Trans-pacific aerosol vertical structure revealed by spaceborne lidar CALIOP

Abstract

The 5-year (2011–2015) spaceborne lidar CALIOP (cloud-aerosol lidar with orthogonal polarization) data were aggregated to obtain a seasonal three-dimensional (3-D) distribution of aerosols over the Northern Pacific (NP). Superior to the Moderate-resolution Imaging Spectroradiometer (MODIS) product, which provided only the bulk Aerosol Optical Depth (AOD), the vertical high-resolution Aerosol Extinction Coefficient (AEC) obtained from the CALIOP product enabled us to determine the AOD for the prescribed layers. By integrating the AEC at the prescribed height interval, the AOD beneath and above 2 km was obtained to characterize the aerosol loading in the surface layer (SL) and the free troposphere (FT), respectively. The AOD in the SL and FT present different seasonal variation patterns over the NP. The SL AOD exhibited the highest level in the winter, which was caused by sea salt emissions driven by the strongest winds. However, a much higher FT AOD occurred over the NP in the springtime compared with that of other seasons, indicating that the most significant transpacific transport occurred in the springtime, mainly in the FT. Fine resolution aerosol distributions in cross section along a transpacific direction were extracted from the CALIOP product as well. Along the cross section, aerosol loading was most abundant below 2 km for all the seasons. However, the detectable aerosol layer may extend to 10 km high in the springtime, which is much higher than that in the other seasons, and the aerosol layers below and above 2 km were separated over the remote NP. The CALIOP data demonstrated that dust plays a role in aloft aerosol transport. In terms of aerosol transport in the FT, an 8-fold decrease in the FT AOD occurred during the journey from East Asia to the Eastern Pacific in the summer, which was the sharpest decrease compared with the smallest one, which has 3-fold decrease occurred in the spring. Aerosol transport in the FT contributed 19% of the AOD over western North America (NA) in the spring, which was much higher than that in other seasons, and the aloft aerosol transport between 2 and 4 km high corresponded to a fine PM concentration of 1–2 μg m−3 as a contributor to air quality over western NA.