Exploring systematic offsets between aerosol products from the two MODIS sensors

Abstract

Abstract. Long-term measurements of global aerosol loading and optical properties are essential for assessing climate-related questions. Using observations of spectral reflectance and radiance, the dark-target (DT) aerosol retrieval algorithm is applied to Moderate Resolution Imaging Spectroradiometer sensors on both Terra (MODIS-T) and Aqua (MODIS-A) satellites, deriving products (known as MOD04 and MYD04, respectively) of global aerosol optical depth (AOD at 0.55 µm) over both land and ocean, and an Ångström exponent (AE derived from 0.55 and 0.86 µm) over ocean. Here, we analyze the overlapping time series (since mid-2002) of the Collection 6 (C6) aerosol products. Global monthly mean AOD from MOD04 (Terra with morning overpass) is consistently higher than MYD04 (Aqua with afternoon overpass) by ∼ 13 % (∼ 0.02 over land and ∼ 0.015 over ocean), and this offset (MOD04 – MYD04) has seasonal as well as long-term variability. Focusing on 2008 and deriving yearly gridded mean AOD and AE, we find that, over ocean, the MOD04 (morning) AOD is higher and the AE is lower. Over land, there is more variability, but only biomass-burning regions tend to have AOD lower for MOD04. Using simulated aerosol fields from the Goddard Earth Observing System (GEOS-5) Earth system model and sampling separately (in time and space) along each MODIS-observed swath during 2008, the magnitudes of morning versus afternoon offsets of AOD and AE are smaller than those in the C6 products. Since the differences are not easily attributed to either aerosol diurnal cycles or sampling issues, we test additional corrections to the input reflectance data. The first, known as C6+, corrects for long-term changes to each sensor's polarization sensitivity and the response versus the scan angle and to cross-calibration from MODIS-T to MODIS-A. A second convolves the detrending and cross-calibration into scaling factors. Each method was applied upstream of the aerosol retrieval using 2008 data. While both methods reduced the overall AOD offset over land from 0.02 to 0.01, neither significantly reduced the AOD offset over ocean. The overall negative AE offset was reduced. A collection (C6.1) of all MODIS Atmosphere products was released, but we expect that the C6.1 aerosol products will maintain similar overall AOD and AE offsets. We conclude that (a) users should not interpret global differences between Terra and Aqua aerosol products as representing a true diurnal signal in the aerosol. (b) Because the MODIS-A product appears to have an overall smaller bias compared to ground-truth data, it may be more suitable for some applications. However (c), since the AOD offset is only ∼ 0.02 and within the noise level for single retrievals, both MODIS products may be adequate for most applications.

Validation, Stability, and Consistency of MODIS Collection 6.1 and VIIRS Version 1 Deep Blue Aerosol Data Over Land

“…For V/A and V/T, these are 35% and 32% for the daily and 56% and 53% for the gridded median offsets, respectively, somewhat lower and largely driven by differences in well‐sampled tropical and midlatitude deserts. For T/A, in most areas, Terra is slightly higher than Aqua; this was also observed by Levy et al (), looking at DT retrieval results. Levy et al () investigated the magnitudes of calibration differences and diurnal variations on the Terra‐Aqua offset and found that neither alone was responsible for it.…”

“…For T/A, in most areas, Terra is slightly higher than Aqua; this was also observed by Levy et al (), looking at DT retrieval results. Levy et al () investigated the magnitudes of calibration differences and diurnal variations on the Terra‐Aqua offset and found that neither alone was responsible for it. DB uses some different bands and also applies 412‐ and 470‐nm band calibration degradation corrections (Sayer, Hsu, Bettenhausen, Jeong, & Meister, ) which DT does not.…”

Estimates of African Dust Deposition Along the Trans‐Atlantic Transit Using the Decadelong Record of Aerosol Measurements from CALIOP, MODIS, MISR, and IASI

“…In this study, we derive daily over‐ocean DOD from the Aqua/MODIS Collection 6 (C6) product (Levy et al, , ) at 1°×1° resolution, following a method first developed by Kaufman et al () and then modified by Yu et al (). Briefly, this method partitions AOD into three components: dust, marine aerosol, and combustion (or pollution) aerosol.…”

Retrieval of Daily PM2.5 Concentrations Using Nonlinear Methods: A Case Study of the Beijing–Tianjin–Hebei Region, China

“…Considering the differences in potential calibration, mission lifespan, and transit time of the Terra and Aqua sensors [50], we trained two sets of models with the same aforementioned variables for both sensors in parallel, and the same training and testing data were employed to deal with each regression model for both datasets, respectively; this allowed for the comparability of the performances of the different models. All of the parameters mentioned in Section 2.1 were used.…”