A decadal regional and global trend analysis of the aerosol optical depth using a data-assimilation grade over-water MODIS and Level 2 MISR aerosol products

Abstract

Using the ten-year (2000–2009) DA quality Terra MODIS and MISR aerosol products, as well as 7 years of Aqua MODIS, we studied both regional and global aerosol trends over oceans. This included both natural and data assimilation grade versions of the products. Contrary to some of the previous studies that showed a decreasing trend in aerosol optical depth (AOD) over global oceans, after correcting for what appears to be aerosol signal drift from the radiometric calibration of both MODIS instruments, we found MODIS and MISR agreed on a statistically negligible global trend of 0.0003/per year. Our study also suggests that AODs over the Indian Bay of Bengal, east coast of Asia, and Arabian Sea show statistically significant increasing trends of 0.07, 0.06, and 0.06 per ten years for MODIS, respectively. Similar increasing trends were found from MISR, but with less relative magnitude. These trends reflect respective increases in the optical intensity of aerosol events in each region: anthropogenic aerosols over the east coast of China and Indian Bay of Bengal; and a stronger influence from dust events over the Arabian Sea. Negative AOD trends are found off Central America, the east coast of North America, and the west coast of Africa. However, confidence levels are low in these regions, which indicate that longer periods of observation are necessary to be conclusive

Observing and understanding the Southeast Asian aerosol system by remote sensing: An initial review and analysis for the Seven Southeast Asian Studies (7SEAS) program

“…For most boundary layer clouds, these appear be largely adequate for MODIS. For SEA, Zhang and Reid (2010) found virtually no difference in mean AOD for all data versus means where additional cloud clearing requirements were applied. Similarly, Myhre et al, 2004Myhre et al, , 2005a found little evidence for cloud bias in the NOAA AVHRR product (though there was bias in the AVHRR Global Aerosol Climatology Project-GACP product), suggesting stringent cloud clearing.…”

“…First and foremost, over ocean agreement of the average product is very close over most of our study domain (within ±0.025). Globally, the mean difference between Terra and Aqua MODIS is 0.015 Li et al, 2009;Zhang and Reid, 2010;; most of this difference can be attributable to the radiance calibration. However, over land the differences can be quite large; differences of −0.025 to −0.075 are visible over much of the land area.…”

“…MISR AOD data, to our knowledge, has not been used specifically in scientific studies about SEA outside of the MODIS-MISR intercomparison of Xiao et al (2009). We will not go into great detail on MISR efficacy as we did with the DTRs, but we can infer its utility in a number of global studies (Kahn et al, 2005Zhang and Reid, 2010;Shi et al, 2011b), as well as regional studies in the nearby Indian Ocean and Subcontinent (e.g., Di Girolamo et al, 2004;Di Girolamo, 2010, 2011).…”

“…MISR has been very well characterized, has stable calibration, and generally verifies better globally than any other single satellite product Zhang and Reid, 2010). For smoke, MISR has been applied to obtain additional microphysical degrees of freedom for size and absorption (e.g., Matichuk et al, 2007;Chen et al, 2008), although the MISR team is still working to improve the smoke model (Olga Kalashnakova, personal communication 2012).…”

Response to “Toward unified satellite climatology of aerosol properties. 3. MODIS versus MISR versus AERONET”

“…This integrated approach is especially needed for long-term trend analysis, which is extremely sensitive to variability in observing conditions, as well as aerosol amount and type variations combined with sampling biases inherent in the global data sets (Section 6 above; see [53] for a demonstration of how this can be done). Similarly, given sampling biases and limited particle microphysical property information in the satellite data, major advances in assessing aerosol direct radiative forcing globally will likely require the application of data from multiple sources and models [12,54].…”