Global aerosol climatology from the MODIS satellite sensors

Abstract

[1] The recently released Collection 5 Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol products provide a consistent record of the Earth's aerosol system. Comparing with ground-based AERONET observations of aerosol optical depth (AOD) we find that Collection 5 MODIS aerosol products estimate AOD to within expected accuracy more than 60% of the time over ocean and more than 72% of the time over land. This is similar to previous results for ocean and better than the previous results for land. However, the new collection introduces a 0.015 offset between the Terra and Aqua global mean AOD over ocean, where none existed previously. Aqua conforms to previous values and expectations while Terra is higher than what had been expected. The cause of the offset is unknown, but changes to calibration are a possible explanation. Even though Terra's higher ocean AOD is unexpected and unexplained, we present climatological analyses of data from both sensors. We find that the multiannual global mean AOD at 550 nm over oceans is 0.13 for Aqua and 0.14 for Terra, and over land it is 0.19 in both Aqua and Terra. AOD in situations with 80% cloud fraction are twice the global mean values, although such situations occur only 2% of the time over ocean and less than 1% of the time over land. Aerosol particle size associated with these very cloudy situations does not show a drastic change over ocean, but does over land. Regionally, aerosol amounts vary from polluted areas such as east Asia and India, to the cleanest regions such as Australia and the northern continents. As AOD increases over maritime background conditions, fine mode aerosol dominates over dust over all oceans, except over the tropical Atlantic downwind of the Sahara and during some months over the Arabian Sea.

Response to “Toward unified satellite climatology of aerosol properties. 3. MODIS versus MISR versus AERONET”

“…Explanations for the most prominent differences between the MISR Standard aerosol products and validation data are given in several of the papers cited above, making use of other studies where individual effects for dust [13], spherical absorbing and non-absorbing particles [7], thin cirrus [17], and algorithmic issues [11] are explored for representative cases with the MISR Research aerosol retrieval algorithm. Detailed analysis of MODIS issues is presented in [2,4,14,20,21,[24][25][26][27].…”

“…AOD Confidence Envelopes. Both the MISR and MODIS teams define confidence envelopes to assess statistical agreement between satellite and near-coincident AERONET sun photometer AOD results [12,20]. For example, the MISR team typically reports the fraction of cases falling within the larger of (0.05 or 0.2 x AOD), and the larger of (0.03 or 0.1 x AOD), representing broader and more restrictive envelopes, respectively [10].…”

“…The MISR and MODIS instrument teams, as well as many other research groups, have performed such evaluations using a variety of methods, ranging from theoretical sensitivity studies to comparisons with other satellite and suborbital data sets [e.g., 2,[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Over the past ten years, the exchange of data, information, and ideas represented by this body of work has yielded better characterization of existing products, as well as algorithm upgrades.…”

“…(4)Validation of either AOD product must rely on direct comparisons with ground-truth, stratified based on observational conditions [e.g., 10,14,20,29] to separate the systematic differences between satellite products that different algorithmic assumptions can produce. Note that retrieved AOD error, when assessed relative to ground truth, is often smaller than the MISR-MODIS AOD deviations, again because of systematic differences in algorithm assumptions [11,12].…”

Characterizing the 2015 Indonesia Fire Event Using Modified MODIS Aerosol Retrievals

“…The pre-calculated LUT only extends to AOD of 5.0 (at 0.55 µm). For an algorithm that aims to retrieve aerosol globally, this AOD cap is reasonable (Remer et al, 2008). The retrieved DT AOD over land has an expected uncertainty of ± 5 (0.05+15%AOD).…”