Rate constants for the reactions of the NO3 radical with HCOOH/HCOO? and CH3COOH/CH3COO? in aqueous solution between 278 and 328 K

Abstract

The kinetics of the aqueous phase reactions of NO3 radicals with HCOOH/HCOO− and CH3COOH/CH3COO− have been investigated using a laser photolysis/long-path laser absorption technique. NO3 was produced via excimer laser photolysis of peroxodisulfate anions (S2O 8 2− ) at 351 nm followed by the reactions of sulfate radicals (SO 4 − ) with excess nitrate. The time-resolved detection of NO3 was achieved by long-path laser absorption at 632.8 nm. For the reactions of NO3 with formic acid (1) and formate (2) rate coefficients ofk 1=(3.3±1.0)×105 l mol−1 s−1 andk 2=(5.0±0.4)×107 l mol−1 s−1 were found atT=298 K andI=0.19 mol/l. The following Arrhenius expressions were derived:k 1(T)=(3.4±0.3)×1010 exp[−(3400±600)/T] l mol−1 s−1 andk 2(T)=(8.2±0.8)×1010 exp[−(2200±700)/T] l mol−1 s−1. The rate coefficients for the reactions of NO3 with acetic acid (3) and acetate (4) atT=298 K andI=0.19 mol/l were determined as:k 3=(1.3±0.3)×104 l mol−1 s−1 andk 4=(2.3±0.4)×106 l mol−1 s−1. The temperature dependences for these reactions are described by:k 3(T)=(4.9±0.5)×109 exp[−(3800±700)/T] l mol−1 s−1 andk 4(T)=(1.0±0.2)×1012 exp[−(3800±1200)/T] l mol−1 s−1. The differences in reactivity of the anions HCOO− and CH3COO− compared to their corresponding acids HCOOH and CH3COOH are explained by the higher reactivity of NO3 in charge transfer processes compared to H atom abstraction. From a comparison of NO3 reactions with various droplets constituents it is concluded that the reaction of NO3 with HCOO− may present a dominant loss reaction of NO3 in atmospheric droplets.