Cloud chemistry in the SE Pacific during the VOCALS Regional Experiment

Cloud water was collected for chemical analysis aboard the NCAR C-130 as part of the autumn 2008 VOCALS Regional Experiment (VOCALS-Rex) off the coast of northern Chile. Samples were collected using the CSU/NCAR airborne cloud collector. This collector, an axial flow cyclone, mounts in a PMS canister attached to an under-wing pod. The collector, which previously flew during the DYCOMS-II campaign, can be remotely opened and closed to permit sampling only during cloud penetration. Up to seven independent cloud water samples can be collected and stored over the course of each research flight. A total of 72 cloud water samples were collected during VOCALS-REx C-130 flights. Sample measurements included pH, major inorganic ions, H2O2, S(IV), HCHO, total organic carbon (TOC), Fe, Mn, and carboxylic acids. To our knowledge, these measurements comprise the first observations of cloud water composition in this part of the Pacific.

Cloud pH covered a wide range from 2.9 to 7.2. Not surprisingly, sea salt-derived species were abundant. Na+, K+, and Mg2+ were found at concentrations corresponding to their relative abundance in seawater, suggesting scavenged sea salt particles were the main contributor of these species to cloud droplets. Cl- was found to be somewhat depleted in abundance, relative to Na+, suggesting prior loss from the aerosol by acid displacement reactions. Sulfate was found in excess of expected sea salt contributions, with an average non-sea salt sulfate concentration of 130 micronormal. The average nitrate concentration was 59 micronormal. Overall, organic carbon comprised only a few percent of total measured cloud solute mass. The most abundant carboxylic acids measured were formic, acetic, and oxalic acids. S(IV) was found at only trace levels, indicating rapid oxidation of sulfur dioxide to sulfate in these clouds. The lack of S(IV) was consistent with abundant H2O2 measured in the cloud water samples, with concentrations reaching as high as 600 micromolar. An analysis of pH and key aqueous phase S(IV) oxidants (H2O2 and O3) and oxidation catalysts (Fe and Mn) revealed H2O2 to be the most important oxidant below pH ~ 5.5 while ozone appeared to be most important in higher pH clouds. The chemical composition of these SE Pacific clouds will be summarized and a discussion presented concerning how cloud composition varied spatially in different VOCALS-Rex missions, including cross-section flights which extended westward from the Chilean coast out into the SE Pacific and studies of “pockets of open cells” (POCs).