**5. Conclusions**

It has been found from the present study that over Taipei City the enhancement of CG lightning activity follows closely the increase of the apparent surface temperature, which indicates the heat island, thus corroborate the thermal hypothesis. In addition to population growth and land use increase, increased pollution and anthropogenic aerosols are also good indicators of urbanization of any city. Therefore, an assessment of the possible impact of aerosol microphysical forcing on the formation of urban CG lightning is equally important. Apart from the evidence in support of thermal hypothesis, a positive linear correlation between the number of CG flashes and a higher concentration of both SO2 and PM10 has also been found, indicating that the aerosol hypothesis is equally important to support the increase of the number of CG flashes. Positive correlations of 0.63 and 0.71 are found for the PM10 and SO2 concentrations, respectively, when compared separately with the number of CG flashes. This higher positive correlation strongly supports that aerosols play a key role in the enhancement of lightning activity over Taipei City. Williams et al. [44] first proposed that under continental and dirty boundary layer conditions, the available liquid water in the storm updraft is shared amongst an innumerable number of small droplets, thereby suppressing the mean droplet size and thwarting the coalescence process. As a result, the cloud water reaches the mixed phase region to participate in creating excess cloud buoyancy, in precipitation formation, and in electric charge separation and increasing the lightning activity. Therefore, based on possible increases of pollution and aerosols, the contribution of aerosol–microphysical forcing to CG lightning enhancement over Taipei city and its suburbs seems reasonable. It is worth mentioning in this context that both hypotheses act on the same way, probably with combined interaction, at least for the enhancement of CG lightning activity.

It is evident from the perspective of local atmospheric dynamics that the UHI, sea breeze and orographic interaction plays a significant role in the formation of urban CG lightning within the New Taipei City. The spatial variability of CG lightning activity over Taipei is also influenced by the aerosol microphysical forcing. This observation corroborates well the findings of Sequera et al. [45] that combined impact of aerosol and UHI plays crucial role in enhancing the lightning activity over Taipei. It is worth mentioning in this context that the aerosol effects or urban heat island may not only be responsible for the observed trends of lightning over Taipei; the synoptic variability can also affect these trends with equal probability. However, it is a challenging task to quantify the individual contributions of these two physical processes based on present observations because of the complexity of urban

modifications of both dynamic and physical processes over this area. Therefore, extensive research is highly needed to fully understand this complex physical mechanism. The present study not only helps to understand the influence of urbanization on the local microclimate system, but also provide valuable environmental information for future urban planning and environmental management.

**Author Contributions:** "Conceptualization, S.K.K.; Y.-A.L.; Methodology, S.K.K.; Formal Analysis, S.K.K.; Investigation, S.K.K.; Data Curation, S.K.K.; Y.-A.L.; Writing-Original Draft Preparation, S.K.K.; Writing-Review & Editing, S.K.K.; Y.-A.L.; Visualization, Y.-A.L.; Supervision, Y.-A.L.; Project Administration, Y.-A.L.; Funding Acquisition, Y.-A.L.

**Funding:** This study was supported by the Ministry of Science and Technology (MOST) of Taiwan for financial support through grant nos. 105-2221-E-008-056-MY3 and 107-2111-M-008-036.

**Acknowledgments:** The authors are thankful to Tai-Power Company of Taiwan for providing lightning data. We are grateful to United States Geological Survey (USGS) for providing Landsat images. Thanks are also given to the Ministry of Science and Technology (MOST) of Taiwan for financial support through grantnos. 105-2221-E-008-056-MY3 and 107-2111-M-008-036. We are also extremely grateful to the anonymous reviewers for their valuable critical comments, which helped us a lot to improve this manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
