Nucleation in the equatorial Pacific during PEM-Tropics B: Enhanced boundary layer H₂SO₄ with no particle production

During the second phase of the NASA Pacific Exploratory Mission to the Pacific Tropics (PEM-Tropics B), regions of unusually high concentrations of sulfuric acid vapor ranging from 10⁷ to 10⁸ molecules cm^-3 were detected near the ocean surface in equatorial regions between Hawaii and Tahiti. No 3-4 nm diameter nanoparticles were observed near the ocean surface where acid concentrations were highest; however, 3-4 nm particles were detected at higher elevations in regions near clouds. Calculations show that in some regions of high acid concentrations newly formed particles would be readily detected by our instruments and thus the lack of nanoparticles suggests that there was no nucleation. In contrast, in the previous PEM-Tropics A mission Clarke et al. [1998a] observed a large nucleation event in the equatorial marine boundary layer under similar temperatures, relative humidity, and sulfuric acid concentrations. Comparison between these two studies further demonstrates that some additional species or unknown process is necessary to significantly enhance nucleation in the remote marine troposphere and that this component is not always present at levels sufficient to sustain nucleation throughout the region, even at a low continuous rate. We speculate that if ammonia is one example of a critical nucleation precursor, tropospheric ternary nucleation (sulfuric acid/ammonia/water) under some conditions requires ammonia concentrations to be greater than typical background concentrations for particle production via this mechanism.