Quantum Yields of Hydroxyl Radical and Nitrogen Dioxide from the Photolysis of Nitrate on Ice

Abstract

Nitrate photolysis proceeds via two major channels at illumination wavelengths above 290 nm: NO 3 -+ hν (+H + ) f NO 2 + • OH (1) and NO 3 -+ hν f NO 2 -+ O( 3 P) (2). A recent study determined the quantum yield of reaction 1 on ice by measuring NO 2 production, but suggested their values might be lower bounds because of incomplete recoveries of NO 2 . We measured the quantum yield of pathway 1 using an alternate approach, i.e., by following the formation of • OH. Our quantum yields for • OH (Φ OH ) at 263 K were independent of nitrate concentration and illumination wavelength (λ > 300 nm), but were dependent upon pH. Values of Φ OH decreased from (3.6 ( 0.6) × 10 -3 at pH 7.0 to (2.1 ( 0.8) × 10 -3 at pH 2.0, where the listed pH values are those of the sample solution prior to freezing. Temperature dependence experiments (239-318 K; pH 5.0) showed that values of Φ OH in ice pellets and aqueous solutions were both well described by the same regression line, ln(Φ OH ) ) ln(Φ 1 ) ) -(2400 ( 480)(1/T) + (3.6 ( 0.8) (where errors represent (1σ), suggesting that the photolysis of nitrate on ice occurs in a "quasi-liquid layer" rather than in the bulk ice. Our ice quantum yields between 268 and 240 K are 3-9 times higher, respectively, than Φ 1 values determined previously in ice. Applying our quantum yields to past field experiments indicates that nitrate photolysis can account for the flux of NO x from sunlit snow in the Antarctic and at Summit, Greenland, but that nitrate was only a minor source of the snowpack NO x measured during the Alert 2000 campaign in the Canadian Arctic. Additional calculations show that the photolysis of nitrate on cirrus clouds in the upper troposphere is a minor source of NO x that cannot account for the apparent underestimation of the ratio of NO x /HNO 3 in current numerical models.

The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts

“…The corresponding average value of F(OH) is (1.1 AE 0.4) Â 10 À2 (see Chu and Anastasio 97 for details of the calculation), which agrees well with past studies reporting a value of B0.01 at room temperature. 82,[90][91][92][93]97 These OH experiments thus suggest that the nature of the cations is not influencing the production of nitrate photo-products in the bulk. However, the bulk-phase measurements were conducted at concentrations that were a factor of 600 (for RbNO 3 ) to 2040 (for NaNO 3 ) times lower than the thin film experiments due to the low solubility of benzene in aqueous solution (23 mM).…”

“…The measured R Phen was converted to the rate of OH formation, R OH , by dividing by the yield of phenol formed from the reaction of OH with benzene (Y Phen = 0.69). 97 For the majority of experiments, the benzene concentration was 10.0 mM which was sufficiently high to trap all of OH generated from the 5.0 mM MNO 3 solutions. To determine the benzene concentration needed to trap all of the generated OH, separate experiments at either constant concentrations of NaNO 3 or Mg(NO 3 ) 2 with varied benzene concentrations were conducted (Fig.…”

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols

“…5b and c show the results of MD simulations that predict that the surface chloride ions pull nitrate ions closer to the interface via formation of a double layer with Na + which attracts NO 3 À . 177 Since the quantum yield for NO 2 production in pure nitrate solutions at wavelengths above 300 nm is only B0.01, 139,155,156 even a relatively small shift in nitrate towards the surface could cause a significant increase in the efficiency, as is observed. Thus, theory again provided molecular level insight into what was initially a surprising experimental result.…”

Using Singlet Molecular Oxygen to Probe the Solute and Temperature Dependence of Liquid-Like Regions in/on Ice

“…For example, liquid-phase kinetics and mechanisms can explain the direct photolysis of nitrate, nitrite, and hydrogen peroxide on/in ice made from slowly frozen solutions, conditions where we expect solutes to be in LLRs. [19][20][21][22][23] However, in many cases liquid-phase chemistry cannot be extrapolated to ice…”