Identification and Yields of 1,4-Hydroxynitrates Formed from the Reactions of C8–C16 n-Alkanes with OH Radicals in the Presence of NOx

Comprehensive Analysis of Products and the Development of a Quantitative Mechanism for the OH Radical-Initiated Oxidation of 1-Alkenes in the Presence of NOx

The reactions of 1-tetradecene and 1-pentadecene, the C₁₄ and C₁₅ linear 1-alkenes, with OH radicals in the presence of NO_x were investigated in a series of environmental chamber experiments. Particle-phase β-hydroxynitrates, dihydroxynitrates, dihydroxycarbonyls, and 1,4-hydroxynitrates and gas-phase aldehydes were sampled and then identified and quantified using a suite of offline analytical techniques that included derivatization, gas and liquid chromatography, and multiple types of mass spectrometry. Measured molar yields of products formed by OH radical addition to the C═C double bond, including β-hydroxynitrates, dihydroxynitrates, dihydroxycarbonyls (which have not been previously directly quantified with high accuracy), and aldehydes were 0.125 ± 0.01, 0.048 ± 0.005, 0.240 ± 0.04, and 0.268 ± 0.03 (0.264 ± 0.02 and 0.271 ± 0.04 for the formaldehyde and tridecanal/tetradecanal co-products of β-hydroxyalkoxy radical decomposition), respectively. These values give a total molar yield of 0.681 ± 0.05, which agrees very well with the results of kinetics measurements that indicate that the fraction of reaction that occurs by OH radical addition is 0.70. The yields were used to calculate branching ratios for all OH radical addition pathways, including a value of 0.18 for the formation of dihydroxynitrates from the reaction of dihydroxyperoxy radicals with NO and values of 0.47 and 0.53 for β-hydroxyalkoxy radical decomposition and isomerization. The results were used with literature data on the yields of aldehydes measured for similar reactions of smaller alkenes, a model for the effect of carbon number on branching ratios for organic nitrate formation, and a mechanism for H atom abstraction derived from studies of linear alkanes to achieve a complete, quantitative gas-phase reaction mechanism for 1-alkenes. The results should also be useful for constructing mechanisms for more complex reactions of volatile organic compounds.

Kinetic Study of the Temperature Dependence of OH-Initiated Oxidation of n-Dodecane

Reaction rate constants for the reaction of n-dodecane with hydroxyl radicals were measured as a function of temperature between 283 and 303 K, using the relative rate method in the CESAM chamber (French acronym for "experimental multiphasic atmospheric simulation chamber"). The rate constants obtained at 283, 293, and 303 K are (1.27 ± 0.31) × 10^-11, (1.33 ± 0.34) × 10^-11, and (1.27 ± 0.40) × 10^-11 cm³ molecule^-1 s^-1, respectively. Rate constants measured were in excellent agreement with the few available data in the literature over the studied temperature range (283-340 K). Rate constants estimated by the structure-activity relationship and transition state theory methods agreed with our experimental data within 14%. From these data combined with previous literature measurement, the following Arrhenius expression, k_DDC+OH = (9.77 ± 6.19) × 10^-11 × exp[(-595 ± 5580)/T] cm³ molecule^-1 s^-1, was found to be valid over a temperature range (283-340 K) of the tropospheric interest.

Identification and Quantitation of Aerosol Products of the Reaction of β-Pinene with NO3 Radicals and Implications for Gas- and Particle-Phase Reaction Mechanisms

Substantial amounts of gas- and particle-phase organic nitrates have been reported in field studies of atmospheric chemistry conducted around the world, and it has been proposed that a significant fraction of these may be formed from the nighttime reaction of monoterpenes with NO₃ radicals. In the study presented here, β-pinene (a major global monoterpene emission) was reacted with NO₃ radicals in an environmental chamber and the molecular and functional group composition of the resulting secondary organic aerosol (SOA) was determined using a variety of methods. Eight products, which comprised ∼95% of the SOA mass, were identified and quantified. More than 90% (by mass) of these consisted of acetal heterodimers and heterotrimers that were apparently formed through acid-catalyzed reactions in phase-separated particles. The molar yield of the major oligomer was 16.7%, and the yields of the other six and the single monomer ranged from 1.1% to 2.9%, for a total yield of 30.7%. From these analyses it was determined that the yields of the two major monomer building blocks were 25.9% and 23.6%, and that those of the other four ranged from 2.0% to 4.8%, for a total monomer yield of 62.4%. The measured SOA mass yield was 88.9% and the O/C, N/C, and H/C ratios, molecular weight, and density of the SOA calculated from the results of functional group analysis of the bulk SOA were 0.40, 0.11, 1.79, 217 g mol^-1, and 1.21 g cm^-3, respectively, similar to values estimated from results of molecular analysis. The results demonstrate the combined importance of RO₂^• + RO₂^• reactions, alkoxy radical decomposition and isomerization, and acid-catalyzed particle-phase reactions in the NO₃ radical-initiated oxidation of β-pinene and subsequent formation of SOA and should be useful for understanding reactions of other monoterpenes and for developing models for the laboratory and atmosphere.

Kinetics Study of the Reactions of OH with n-Undecane and n-Dodecane Using the RR/DF/MS Technique

The kinetics of the reactions of hydroxyl radical with n-undecane (n-C₁₁H₂₄) and n-dodecane (n-C₁₂H₂₆) has been studied at 240-340 K and a total pressure of 1 Torr using the relative rate/discharge flow/mass spectrometry (RR/DF/MS) technique. The rate constants at 298 K for these reactions were determined to be k_{n-undecane+OH} = (1.59 ± 0.24) × 10^-11 cm³ molecule^-1 s^-1 and k_{n-dodecane+OH} = (1.83 ± 0.26) × 10^-11 cm³ molecule^-1 s^-1, respectively. The rate constants of these reactions were found to positively dependent on temperature at 277-340 K, and negatively dependent on temperature at 240-277 K. The atmospheric lifetime of these compounds are estimated to be 25.8 and 19.8 h for n-undecane and n-dodecane, respectively, based the kinetics results in the present study.

Formation of Low Volatility Organic Compounds and Secondary Organic Aerosol from Isoprene Hydroxyhydroperoxide Low-NO Oxidation

Gas-phase low volatility organic compounds (LVOC), produced from oxidation of isoprene 4-hydroxy-3-hydroperoxide (4,3-ISOPOOH) under low-NO conditions, were observed during the FIXCIT chamber study. Decreases in LVOC directly correspond to appearance and growth in secondary organic aerosol (SOA) of consistent elemental composition, indicating that LVOC condense (at OA below 1 μg m(-3)). This represents the first simultaneous measurement of condensing low volatility species from isoprene oxidation in both the gas and particle phases. The SOA formation in this study is separate from previously described isoprene epoxydiol (IEPOX) uptake. Assigning all condensing LVOC signals to 4,3-ISOPOOH oxidation in the chamber study implies a wall-loss corrected non-IEPOX SOA mass yield of ∼4%. By contrast to monoterpene oxidation, in which extremely low volatility VOC (ELVOC) constitute the organic aerosol, in the isoprene system LVOC with saturation concentrations from 10(-2) to 10 μg m(-3) are the main constituents. These LVOC may be important for the growth of nanoparticles in environments with low OA concentrations. LVOC observed in the chamber were also observed in the atmosphere during SOAS-2013 in the Southeastern United States, with the expected diurnal cycle. This previously uncharacterized aerosol formation pathway could account for ∼5.0 Tg yr(-1) of SOA production, or 3.3% of global SOA.