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

The interaction of NO2 on carbonaceous aerosol particles in an NO2 concentration range relevant for the troposphere was studied. The adsorption as a function of NO2 concentration (2.5–65 ppb) was investigated along with the dependence on time (1–600 s) and particle concentration. The results exhibit a zero-order process in NO2 for the chemisorption over the measured time and concentration range. The results suggest that the chemisorption reaction is limited by a rapidly established steady-state coverage of a precursor in the form of reversibly adsorbed NO2 which seems to be constant over the whole investigated NO2 concentration range. Within the first 20 s, a chemisorption rate of 2.5×1011 molecules cm-2 s-1 was calculated. To estimate a saturation value for the NO2 adsorption on carbonaceous aerosol particles, 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 adsorption of about 1×1014 molecules cm-2 of particle surface area. The measurements show that the concept of the often used sticking coefficient γ (i.e. the number of adsorbed molecules per number of the total gas–surface collisions) is not a useful parameter to describe the chemisorption of NO2 at low ppb concentration on such complex surfaces as carbonaceous aerosol particles.