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Retrieving aerosol in a cloudy environment: aerosol availability as a function of spatial and temporal resolution

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Author(s): L. A. Remer | S. Mattoo | R. C. Levy | A. Heidinger | R. B. Pierce | M. Chin

Journal: Atmospheric Measurement Techniques Discussions
ISSN 1867-8610

Volume: 5;
Issue: 1;
Start page: 627;
Date: 2012;
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ABSTRACT
The challenge of using satellite observations to retrieve aerosol properties in a cloudy environment is to prevent contamination of the aerosol signal from clouds, while maintaining sufficient aerosol product yield to satisfy specific applications. We investigate aerosol retrieval availability at different instrument pixel resolutions, using the standard MODIS aerosol cloud mask applied to MODIS data and a new GOES-R cloud mask applied to GOES data for a domain covering North America and surrounding oceans. Aerosol availability is not the same as the cloud free fraction and takes into account the technqiues used in the MODIS algorithm to avoid clouds, reduce noise and maintain sufficient numbers of aerosol retrievals. The inherent spatial resolution of each instrument, 0.5 × 0.5 km for MODIS and 1 × 1 km for GOES, is systematically degraded to 1 × 1 km, 2 × 2 km, 4 × 4 km and 8 × 8 km resolutions and then analyzed as to how that degradation would affect the availability of an aerosol retrieval, assuming an aerosol product resolution at 8 × 8 km. The results show that as pixel size increases, availability decreases until at 8 × 8 km 70% to 85% of the retrievals available at 0.5 km have been lost. The diurnal pattern of aerosol retrieval availability examined for one day in the summer suggests that coarse resolution sensors (i.e. 4 × 4 km or 8 × 8 km) may be able to retrieve aerosol early in the morning that would otherwise be missed at the time of current polar orbiting satellites, but not the diurnal aerosol properties due to cloud cover developed during the day. In contrast finer resolution sensors (i.e. 1 × 1 km or 2 × 2 km) have much better opportunity to retrieve aerosols in the partly cloudy scenes and better chance of returning the diurnal aerosol properties. Large differences in the results of the two cloud masks designed for MODIS aerosol and GOES cloud products strongly reinforce that cloud masks must be developed with specific purposes in mind and that a generic cloud mask applied to an independent aerosol retrieval will likely fail.

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