Large‐scale air mass characteristics observed over the remote tropical Pacific Ocean during March‐April 1999: Results from PEM‐Tropics B field experiment

Eighteen long‐range flights over the Pacific Ocean between 38°S to 20°N and 166°E to 90°W were made by the NASA DC‐8 aircraft during the NASA Pacific Exploratory Mission (PEM) Tropics B conducted from March 6 to April 18, 1999. Two lidar systems were flown on the DC‐8 to remotely measure vertical profiles of ozone (O 3 ), water vapor (H 2 O), aerosols, and clouds from near the surface to the upper troposphere along their flight track. In situ measurements of a wide range of gases and aerosols were made on the DC‐8 for comprehensive characterization of the air and for correlation with the lidar remote measurements. The transition from northeasterly flow of Northern Hemispheric (NH) air on the northern side of the Intertropical Convergence Zone (ITCZ) to generally easterly flow of Southern Hemispheric (SH) air south of the ITCZ was accompanied by a significant decrease in O 3 , carbon monoxide, hydrocarbons, and aerosols and an increase in H 2 O. Trajectory analyses indicate that air north of the ITCZ came from Asia and/or the United States, while the air south of the ITCZ had a long residence time over the Pacific, perhaps originating over South America several weeks earlier. Air south of the South Pacific Convergence Zone (SPCZ) came rapidly from the west originating over Australia or Africa. This air had enhanced O 3 and aerosols and an associated decrease in H 2 O. Average latitudinal and longitudinal distributions of O 3 and H 2 O were constructed from the remote and in situ O 3 and H 2 O data, and these distributions are compared with results from PEM‐Tropics A conducted in August‐October 1996. During PEM‐Tropics B, low O 3 air was found in the SH across the entire Pacific Basin at low latitudes. This was in strong contrast to the photochemically enhanced O 3 levels found across the central and eastern Pacific low latitudes during PEM‐Tropics A. Nine air mass types were identified for PEM‐Tropics B based on their O 3 , aerosols, clouds, and potential vorticity characteristics. The data from each flight were binned by altitude according to air mass type, and these results showed the relative observational frequency of the different air masses as a function of altitude in seven regions over the Pacific. The average chemical composition of the major air mass types was determined from in situ measurements in the NH and SH, and these results provided insight into the origin, lifetime, and chemistry of the air in these regions.