Investigation of the relationship between air pollution and lightning during thunderstorm events of the years 2009-2013 in Tehran

Expanded Abstract
Introduction
Atmospheric pollutants affect the various development and formation of local weather systems. It is not well known how the variation of atmospheric pollutants affects the different stages of weather conditions. Many studies have been conducted to explore the physical relationship between lightning occurrence and air pollution. The influence of atmospheric pollutants on cloud systems includes the direct effect on radiation budget and the indirect effect on the cloud dynamics and microphysical properties. Atmospheric pollutants modify weather and climate. Some research has reported the significant effect of aerosols on cloud microphysics on a global scale. Many studies have been dedicated to the effect of the urban area on the local weather systems with a focus on the effect of the urban heat island circulation and its possible role in air pollution.
Lightning clouds in intensely populated areas as large cities can have damaging effects in such urban areas. The main aim of this study is to consider the effect of the increase in the concentration of PM10 on lightning events in the Tehran area. Usually, higher atmospheric aerosol concentration contributes to more Cloud Condensation Nuclei (CCN) formation, reducing the mean cloud droplet size by decreasing the coalescence and droplet collision efficiency. Thus, more supercooled water is likely to exist at greater depths inside the cloud. The abundance of supercooled water may generate large cloud graupels, leading to an enhanced lightning activity over urban regions. Studies show that lightning activity is more vigorous at downwind of an urban area than in suburban areas, and there is a positive relationship between PM10 and SO2 concentration and lightning frequency. This issue verifies the crucial role of aerosols in increasing the liquid water content and ice particles in the phase change zone of clouds. Also, the results of a study over Kolkata show that following the outbreak of Covid-19 disease, the concentration of surface pollutants has decreased by more than 40% compared to the pre-lockdown period in this city. Hence, decreasing the concentration of surface pollutants has a more significant impact on lightning in the pre-monsoon period.
Materials and Methods
In this study, firstly, thunderstorm reports are collected from the Iran Meteorological Organization (IRIMO) in the Tehran region from 2009 to 2013. Then the days of thunderstorms with lightning occurrence are obtained by checking the current weather code. The lightning events for the desired days are obtained from the Lightning Imaging Sensor (LIS). The LIS is a space-based tool which is used to detect the distribution and variability of total lightning including Cloud-to-Ground (CG) and Intra-Cloud (IC) lightning. It measures the amount, rate, and radiant energy of lightning during both day and night. Moreover, to investigate the relationship between lightning and surface aerosol concentrations in the Tehran region, PM10 concentration data (including daily and annual mean) are obtained from Tehran Air Quality Control Center (AQCC) for several pollution stations in the Tehran area, including Fatah, Sharif, Shadabad, and Aqdasiyeh stations. The air pollution stations are selected due to the completeness of the required data during the study period and the proximity to the meteorological stations under investigation (Mehrabad and Shemiran stations).
The analysis section is dedicated to investigating the mean annual number of lightning changes with the annual mean of PM10 concentration during the studied years. Moreover, the scatterplots of PM10 concentration and the number of lightning flashes (for the studied stations) during these years are presented and examined.
Results and Discussion
From 2009 to 2013, Tehran experienced many thunderstorms caused by local convective storms that resulted in many lightning events. The relationship between cloud base (Lifted Condensation Level; LCL) height and the number of lightning flashes taken from LIS data at Mehrabad station in Tehran confirm previous observations. Specifically, this study shows that more supercooled water is expected to be present at higher altitudes in the clouds, leading to higher lightning activity.
The trend of PM10 changes is almost similar to the trends of changes in the mean annual number of lightning in the studied years for the Aqdasiyeh and Shadabad stations. The maximization of lightning clearly shows that the increase in PM10 concentration can enhance the lightning activity over the polluted area. This effect may be attributed to enhanced precipitation, which usually co-occurs with and after atmospheric electrical activity. Such an effect is usually related to the general precipitation washing effect of the atmosphere.
The distribution of the mean-daily concentration of PM10 versus the number of lightning flashes shows that the daily PM10 concentration has a moderate positive correlation with the number of lightning (based on the correlation coefficient, which ranges from 0.36 to 0.85 for the studied pollution monitoring stations). The values of correlation coefficient greater than 0.7 indicate a strong positive correlation between the two correlated variables, even if their scatterplots are highly scattered. When correlation coefficient is obtained between 0.3 and 0.7, it indicates a moderately positive relationship between the two variables. Therefore, the pollution has intensified the storm and the lightning activity in these stations. Also, the seasonal study of overall data from the stations of Shadabad, Sharif and Fatah showed an almost stronger correlation between the daily concentration of PM10 and the number of lightning flashes in the warm period of the studied years compared to the whole years according to the obtained correlation coefficients.
Conclusions
The Tehran urban area results show the trend of PM10 and mean annual number of lightning changes are almost similar in the studied years for the Aqdasiyeh and Shadabad stations. Moreover, there is a good correlation between daily-mean PM10 concentration and lightning occurrence in this region for the studied pollution monitoring stations during the precipitating storm activity. In other words, near-surface PM10 significantly contributes to the increasing concentration of atmospheric aerosols. It, therefore, can act as cloud CCN which can affect a high frequency of atmospheric lightning activity. The results are also consistent with other parameters, including the correlation of the increase in cloud base height and the number of lightning flashes. Subsequently, the seasonal study of overall data from the stations of Shadabad, Sharif and Fatah showed an almost stronger correlation between the daily PM10 concentration and the number of lightning flashes in the warm period of the studied years than the entire years.
Keywords: Lightning; Air pollution; PM10; LIS