Physical and chemical properties of surface and column aerosols at a rural New England site during MODIS overpass

Abstract

Surface-based measurements of aerosol optical depth at a rural site in southern New Hampshire (43.11°N, 70.95°W) are compared to retrievals of the same parameter by the Moderate Resolution Imaging Spectrometer (MODIS) during April–August, 2001. Hourly averages of aerosol optical depth (AOD) were derived using a multi-filter rotating shadowband radiometer (MFRSR) at the time of NASA's Terra satellite overpass. The MODIS Level 2 aerosol product at a wavelength of 550 nm was directly compared to the MFRSR interpolated AOD at 550 nm. We were able to compare the two AOD measurement platforms on 46 days (out of a possible 128 days) and observed a good agreement between the two methods ( R=0.81; slope=0.95±0.10). However, there were 11 days during this study period when MODIS measured AOD at the site, but the MFRSR did not due to excessive cloud cover. There were also 7 days when clear skies prevailed at the site during the time of MODIS overpass, but there was no AOD retrieved by MODIS. Surface measurements of fine particle (PM 2.5) mass, chemical composition, and optical properties were also performed during summer 2001. A good correlation ( R=0.87) between fine particle mass and AOD measured by the MFRSR was observed. A comparison between fine particle light extinction at the surface and MFRSR AOD (at the same wavelength) also showed good agreement ( R=0.80). Aerosol chemical analysis revealed that ammonium sulfate was the main aerosol component during times of very high turbidity, while organic carbon dominated during times of below-average turbidity.