisions to the product yields for organic peroxy radical-organic peroxy radical re actions were implemented in the mechanism of Stockwell et al. [19901. The revised mechanism yields significantly different concentrations of peroxyacetyl nitrate, higher organic hydroperoxides and peroxyacetic acid concentrations. Nighttime concentrations of organic peroxy radicals, HO2, HO, and NO3 are also strongly affected by organic peroxy radical-organic peroxy radical reactions and the reactions of organic peroxy radicals with NO3. Our results suggest that the reactions of organic peroxy radicals with NO3 are more important than organic peroxy radical-organic peroxy radical reactions in the nighttime atmosphere