The Collection 6 MODIS aerosol products over land and ocean

Abstract

Abstract. The twin Moderate resolution Imaging Spectroradiometer (MODIS) sensors have been flying on Terra since 2000 and Aqua since 2002, creating an extensive data set of global Earth observations. Here, we introduce the Collection 6 (C6) algorithm to retrieve aerosol optical depth (AOD) and aerosol size parameters from MODIS-observed spectral reflectance. While not a major overhaul from the previous Collection 5 (C5) version, there are enough changes that there are significant impacts to the products and their interpretation. The C6 aerosol data set will be created from three separate retrieval algorithms that operate over different surface types. These are the two "Dark Target" (DT) algorithms for retrieving (1) over ocean (dark in visible and longer wavelengths) and (2) over vegetated/dark-soiled land (dark in the visible), plus the "Deep Blue" (DB) algorithm developed originally for retrieving (3) over desert/arid land (bright in the visible). Here, we focus on DT-ocean and DTland (#1 and #2). We have updated assumptions for central wavelengths, Rayleigh optical depths and gas (H 2 O, O 3 , CO 2 , etc.) absorption corrections, while relaxing the solar zenith angle limit (up to ≤ 84 • ) to increase poleward coverage. For DT-land, we have updated the cloud mask to allow heavy smoke retrievals, fine-tuned the assignments for aerosol type as function of season/location, corrected bugs in the Quality Assurance (QA) logic, and added diagnostic parameters such topographic altitude. For DT-ocean, improvements include a revised cloud mask for thin-cirrus detection, inclusion of wind speed dependence on the surface reflectance, updates to logic of QA Confidence flag (QAC) assignment, and additions of important diagnostic information.At the same time, we quantified how "upstream" changes to instrument calibration, land/sea masking and cloud masking will also impact the statistics of global AOD, and affect Terra and Aqua differently. For Aqua, all changes will result in reduced global AOD (by 0.02) over ocean and increased AOD (by 0.02) over land, along with changes in spatial coverage. We compared preliminary data to surface-based sun photometer data, and show that C6 should improve upon C5. C6 will include a merged DT/DB product over semi-arid land surfaces for reduced-gap coverage and better visualization, and new information about clouds in the aerosol field. Responding to the needs of the air quality community, in addition to the standard 10 km product, C6 will include a global (DT-land and DT-ocean) aerosol product at 3 km resolution.

Comparison of aerosol optical depth from satellite (MODIS), sun photometer and broadband pyrheliometer ground-based observations in Cuba

“…Under these conditions, the DB band provides a better signal than the visible wavelengths, improving the information content for aerosol retrievals (Hsu et al, 2004(Hsu et al, , 2006 due to lower surface albedo at this short wavelength. Levy et al (2013) provide a detailed explanation of basic MODIS retrieval concepts and improvements to the DT algorithm in Collection 6 for aerosol products. In addition, Hsu et al (2013) give a detailed explanation of the DB algorithm improvements in Collection 6.…”

“…In addition, we selected the AE retrieved over land from the DB algorithm using the corresponding pairs of AOD values (412/470 or 470/650 nm) with the highest quality (quality flag = 2, 3), since the DT algorithm only retrieves the AE over the ocean (Table B1 in Levy et al, 2013). Table 1 lists the aerosol products used in the present study.…”

“…Following Levy et al (2013), we summarise the MODIS calculus chain. MODIS Level 0 (L0) is the basic data file containing raw observations from the sensors.…”

“…These are the two most common parameters for describing atmospheric aerosol optical properties. Based on the assumptions about the properties of the Earth's surface and the aerosol type expected over these surfaces, the MODIS Atmosphere team developed three algorithms for processing MODIS observations (Levy et al, 2013). Regions which appear visually dark from space, referred to as dark target (DT), include the algorithm assumptions for vegetated land surfaces (Kaufman et al, 1997a, b) and for remote ocean regions (Tanré et al, 1997).…”

“…This comparative analysis does not aim to be a validation study of the MODIS sensor since many works during the long history of the MODIS sensor on the Terra and Aqua platforms have sought to improve its features (these include Kaufman et al, 1997a, b;Tanré et al, 1997;Remer et al, 2002Remer et al, , 2005Remer et al, , 2006Hsu, et al, 2004Hsu, et al, , 2006Hsu, et al, , 2013Levy et al, 2007Levy et al, , 2009Levy et al, , 2010Levy et al, , 2013Levy et al, , 2015Sayer et al, 2013Sayer et al, , 2014 https://darktarget.gsfc.nasa.gov/atbd/overview, last access: 9 April 2018). However, compared to other areas of the world, no studies have been reported in the Caribbean region and in Cuba in particular (Papadimas et al, 2009;Mishchenko et al, 2010;Kahn et al, 2011;Bennouna et al, 2011Bennouna et al, , 2013Witte et al, 2011;Gkikas et al, 2013Gkikas et al, , 2016Levy et al, 2015).…”