Wet annular denuder measurements of nitrous acid:: laboratory study of the artefact reaction of NO2 with S(IV) in aqueous solution and comparison with field measurements

Within this study denuder laboratory and field experiments as well as direct kinetic laboratory studies were performed in order to quantify an artefact by the aqueous phase formation of nitrous acid from dissolved NO2 and SO2 at wetted denuder walls. A wet annular denuder with a stripping solution of K2CO3 (1 mM, pH = 10) with high collection efficiency (> 97%) for measurements of acid tropospheric gases (HNO2, HNO3 and HQ without substantial influence of dispersed particulate matter is used. The samples from the denuder are analysed by ion chromatography. To study the reaction of NO2(aq) with SO3(aq)2- in the temperature interval between 288 and 328 K for the systemWithin this study denuder laboratory and field experiments as well as direct kinetic laboratory studies were performed in order to quantify an artefact by the aqueous phase formation of nitrous acid from dissolved NO2 and SO2 at wetted denuder walls. A wet annular denuder with a stripping solution of K2CO3 (1 mM, pH = 10) with high collection efficiency (> 97%) for measurements of acid tropospheric gases (HNO2, HNO3 and HCL) without substantial influence of dispersed particulate matter is used. The samples from the denuder are analysed by ion chromatography. To study the reaction of NO2(aq) with SO3(aq)2- in the temperature interval between 288 and 328 K for the system NO2(aq) + SO3(aq)2- reversible arrow(k20,k-20) [NO2 - SO3](2-), [NO2 - SO3](2-) -->(k21) NO2- + SO3- excimer laser photolysis-long path absorption was applied. The rate constants were measured following the decay of absorbance of the NO2 radical at 400 nm. The rate constant for the adduct formation from NO2(aq) with SO3(aq)2- is found to be independent of temperature with k(20) (288less than or equal toTless than or equal to328K)=(1.38+/-0.2)x 10(-7) lmol(-1) s(-1). The following temperature-dependencies for the forward k(22), the reverse reactions k(-20), k(-22) and for the equilibrium constants K-20 and K-22 were obtained: At pH = 10.0: k(-20)(T) = (3.5 +/- 0.6). 10(6) exp [-(2440 +/- 800) K/T]s(-1), K-20(T) = (1.06 +/- 900) exp [-(2833 +/- 2000) K/T] lmol(-1). At pH = 4.5: k(22)(T) = (5.3 +/- 0.1) x 10(12) exp [-(4500 +/- 1000) K/T] lmol(-1) s(-1), k(-22)(T) = (3.7 +/- 0.6) x 10(6)exp [-(3550 +/- 800) K/T]s(-1), K-22(T) = (1.5 +/- 1.8) x 10(6) exp [(950 +/- 300)K/T] lmol(-1). These latter data are of interest for tropospheric multiphase modelling. The reaction involves the formation of a long-lived intermediate [NO2-SO3](2-) which decays into NO2(aq)- and SO3(aq)-. To obtain the decay rate constant of this intermediate results at pH = 10.0 of the laboratory denuder measurements were fitted to the above mechanism combined to a description of phase transfer resulting in k(21) = (8.4 +/- 0.1) x 10(-3) s(-1) at T = 298 K. The overall HNO2 artefact formation in a wet annular denuder with a solution volume of 10ml is described by (T = 298 K) [HNO2]art(k) = 0.0056[NO2] + (0.0022/ppb)[NO2] [SO2]. The first term accounts for HNO2 formation from NO2 and water alone and the second term quantifies the aqueous phase reaction of dissolved NO2 and SO2. This equation was used for a correction of HNO2 field measurements with the described denuder technique at the IfT-research station Melpitz (Germany) and the results are discussed. Corrected HNO2 measurements were compared with HNO2-DOAS measurements. Generally, the agreement with DOAS and denuder HNO2 measurements is improved after correction of the denuder data according to the above equation. Field measurements are discussed in view of NO2 concentration and meteorological conditions. (C) 2003 Elsevier, Science Ltd. All rights reserved.