Retrieving tropospheric nitrogen dioxide from the Ozone Monitoring Instrument: effects of aerosols, surface reflectance anisotropy, and vertical profile of nitrogen dioxide

摘要

Retrievals of tropospheric nitrogen dioxide (NO₂) from the OzoneMonitoring Instrument (OMI) are subject to errors in the treatments ofaerosols, surface reflectance anisotropy, and vertical profile of NO₂.Here we quantify the influences over China via an improved retrievalprocess. We explicitly account for aerosol optical effects (simulated bynested GEOS-Chem at 0.667° long. × 0.5° lat.and constrained by aerosol measurements), surface reflectance anisotropy,and high-resolution vertical profiles of NO₂ (simulated by GEOS-Chem).Prior to the NO₂ retrieval, we derive the cloud information usingconsistent ancillary assumptions.We compare our retrieval to the widely used DOMINO v2 product, using MAX-DOAS measurements at three urban/suburban sites in East China asreference and focusing the analysis on the 127 OMI pixels (in 30 days) closest to theMAX-DOAS sites. We find that our retrieval reduces the interference ofaerosols on the retrieved cloud properties, thus enhancing the number ofvalid OMI pixels by about 25%. Compared to DOMINO v2, our retrievalbetter captures the day-to-day variability in MAX-DOAS NO₂ data(R2 = 0.96 versus 0.72), due to pixel-specific radiative transfercalculations rather than the use of a look-up table, explicit inclusion ofaerosols, and consideration of surface reflectance anisotropy. Our retrievedNO₂ columns are 54% of the MAX-DOAS data on average, reflecting theinevitable spatial inconsistency between the two types of measurement,errors in MAX-DOAS data, and uncertainties in our OMI retrieval related toaerosols and vertical profile of NO₂.Sensitivity tests show that excluding aerosol optical effects can eitherincrease or decrease the retrieved NO₂ for individual OMI pixels withan average increase by 14%. Excluding aerosols also complexly affects theretrievals of cloud fraction and particularly cloud pressure. Employingvarious surface albedo data sets slightly affects the retrieved NO₂ onaverage (within 10%). The retrieved NO₂ columns increase when theNO₂ profiles are taken from MAX-DOAS retrievals (by 19% on average)or TM4 simulations (by 13%) instead of GEOS-Chem simulations. Ourfindings are also relevant to retrievals of other pollutants (e.g., sulfurdioxide, ormaldehyde, glyoxal) from UV–visible backscatter satelliteinstruments.