Large and giant aerosols in the marine boundary layer during WATOX, January 4‐9, 1986

Abstract

Aircraft measurements of large and giant aerosols (1‐45 μm diameter) in the marine boundary layer over the western North Atlantic (32°‐40°N) under offshore flow conditions during the Western Atlantic Ocean Experiment on January 4, 6, and 9, 1986 (WATOX‐86), showed a domination by sea salt, with volume spectra peaks between 7 and 9 μm diameter. Aerosol mass increased rapidly when short‐term averaged wind speeds, ū, exceeded 17 m s−1. Aerosol mass also showed a qualitative dependence on relative humidity that was not as strong as the dependence on wind speed. Both wind speed and humidity variations can be used to explain the spatial variations in volume spectra along the flight track. Most models of sea salt mass or volume in the literature consider near‐surface aerosol sizes smaller than 2.0‐μm diameter and are not applicable to our data. At altitudes of 200‐600 m above the sea surface our WATOX data show a strong nonlinear dependence between coarse aerosol mass and volume, and short‐term wind speed. The best fit is a ū3 dependence (dry mass = 8.7 + 0.0065 ū3, R2 = 0.95; volume = 8.24 + 0.0065 ū3 R2 = 0.92). This dependence may have been enhanced by the offshore flow conditions that prevailed, and the location of the aircraft within 300 km of the coast.