Aircraft Measurements of the Latitudinal and Seasonal Variations of Nonmethane Hydrocarbons and Selected Halocarbons During ACE-1

Nicola J. Blake, Oliver W. Wingenter, Donald. R. Blake, and F. Sherwood Rowland (Department of Chemistry, University of California, Irvine, CA 92697-2025; 714-824-4854; e-mail:

A suite of nonmethane hydrocarbon (NMHC) and halocarbon gases were measured in more than 2500 whole air samples collected aboard the NCAR C-130 aircraft between November 2 and December 21, 1995 as part of the Southern Hemisphere Marine Aerosol Characterization Experiment (ACE-1). The transit flights to Tasmania, Australia in November included a unique airborne latitudinal profile extending north from Anchorage to 76°N and south from Christchurch, New Zealand to 60°S, via Hawaii, Christmas Island, and Western Samoa. This profile revealed relatively high mixing ratios of NMHCs, including ethane, propane, ethyne, and benzene, at high latitudes of the northern hemisphere, where their major anthropogenic sources are located, and exhibited significant latitudinal and vertical gradients. The return transit in December from Tasmania to 40°N revealed evidence for the northern hemisphere winter build-up of NMHCs typical of the fall/winter period. The NMHC mixing ratios in the southern hemisphere were much lower and more uniformly distributed, both latitudinally and vertically, within this hemisphere. During the month-long period of flights out of Hobart, Tasmania (November 15 to December 15), significant decreases in the background mixing ratios of NMHCs including ethane, ethyne, and propane were observed. This change was most likely caused by the seasonal increase in their HO radical sink as the southern hemisphere spring advanced. CH3I demonstrated strong vertical gradients in both hemispheres, with the highest CH3I mixing ratios measured at low altitude over the ocean during the transit flight from Western Samoa to New Zealand. These latitudinal and seasonal variations, as well as those for other gases, will be compared to surface measurements collected in the Pacific and will be discussed in terms of their source and sink regions.