New Particle Formation in the Remote Troposphere: A Comparison of Observations at Various Sites

Abstract

Measurements show that new particles are formed by homogenous nucleation over a wide range of conditions in the remote troposphere. In our studies, large nucleation events are found exclusively in regions of enhanced sulfuric acid vapor (H 2 SO 4g ) concentrations, with maximum concentrations never exceeding 5×10 7 molecules cm −3 . Although these data suggest that H 2 SO 4g participated, comparisons between ambient conditions in regions of nucleation to conditions necessary for binary H 2 SO 4 water (H 2 O) nucleation indicate that the mechanism may vary with elevation. In remote marine regions, at altitudes greater than ∼4 km above sea level, observations of nucleation in clear air along cloud perimeters are in fair agreement with current classical binary nucleation models. In these regions, the low temperatures associated with high altitudes may produce sufficiently saturated H 2 SO 4 for the production of new H 2 SO 4 /H 2 O particles. However, uncertainties with current binary nucleation models limit decisive comparisons. In warmer regions, closer to the earth's surface, measured H 2 SO 4 concentrations are clearly insufficient for binary nucleation. Conditions at these sites are similar to those observed in an earlier study where there was circumstantial evidence for a ternary mechanism involving H 2 SO 4 , H 2 O, and ammonia (NH 3 ) [ Weber et al. , 1998], suggesting that this may be a significant route for particle production at lower altitudes where surface‐derived species, like NH 3 , are more apt to participate.