Dust and sulfate aerosols off U.S. east coast bias remotely sensed ocean
color measurements
Stephanie Schollaert
The influence of two dominant aerosol species on satellite ocean color
retrievals was examined off the U.S. east coast in the western Sargasso
Sea. With appropriate atmospheric correction, satellite-derived
water-leaving radiance (nLw) should be insensitive to the presence of
atmospheric aerosols, but we found the Sea-viewing Wide Field-of-view
Sensor (SeaWiFS) spectra to be sensitive to the species and optical
thickness of aerosols, as modelled by the Navy's Aerosol Analysis and
Prediction System (NAAPS) and derived from the SeaWiFS satellite sensor
respectively. Since chlorophyll a estimates are derived from a band
ratio of nLw(442, 490, or 510 nm)/nLw(555), their accuracy depends upon
a successful atmospheric correction. When aerosols were negligible, nLw
spectra were consistent with in situ reference data between 443 and 510
nm. With increasing aerosol optical thickness during dust events, most
common during summer, the nLw spectra were lowered between 412 and 510
nm, resulting in artificially high chlorophyll a estimates.
Sulfate-dominated pixels were associated with elevated nLw spectra
between 412 and 555 nm. Increasing sulfate optical thickness also
biased the chlorophyll a estimates high. The effect of sulfate upon
ocean color retrievals is more problematic than dust off the U.S. east
coast because of the nearly constant presence of sulfate along with
commonly colocated pollutant aerosols that confound atmospheric
correction algorithms.