NO2 and NO (together referred to as NOx) are trace gases important in ozone chemistry in both the troposphere and stratosphere. Worldwide, anthropogenic emissions of NOx dominate the NOx budget. However, considerable uncertainty surrounds emission rates from natural sources (lightning and soil). Lightning is the largest nonanthropogenic source of NOx in the free troposphere (hereafter, we refer to lightning-generated NOx as LNOx). The most accepted estimates of global LNOx production range from 2 to 8 Tg (N) yr−1 [Schumann and Huntrieser, 2007], or about 10–15% of the total NOx budget. The effects of lightning are felt most strongly in the middle and upper part of the troposphere, where this source plays the dominant role in controlling NOx and ozone amounts, despite the greater overall magnitude of the anthropogenic NOx emissions [R. Zhang et al., 2003]. In this region, NOx has a lifetime of 5–10 times longer than the ∼1 day lifetime in the lower troposphere [Jaeglé et al., 1998; Martin et al., 2007] so that a given amount of LNOx in the upper troposphere can have a greater impact on ozone chemistry. Ozone production can proceed at rates of up to 10 ppbv per day in the lightning-enhanced convective outflow plumes of ozone precursors [DeCaria et al., 2005; Ott et al., 2007; Pickering et al., 1996]. Ozone is the third most important greenhouse gas, and ozone enhancements near the tropopause have the greatest effect on its radiative forcing. Therefore, additional ozone produced downwind of thunderstorm events is particularly effective in climate forcing.
 Recent studies have attempted to constrain the magnitude of the global LNOx source using satellite observations. Bond et al.  combined satellite measurements of lightning with models based on climatological parameterizations of LNOx production to infer a global production rate of 6.3 Tg (N) yr−1. Other studies have used satellite measurements of NO2 directly in their calculations. Beirle et al.  used Global Ozone Monitoring Instrument (GOME) NO2 column densities over Australia and data from the Lightning Imaging Sensor (LIS) to estimate that lightning produces 2.8 Tg (N) yr−1, but the range of uncertainty was large (0.8–14 Tg (N) yr−1). Beirle et al.  studied LNOx production from a storm system in the Gulf of Mexico using GOME data and National Lightning Detection Network (NLDN) observations. Extrapolating their findings to the global scale, they estimated an LNOx source of 1.7 Tg (N) yr−1 with a range of uncertainty from 0.6 to 4.7 Tg (N) yr−1. Boersma et al.  used GOME NO2 observations and the TM3 global chemical transport model with two different LNOx parameterizations and concluded that LNOx production was between 1.1 and 6.4 Tg (N) yr−1. In their study, stratospheric NO2 was estimated and removed from the data by an assimilation approach using the TM3 model. Martin et al.  used Goddard Earth Observing System chemistry model (GEOS-Chem) simulations in conjunction with space-based observations of NOx, ozone, and nitric acid to estimate LNOx production of 6 ± 2 Tg (N) yr−1. Their NO2 data were obtained using the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography/chemistry (SCIAMACHY) instrument and analyzed with methods similar to those described in the work of Martin et al. . In general, satellite observations of LNOx are challenging because of issues of cloud cover and because most upper tropospheric NOx exists in the form of NO, which is not directly detectable from space. Beirle et al.  have demonstrated, through the use of cloud/chemistry and radiative transfer modeling, that nadir-viewing satellites likely have a sensitivity near or less than 50% for LNOx produced in a typical marine convective system. Therefore, when satellite data are used to estimate LNOx, this sensitivity factor must be taken into account.
 A critical quantity in many studies that attempt to infer global production rates is the rate of NOx generation in individual thunderstorms, often expressed as the number of moles of NOx produced per lightning flash. Estimates for this NOx generation can vary by at least an order of magnitude [X. Zhang et al., 2003], with many estimates between 50 and 700 mol/flash [Ott et al., 2007, 2010, and references therein]. From studies of individual storms, these estimates have been extrapolated to provide global LNOx production rates. However, such extrapolations are complicated by variations in pressure level, intensity, and length of lightning strokes for tropical versus midlatitude storms. The satellite investigation by Beirle et al.  found that, on average, lightning in the Gulf of Mexico system produced 90 mol/flash NO. Modeling studies [e.g., Ott et al., 2010] have examined how these parameters vary for intracloud (IC) and cloud-to-ground (CG) flashes in different latitude regions. The variations may result in different LNOx production rates, PIC and PCG, for IC and CG flashes, respectively. Although early investigations [e.g., Price et al., 1997] suggest that the value of the ratio PIC/PCG is much less than 1 (∼0.1), more recent studies provide evidence that the value may be near unity or even greater [DeCaria et al., 2005; Fehr et al., 2004; Ott et al., 2007, 2010; X. Zhang et al., 2003]. Huntrieser et al.  suggest that overall production of LNOx per flash, PIC+CG, may be 2–8 times larger in subtropical and midlatitude storms than in tropical storms. This result may be due to longer flash channel lengths outside the tropics in regions of greater vertical wind shear.
 In this paper we examine four tropical convective events from the NASA Tropical Composition, Clouds, and Climate Coupling (TC4) campaign [Toon et al., 2010] and compute the number of moles of LNOx per flash using a combination of data from the Ozone Monitoring Instrument (OMI) instrument on the Aura satellite, in situ observations from the DC-8 aircraft, global chemical transport model output, and ground-based lightning flash observations. Our approach differs from those of previous satellite investigations in the methods used to remove the stratospheric and tropospheric background (as described later in this paper), and because we derive LNOx production per flash directly from an estimate of accumulated LNOx and lightning flash counts, rather than by adjusting model parameters to match the satellite data. Our use of OMI data is better suited to individual case studies than are the lower-resolution GOME and SCIAMACHY data. We also focus exclusively on tropical-latitude storms that occurred over ocean regions. In these regions convection is less tied to late afternoon diurnal cycles (and hence more likely to occur before or near the OMI overpass time of ∼1345 local time [LT]), and NO2 contamination from anthropogenic sources is less [Beirle et al., 2009]. We use measured OMI NO2 columns and CG flash counts. From these we estimate the LNOx columns and the total flashes (IC + CG) and combine results to obtain the PCG+IC for the storms on the 4 days studied. We then examine our results in the context of estimates of LNOx per flash from other studies.
 Section 2 describes the data we used in our analyses. Section 3 details the calculations that were performed in the LNOx retrieval process and describes how we used the retrieved LNOx values, in combination with flash rates, to estimate production per flash. Results are presented in section 4. We discuss the implications of the derived values and their uncertainties in section 5 and draw conclusions in section 6.