Abstract
Abstract. MODerate resolution Imaging Spectroradiometer (MODIS) instruments aboard the Terra and Aqua satellites have provided a rich dataset of aerosol information at a 10 km spatial scale. Although originally intended for climate applications, the air quality community quickly became interested in using the MODIS aerosol data. However, 10 km resolution is not sufficient to resolve local scale aerosol features. With this in mind, MODIS Collection 6 includes a global aerosol product with a 3 km resolution. Here, we evaluate the 3 km product over the Baltimore–Washington D.C., USA, corridor during the summer of 2011 by comparing with spatially dense aerosol data measured by airborne High Spectral Resolution Lidar (HSRL) and a network of 44 sun photometers (SP) spaced approximately 10 km apart, collected as part of the DISCOVER-AQ field campaign. The HSRL instrument shows that AOD can vary by over 0.2 within a single 10 km MODIS pixel, meaning that higher resolution satellite retrievals may help to better characterize aerosol spatial distributions in this region. Different techniques for validating a high-resolution aerosol product against SP measurements are considered. Although the 10 km product is more statistically reliable than the 3 km product, the 3 km product still performs acceptably with nearly two-thirds of MODIS/SP collocations falling within an expected error envelope with high correlation (R > 0.90), although with a high bias of ~ 0.06. The 3 km product can better resolve aerosol gradients and retrieve closer to clouds and shorelines than the 10 km product, but tends to show more noise, especially in urban areas. This urban degradation is quantified using ancillary land cover data. Overall, we show that the MODIS 3 km product adds new information to the existing set of satellite derived aerosol products and validates well over the region, but due to noise and problems in urban areas, should be treated with some degree of caution.
Highlights
Hydrology andFor over 12 yr, the MODEeraatrethresSoluytsiotneImmaging Spectroradiometer (MODIS) instrumenStsc, ifleynincg eabsoard the Terra and Aqua satellites, have been collecting data concerning the global distribution of climate related variables
Because much of the value of the 3 km resolution retrieval is better at resolving smaller scale aerosol features, we first look at two case studies from differing days in the DISCOVERAQ study
HYSPLIT back trajectories, using winds from the NAM model at 12 km resolution, show that the lower aerosol optical depth (AOD) air mass originated in the Midwestern United States, while the higher AOD air mass was coming from southern Virginia and North Carolina, which had active biomass burning nearby, and passed over more urbanized regions
Summary
For over 12 yr, the MODEeraatrethresSoluytsiotneImmaging Spectroradiometer (MODIS) instrumenStsc, ifleynincg eabsoard the Terra and Aqua satellites, have been collecting data concerning the global distribution of climate related variables. Several of these variables describe aerosols, which have been shown tsoysbteemon(eIPoCf Cth,e2l0a0rg7e)s. Munchak et al.: MODIS 3 km aerosol product: applications over land air quality conditions The MODIS instruments are appealing for air quality applications because the broad (2330 km) swath of the instrument allows most global locations to be monitored on a nearly daily basis. The 10 km resolution of the MODIS aerosol products is insufficient to resolve small-scale aerosol features, including point sources in urban areas Several research techniques exist to retrieve aerosols at higher spatial scales (100 m to 1 km) from MODIS observations (e.g. Lyapustin et al, 2011; Li et al, 2012), these have not been produced globally in an operational environment
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