Absolute rate coefficients for the gas-phase reactions of NO3 radical witha series of monoterpenes at T=298 to 433 K

The aim of this work is to study the reactivity of some naturally emitted t erpenes, 2-carene, sabinene, myrcene, alpha-phellandrene, d-limonene, terpi nolene and gamma-terpinene, towards NO3 radical to evaluate the importance of these reactions in the atmosphere and their atmospheric impact. The experiments with these monoterpenes have been carried out under second- order kinetic conditions over the range of temperature 298-433 K, using a d ischarge flow system and monitoring the NO3 radical by Laser Induced Fluore scence (LIF). This work is the first temperature dependence study for the r eactions of the nitrate radical with the above-mentioned monoterpenes. The measured rate constants at 298 K for the reaction of NO3 with such terpenes are as follows: 2-carene, 16.6 +/- 1.8, sabinene 10.7 +/- 1.6, myrcene 12. 8 +/- 1.1, alpha-phellandrene 42 +/- 10, d-limonene 9.4 +/- 0.9, terpinolen e 52 +/- 9 and gamma-terpinene 24 +/- 7, in units of 10(-12) cm(3) molecule (-1) s(-1). The proposed Arrhenius expressions, for the reactions of NO3 wi th 2-carene, sabinene, myrcene and alpha-phellandrene are, respectively k(1 ) = (1.4 +/- 0.7) x 10(-12) exp[(741 +/- 190/T)] (cm(3) molecule(-1) s(-1)) , k(2)=(2.3 +/- 1.3) x 10(-10) exp[-(940 +/- 200/T)] (cm(3) molecule(-1) s( -1)), k(3) = (2.2 +/- 0.2) x 10(-12) exp[(523 +/- 35/T)] (cm(3) molecule(1) s(-1)) and k(4) = (1.9 +/- 1.3) x 10(-9) exp[-(1158 +/- 270/T)] (cm(3) mol ecule(-1) s(-1)). A decrease in the rate constants when raising the tempera ture has also been found for the reaction of d-limonene with NO3 while an i ncrease in the rate constant with temperature has been observed for the rea ctions of terpinolene and gamma-terpinene with NO3. Tropospheric half-lives for these terpenes have been calculated at night and during the day for ty pical NO3 and OH concentrations showing that both radicals provide an effec tive tropospheric sink for these compounds and that the night-time reaction with NO3 radical can be an important, if not dominant, loss process for th ese naturally emitted organics and for NO3 radicals.