Adsorption of NO2 on carbon aerosol particles in the low ppb range

The interaction of NO2 on carbonaceous aerosol particles in an NO2 concentr ation range relevant for the troposphere was studied. The adsorption as a f unction of NO2 concentration (2.5-65 ppb) was investigated along with the d ependence on time (1-600 s) and particle concentration. The results exhibit a zero-order process in NO2 for the chemisorption over the measured time a nd concentration range. The results suggest that the chemisorption reaction is limited by a rapidly established steady-state coverage of a precursor i n the form of reversibly adsorbed NO2 which seems to be constant over the w hole investigated NO2 concentration range. Within the first 20 s, a chemiso rption rate of 2.5 x 10(11) molecules cm(-2) s(-1) was calculated. To estim ate a saturation value for the NO2 adsorption on carbonaceous aerosol parti cles, bulk experiments were performed where the aerosol was deposited on a filter before exposure to NO2. This gives a lower limit for the total NO2 a dsorption of about 1 x 10(14) molecules cm(-2) of particle surface area. Th e measurements show that the concept of the often used sticking coefficient gamma (i.e. the number of adsorbed molecules per number of the total gas-s urface collisions) is not a useful parameter to describe the chemisorption of NO2 at low ppb concentration on such complex surfaces as carbonaceous ae rosol particles. (C) 1999 Elsevier Science Ltd. All rights reserved.