Natural and anthropogenic aerosols play a major role in the development of convective clouds and more specifically thunderstorms. On the one hand it is possible that aerosol loading can change the microphysics of clouds and hence the convection intensity (lightning activity) while on the other hand aerosols can impact the stability of the atmosphere itself. To understand the link between aerosols and electric discharges, Scientists must unravel the mysteries of clouds. In Greek Mythology, Zeus had dominion over the creation of lightning. Thousands of years later, humans have begun to assume that role. Scientists have already linked aerosol emissions to increase in lightning over areas of amazon prone to forest fire as well as regions with thick air pollution. The clearest example, yet of humanity’s influence on atmospheric electrostatic discharges however, surfaced recently when researchers discovered dense trails of lightning in the soot-filled skies over two of the world’s busiest shipping routes in the Indian Ocean and south China sea. Poring over 12 years of detailed data, atmospheric scientist “Joel Thornton” at the University of Washington, postdoc” Katrina Virts” of NASA Marshall Space Flight Center and their colleagues, found lightning flashes occurs nearly twice as often directly above heavily trafficked shipping lanes as they do elsewhere over the ocean. Oceanic lightning can be a sensitive indicator of aerosol effects on lightning characteristics as thermodynamic contrast is indistinct over adjacent oceanic regions and this makes oceanic lightning. The increased frequency of lightning follows the exhaust from ships and cannot be explained by meteorological factors such as winds or the atmosphere’s temperature structure, according to a study published in Geophysical Research Letters in last September.
The effect was easier to see over water than land, because in general, the atmosphere above the Ocean is relatively low in aerosols- tiny liquid or solid particles that float in the air. Scientists noticed a greater density of lightning in locations where ships blast emissions, including sulfur and nitrogen dioxide into the air. Then the researchers tracked instances of lightning using the World Wide Lightning locations network (WWLLN)- a system of acoustic sensors that detect electrical disturbances all around the globe. In fact, aerosol is an important player in atmospheric electrification. The relationship between aerosol and lightning over the Indo-Gangetic Plain(IGP), India has been evaluated by utilizing aerosol optical depth(AOD), cloud droplet effective radius and cloud fraction imaging Spectroradiometer. Lightning flashes have been overseen by the lightning Imaging Sensor on the board of Tropical Rainfall and measuring emission and humidity from modern era retrospective – analysis for research and applications for the period of 2001-2012. The role of aerosol in lightning generation over the north –west sector of IGP has been revealed.It is found that lightning activity increased (decreased) with increasing aerosol during normal (deficient) monsoon rainfall years. However lightning increases with increasing aerosol during deficient rainfall years, when the average value of AOD is less than 0.88. Even as the evidence of humanity’s role in generating lightning mounts, the proposed correlation creates more questions than they answer. Why would aerosol have this effect? And, are there ways of analyzing clouds that can help predict future lightning strikes? Lightning is a natural features of storms that occurs when certain conditions are met. Particles in a cloud rub together, gathering opposite charges that eventually separate into positive and negative regions. These two poles create a space across which a transfer of charge- or lightning bolt may then occur. Sometimes the bolts transmit the charge to the ground and lightning strikes. But we are now learning the amount of lightning generated may be influenced by the factors that go beyond natural meteorology, including aerosols. Other recent studies have given evidence to the idea, aerosols are linked to more lightning, since Thornton’s and his colleagues, study was published. Ilan Koren and Orit Altaratz at the Weizmann Institute of Science in Israel and colleagues have found, using WWLLN, that more intense lightning is connected with aerosol sources over land. The result is consistent globally and the effect appears over large continental region “on global scale”. Lightning can damage buildings and vehicles and is responsible for thousands of death every year, many of them in developing countries. It may be premature to suggest a direct link between pollution and electrical destruction, but keeping an eye on increased lightning activity could be helpful to those working on preventive measures.
Scientists have some idea of how aerosols change a cloud’s inner working but the microphysics of charge separation and lightning generation are still not fully understood. Clouds droplets form when water vapor condenses on to aerosols. If there are more condensation sites- called cloud condensation nuclei – so the water is distributed among them to make smaller droplets. These little droplets are especially light and so it is easier for them to rise to higher levels of clouds on thermal updrafts. They eventually freeze partially forming what’s known as graupel and begin colliding with ice crystals also floating in the cloud. Those collisions are what make lightning possible. It’s a classic example of creating an electric charge via friction- just like when you rub a balloon to create a static charge. But this is also where the story gets murky. It is not known why the graupel tends to become negatively charged and the ice crystals positively charged. A key question facing researchers is whether increased electrification is the result of more aerosols or an abundance of warm air. The hot air may, on its own, help the droplets rise to that crucial upper layer before they fall out of the clouds as rain. This is known as the thermodynamic effect.A 2013 computer simulation of this process found increased aerosols alone did result in more lightning due to ice crystal collisions, although at very large aerosols volumes the effect was muted. At extreme aerosols contents (the droplets were) too small, they travelled through the air at slower speeds, meaning they may have been less likely to collide with ice crystals, rubbing against them and causing that all- important charge separation.
A comprehensive understanding of cloud microphysics- such as those complex interactions between graupel and ice crystal – may some way off. But the scientist at “The Hebrew University” of Jerusalem, suggest that more accurately quantifying how many aerosols are present at the base of clouds could lead to a better understanding of aerosols, impact on electrification. Rosenfeld of MIT’s and his students have developed an aerosols quantification method that use satellite – based measurement of infrared light reflected by clouds. Clouds droplets absorb certain wavelengths of light depending on their size, so noting which wavelength are missing in readings, reveals the size of droplets present. It is a bit like knowing whether a gemstone appears to be a ruby or an emerald. Rosenfeld than divides the total volume of water in the cloud by the droplets size to reveal an estimate of the total number of clouds droplets. Additional measurements help estimates the effect of hot air updrafts. If aerosols quantities are known, they can of course be compared with how much lightning is later produced by the cloud in question. Clouds remains complex and mysterious systems. Scientists are gradually discovering how they work-and what factors influence the creation of lightning, one of nature’s more dramatic calling cards. That human beings may take some credit for the generation of those bolts may come as something of a surprise but future measurements will reveal the true significance of the role we play. One thing is certain: Zeus is may be off the book.
The writer is Asst.Prof., JCRE Global College, Babupara Imphal. He can be reached to:[email protected]