Measurements of OH aboard the NASA P-3 during PEM-Tropics B

Airborne measurements of the hydroxyl radical were performed aboard the NASA P-3B using the Selected Ion Chemical Ionization technique during the Pacific Exploratory Mission (PEM)-Tropics B study. Typical midday boundary layer OH concentrations of 6-7 × 10⁶ molecule cm^-3 were observed in the vicinity of Christmas Island (0°-3° N), and 7-8 × 10⁶ molecule cm^-3 in the vicinity of Tahiti (17°-22° S). Photochemical box model calculations of the concentration of OH ([OH]) throughout the entire mission yield generally good agreement (slope = 1.26) with a tendency for the model to slightly overestimate the measured OH. Boundary layer measurements of OH in an air mass containing low ozone concentrations (<5 ppbv) yielded a midday [OH] of ∼5 × 10⁶ molecule cm^-3, some 45% lower than 7 × 10⁶ molecule cm^-3 observed in air masses at the same latitude with more typical ozone concentrations (30-40 ppbv). Measurements performed under clouds yielded [OH] of ∼2.5 × 10⁶ molecule cm^-3, a value which is a factor of 2 lower when compared to data taken on either side of the cloud. This decrease corresponds to the decrease in j(O₃) to produce O(¹D).