# On the Potential of the RST-FLARE Algorithm for Gas Flaring Characterization from Space
## 5 Discussion
Besides the comparison to independent data sets, even the analysis of the temporal trend of the detected flaring sites (black line in Figure 5), seem to confirm the accuracy of the results, allowing for a reconstruction of the flaring history in the region. The Niger Delta basin is strongly affected by onshore gas flaring, with a number of flaring sites almost double respect to the ones identified offshore. In both cases, an overall general reduction in the number of active flare sites between 2000 and 2014 was observed, with a significant decreasing trend that started in 2003 onshore and in 2004 offshore, respectively. Especially for the onshore sites, the general downward trend in the number of flares is due to several causes, mainly related to the operations carried out since 2002 for the installation of associated gas gathering infrastructures at various oilfields [12; 45; 46; 47]. Moreover, the Niger Delta militants' actions occurred between 2006 and 2009, which severely disrupted oil and gas production in the region, may explain the observed reduction in total number of flaring sites in the same period [48]. Since 2014 (for onshore) and 2015 (for offshore), a new increasing in flaring sites seems to be started (of about 64% for land and 36% for sea areas). No information on gas flaring sites, both from official sources and independent satellite methods, have been found to explain such a significant relative variation. It can be probably due to new investments in oil plants equipped with modern infrastructures able to collect, treat, transport and utilize the associated gases. Of course, further analyses have to be carried out to assess these results. Besides, the satisfactory agreement with MODET (between in 74% and 95%) strengthen the RST achievements, confirming the high variability among the years in the number of operational flares, even if detected with a different level of accuracy by the two algorithms. The different aggregation scheme, bi-monthly (only December and January) over 15 years (2000-2014) for MODET, monthly (for each calendar month) for the 2000-2016 period for RST-FLARE, may be one of the possible causes of the observed discrepancies together with the different detection methods and thresholds. Concerning the flaring sites localization, RST-FLARE shows again a very good agreement (95% of spatial matching) when compared to results derived by means of VNF, a fully independent product based on VIIRS data offering improved spectral (with a SWIR band capability) and spatial (~750 m of pixel size in SWIR) resolution. Such a cross-validation between two independent satellite-based products reveals to be helpful in a twofold perspective: (i) common detections strengthened the accuracy of each system in providing reliable and consistent information; (ii) on the other hand, unmatched detections might reveal possible false positives (a couple of examples are shown in Figure 13), allowing for further \"fine tunings\" and optimization of the products. 2. A further possible issue in the flaring sites characterization may be due to the variability in sampling and cloud cover, which could affect the RST-FLARE results consistence between years. In order to reduce these effects, a coefficient \(\alpha_{j,y}\) has been defined and computed for each of the detected flaring sites within the single year. It allows normalizing, between investigated years, the source emissive power (quantified in terms of Brightness Temperature Excess, BTE, see Equation (2)) according to the imagery sampling caused by the cloud coverage and the satellite view geometry. 3. Additionally, the MIR MODIS signal, used in the BTE metric computation, may be affected by the atmosphere. Although a number of papers in literature [49; 50] do not use atmospherically corrected MODIS MIR radiances and, more recently, [51] suggested considering the atmospheric transmittance of the 4-\(\upmu\)m channel (MODIS band 22) equal to 1 when computing the Collection 6 MODIS fire product, atmospheric effects might be not completely negligible.