ISSN 1990-7931, Russian Journal of Physical Chemistry B, 2010, Vol. 4, No. 3, pp. 399–407. © Pleiades Publishing, Ltd., 2010. Original Russian Text © V.V. Zelenov, E.V. Aparina, A.V. Chudinov, S.A. Kashtanov, 2010, published in Khimicheskaya Fizika, 2010, Vol. 29, No. 5, pp. 39–47. 399 INTRODUCTION The nitrate radical is the main nighttime oxidizer in the atmosphere. According to recent systematic in- field measurements, its concentration in the near- Earth air layer ranges from 1–6 ppt in coastal areas to ~90 ppt in continental air masses [1, 2]. The main channel of NO 3 sink in nighttime hours is its uptake by cloud droplets and sea salt aerosol particles. The mean NO 3 lifetime decreases from 45 to 10 min as the relative humidity increases from 10 to 40% [3]. The results of laboratory measurements of the coefficient of uptake of NO 3 on the surface of water and aqueous solutions of salts were given in the reviews [4, 5]. Heterogeneous reactions of NO 3 with the surface of NaCl, NaBr, and NaI solid salts have been studied in the number of works [6–12]. In these studies, NO 3 and gaseous products were monitored by mass spectrometry in a flow reactor at [NO 3 ] = 1 × 10 11 –8 × 10 12 cm –3 and Т = 293–300 K. The coefficient of steady-state uptake γ s of NO 3 was found to be 0.5 × 10 –3 [9] and 5 × 10 –2 [6–8] on NaCl, 3 × 10 –3 –0.1 [7–9] on NaBr under the same conditions, and 0.3 on KI and NaI [7, 8] irre- spective of the state of the substrate (powder, grains, thin deposited film, or monocrystal). Among the gas-phase products, Br and I atoms were detected, which were formed by the reactions NO 3 + NaBr → NaNO 3 + Br and NO 3 + NaI → NaNO 3 + I; at the same time no Cl atoms formed by NO 3 + NaCl → NaNO 3 + Cl, an hypothetical analogous reaction, were detected [7, 10]. The uptake of NO 3 on X–NaCl mixtures of salts (X = NaBr, NaI, MgBr 2 ⋅ 6H 2 O, and MgCl 2 ⋅ 6H 2 O) at various X-to-NaCl mass ratios under “dry” conditions (without supply of water vapor) was studied in our pre- vious works [13–15] in connection with the discovery of surface segregation of ionic crystals. This effect gives rise to a layered structure of sea aerosol particles, with the external layers being composed of salts more reactive than NaCl. The choice of the indicated binary salts was motivated by the possibility that halogens released can contribute to the overall halogen-pro- moted activation of the troposphere. In [13–15], the dependences of the probability of uptake of NO 3 on X individual salts on the NO 3 concentration (10 11 – 10 13 cm –3 ) and on the X content in the binary mixture were obtained. The experimental dependences were described by parametric functions, which make it pos- sible to extrapolate the uptake probability to low NO 3 concentrations characteristic of the troposphere. It is known that the relative humidity (RH) of the air basin varies within a wide range, reaching 90% and more in coastal areas, where sea aerosols are moist. At RHs substantially higher that the relative humidity of deliquescence (RHD) (RHD =75% for NaCl and 32– 35% for MgCl 2 [16]) aerosol particles transform into Effect of the Humidity on the Uptake of NO 3 on Coatings Composed of MgCl 2 ⋅ 6H 2 O and MgBr 2 ⋅ 6H 2 O and Mixtures Thereof with NaCl V. V. Zelenov a , E. V. Aparina a , A. V. Chudinov a , and S. A. Kashtanov b a Institute of Energy Problems of Chemical Physics (Chernogolovka Branch), Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia b Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia e-mail: zelenov@binep.ac.ru Received February 16, 2009 Abstract—The reactive uptake of NO 3 radicals on the surface of wetted individual X salts and of wetted X–NaCl salts (X = MgCl 2 ⋅ 6H 2 O and MgBr 2 ⋅ 6H 2 O) at [H 2 O] = 2 × 10 12 –2 × 10 15 cm –3 and NO 3 (4.8 × 10 12 cm –3 ) was studied using a reactor with a movable insert covered with a salt coating in combination with a mass spectrometer for monitoring the initial reactant and products. The probabilities of NO 3 uptake γ on X–NaCl binary salts as functions of the content of doping salt were determined. A parametric approximation of the experimental data was proposed, which makes it possible to quantitatively predict the extent of surface enrichment of a wetted binary salt coating in doping salt and its dependence on the humidity and the content of this salt in the binary mixture. It was established that the relative surface density σ X of X doping salt depends on its mole fraction μ X in the X–NaCl binary salt as σ X = aμ X (a = 2.2 for MgBr 2 and 13.1 for MgCl 2 ) over the entire humidity range covered. The contributions of the X salts to the overall uptake of NO 3 at NO 3 con- centration typical of the tropospheric conditions ([NO 3 ] ~ 10 7 cm –3 and relative humidities of RH ≤ 20%) were estimated. DOI: 10.1134/S1990793110030061 KINETICS AND MECHANISM OF CHEMICAL REACTIONS. CATALYSIS