Night-time Atmospheric Loss Process for Unsaturated Aldehydes: Reaction with NO3 Radicals

The rate constants for the gas-phase reactions of the NO 3 radical with a series 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 NO 3 radical was monitored. The kinetic study was conducted in the temperature range from 298 to 433 K to investigate the temperature dependence of these reactions. This work is the first temperature-dependence study for the reactions of the nitrate radical with the above-mentioned aldehydes. The measured room-temperature rate constants for the reaction of NO 3 with such unsaturated compounds (in units of 10 -14 cm 3 molecule -1 s -1 ) are as follows: acrolein, 0.25 ± 0.04; crotonaldehyde, 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 NO 3 are, respectively, k 1 = (1.7 ± 3.2) × 10 -11 exp[−(3232 ± 355)/ T ] (cm 3 molecule -1 s -1 ), k 2 = (5.52 ± 0.82) × 10 -11 exp[−(2418 ± 57)/ T ] (cm 3 molecule -1 s -1 ), k 3 = (5.4 ± 0.3) × 10 -12 exp[−(1540 ± 200)/ T ] (cm 3 molecule -1 s -1 ), k 4 = (1.20 ± 0.3) × 10 -11 exp[−(926 ± 85)/ T ] (cm 3 molecule -1 s -1 ), k 5 = (0.8 ± 0.2) × 10 -12 exp[−(632 ± 47)/ T ] (cm 3 molecule -1 s -1 ), and k 6 = (0.2 ± 0.1) × 10 -11 exp[−(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 typical NO 3 and OH concentrations showing that both radicals provide an effective tropospheric sink for these compounds and that the night-time reaction with NO 3 radical can be an important loss process for these emitted organics and for the NO 3 radicals. The present work aims to evaluate the importance of these reactions in the atmosphere and to contribute new data to the study of NO 3 reactivity.