Abstract
Abstract. The accuracy of space-based nitrogen dioxide (NO2) retrievals from solar backscatter radiances critically depends on a priori knowledge of the vertical profiles of NO2 and aerosol optical properties. This information is used to calculate an air mass factor (AMF), which accounts for atmospheric scattering and is used to convert the measured line-of-sight "slant" columns into vertical columns. In this study we investigate the impact of biomass burning emissions on the AMF in order to quantify NO2 retrieval errors in the Ozone Monitoring Instrument (OMI) products over these sources. Sensitivity analyses are conducted using the Linearized Discrete Ordinate Radiative Transfer (LIDORT) model. The NO2 and aerosol profiles are obtained from a 3-D chemistry-transport model (GEOS-Chem), which uses the Fire Locating and Monitoring of Burning Emissions (FLAMBE) daily biomass burning emission inventory. Aircraft in situ data collected during two field campaigns, the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) and the Dust and Biomass-burning Experiment (DABEX), are used to evaluate the modeled aerosol optical properties and NO2 profiles over Canadian boreal fires and West African savanna fires, respectively. Over both domains, the effect of biomass burning emissions on the AMF through the modified NO2 shape factor can be as high as −60%. A sensitivity analysis also revealed that the effect of aerosol and shape factor perturbations on the AMF is very sensitive to surface reflectance and clouds. As an illustration, the aerosol correction can range from −20 to +100% for different surface reflectances, while the shape factor correction varies from −70 to −20%. Although previous studies have shown that in clear-sky conditions the effect of aerosols on the AMF was in part implicitly accounted for by the modified cloud parameters, here it is suggested that when clouds are present above a surface layer of scattering aerosols, an explicit aerosol correction would be beneficial to the NO2 retrieval. Finally, a new method that uses slant column information to correct for shape-factor-related AMF error over NOx emission sources is proposed, with possible application to near-real-time OMI retrievals.
Highlights
Nitrogen oxides (NOx = NO2 + NO) play a key role in tropospheric chemistry by affecting ozone, atmospheric oxidation, and aerosol formation (Logan, 1983)
The sensitivity of the air mass factor (AMF) to biomass burning emissions is analyzed in order to better characterize and understand NO2 retrieval uncertainties over fire areas
In clear-sky conditions, the effect of biomass burning emissions on the AMF through the modified NO2 shape factor ranges from −60 to 0 % over both boreal and savanna fires
Summary
Recent studies showed that using highresolution terrain information and NO2 profile simulations can result in large differences (more than a factor of 2) in the retrieved NO2 columns compared to products that use a priori data at typical global chemical transport model resolution (Zyrichidou et al, 2013; Heckel et al, 2011; Boersma et al, 2011; Russell et al, 2011; Hains et al, 2010; Zhou et al, 2009) Another approach to tackle the representativeness problem is to use information from the observations themselves, which has been proposed by Lamsal et al (2008) in the context of top-down estimate of surface NO2 concentrations. DABEX, part of the African Monsoon Multidisciplinary Analysis (AMMA) experiment, took place in January 2006 over West Africa (Johnson et al, 2008a; Capes et al, 2008; Redelsperger et al, 2006) These two campaigns provide an unprecedented data set to evaluate modeled aerosol and NO2 profiles that are used in the AMF calculation over different biomass burning regions.
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