TROPOMI, operating since 13 October 2017 from a low Earth polar orbit onboard the European Space Agency Sentinel-5 Precursor satellite, is a passive imaging spectrometer with eight spectral bands from ultraviolet (UV) to shortwave infrared (SWIR). It provides daily global measurements of cloud properties and several trace gases , with overpasses at approximately 13:30 local solar time.

In this study, we used the TROP-DLR operational cloud and NO₂ research products , which were recently applied by for LNO_x PE estimates. The TROP-DLR operational cloud and NO₂ research products provide the slant column density (SCD) of NO₂, the associated SCD error, the quality assurance (QA) value, the stratospheric vertical column density (VCD) of NO₂, the stratospheric air mass factor (AMF) of NO₂, the cloud fraction (CF), the cloud optical thickness (COT), and the optical centroid pressure (OCP), with a horizontal resolution at nadir of 3.5km×5.5km. The TROP-DLR NO₂ research product is better suited for estimating LNO_x than the operational TROPOMI NO₂ product, as it provides more reliable cloud properties and NO₂ retrievals over bright surfaces, such as thunderclouds. Cloud properties are derived using the OCRA/ROCINN algorithms within the Clouds-As-Layers (CAL) model , which treats clouds as optically uniform layers and employs a more realistic cloud scattering approach than the Cloud as Reflecting Boundaries (CRB) model implemented in the operational TROPOMI NO₂ product .

The TROP-DLR NO₂ research product further applies the Directionally dependent STRatospheric Estimation Algorithm from Mainz (DSTREAM) to separate the contributions of the troposphere and stratosphere to the NO₂ column density , without relying on atmospheric model inputs. Following , we selected TROPOMI pixels with SCD NO₂ errors below 2×1015molec.cm-2. Pixels with deep convection were defined as those with an effective cloud fraction greater than 0.95 and an OCP below 534 hPa .

The MTG-LI onboard Meteosat-12 satellite was launched on 13 December 2022. Pre-operational Meteosat 12 LI (previously Meteosat Third Generation, MTG) L2 data were publicly released 4 July 2024, and are declared operational on 31 October 2024 at 10:00 UTC . Meteosat 12 LI detects continuously optical signal emitted by lightning from geostationary orbit focused around 0° longitude, providing near-continuous views of the Earth's full disk, including Europe, Africa, the Atlantic Ocean and parts of South America . The CMOS detector of Meteosat 12 LI detects light at 1 kHz sampling frequency within a 1.9 nm wide band centered on the atomic oxygen triplet at 777 nm, with a 4.5 km resolution at sub-satellite point (0° latitude, 0° longitude) . Meteosat 12 LI L2 data include lightning detections at three hierarchical levels: events, groups, and flashes. Events are pixels illumined by lightning during one frame. Events are clustered into a group if they are within one LI frame and spatially contiguous. Finally, flashes are formed by groups separated by less than 330 ms and within about 16 km of each other. Relevant physical information about flashes and groups provided by Meteosat 12 LI L2 data is used in this study, such as flash and groups id, flash and group time, flash duration, flash and group coordinates, total flash radiance, number of groups in a flash, and flash footprint. It is important to note that flash and group properties, such as radiance, duration, and footprint, are influenced not only by the intrinsic characteristics of the lightning but also by the cloud structure . Apart from the flash and group explicitly included in the Meteosat 12 LI L2 data, we have produced two new variables to account for the time continuity of the optical signal of flashes detected by Meteosat 12 LI. These variables are the total number of time-contiguous groups per flash and the ratio of flashes containing at least 5 ms of time-contiguous optical signal to the total number of flashes reported by Meteosat 12 LI in each flash. The reason for defining these new variables is that the duration of flashes reported by Meteosat 12 LI does not contain information about the total time in which the discharge is optically active. It is important to note that time contiguous optical signal of flashes reported by Meteosat 12 LI could be a proxy for continuing current in CG, but also for in-cloud lightning activity. classified lightning flashes reported by the Lightning Imaging Sensor (LIS) as CG flashes with continuing current using a criterion of at least 10 ms of time-continuous signal. However, the GLM, in geostarionary orbit as Meteosat 12 LI, has a low detection efficiency for the optical signal emitted by continuing currents and is more sensitive to in-cloud activity associated with CG lightning . Therefore, we cannot use Meteosat 12 LI solely to distinguish the LNO_x produced by continuing currents or by time contiguous in-cloud activity to determine if continuing currents lasting more than 5 ms produce more LNO_x in CG strokes than shorter CG strokes, as proposed by . However, we can analyze possible relationships between LNO_x and the time continuity of the optical signal of lightning detected by Meteosat 12 LI to better understand the role of continuing currents and in-cloud continuous activity in the emissions of LNO_x. The total number of time-contiguous groups per flash and the ratio of flashes containing at least 5 ms of time-contiguous optical signal to the total number of flashes are determined by using Meteosat 12 LI L2 group data. A time-contiguous group is defined as a set of groups within the same flash where at least one group occurs with a time difference of less than 1.1 ms.

The Ebro Lightning Mapping Array (eLMA) is a VHF time-of-arrival network that was initially established in the Ebro Valley in 2011 and has been operational since then. Between 2023 and 2025, the eLMA network expanded up to 30 stations. The now so-called XCALMA (eXtended CAtalonia Lightning Mapping Array) extends the eLMA coverage mostly to the northern and eastern regions of Catalonia, with 15 new rev. 5 stations deployed by the Meteorological Service of Catalonia . Furthermore, the initial 15 eLMA stations were enhanced to the LMA rev. 5 version . In this study, we use lightning data reported by XCALMA between 4 July 2024 and 4 May 2025, based on measurements from the 24 active stations shown in Fig. . Lightning sources are located using a minimum of 5 or 6 stations. The 3-dimensional locations of VHF sources in time reported by XCALMA are clustered into flashes in a different way than the previous study , which used eLMA data. The data are initially processed using a simple filter to remove scattered sources (fewer than 3 sources in a 7×7×2 km grid cell within a 100 ms interval). A DBSCAN-based flash clustering method was refined using two passes in order to perform better for both convective and stratiform flashes. The first pass groups sources at 300 ms and 4 km separation criteria. The second pass identifies convective and stratiform flash regions, and proceeds to recluster the associated sources with longer time and distance criteria to prevent the large stratiform flashes from being broken into multiple small flashes. At greater distances from the LMA network continuous leader activity within a flash is no longer detectable, and the vertical position accuracy is reduced. For this reason, maps of quality metrics were produced to select storms used for this study. compared lightning detection between XCALMA and Meteosat 12 LI, revealing that the DE of Meteosat 12 LI and XCALMA are comparable for thunderstorms with low lightning activity (below 15 flashes per minute). However, the DE of Meteosat 12 LI decreases significantly for more active thunderstorms. Conversely, the study also reported an increase in the DE of Meteosat 12 LI for higher intracloud (IC) flash rates.

The variables of each flash extracted from XCALMA for this study include the median flash latitude and longitude, flash start time, flash channel length, flash volume, flash duration, and the 98th percentile vertical density score (vdensity98). The vdensity is calculated using the standard deviation of altitude in 10 ms time steps. It clusters horizontally within every 10 ms, and for each cluster, the metric is defined as the square root of the number of sources in the cluster divided by the variance of their altitudes. This metric correlates inversely with the horizontal and, in particular, the vertical dispersion of points around the true location. Subsequently, vdensity98 is calculated per flash, indicating the quality of the best part of the flash. The vdensity98 ranges approximately up to 100, with higher values indicating better quality. In Fig. , we show the 98th percentile vertical density score averaged over all the flashes analyzed in this study and contained in each 0.1°×0.1° latitude and longitude grid boxes.

We incorporate CG lightning data provided by the Lightning Location System (LLS) operated by the Meteorological Service of Catalonia (SMC) . In particular, we calculate the ratio of CG flashes detected by the SMS-LLS to the total number of flashes reported by XCALMA. The SMC-LLS is composed of four LS8000 and three LS-7002 stations (Vaisala Inc.) covering the region of Catalonia, including the Ebro Valley. CG strokes are detected by a Low Frequency (LF) sensor and located using a combination of the Time-of-Arrival/Magnetic Direction Finding techniques . Like other Vaisala's LLSs based on this technology, the flash algorithm implemented in the SMC-LLS relies on the criteria of the U. S. National Lightning Detection Network (NLDN). Throughout the years of operation, the SMC-LLS performance has been experimentally evaluated in successive campaigns by means of electromagnetic field measurements and video recordings of natural lightning , with a CG flash detection efficiency around 90 % and a location accuracy below 1 km. The method selected for grouping CG strokes into flashes significantly impacts the annual count of lightning flashes. Grouping algorithms primarily depend on two key characteristics of a lightning flash: its spatial extent and total duration. Since the mid-1990s upgrade of the U.S. NLDN, other LLSs based on the same technology have adopted the “NLDN” grouping criteria . Under these criteria, a subsequent stroke is grouped with the first return stroke to form a flash if the stroke occurs within 1 s of the first return stroke, its location is within 10 km of the first return stroke, and the time interval between successive strokes is less than or equal to 500 ms. These criteria have remained largely unchanged through subsequent processor upgrades and changes in NLDN ownership . More recently, the International Standard IEC 62858 has also adopted this flash algorithm.

The case study selection is done by the following method. First, we identify candidate dates by filtering days between 4 July 2024, and 4 May 2025, with at least 650 flashes reported by Meteosat 12 LI within the latitude range of 39.5 to 42.9° N and the longitude range of 1.5° W to 4.5° E, and between 07:00 and 14:00 UTC. This spatial region is chosen as it encompasses most of the area where the LMA vdensity98 exceeds 15 (see Fig. ). The 07:00–14:00 UTC time window is selected because the TROPOMI overpass occurs approximately between 12:00 and 14:00 UTC in all cases. This yields 35 candidate dates, as shown in Table S1 in the Supplement.

After this preselection of dates, we analyze the spatial distribution of lightning activity and discard dates where no electrical activity cores are centered in the area where the LMA vdensity98 exceeds 15. Twelve dates are excluded based on this criterion, as detailed in Table S1. We further analyze the remaining dates and exclude those with fewer than 650 flashes reported by Meteosat 12 LI in the 5 h preceding the TROPOMI overpass, or where the average flash age exceeds 3.5 h, thereby excluding an additional 4 dates (see Table S1). The age of each flash defined as the hours between the flash occurrence and the overpass of TROPOMI . Finally, we exclude the date 17 October 2024, as TROPOMI NO₂ observations indicated that the LNO_x was advected outside the LMA-covered area, where most lightning occurred. This results in the selection of the 18 dates shown in Table S1. Finally, we define the region of interest (ROI) for each date based on the more active thunderstorm within the latitude range of 39.5 to 42.9° N and the longitude range of 1.5° W to 4.5° E from the spatial distribution of Meteosat 12 LI lightning flash activity, CF and OCP.

The calculation of the LNO_x PE is based on a modification of the algorithm proposed by and later used by and . The LNO_x PE in units of mol of NO_x produced per flash is calculated as 1LNOxPE(molm-1)=VtropLNOx×A/DE-1×NA×∑iFexp⁡(ti/τ), where F is the total number of flashes reported by Meteosat 12 LI or the LMA in the analyzed region and up to 5 h before the overpass of TROPOMI, ti is the age of each flash defined as the hours between the flash occurrence and the overpass of TROPOMI, A is the area (cm⁻²) of the thunderstorm, defined as the area of TROPOMI pixels satisfying the Deep Convective Constraint (DCC), which are the pixels with a cloud fraction equal or greater than 0.95 , and OCP equal or lower than 534 hPa or with undefined OCP. NA is the Avogadro's number (molec. mol⁻¹). τ is the lifetime of NO_x in the near-field of convection, in hours. The value of τ is uncertain and can vary between 2 h and 2 d depending on the altitude of LNO_x emissions within the particular thunderstorm and subsequent transport. In this study, we primarily assume τ to be 5 h, as it is a commonly employed value in the literature , which also consider τ=3 h, and aligns with the temporal window of flashes included in Eq. (). To account for the variability in results due to this choice, we also performed calculations using τ=3 h. DE stands for the flash detection efficiency, ranging between 0 and 1. We assume a DE of 1 for the LMA and 0.75 for Meteosat 12 LI . Finally, VtropLNOx is the VCD NO_x produced by recent lightning (molec. cm⁻²). and calculated VtropLNOx as the median VCD NO_x over TROPOMI pixels satisfying the DCC criteria minus a given value of background derived from measurements or TROPOMI pixels without flashes in the same day or before, or from previous aircraft measurements close to the investigated region. In this study, we calculate VtropLNOx by using the WRF-Chem simulations as 2VtropLNOx=Nf×VtropNOxf×Rf+Nnf×VtropNOxnf×Rnf/Nf+Nnf, where Nf and Nnf are the total number of TROPOMI pixels satisfying the DCC criteria that contain and do not contain Meteosat 12 LI flash groups, respectively. Rf and Rnf are derived from WRF-Chem simulations (Table ). The advantage of using high-resolution WRF-Chem simulations over other methods is that, by emitting LNO_x at the exact locations where lightning has been reported, the model faithfully reproduces the advection of LNO_x and its interaction with other chemical species. This allows for the determination of the LNO_x ratio relative to the total tropospheric NO_x at each grid point. Although the horizontal resolution of the simulations and TROPOMI pixels is not exactly the same as the horizontal resolution of WRF-Chem simulations, we utilized the coefficients from the table because TROPOMI pixels do not exhibit significant variations from case to case. Additionally, their resolutions are of the same order of magnitude (2km×2km vs 3.5km×5.5km). VtropNOxf and VtropNOxnf are the median values of VCD NO_x over pixels satisfying the DCC criteria that contain and do not contain Meteosat 12 LI flash groups, respectively. Given that VtropNOx is the union of VtropNOxf and VtropNOxnf, it is calculated as 3VtropNOx=SNO2-avgVstratNO2×AMFstrat/AMFLNOx, where SNO2 and AMFstrat are the SCD of NO₂ and the stratospheric AMF from the TROP-DLR NO₂ research product, respectively. Only TROPOMI pixels with SCD NO₂ errors below 2×1019molec.m-2 are used . Finally, AMFLNOx is the AMF of LNO_x derived from WRF-Chem simulations and using the algorithm of (see Sect. ). avgVstratNO2×AMFstrat stands for the average of the product VstratNO2×AMFstrat.

Additionally, we have estimated the averaged LNO_x PE in terms of molec NO_x per meter, per second or per unit of radiance (mW m⁻² sr⁻¹) by modifying Eq. () to better account for the metrics of individual flashes and the influence of their ages as 4LNOxPE(molec.m-1)=VtropLNOx×A/DE-1×∑iFMiexp⁡(ti/τ), where Mi is the metric representing the flash channel length, the radiance, or the duration of the flash i reported by Meteosat 12 LI or the LMA.

As an alternative approach, we modified Eq. () to calculate VtropLNOx as the difference between the median VCD NO_x over TROPOMI pixels satisfying the DCC criteria and the 10th and the 30th percentiles of the tropospheric VCD NO_x over TROPOMI non-flashing pixels (i.e., pixels without detected lightning activity), as previously proposed by . This alternative formulation facilitates comparison with the results from and for the same region.

Figure  shows the TROP-DLR products, lightning measurements and calculated VtropNOx for the case 6 July 2024, while the figures showing the rest of the analyzed dates are provided in Figs. S1–S51. In the majority of the analyzed cases, a distinct enhancement in the SCD NO₂ (Fig. a, b) is observed in or near TROPOMI pixels characterized by high lightning activity in the last 5 h, as in the TROPOMI orbits analyzed in other studies . However, this enhancement is not evident in all cases, particularly on 6 July, 23 August, 25 August, 2 September, and 31 October 2024. Nevertheless, when the VCD NO_x (Fig. e) is calculated using the AMFLNOx, which incorporates cloud properties (Fig. c, d), observation geometry, and simulated LNO_x vertical profiles, the enhancement in TROPOMI pixels with or near lightning activity becomes apparent. In turn, a clear similarity between the spatial distribution of the OCP (Fig. d) and the lightning activity (Fig. f, g) can be seen. The spatial distribution of flashes detected by Meteosat 12 LI and the LMA do not show large discrepancies (Fig. f, g), with the small differences observed likely attributable to the parallax correction applied in Meteosat 12 LI . In fact, larger differences emerge when comparing the spatial distribution of individual flashes (Fig. f, g) and groups detected by Meteosat 12 LI (Fig. h). These differences suggest that relying solely on the flash product may introduce inaccuracies in distinguishing between TROPOMI pixels with and without lightning activity, as lightning flashes coordinates are estimated as the radiance-weighted geometrical centroid of the groups detected by Meteosat 12 LI or by the median coordinates of the sources detected by the LMA. However, each lightning flash may not be fully contained within a single TROPOMI pixel. By using the flash groups reported by Meteosat 12 LI, we can access information about the TROPOMI pixels that actually contain at least a portion of a flash. Note that the VCD NO_x (Fig. e) can have negative values, indicating that the average stratospheric column exceeds the local vertical column . The TROP-DLR products, lightning measurements and calculated VtropNOx for other cases analyzed in this study can be seen in the Supplement (Figs. S1–S51).

The comparison of WRF-Chem simulation and TROPOMI measurements indicates that the CTP (Fig. b) and the OCP (Fig. d) are nearly in agreement, with a slight displace of the simulated high clouds westward. The simulated tropospheric VCD NO_x in areas with high convection (Fig. c) is consistently displaced westward, and lower in about 1 order of magnitude than the tropospheric VCD NO_x calculated from measurements (Fig. e). The obtained disagreement in the maximum simulated and measurements-based tropospheric VCD NO_x can be due to different horizontal resolutions in simulations and measurement, as well as the inherent uncertainties in the WRF-Chem model, such as emission inventories, chemical reaction rates, meteorological inputs, or parameterizations of physical processes.

Figure  and Table S2 shows the basic measurements for each analyzed date. The LMA detects 1.3 to 6.5 times more flashes than the Meteosat 12 LI. The ratio of CG flashes reported by the LLS-SMC to total flashes reported by the LMA ranges between 0.01 and 0.39. There are no significant differences in the average age of flashes reported by Meteosat 12 LI and the LMA. The average OCP ranges between 323.07 and 499.34 hPa, which are typical values in thunderstorms . The percentage of valid TROPOMI pixels satisfying the DCC that contain Meteosat 12 LI lightning flashes with respect to pixels without flashes ranges between 9 % and 46 %. In turn, the ratio of the total number of TROPOMI pixels satisfying the DCC and containing flashes according to the Meteosat 12 LI to the pixels containing flashes according to the LMA ranges between 0.7 and 1.4, indicating that there are no significant differences in the spatial distribution of flashes detected by both instruments.

The characteristics of lightning flashes analyzed in this study and the LNO_x PE estimates are shown in Table . There are notable variations in the characteristics of lightning flashes detected by Meteosat 12 LI. The average footprint ranges from 4.88 to 14.59 Meteosat 12 LI pixels, with a mean value of approximately 7.71 pixels, indicating spatial variability in the optical extent of detected events. The average radiance spans a broad range from 152.69 to 910.57 mW m⁻² sr⁻¹, reflecting substantial differences in the optical intensity of the flashes. This variability may be attributed to differences in the energy released during lightning events and to differences in the cloud opacity. Regarding the average duration of Meteosat 12 LI detected flashes, values range from 159 to 374 ms, with a mean duration of around 230 ms. The average flash channel length reported by the LMA ranges from 12.33 to 75.48 km, and the average flash volume spans from 8.60 to 64.93 km³ (number of 1×1×0.25 km boxes), indicating substantial variability in the spatial extent of lightning channels. Additionally, the average duration of LMA-detected flashes ranges from 170 to 420 ms, with a mean duration of approximately 295 ms, slightly longer than the average duration provided by Meteosat 12 LI. The obtained average LNO_x PE ± the standard deviation with Meteosat 12 LI flash data and τ=5 h is 37 ± 25 mol NO_x per flash, while it is 31 ± 24 mol NO_x per flash when using the LMA lightning flash data. When setting τ=3 h, we obtain 41 ± 29 mol NO_x and 38 ± 31 per flash when using Meteosat 12 LI and LMA lightning data, respectively. The approximately similar LNO_x PE obtained with both lightning data sources suggests that the WRF-Chem simulations used in the estimation of LNO_x PE help mitigate the impact of the difference in the total number of flashes reported by Meteosat 12 LI and the LMA (see Fig. ). The averaged LNO_x PE obtained in this study falls at the lower end of the range of previous estimations in the same region, ranging between 42 and 108 mol NO_x per flash . In turn, the obtained LNO_x PE falls at the lower end of the widely consensus value, ranging between 100 and 400 mol NO_x per flash . However, it is important to mention that the obtained averaged LNO_x PE value is significantly influenced by the production of NO_x per flash assumed in WRF-Chem simulations, as shown in the last four columns of Table . Alternatively, we approximated the background-NO_x by considering the 10th and the 30th percentiles of the tropospheric VCD NO_x over TROPOMI non-flashing pixels (i.e., pixels without detected lightning activity) . Based on LMA lightning data, we obtained 116 ± 89 and 49 ± 47 mol NO_x per flash by considering the 10th and the 30th percentiles, respectively. These values are in agreement with , who employed similar methods to estimate the background-NO_x.

Table  shows the Spearman and Pearson correlation coefficients and corresponding p-values between VtropLNOx (as defined in Eq. ) and various lightning flash characteristics, computed across all TROPOMI pixels included in the 18 analyzed dates. Only pixels with values of VtropLNOx between the 5th and the 95th percentiles are included in the calculation of the correlation coefficients in order to exclude outliers. The lightning characteristics for each TROPOMI pixel were derived by averaging the respective values over all lightning flashes occurring within that pixel. The Spearman correlation coefficients presented in Table  reveal measurable, albeit weak, associations between lightning flash characteristics and VtropLNOx. Among the analyzed variables, footprint exhibits the weakest correlation with VtropLNOx, suggesting a less pronounced influence compared to other flash characteristics. Nevertheless, the results indicate that the properties of lightning flashes, such as length, volume, duration, and radiance, play an appreciable role in the production of LNO_x. The data employed to calculate the correlations presented in Table  are shown in Fig. S52.

Figure  shows a Spearman monotonic correlation heatmap of the most important lightning characteristics and LNO_x PE estimates with τ=5 h. There are strong direct relationships between the average flash channel length (LMA – av. Length), median flash channel length (LMA – med. length) flash volume (LMA – volume) and flash duration (LMA – duration) estimates by the LMA and the average footprint (MTG – footprint), radiance (MTG – rad) and duration (MTG – duration) reported by Meteosat 12 LI. These correlations indicates consistency in the methods used to cluster LMA sources into flashes and Meteosat 12 LI groups into flashes. In turn, the high correlation between durations and geometric variables, such as flash channel length, flash volume and duration, suggests that larger flashes then to have longer durations. There is a significant direct correlation between the flash length and the age of flashes (Flash age), showing that flashes that occur near the sunrise are longer than flashes that occur near the mid-day, which is consistent with previous findings of shorter flashes in more active thunderstorms . In the same manner, there is an even stronger correlation between the age of the flashes and the footprint, radiance and duration reported by Meteosat 12 LI, indicating that Meteosat 12 LI is capable of reporting a longer and brighter signal of flashes during early morning than during the mid-day, due to the lower DE when the sun is close to the zenith. Conversely, a higher DE during the early morning could allow Meteosat 12 LI to capture more weak flashes that would otherwise be missed when the DE is lower. However, the effect of observing a larger portion of bigger flashes when the DE is higher ultimately dominates. As shown by the LMA measurements reported in , small flashes are typically less numerous than large flashes early in the day. The total number of time contiguous groups (MTG – Cont. Groups) and the ratio of flashes with at least 5 ms of time contiguous optical signal to the total number of Meteosat 12 LI (MTG – CC (5 ms)/f) flashes is moderately correlated with the flash channel length, the flash volume and the flash duration provided by the LMA, while they are strongly correlated with the footprint, radiance and duration measured by Meteosat 12 LI.

The ratio of CG flashes to total flashes (CG/f) is strongly correlated with the flash channel length, flash volume, duration, footprint and radiance (Fig. ), indicating that CG flashes are longer in duration, larger and brighter from space than IC flashes. Similar results were obtained by for GLM lightning measurements. The ratio of CG flashes to total flashes reported is moderately correlated with the total number of time contiguous groups and with the ratio of flashes with at least 5 ms of time contiguous optical signal, indicating that CG flashes reported by Meteosat 12 LI tend to have longer time contiguous optical signals detected by Meteosat 12 LI. This may be due to the presence of continuing currents in CG, or to longer in-cloud stroke activity in CG flashes than in IC flashes.

The ratio of the total number of flashes reported by the LMA (N LMA flashes) to the total flashes reported by Meteosat 12 LI (N MTG flashes) is inversely correlated with the OCP (TROP-DLR OCP) (Fig. ), indicating that the detection efficiency of Meteosat 12 LI with respect to the LMA is higher when the cloud tops are lower, possibly due to lower cloud opacity in the travel of the light between the emitting source and Meteosat 12 LI. In turn, the ratio of the total number of flashes reported by the LMA to the total flashes reported by Meteosat 12 LI is inversely correlated with the average footprint and with the ratio of CG flashes to total flashes, indicating that the detection efficiency of Meteosat 12 LI with respect to the LMA is higher for brighter flashes, particularly for CG flashes. The positive correlation observed between the total number of flashes detected by the LMA and the ratio of this quantity to the total number of flashes reported by Meteosat 12 LI suggests that the DE of Meteosat 12 LI decreases for more active thunderstorms, in agreement with .

The analysis of the flash initiation altitude (LMA – start altitude) reveals that lightning initiated at lower altitudes exhibits greater spatial extent, longer duration, and higher radiance as observed by MTG, likely due to the predominance of CG flashes, which tend to be more energetic and prolonged (Fig. ). Additionally, lower initiation altitudes correlate with a higher number of flashes lasting at least 5 ms and elevated OCP values (indicating lower cloud tops). Conversely, lightning initiated at higher altitudes, though typically smaller in size , is more effectively detected by LMA than MTG, suggesting higher DE of LMA than MTG for smaller flashes.

The LNO_x PE is positively correlated with the flash channel length, the flash volume and the flash duration estimates by the LMA and the footprint, radiance, duration and total number of contiguous groups per flash reported by Meteosat 12 LI (Fig. ), indicating that larger, brighter and longer duration flashes produce more LNO_x. The stronger correlations are obtained when comparing the LNO_x PE based on LMA flash measurements with the average flash channel length, the flash volume and the flash duration, in agreement with previous studies . There is a weak positive correlation between the LNO_x PE and the total number of Meteosat 12 LI time contiguous groups, as well as with the ratio of flashes with at least 5 ms of time contiguous optical signal, indicating that time contiguous emissions could contribute to a larger LNO_x PE.

The lack of significant correlations between the LNO_x PE and the average flash age indicates that the assumption of a temporal window of 5 h and a τ=5 h do not introduce a significant bias in the LNO_x PE estimates (Fig. ). In turn, the lack of significant correlations between the LNO_x PE and the average OCP suggests that the calculation of the AMFLNOx does not introduce a significant bias (Figure ). The Spearman monotonic correlation heatmap with τ=3 h is shown in Fig. S53, indicating minor differences with the results obtained by using τ=5 h.

Figure  shows the Pearson linear correlations between the average LNO_x PE and the most important lightning characteristics with τ=5 h, while nearly similar results are shown in Fig. S54 with τ=3 h. The average LNO_x PE is obtained as the average between the LNO_x PE estimates by using both Meteosat 12 LI and the LMA. Figure a, b and c shows a positive linear correlation between the LNO_x PE and the average flash channel length, volume and duration estimated from LMA measurements, respectively. In turn, Figure d, e and f show a positive but weaker linear correlation between the LNO_x PE and the average footprint, radiance and duration based on the Meteosat 12 LI flash product, respectively. reported that the correlation between LNO_x PE and the flash energy reported by GLM ranged between 0.18 and 0.57, which is consistent with the 0.3578 correlation reported in this study (Fig. e).

There is a positively and moderate correlation between the total number of contiguous Meteosat 12 LI groups per flash and the LNO_x PE Fig. d. The correlation between the LNO_x PE and the total number of contiguous groups per flash is lower than the correlation with the total duration measured by the LMA (Fig. c, g). Geostationary lightning detection optical systems detect more efficiently in-cloud activity than cloud-to-ground processes, especially during the occurrence of continuing currents in CG strokes . The lower correlation of the LNO_x PE with the total number of contiguous groups per flash than with the total duration reported by the LMA (0.36 vs 0.68) indicates that there may be lightning activity below clouds not detected by Meteosat 12 LI that contributes to the production of LNO_x. Finally, Fig. h indicates a positive linear correlation between LNO_x PE and the ratio of CG to total flashes.

The linear positive correlations between the LNO_x PE and the characteristics of lightning measured by the LMA (Fig. a–c) are stronger than the correlations with the lightning metrics reported by Meteosat 12 LI (Fig. d–g) especially due to differences in two particular cases, that are the 14 August 2024 and the 4 May 2025. These cases are characterized by medium flashes in terms of flash channel length, volume, duration from the LMA and LNO_x PE, but small and short in terms of the optical signal detected by Meteosat 12 LI. This suggests that the lightning characteristics measured by the LMA are better proxies for the LNO_x PE than Meteosat 12 LI flash properties, potentially due to the effect of cloud cover in the optical signals detected from space.

The obtained LNO_x PE in molecules of NO_x per meter obtained as the LNO_x PE averaged over the 18 analyzed cases divided by the averaged flash channel length with τ=5 h (Table ) is 0.56 × 10²¹ molec. NO_x m⁻¹, which falls in the lower range of previous estimates based on laboratory and aircraft measurements , with theoretical results based on electrodynamical models ranging between 0.045 and 0.106 × 10²¹ molec. NO_x m⁻¹ , and with TROPOMI based estimates in the same region, ranging between 0.8 × 10²¹ and 1.6 × 10²¹ molec. NO_x m⁻¹ . In turn, we have estimated the averaged LNO_x PE in terms of molec NO_x per meter from Eq. (), obtaining 0.48 × 10²¹ molec. NO_x m⁻¹. Additionally, we have estimated the averaged LNO_x PE in terms of molec NO_x per second of discharge by using Eq. (), yielding 6.53 × 10²⁵ molec. NO_x s⁻¹ of duration reported by Meteosat 12 LI, and 6.02 × 10²⁵ molec. NO_x s⁻¹ of duration reported by the LMA. Finally, we have calculated the averaged LNO_x PE in terms of molec NO_x per flash radiance seen by Meteosat 12 LI from Eq. (), obtaining a production of 9.79 × 10²² molec. NO_x per mW m⁻² sr⁻¹.