Comparison of aerosol optical depths from the Ozone Monitoring Instrument (OMI) on Aura with results from airborne sunphotometry, other space and ground measurements during MILAGRO/INTEX-B

Abstract

Airborne sunphotometer measurements are usedto evaluate retrievals of extinction aerosol optical depth(AOD) from spatially coincident and temporally nearcoincidentmeasurements by the Ozone Monitoring Instrument(OMI) aboard the Aura satellite during the March 2006Megacity Initiative-Local And Global Research Observations/Phase B of the Intercontinental Chemical TransportExperiment (MILAGRO/INTEX-B). The 14-channel NASAAmes Airborne Tracking Sunphotometer (AATS) flew onnine missions over the Gulf of Mexico and four in or near theMexico City area. Retrievals of AOD from near-coincidentAATS and OMI measurements are compared for three flightsover the Gulf of Mexico for flight segments when the aircraftflew at altitudes 60-70m above sea level, and forone flight over the Mexico City area where the aircraftwas restricted to altitudes 320-800m above ground levelover the rural area and 550-750m over the city. OMImeasuredtop of atmosphere (TOA) reflectances are routinelyinverted to yield aerosol products such as AOD andaerosol absorption optical depth (AAOD) using two differentretrieval algorithms: a near-UV (OMAERUV) and amultiwavelength (OMAERO) technique. This study usesthe archived Collection 3 data products from both algorithms.In particular, AATS and OMI AOD comparisonsare presented for AATS data acquired in 20 OMAERUVretrieval pixels (15 over water) and 19 OMAERO pixels(also 15 over water). At least four pixels for one of theover-water coincidences and all pixels for the over-land casewere cloud-free. Coincident AOD retrievals from 17 pixelsof the Moderate Resolution Imaging Spectroradiometer(MODIS) aboard Aqua are available for two of the overwaterflights and are shown to agree with AATS AODs towithin root mean square (RMS) differences of 0.00-0.06, dependingon wavelength. Near-coincident ground-based AODmeasurements from ground-based sun/sky radiometers operatedas part of the Aerosol Robotic Network (AERONET)at three sites in and near Mexico City are also shown andare generally consistent with the AATS AODs (which excludeany AOD below the aircraft) both in magnitude andspectral dependence. The OMAERUV algorithm retrievesAODs corresponding to a non-absorbing aerosol model forall three over-water comparisons, whereas the OMAERO algorithmretrieves best-fit AODs corresponding to an absorbingbiomass-burning aerosol model for two of the three overwatercases. For the four cloud-free pixels in one overwatercoincidence (10 March), the OMAERUV retrievalsunderestimate the AATS AODs by 0.20, which exceedsthe expected retrieval uncertainty, but retrieved AODs agreewith AATS values within uncertainties for the other twoover-water events. When OMAERO retrieves AODs correspondingto a biomass-burning aerosol over water, the valuessignificantly overestimate the AATS AODs (by up to0.55). For the Mexico City coincidence, comparisons arepresented for a non-urban region 50-70 km northeast of thecity and for a site near the center of the city. OMAERUVretrievals are consistent with AERONET AOD magnitudesfor the non-urban site, but are nearly double the AATS andAERONET AODs (with differences of up to 0.29) in the centerof the city. Corresponding OMAERO retrievals exceedthe AATS and/or AERONET AODs by factors of 3 to 10.