Comparison and evaluation of the MODIS Collection 6 aerosol data in China

Abstract

Several important improvements have been made in recent Moderate Resolution ImagingSpectroradiometer (MODIS) Collection 6 aerosol retrievals, but few regional validations are conducted. We compared Deep Blue (DB) and Dark Target (DT) retrievals over China and evaluated their performance with ground observations. Sampling frequency of DB was much higher than that of DT, which was mainly caused by unavailable retrieval of DT in bright surface and heavy pollution conditions. It is found that merged aerosol optical depth (AOD) will miss some cases if it was determined only by vegetation density. Collocated comparison shows that DT AOD was substantially higher than DB with seasonal difference exceeding 0.3-0.4 over eastern China. However, when all available retrievals were considered, DT AOD was obviously lower than DB in all the seasons except spring due to their large difference in spatial coverage in high-AOD conditions. DB can well reveal spatial extent of the widespread haze pollution while there were few values in DT. More than one half of the situations with AOD > 1.0 was missed by DT, which largely underestimated the common regional haze pollution in eastern China. Ground validations show that DT tended to overestimate the aerosol loading by underestimation of the surface contribution and single scattering albedo of aerosols. DB performed generally well with higher accuracy in northern China but exhibited obvious underestimation in northwestern and southern China. Despite of slight decrease in accuracy, DB retrievals with all quality enable a large increase in spatial coverage, especially when dense haze clouds existed.

Evaluation of the MODIS C6 Aerosol Optical Depth Products over Chongqing, China

“…Even though considering a relaxed EE (±(0.05 + 20%)), the f for the DT algorithm does not exceed 64% (not showing in Table 1). When comparing with validations of MODIS C6 products in other regions of China [19,22,52], the correlation coefficients are relatively higher for both of the validated algorithms in Chongqing. The RMSE for the DT algorithm are lower than those in most of the other regions, while higher RMSE are identified for the DB algorithm.…”

“…Hence, cautions should be taken while comparing these annual means with those in other regions. Secondly, the overestimation of surface reflectance in the DB algorithm reduced the confidence of the surface reflectance assumptions, as verified in several cities of China [22]. This evokes the demand of validating the DB AOD products in wider regions carefully.…”

“…The intercepts are −0.33 and −0.16 for Terra and Aqua, respectively. Tao et al [22] gave regression coefficients for the 10 km DT and DB algorithm when the two algorithms both provided matchup. The intercepts for the 10 km DT algorithm were about 0.13 at Beijing (in northern China), 0.09 at Hangzhou (in eastern China), and 0.05 at Hong Kong (in southern China).…”

Improving Spatial Coverage for Aqua MODIS AOD using NDVI-Based Multi-Temporal Regression Analysis

“…The MODIS Collection 6 (C6) datasets were released in 2014, and the original DB AOD products were replaced by the enhanced DB AOD products, which provide better coverage on vegetated land surfaces, urban areas, and brighter deserts. It has been proven that the deep blue (DB) and dark target (DT) retrievals exhibited very poor consistency even on clear days and that DB AOD products possess better spatial coverage than DT, especially in winter [32]. Thus, the MODIS DB AOD, with all quality assurance flags, was utilized in this study for the maximum spatial coverage, where MOD04_L2 (Terra) and MYD04_L2 (Aqua) with 10 km spatial resolution were distributed in Hierarchical Data Format (HDF) available on the NASA Level 1 and Atmosphere Archive and Distribution System (LAADS) website [33].…”

Integrated Analysis of Dust Transport and Budget in a Severe Asian Dust Event

“…The DB algorithm is applied to retrieve the aerosol parameters of bright surfaces by using blue (0.412 and 0.47 µm) channels (Hsu et al, 2004). Collection 6 (C6) of MODIS aerosol products contains merged AOD data combining DB and DT retrievals over land and ocean (Levy et al, 2013;Tao et al, 2015). In this study, the Aqua MODIS C6 level 3 merged AOD at 1° × 1° resolution was used.…”