MEASURED ATMOSPHERIC NEW PARTICLE FORMATION RATES - IMPLICATIONS FOR NUCLEATION MECHANISMS

Measured production rates of tropospheric ultrafine particles (similar to 3 nm diameter) are reported for the first time and are shown to be orders of magnitude greater than nucleation rates predicted by the bi nary theory of homogeneous nucleation for sulfuric acid and water. Fur thermore, the functional dependence of observed particle formation rat es on sulfuric acid vapor concentrations is much weaker than predicted by binary theory. We present arguments to show that these discrepanci es might be due to the participation of a species such as ammonia whic h could stabilize subcritical clusters, thereby enhancing nucleation r ates. The data suggest that atmospheric nucleation may occur by a coll ision-limited process, rather than by a condensation/evaporation contr olled process as is assumed in the classical theory.