Aerosol Climatology of the Pacific: Production, Transport, Evolution and Mixing Evident in Two Decades of Aerosol Measurements

Reanalysis of our extensive aerosol data collected during the past two decades around the Pacific Basin is been used to develop a climatology of aerosol microphysics for this region. Our aircraft and ship measurements include aerosol size distributions, optical properties (light scattering and light absorption) and chemistry. The aerosol measurement systems were able to characterize aerosol concentrations and properties over all size ranges of primary interest to processes in atmospheric chemistry and aerosol physics (i.e. 0.003 to 20 µm). These included processes ranging from aerosol nucleation and evolution of the size distribution to mass burdens and aerosol radiative effects. Size-distributions were established with a combination of laser optical particle spectrometer, aerodynamic particle sizer (APS), forward scattering spectrometer probe (FSSP), a differential mobility analyzer (DMA) and several condensation nuclei counters. Thermal analysis (volatility) of size distributions allowed inference of aerosol physicochemistry and can distinguish air masses and aerosol with continental vs. "clean" characteristics.

Variations in the smaller sizes reveal processes related to the formation and evolution of aerosol while the larger sizes reveal features that dominate aerosol radiative effects and optical depth. Analysis of data sets taken in the Pacific free troposphere (FT) has revealed regimes with distinct characteristics. These include the tropics with low aerosol mass but high number and a volatile (at 300C) aerosol. These aerosol are often naturally formed from sulfuric acid (nucleation) at very high numbers near cloud edges associated with ITCZ convection but appear to grow in size during subsidence. Details of the process involve a complex interplay between source regions, cloud dynamics, chemistry, microphysics and regional meteorology that will be discussed.

We also include some recent findings for direct forcing , closure studies and optical characterization of pollution plumes during INDOEX (1999) and related efforts during NSF-ACE 1 and NASA- PEMT experiments. Vertical profiles and horizontal legs in the marine boundary layer and the free troposphere were obtained for diverse regions. These included regions of clean air as well as regions influenced by "layers" or "rivers" of continental aerosol at various altitudes above the MBL. Sources included dust, biomass burning, continental pollution as well as natural emissions including volcanic sulfates and sea-salt.

Fewer number but larger aerosol are more characteristic of the mid-latitude FT. These continental aerosol are often internally mixed with a non-volatile residual indicative of soot with volatile surface components (sulfate etc.). In the springtime North Pacific these combustion derived aerosol are also found associated with the same meteorology that transports "dust events". They can also dominate the scattering and absorption properties of the aerosol even though the increase in large dust particles in such events generally dominate the mass. The FT in the subtropics tends to show frequent and marked transitions and mixing between these clean and continental aerosol types. Some climatological, microphysical and optical characteristics of these aerosol types and aerosol river/layer structures will be described.