Reactive quenching of electronically excited NO<sub>2</sub><sup>∗</sup> and NO<sub>3</sub><sup>∗</sup> by H<sub>2</sub>O as potential sources of atmospheric HO<sub><i>x</i></sub> radicals

Abstract

Abstract. Pulsed laser excitation of NO2 (532–647 nm) or NO3 (623–662 nm) in the presence of H2O was used to initiate the gas-phase reaction NO2∗+H2O → products (Reaction R5) and NO3∗+H2O → products (Reaction R12). No evidence for OH production in Reactions (R5) or (R12) was observed and upper limits for OH production of k5b/k5<1×10-5 and k12b/k12<0.03 were assigned. The upper limit for k5b∕k5 renders this reaction insignificant as a source of OH in the atmosphere and extends the studies (Crowley and Carl, 1997; Carr et al., 2009; Amedro et al., 2011) which demonstrate that the previously reported large OH yield by Li et al. (2008) was erroneous. The upper limit obtained for k12b∕k12 indicates that non-reactive energy transfer is the dominant mechanism for Reaction (R12), though generation of small but significant amounts of atmospheric HOx and HONO cannot be ruled out. In the course of this work, rate coefficients for overall removal of NO3∗ by N2 (Reaction R10) and by H2O (Reaction R12) were determined: k10=(2.1±0.1)×10-11 cm3 molecule−1 s−1 and k12=(1.6±0.3)×10-10 cm3 molecule−1 s−1. Our value of k12 is more than a factor of 4 smaller than the single previously reported value.

Theoretical Investigation of the Photoexcited NO2+H2O reaction at the Air–Water Interface and Its Atmospheric Implications

“…Atmospheric concentrations of NO 2 are very variable but in polluted urban areasa nnualm ean concentrations can reach severalt ens of ppb (1 ppb = 2.5 10 10 molecule cm À3 ), and even several hundreds of ppb in 1hourp eaks. In agreement with Dillon et al, [10] we predict an OH productionr ate in air significantly smaller than the ozone reference (we assumea 20 %r elative humidity), and only at [NO 2 ]c oncentrations higher than 100 ppb v OH would attain that level. The main result of our work, however,i st hat the effect of adsorption at the interface produces an enhancement of v OH by about two orders of magnitude,r oughlyafactor 3.1 10 2 .T hus, the productionrate of OH on water surfaces from NO 2 photochemistry attains ozone reference values for NO 2 concentrationsg reater than af ew ppb, corresponding to slightly polluted rural areas (the precise NO 2 concentration depends on the value taken for k 5b but is comprised between 0.4 and 13.8 ppb).…”

“…Calculations in the gas phase (light grey) and air-water interface(dark grey) using k 5b from reference. [10] The plain and dashed red lines correspond, respectively, to the gas phase and interface values using k 5b from Li et al [7] Gas-phase valuesa ssume ar elative humidity of 20 %. The horizontal plainline represents atypical OH production rate from ozonep hotolysis.…”

“…Somew orks [8] claimed that this finding could be an artifact due to multiphoton effects present in the experimentsb ut Li et al reported further experimental data contradicting this interpretation. [9] Very recently,n ew experiments by Dillon and Crowley [10] revisited the reaction in the wavelengths previously considered by Li et al [7] but avoiding possible complications due to multiphoton processes. The authors confirm the possibility for excited NO 2 * to react with water but they report small OH yields [10] k 5b /k 5 = 6 10 À6 -1.4 10 À4 ,f rom which one deduces k 5b % 10 À15 -3.4 10 À14 cm 3 molecule À1 ·s À1 ,t hat is one to two orders of magnitude smaller than the value of Li et al [7] (we assume k 5 % k 5a % 1.7 10 À10 cm 3 molecule À1 s À1 ).…”

“…[9] Very recently,n ew experiments by Dillon and Crowley [10] revisited the reaction in the wavelengths previously considered by Li et al [7] but avoiding possible complications due to multiphoton processes. The authors confirm the possibility for excited NO 2 * to react with water but they report small OH yields [10] k 5b /k 5 = 6 10 À6 -1.4 10 À4 ,f rom which one deduces k 5b % 10 À15 -3.4 10 À14 cm 3 molecule À1 ·s À1 ,t hat is one to two orders of magnitude smaller than the value of Li et al [7] (we assume k 5 % k 5a % 1.7 10 À10 cm 3 molecule À1 s À1 ). [6] The reaction mechanism of Equation (5 b) was theoretically studied by Minaev et al, [11] who described the process as hydrogen transfer from water to distorted NO 2 in the third 2 A'' electronic state, after formation of an intermediate complex that prevents vibronic relaxation to the ground state.…”

“…Using thermodynamic data reportedb yt hese authors, we have estimated ak inetic constant (including tunneling corrections) of k 5b % 8 10 À16 cm 3 molecule À1 s À1 ,w hichi sn ot far from the lower limit (10 À15 cm 3 molecule À1 s À1 ,s ee above)o ft he most recentm easurements reported by Dillon and Crowley. [10] Another theoretical study by Fang et al [12] suggested ad ifferent mechanism involving relaxation through conical intersections into ah igh vibrational level of the electronicg round state, which would permit the system to overcome the abstractionb arrier for H-atom.T he same group [13] reported calculations on two photon mechanism showing that it should make negligible contributions to OH formation duet ol ow quantum yields. These works illustrate the high complexity of the mechanism(s) leadingt othe formation of OH radicals.…”