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 ∼6 km above sea level in a remote marine region are reported. Two distinct locations situated near the cloud were found to have enhanced concentrations of sulfuric acid vapor (H 2 SO 4 ) and freshly formed 3–4 nm particles. Both were in droplet‐free air situated above cloudy regions. No evidence for enhanced H 2 SO 4 or nucleation was observed in clear air far from the cloud. In the nucleating region the aerosol size distribution from 3 nm to 600 μm 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 particle production in the free troposphere occurs preferentially in clear air near clouds via enhanced photochemical production of nucleation precursor gases and that H 2 SO 4 participates. A model simulation suggested that a doubling of ultraviolet intensities above the cloud due to cloud enhanced up‐welling radiation or reasonable enhancements in sulfur dioxide concentrations could account for the higher H 2 SO 4 concentration observed near the cloud. In the nucleation regions, H 2 SO 4 and water vapor concentrations were too low for binary nucleation of sulfuric acid and water, according to current nucleation models. The mechanisms of particle formation and growth remain uncertain. The measurements were part of the first Aerosol Characterization Experiment (ACE 1) conducted in the remote South Pacific Ocean (153°E, 47°S) on November 27, 1995.