No abstract

Although OH uptake by carbonaceous surfaces is very efficient, it is thought to be negligible to affect the OH and O 3 balance in the lower troposphere. To estimate its contribution in the upper troposphere, we have studied the temperature dependence of OH uptake by carbonaceous surfaces of atmospheric impor tance over the temperature range from 218 to 298 K. We found that the OH uptake coefficient is weakly dependent of temperature, ranging from 0.1 to 1, as was measured under flow conditions using chemical ion ization mass spectrometry. This finding was further used to estimate OH loss on carbonaceous aerosol under upper tropospheric conditions. Our calculations suggested that OH heterogeneous loss may represent a nota ble sink for OH in the upper troposphere, which is due to both the significantly lower diffusion limitation and weakly temperature dependent OH uptake. The obtained results may be thus important for aerosol chemistry of the upper troposphere.

Kinetic studies and the mechanism determination of ClONO2 uptake on polycrystalline NaCl were carried out using a coated-insert flow tube reactor combined with high-resolution, low-energy electron-impact mass spectrometer under the following conditions: p = 1-2 Torr, linear flow velocity v = 3.5-75 m s(-1), T = 293 and 387 K, [ClONO2] = (0.5-25) x 10(12) molecules cm(-3). The salt was deposited as a film from nonsaturated aqueous solution on the sliding rod. The temporal dependences of the uptake coefficient and the partial uptake coefficients leading to a formation of the prime Cl2 and HOCl products were measured for different ClONO2 concentrations. These dependences are established to be described by gamma = gamma0 exp(-t/tau) + gamma(s), gamma(0,s)(-1) = a(0,s) + b(0,s)[ClONO2]. In the framework of the proposed kinetic model, the data are explained and the main elementary kinetic parameters of the uptake are evaluated. The model is based on a combination of Langmuir adsorption, formation of surface complexes on initial active sites, Z(ch), followed by their unimolecular decomposition. Decomposition is proposed to proceed concurrently in two channels, one of which is a released surface site that conserves the properties of the initial site. In the other channel, the initial Z(ch) transforms into Z(ph) followed by steady-state uptake and reproduction of final Z(ph). The model gives an analytical expression for experimental parameters gamma0, gamma(s), and tau in terms of elementary rate constants and the reactant volume concentration. The final objective of the proposed model is the extrapolation of gamma0, gamma(s), and tau parameters to real tropospheric conditions.

No abstract

No abstract