Vertical profiles of lightning-produced NO₂ enhancements in the upper troposphere observed by OSIRIS

Abstract. The purpose of this study is to perform a global search of the upper troposphere (z≥10 km) for enhancements of nitrogen dioxide and determine their sources. This is the first application of satellite-based limb scattering to study upper tropospheric NO₂. We have searched two years (May 2003–May 2005) of OSIRIS (Optical Spectrograph and Infrared Imager System) operational NO₂ concentrations (version 2.3/2.4) to find large enhancements in the observations by comparing with photochemical box model calculations and by identifying local maxima in NO₂ volume mixing ratio. We find that lightning is the main production mechanism responsible for the large enhancements in OSIRIS NO₂ observations as expected. Similar patterns in the abundances and spatial distribution of the NO₂ enhancements are obtained by perturbing the lightning within the GEOS-Chem 3-dimensional chemical transport model. In most cases, the presence of lightning is confirmed with coincident imagery from LIS (Lightning Imaging Sensor) and the spatial extent of the NO₂ enhancement is mapped using nadir observations of tropospheric NO₂ at high spatial resolution from SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) and OMI (Ozone Monitoring Instrument). The combination of the lightning and chemical sensors allows us to investigate globally the role of lightning to the abundance of NO₂ in the upper troposphere (UT). Lightning contributes 60% of the tropical upper tropospheric NO₂ in GEOS-Chem simulations. The spatial and temporal distribution of NO₂ enhancements from lightning (May 2003–May 2005) is investigated. The enhancements generally occur at 12 to 13 km more frequently than at 10 to 11 km. This is consistent with the notion that most of the NO₂ is forming and persisting near the cloud top altitude in the tropical upper troposphere. The latitudinal distribution is mostly as expected. In general, the thunderstorms exhibiting weaker vertical development (e.g. 11≤z≤13 km) extend latitudinally as far poleward as 45° but the thunderstorms with stronger vertical development (z≥14 km) tend to be located within 33° of the equator. There is also the expected hemispheric asymmetry in the frequency of the NO₂ enhancements, as most were observed in the northern hemisphere for the period analyzed.