First estimates of global free-tropospheric NOsub2/sub abundances derived using a cloud-slicing technique applied to satellite observations from the Aura Ozone Monitoring Instrument (OMI)

摘要

We derive free-tropospheric NO₂ volume mixing ratios (VMRs)by applying a cloud-slicing technique to data from the Ozone Monitoring Instrument (OMI)on the Aura satellite. In the cloud-slicing approach, the slope ofthe above-cloud NO₂ column versus the cloud scene pressure isproportional to the NO₂ VMR. In this work, we use a sampleof nearby OMI pixel data from a single orbit for the linear fit. TheOMI data include cloud scene pressures from the rotational-Ramanalgorithm and above-cloud NO₂ vertical column density (VCD)(defined as the NO₂ column from the cloud scene pressure tothe top of the atmosphere) from a differential optical absorptionspectroscopy (DOAS) algorithm.We compare OMI-derived NO₂ VMRs with in situaircraft profiles measured during the NASA Intercontinental ChemicalTransport Experiment Phase B (INTEX-B) campaign in 2006. Theagreement is generally within the estimated uncertainties whenappropriate data screening is applied. We then derive a globalseasonal climatology of free-tropospheric NO₂ VMR in cloudyconditions. Enhanced NO₂ in the free troposphere commonlyappears near polluted urban locations where NO₂ produced inthe boundary layer may be transported vertically out of the boundarylayer and then horizontally away from the source. Signatures oflightning NO₂ are also shown throughout low and middlelatitude regions in summer months. A profile analysis of our cloud-slicing data indicates signatures oflightning-generated NO₂ in the upper troposphere.Comparison of the climatology with simulations from the globalmodeling initiative (GMI) for cloudy conditions (cloud opticaldepth > 10) shows similarities in the spatial patterns ofcontinental pollution outflow. However, there are also somedifferences in the seasonal variation of free-troposphericNO₂ VMRs near highly populated regions and in areas affectedby lightning-generated NO_x.