HETEROGENEOUS INTERACTIONS OF OH AND HO2 RADICALS WITH SURFACES CHARACTERISTIC OF ATMOSPHERIC PARTICULATE MATTER

The heterogeneous interactions of OH and HO2 radicals with a number of surfaces characteristic of atmospheric particulate matter have been s tudied by using a low-temperature flow tube coupled to a resonance flu orescence detector. In particular, the mass accommodation coefficients (a) of both OH and HO2 on supercooled sulfuric acid solutions have be en measured: for OH, alpha > 0.2 for 45-96 wt % solutions from 220 to 295 K, and for HO2, alpha > 0.2 on 55 wt % solutions doped with 0.1 M CuSO4 at 223 K. Radical uptake coefficients (gamma) were also measured on a variety of solid surfaces prevalent in the atmosphere: water ice , NH4HSO4 and (NH4)(2)-SO4. On water ice, although it was found that t he uptake coefficients of both OH and HO2 were relatively small on con ditioned surfaces (gamma(OH) = 0.03 +/- 0.02 from 205 to 230 K, gamma( HO2) = 0.025 +/- 0.005 at 223 K), it was observed that the OH uptake c oefficient could be significantly increased by either adsorbing HNO3 t o the surface or melting the ice surface by exposure to relatively hig h partial pressures of HCl. Similarly, on conditioned (NH4)(2)SO4 and NH4HSO4 surfaces at room temperature, the OH uptake coefficient is rel atively small, gamma(OH) < 0.03. Nevertheless, the uptake coefficient can be significantly increased (gamma(OH) > 0.2) by exposing the surfa ce to relatively low partial pressures of an organic species, 1-hexano l.