Measurements of OH during PEM-Tropics A

Results are presented for measurements of the concentration of OH performed aboard the NASA P3-B aircraft using the Selected Ion Chemical Ionization M ass Spectrometry technique during the Pacific Exploratory Mission Tropics ( PEM) study. Typical midday boundary layer OH concentrations of 6-8 x 10(6) molecules cm(-3) were observed. Diurnally averaged boundary layer OH values obtained near Christmas Island, Kiribati, combined with SO2 and filter mea surements of non-sea-salt sulfate reveal that only 12% of the sulfate found in the particulate form can be attributed to gas phase H2SO4 production/co ndensation. Measurements performed over clouds yielded [OH] 2-3 times large r than those typically observed under similar cloudless conditions.