Pacific Atmospheric Sulfur Experiment (PASE): dynamics and chemistry of the south Pacific tropical trade wind regime

• Published in: Journal of atmospheric chemistry 2011-03, Vol.68 (1), p.5-25
• Main Authors: Bandy, Alan, Faloona, Ian C., Blomquist, Byron W., Huebert, Barry J., Clarke, Antony D., Howell, Steven G., Mauldin, R. L., Cantrell, Christopher A., Hudson, James G., Heikes, Brian G., Merrill, John T., Wang, Yuhang, O’Sullivan, Daniel W., Nadler, Wolfgang, Davis, Douglas D.
• Format: Article
• Language: English
• Subjects: Aerosols; Albedo; Atmosphere; Atmospheric aerosols; Atmospheric chemistry; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Boundary layers; Bromine; Chemical properties; Chemistry; Clouds; Dynamic meteorology; Earth and Environmental Science; Earth Sciences; Intertropical convergence zone; Nuclear radiation; Ocean-atmosphere interaction; Oxidation; Rain and rainfall; Sulfur; Sulfur compounds; Sulfur dioxide; Supersaturation; Trade winds; Wind
• ISSN: 0167-7764; 1573-0662
• DOI: 10.1007/s10874-012-9215-8

The Pacific Atmospheric Sulfur Experiment (PASE) was a comprehensive airborne study of the chemistry and dynamics of the tropical trade wind regime (TWR) east of the island of Kiritibati (Christmas Island, 157º, 20′ W, 2º 52′ N). Christmas Island is located due south of Hawaii. Geographically it is in the northern hemisphere yet it is 6–12º south of the intertropical convergence zone (ITCZ) which places it in the southern hemisphere meteorologically. Christmas Island trade winds in August and September are from east south east at 3–15 ms −1 . Clouds, if present, are fair weather cumulus located in the middle layer of the TWR which is frequently labeled the buffer layer (BuL). PASE provided clear support for the idea that small particles (80 nm) were subsiding into the tropical trade wind regime (TWR) where sulfur chemistry transformed them to larger particles. Sulfur chemistry promoted the growth of some of these particles until they were large enough to activate to cloud drops. This process, promoted by sulfur chemistry, can produce a cooling effect due to the increase in cloud droplet density and changes in cloud droplet size. These increases in particle size observed in PASE promote additional cooling due to direct scattering from the aerosol. These potential impacts on the radiation balance in the TWR are enhanced by the high solar irradiance and ocean albedo of the TWR. Finally because of the large area involved there is a large factional impact on earth’s radiation budget. The TWR region near Christmas Island appears to be similar to the TWR that persists in August and September, from southwest of the Galapagos to at least Christmas Island. Transport in the TWR between the Galapagos and Christmas involves very little precipitation which could have removed the aerosol thus explaining at least in part the high concentrations of CCN (≈300 at 0.5% supersaturation) observed in PASE. As expected the chemistry of sulfur in the trade winds was found to be initiated by the emission of DMS into the convective boundary layer (BL, the lowest of three layers). However, the efficiency with which this DMS is converted to SO 2 has been brought into further question by this study. This unusual result has come about as result of our using two totally different approaches for addressing this long standing question. In the first approach, based on accepted kinetic rate constants and detailed steps for the oxidation of DMS reflecting detailed laboratory studies, a DMS to