Night-time atmospheric loss process for unsaturated aldehydes: Reaction with NO3 radicals

The rate constants for the gas-phase reactions of the NO3 radical with a se ries of unsaturated aldehydes (acrolein, crotonaldehyde, trans-2-pentenal, trans-2-hexenal, trans-2-heptenal, and cis-4-heptenal) have been. measured directly using a flow tube system coupled to a laser-induced fluorescence ( LIF) detection system where the NO3 radical was monitored. The kinetic stud y was conducted in the temperature range from 298 to 433 K to investigate t he temperature dependence of these reactions. This work is the first temper ature-dependence study for the reactions of the nitrate radical with the ab ove-mentioned aldehydes. The measured room-temperature rate constants for t he reaction of NO3 with such unsaturated compounds (in units of 10(-14) cm( 3) molecule(-1) s(-1)) are as follows: acrolein, 0.25 +/- 0.04; crotonaldeh yde, 1.61 +/- 0.19; trans-2-pentenal, 2.88 +/- 0.29; trans-2-hexenal, 5.49 +/- 0.95; trans-2-heptenal, 9.59 +/- 0.19; cis-4-heptenal, 26.40 +/- 0.40. The proposed Arrhenius expressions for such reactions of NO3 are, respectiv ely, k(1) = (1.7 +/- 3.2) x 10(-11) exp[-(3232 +/- 355)/T] (cm(3) molecule( -1) s(-1)), k(2) = (5.52 +/- 0.82) x 10(-11) exp[-(2418 +/- 57)/T] (cm(3) m olecule(-1) s(-1)), k(3) = (5.4 +/- 0.3) x 10(-12) exp[-(1540 +/- 200)/T] ( cm(3) molecule(-1) s(-1)), k(4) = (1.20 +/- 0.3) x 10(-11) exp[-(926 +/- 85 )/T] (cm(3) molecule(-1) s(-1)), k(5) = (0.8 +/- 0.2) x 10(-12) exp[-(632 /- 47)/T] (cm(3) molecule(-1) s(-1)), and k(6) = (0.2 +/- 0.1) x 10(-11) ex p[-(657 +/- 6.0)/T] (cm(3) molecule(-1) s(-1)). Tropospheric lifetimes for these aldehydes have been calculated at night and during the daytime for ty pical NO3 and OH concentrations showing that both radicals provide an effec tive tropospheric sink for these compounds and that the nighttime reaction with NO3 radical can be an important loss process for these emitted organic s and for the NO3 radicals. The present work aims to evaluate the importanc e of these reactions in the atmosphere and to contribute new data to the st udy of NO3 reactivity.