Revisiting AVHRR Tropospheric Aerosol Trends using Principal Component Analysis

The Advanced Very High Resolution Radiometer (AVHRR) satellite instrument provides a nearly 25-year continuous record of global aerosol properties over the ocean. It offers valuable insights into the long-term change in global aerosol loading. However, the AVHRR data record is heavily influenced by two volcanic eruptions, El Chichon in March 1982 and Mt. Pinatubo in June 1991. The gradual decay of volcanic aerosols may last years after the eruption, which potentially masks the estimation of aerosol trends in the lower troposphere, especially those of anthropogenic origin. In this study, we show that a Principal Component Analysis (PCA) approach effectively isolates the variability of volcanic aerosols into a single mode. The spatial pattern and time series of this mode well match the global distribution and decay of volcanic aerosols. We further reconstruct the data set by removing the volcanic aerosol component and re-estimate global and regional aerosol trends. Globally, the reconstructed data set reveals an increase of aerosol optical depth (AOD) from 1985 to 1990, and decreasing trend from 1994 to 2006. Regionally, in the 1980's, positive trends are observed over the North Atlantic and Arabian Sea, while negative tendencies are present off the West Africa coast. During the 1994 to 2006 period, North America, Europe and North Africa exhibit decreasing trends, while East and South Asia, the Sahel region and South America show increasing trends. The newly estimated trends are consistent with those inferred from Barbados dust record, surface radiance measurements and MODIS and MISR aerosol retrievals.