Based on a small test system, (R)-CH(OH)(OO·)CHCHO, we have developed a cost-effective approach to the practical implementation of multiconformer transition state theory for peroxy radical hydrogen shift reactions at atmospherically relevant temperatures. While conformer searching is crucial for accurate reaction rates, an energy cutoff can be used to significantly reduce the computational cost with little loss of accuracy. For the reaction barrier, high-level calculations are needed, but the highest level of electronic structure theory is not necessary for the relative energy between conformers. Improving the approach to both transition state theory and electronic structure theory decreases the calculated reaction rate significantly, so low-level calculations can be used to rule out slow reactions. Further computational time can be saved by approximating the tunneling coefficients for each transition state by only that of the lowest-energy transition state. Finally, we test and validate our approach using higher-level theoretical values for our test system and existing experimental results for additional peroxy radical hydrogen shift reactions in three slightly larger systems.

Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary aerosols that affect Earth’s radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH2SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate.

S1. Peak identification of α-pinene and β-pinene hydroxy nitrates 26 The structural assignment of α-pinene and β-pinene hydroxy nitrate isomers is achieved by the 27 collection of several chromatograms. Firstly, the ring-opened HNs (i.e., 3-OH,8-ONO2 for α-28 pinene and 1-OH,8-ONO2 for β-pinene) are identified by adding O3 to the chamber after photooxidation. The ring-opened HNs are unsaturated and thus react with O3 while the ring-30 retained HN isomers are saturated and will not do so. After the photooxidation ceases, we remove 31 the chamber content through a cold trap (i.e., 6.5 m ¼ inch PFA tubing submerged in-60°C isopropanol + liquid nitrogen bath). The oxidation products are trapped, but volatile compounds including precursor hydrocarbon, NO, NO2 are not. Then, we remove the tubing from the trap and 34 use a flow of zero air to send the trapped analytes back to chamber. 2-4 ppmv O3 is added to chamber using an O3 generator. We also inject approximately 90 ppmv cyclohexane, which serves 36 as an OH scavenger. As shown in Figure S 1, the last peaks for both monoterpenes disappeared 37 after O3 addition, indicating that they are the ring-opened HNs. The HNs with the-ONO2 group on the less-substituted carbon (2-OH,3-ONO2 for α-pinene 39 and 2-OH,1-ONO2 for β-pinene) are identified from NO3 radical oxidation experiments. This 40 approach is based on the assumption that NO3 radicals react with alkenes by primarily adding to the less-substituted olefinic carbon 1. Oxygen adds to the alkyl radical and these RO2 react with other RO2 to produce hydroxy nitrate (Scheme S 3). We perform NO3 oxidation experiments by 43 mixing ~200ppb NO and ~300ppb O3 in an 800 L chamber, waiting for 1 hr to produce NO3 and N2O5, and injecting about 80 ppbv α-pinene or β-pinene. The chromatograms of HNs from NO3 oxidation experiments are shown in Figure S 2. For both α-pinene and β-pinene, only one peak is resolved from NO3 oxidation experiments and the retention time of this peak matches that of the first in OH oxidation experiments. The remaining peak in the OH oxidation is identified based on previous finding that the retention order for HNs with similar structures is generally tertiary-OH, secondary-OH, and then primary-OH for the same GC column 2. This observation has a plausible rational as the primary 51-OH likely has stronger interaction with GC column than secondary or tertiary OH due to less 52 shielding effects. Thus, the second peak in the α-pinene chromatogram is assigned to 3-OH,2-53 ONO2, which has a secondary-OH and elutes later than 2-OH,3-ONO2 with a tertiary-OH. Similarly, the second peak in the β-pinene chromatogram is assigned to 1-OH,2-ONO2. We further S4 verify the structural assignment of HN peaks by comparing the chromatograms between α-pinene 56 and β-pinene using the same GC temperature profile (Figure S 3). α-pinene 3-OH,2-ONO2 elutes between β-pinene 2-OH,1-ONO2 and 1-OH,2-ONO2, which is consistent with the rule of thumb 58 described above. The ring-opened HNs elute later than ring-retained HNs because the...

With an annual emission of about 500 Tg, isoprene is an important molecule in the atmosphere. While much of its chemistry is well constrained by either experiment or theory, the rates of many of the unimolecular peroxy radical hydrogen-shift (H-shift) reactions remain speculative. Using a high-level multiconformer transition state theory (MC-TST) approach, we determine recommended temperature dependent reaction rate coefficients for a number of the H-shift reactions in the isoprene oxidation mechanism. We find that most of the (1,4, 1,5, and 1,6) aldehydic and (1,5 and 1,6) α-hydroxy H-shifts have rate constants at 298.15 K in the range 10 −2 to 1 s −1 , which make them competitive with bimolecular reactions in the atmosphere under typical atmospheric conditions. In addition, we find that the rate coefficients of different diastereomers can differ by up to 3 orders of magnitude, illustrating the importance of chirality. Implementation of our calculated reaction rate coefficients into the most recent GEOS-Chem model for isoprene oxidation shows that at least 30% of all isoprene molecules emitted to the atmosphere undergo a minimum of one peroxy radical hydrogen-shift reaction during their complete oxidation to CO 2 and deposited species. This highlights the importance of peroxy radical H-shifts reactions in atmospheric oxidation.

Oxidation of monoterpenes (CH) by nitrate radicals (NO) constitutes an important source of atmospheric secondary organic aerosol (SOA) and organonitrates. However, knowledge of the mechanisms of their formation is incomplete and differences in yields between similar monoterpenes are poorly understood. In particular, yields of SOA and organonitrates from α-pinene + NO are low, while those from Δ-carene + NO are high. Using computational methods, we suggest that bond scission of the nitrooxy alkoxy radicals from Δ-carene lead to the formation of reactive keto-nitrooxy-alkyl radicals, which retain the nitrooxy moiety and can undergo further reactions to form SOA. By contrast, bond scissions of the nitrooxy alkoxy radicals from α-pinene lead almost exclusively to the formation of the relatively unreactive and volatile product pinonaldehyde (CHO), thereby limiting organonitrate and SOA formation. This hypothesis is supported by laboratory experiments that quantify products of the reaction of α-pinene + NO under atmospherically relevant conditions.