Measurements of enhanced H2SO4 and 3-4 nm particles near a frontal cloud during the First Aerosol Characterization Experiment (ACE 1)

Observations of new particle production recorded near a frontal cloud at si milar to6 km above sea level in a remote marine region are reported. Two di stinct locations situated near the cloud were found to have enhanced concen trations of sulfuric acid vapor (H2SO4) and freshly formed 3-4 nm particles . Both were in droplet-free air situated above cloudy regions. No evidence for enhanced H2SO4 or nucleation was observed in clear air far from the clo ud. In the nucleating region the aerosol size distribution from 3 nm to 600 mum was observed to be trimodal, with a prominent ultrafine mode, and was qualitatively, similar to surface-based measurements recorded in regions of postfrontal subsidence. The measurements support the notion that new parti cle production in the free troposphere occurs preferentially in clear air n ear clouds via enhanced photochemical production of nucleation precursor ga ses and that H2SO4 participates. A model simulation suggested that a doubli ng of ultraviolet intensities above the cloud due to cloud enhanced up-well ing radiation or reasonable enhancements in sulfur dioxide concentrations c ould account for the higher H2SO4 concentration observed near the cloud. In the nucleation regions, H2SO4 and water vapor concentrations were too low for binary nucleation of sulfuric acid and water, according to current nucl eation models. The mechanisms of particle formation and growth remain uncer tain. The measurements were part of the first Aerosol Characterization Expe riment (ACE 1) conducted in the remote South Pacific Ocean (153 degreesE, 4 7 degreesS) on November 27, 1995.