MODEL STUDY OF TROPOSPHERIC TRACE SPECIES DISTRIBUTIONS DURING PEM-WEST-A

A three-dimensional mesoscale transport/photochemical model is used to study the transport and photochemical transformation of trace species over eastern Asia and western Pacific for the period from September 2 0 to October 6, 1991, bf the Pacific Exploratory Mission-West A experi ment. The influence of emissions from the continental boundary layer t hat was evident in the observed trace species distributions in the low er troposphere over the ocean is well simulated by the model, In the u pper troposphere, species such as O-3, NOy (total reactive nitrogen sp ecies), and SO2 which have a significant source in the stratosphere ar e also simulated well in the model, suggesting that the upper troposph eric abundances of these species are strongly influenced by stratosphe ric fluxes and upper tropospheric sources. In the case of SO2 the stra tospheric flux is identified to be mostly from the Mount Pinatubo erup tion, Concentrations in the upper troposphere for species such as CO a nd hydrocarbons,which are emitted in the continental boundary layer an d have a sink in the troposphere, are significantly underestimated by the model, Two factors have been identified to contribute significantl y to the underestimate: one is emissions upwind of the model domain (e astern Asia and western Pacific); the other is that vertical transport is underestimated in the model, Model results are also grouped by bac k trajectories to study the contrast between compositions of marine an d continental air masses, The model-calculated altitude profiles of tr ace species in continental and marine air masses are found to be quali tatively consistent with observations, However, the difference in the median values of trace species between continental air and marine air is about twice as large for the observed values as for model results, This suggests that the model underestimates the outflow fluxes of trac e species from the Asian continent and the Pacific rim countries to th e ocean, Observed altitude profiles for species like CO and hydrocarbo ns show a negative gradient in continental air and a positive gradient in marine air, A mechanism which may be responsible for the altitude gradients is proposed.