On the significance of regional trace gas distributions as derived from aircraft campaigns in PEM-West A and B

With the help of a chemical transport model we calculate the global distributions of three synthetic tracers with uniform emission rates over the continents, zero emissions over the ocean, and constant lifetimes of 5, 20, and 100 days. These distributions serve as a first approximation to those of the lower alkanes and are analyzed in detail for the region between 115 degrees E and 155 degrees E longitude, 0 degrees N and 56 degrees N latitude, within which the Pacific Exploratory Mission-West (PEM-West) A and B campaigns took place. We show that the longitudinally averaged latitude by altitude distributions of the tracers over that domain obtained from sampling the model data along the flight tracks closely represent those obtained from complete averages over the full data sets for the months March and September when the campaigns took place. We further show that for each campaign the modeled distributions closely resemble those measured for ethane and propane, although these distributions change significantly between the two seasons. Taken together, this demonstrates that the average distributions of ethane and propane measured during PEM-West A and B describe reasonably well the true pattern of these gases between 0 degrees N and 50 degrees N latitude and 115 degrees E and 155 degrees E longitude in both seasons, despite incomplete and irregular sampling of that domain. In turn, the measured distributions can be used to test the model, mainly for correct parameterization of transport.