RATE CONSTANTS FOR THE REACTIONS OF THE NO3 RADICAL WITH HCOOH/HCOO- AND CH3COOH/CH3COO- IN AQUEOUS-SOLUTION BETWEEN 278-K AND 328-K

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 (S2O8(2-)) at 351 nm followed by the reactions of sulfate radicals (SO4-) 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 of k1 = (3.3 +/- 1.0) x 10(5) l mol-1 s-1 and k2 = (5.0 +/- 0.4) x 10(7) l mol-1 s-1 were found at T = 298 K and I = 0.19 mol/l. The following Arrhenius expressions were derived: k1(T) = (3.4 +/- 0.3) x 10(10) exp[-(3400 +/- 600)/T] l mol-1 s-1 and k2(T) = (8.2 +/- 0.8) x 10(10) exp[-(2200 +/- 700)/T] l mol-1 s-1. The rate coefficients for the reactions of NO3 with acetic acid (3) and acetate (4) at T = 298 K and I = 0.19 mol/l were determined as: k3 = (1.3 +/- 0.3) x 10(4) l mol-1 s-1 and k4 = (2.3 +/- 0.4) x 10(6) l mol-1 s-1. The temperature dependences for these reactions are described by: k3(T) = (4.9 +/- 0.5) x 10(9) exp[-(3800 +/- 700)/T] l mol-1 s-1 and k4(T) = (1.0 +/- 0.2) x 10(12) 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.