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Airborne sunphotometer measurements are used to evaluate retrievals ofextinction aerosol optical depth (AOD) from spatially coincident andtemporally near-coincident measurements by the Ozone Monitoring Instrument(OMI) aboard the Aura satellite during the March 2006 MegacityInitiative-Local And Global Research Observations/Phase B of theIntercontinental Chemical Transport Experiment (MILAGRO/INTEX-B). The14-channel NASA Ames Airborne Tracking Sunphotometer (AATS) flew on ninemissions over the Gulf of Mexico and four in or near the Mexico City area.Retrievals of AOD from near-coincident AATS and OMI measurements arecompared for three flights over the Gulf of Mexico for flight segments whenthe aircraft flew at altitudes 60–70 m above sea level, and for one flightover the Mexico City area where the aircraft was restricted to altitudes~320–800 m above ground level over the rural area and ~550–750 mover the city. OMI-measured top of atmosphere (TOA) reflectances areroutinely inverted to yield aerosol products such as AOD and aerosolabsorption optical depth (AAOD) using two different retrieval algorithms: anear-UV (OMAERUV) and a multiwavelength (OMAERO) technique. This study usesthe archived Collection 3 data products from both algorithms. In particular,AATS and OMI AOD comparisons are presented for AATS data acquired in 20OMAERUV retrieval pixels (15 over water) and 19 OMAERO pixels (also 15 overwater). At least four pixels for one of the over-water coincidences and allpixels for the over-land case were cloud-free. Coincident AOD retrievalsfrom 17 pixels of the Moderate Resolution Imaging Spectroradiometer (MODIS)aboard Aqua are available for two of the over-water flights and are shown toagree with AATS AODs to within root mean square (RMS) differences of0.00–0.06, depending on wavelength. Near-coincident ground-based AODmeasurements from ground-based sun/sky radiometers operated as part of theAerosol Robotic Network (AERONET) at three sites in and near Mexico City arealso shown and are generally consistent with the AATS AODs (which excludeany AOD below the aircraft) both in magnitude and spectral dependence. TheOMAERUV algorithm retrieves AODs corresponding to a non-absorbing aerosolmodel for all three over-water comparisons whereas the OMAERO algorithmretrieves best-fit AODs corresponding to an absorbing biomass-burningaerosol model for two of the three over-water cases. For the four cloud-freepixels in one over-water coincidence (10 March), the OMAERUV retrievalsunderestimate the AATS AODs by ~0.20, which exceeds the expectedretrieval uncertainty, but retrieved AODs agree with AATS values withinuncertainties for the other two over-water events. When OMAERO retrievesAODs corresponding to a biomass-burning aerosol over water, the valuessignificantly overestimate the AATS AODs (by up to 0.55). For the MexicoCity coincidence, comparisons are presented for a non-urban region~50–70 km northeast of the city and for a site near the center of the city.OMAERUV retrievals are consistent with AERONET AOD magnitudes for thenon-urban site, but are nearly double the AATS and AERONET AODs (withdifferences of up to 0.29) in the center of the city. Corresponding OMAEROretrievals exceed the AATS and/or AERONET AODs by factors of 3 to 10

atmospheric chemistry and physics

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