Title: Asian pollution subsidence and MOPITT tropical validation  

Objectives: (1) to sample aged Asian pollution subsiding in the tropics, (2) to conduct a MOPITT tropical validation, (3) to intercompare instruments on the DC-8 and P-3 aircraft.  

Execution: The intercomparison, planned as a boundary layer run out of Kona with the two aircraft flying parallel tracks, had to be canceled because of lack of clearance.  The DC-8 followed a straight flight track from Kona to Guam, with extensive vertical profiling.  A MOPITT validation experiment was conducted at (18N, 175W, 2235 UTC) with a spiral from 31Kft to 0.5Kft above a broken cumulus deck (tops at 6Kft) in a strongly subsiding atmosphere.  

Results: Our first two objectives were met.  (1) We observed and sampled repeatedly a layer of Asian pollution at 8-12Kft (CO up to 210 ppbv, ozone up to 80 ppbv) with background air above and below.  This pollution was in a strongly subsiding layer on top of the trade wind inversion, and appeared to contain both fossil fuel and biomass burning influence. Through mapping with DIAL it was determined to extend from 170W to 162E. The layer was predicted in the chemical forecast where it was due to Asian outflow transported rapidly to the central Pacific, and then strongly subsiding and stagnating west of Hawaii, although the observed layer extended further west than forecast.  Formaldehyde was elevated (up to 700 pptv), in contrast to the Asian plumes sampled on the Dryden-Kona flight, reflecting presumably the stronger photochemical activity.  (2) the MOPITT validation experiment was a success.  Skies were 100% clear above 6Kft.  Although the aircraft ceiling was 31Kft, DIAL observations showed very clean air above.  The dominant feature of the profile was the strong layer of Asian pollution at 8-12Kft.

Meteorological Summary DC-8 Kona to Guam

Relevant Flow Patterns
         Lower troposphere-Subtropical high pressure areas dominated the flight track. One center was near Hawaii, while a second was located over the western Pacific Basin.           

Middle troposphere-Subtropical highs continued to dominate the area.         

Upper troposphere-Continued high pressure over the area.  The jet stream was centered near 35oN-well north of the flight track.

Relevant Cloud Features
         The flight track mostly was devoid of middle and upper level clouds.  However, scattered to broken cumulus and/or stratus covered much of the flight area.

         Water vapor imagery showed that the flight area was quite dry in the middle and upper troposphere.  The dryness was associated with widespread subsidence associated with the subtropical highs described above.  In situ soundings revealed classical subsidence-type temperature/dew point profiles. Chemical data indicated that the moisture and ozone/CO concentrations were consistently anti-correlated with each other.

         The MOPITT evaluation area exhibited a scattered, thin layer of stratus with tops ~6,000 ft.  There also was a broken cumulus deck with bases near 1500 ft, and tops reaching ~2500 ft.  The clouds in the MOPITT area today were not as uniform as encountered yesterday.

         Slight, brief rain rainshowers were encountered near Guam.