Extension, validation, and analysis of the multi-decadal GACP/AVHRR aerosol optical thickness record

Abstract:

The main product of the Global Aerosol Climatology Project (GACP) is a continuous record of the aerosol optical thickness (AOT) over the oceans based on channel-1 and -2 radiances from successively flown AVHRR instruments. We extend the previous GACP dataset by four years though the end of 2009 using NOAA-17 and -18 AVHRR data recalibrated against MODIS radiances according to Heidinger et al. (2010), thereby making the GACP record almost three decades long. The temporal overlap of the new NOAA-17 and the previous NOAA-16 record reveals an excellent agreement of the corresponding global monthly mean AOT values, thereby confirming the robustness of the vicarious radiance calibration used in the original GACP product.

A comprehensive set of monthly mean AOT data from coastal and insular AERONET stations was used to validate GACP retrievals for the period 1995–2009. To put the GACP performance in broader perspective, we also compared AERONET and MODIS Aqua level-2 data for 2003–2009 using the same methodology. Monthly mean AOTs from the two over-the-ocean satellite datasets are well correlated with the ground-based values, the correlation coefficients being 0.81–0.85 for GACP and 0.74–0.79 for MODIS. Regression analyses demonstrated that the GACP mean AOTs are approximately 17%–27% lower than the AERONET values on average, while the MODIS mean AOTs are 5%–25% higher.

The previously identified negative trend in the global GACP AOT which started in the late 1980s and continued into the early 2000s was confirmed. Its magnitude and duration indicate that it was caused by changes in tropospheric aerosols. The latest multi-satellite segment of the GACP record shows that this trend tapered off, with no noticeable AOT change after 2002. This result is consistent with the MODIS and MISR AOT records as well as with the recent gradual reversal from brightening to dimming revealed by surface flux measurements in many aerosol producing regions. Thus the robustness of the GACP record is confirmed, increasing our confidence in the validity of the negative trend. Although the nominal negative GACP AOT trend could partially be an artifact of increasing aerosol absorption, we argue that the time dependence of the GACP record, including the latest flat period, is more consistent with the actual decrease in the tropospheric AOT.