Airborne sampling of aerosol particles: Comparison between surface sampling at Christmas Island and P‐3 sampling during PEM‐Tropics B

Bulk aerosol sampling of soluble ionic compounds from the NASA Wallops Island P‐3 aircraft and a tower on Christmas Island during PEM‐Tropics B provides an opportunity to assess the magnitude of particle losses in the University of New Hampshire airborne bulk aerosol sampling system. We find that most aerosol‐associated ions decrease strongly with height above the sea surface, making direct comparisons between mixing ratios at 30 m on the tower and the lowest flight level of the P‐3 (150 m) open to interpretation. Theoretical considerations suggest that vertical gradients of sea‐salt aerosol particles should show exponential decreases with height. Observed gradients of Na + and Mg 2+ , combining the tower observations with P‐3 samples collected below 1 km, are well described by exponential decreases ( r values of 0.88 and 0.87, respectively), though the curve fit underestimates average mixing ratios at the surface by 25%. Cascade impactor samples collected on the tower show that >99% of the Na + and Mg 2+ mass is on supermicron particles, 65% is in the 1–6 micron range, and just 20% resides on particles with diameters larger than 9 microns. These results indicate that our airborne aerosol sampling probes must be passing particles up to at least 6 microns with high efficiency. We also observed that nss SO 4 2− and NH 4 + , which are dominantly on accumulation mode particles, tended to decrease between 150 and 1000 m, but they were often considerably higher at the lowest P‐3 sampling altitudes than at the tower. This finding is presently not well understood.