Variations in the total aerosol covered area and its spatial pattern reflect a close
relationship with the spatial patterns of surface temperatures and rainfall
The thickest aerosols in Southern India are observed in May and in Central and Northern India in June 2013.
Variations in the total aerosol covered area and its spatial pattern reflect a close relationship with the spatial patterns of surface temperatures and rainfall
A World Health Organization (WHO) study of 1,600 cities released in May 2014 found New Delhi had the world’s dirtiest air. New Delhi recorded six times the safe level of ‘particulate matters’ (PM2.5) in air. What was more disquieting is that WHO stated that 13 of the dirtiest 20 cities worldwide were in India.
Regardless of how vociferously the Indian government may contest the WHO report, it is a known-fact that air quality in the northern part of India has deteriorated substantially in the last few years. System of Air Quality Weather Forecasting and Research (SAFAR) has blamed the peaking aerosol levels in the air in north India on the burning of farm waste in Punjab and Haryana. Farm waste combustion, which is a common method of clearing land and consuming farm waste in India, yields smoke that comprises mainly organic carbon and black carbon which result in the formation of aerosols. The National Green Tribunal has even directed the Union Government to finalise a national policy for controlling and preventing pollution resulting from burning of agricultural waste in open fields. Aerosols comprise commonly visible smoke, haze, ash, soot, dust etc. in a variety of forms, shapes and sizes as well as a range of chemical elements suspended in the air.
Effect of Aerosols
Aerosols play a significant role in the global climatic phenomena as well as have a huge potential of affecting human health adversely. Their nature and composition affect the way these interact with various gases and other particles present in the atmosphere. Depending on the types of aerosols — and the clouds seeded by them — they influence the radiation balance of the earth by reflecting or absorbing a quarter of the Sun’s energy that drives earth’s climate. Aerosols act as cloud condensation nuclei and cause alterations in the microphysical and other properties of clouds. This means, their presence, in combination with greenhouse gases, may either cause warming in the upper air by reflecting, scattering and absorbing insolation, or cooling in the surface temperatures by obstructing the infiltration of insolation.
Therefore, it is important to study the spatio-temporal pattern of aerosols in the atmosphere. In this respect, a research studying the spatial pattern and the dynamics of aerosols optical thickness in India is currently underway at Interfaculty Department of Geoinformatics – Z_GIS, University of Salzburg, Austria.
According to the preliminary findings of the study, aerosols cover 80 to 85% area of India from January to May as well as from September to December, whereas it is only 20 to 30% during July and August. This means that the area covered under thicker aerosols increases from January to May and starts decreasing from June but increases after August. The thickest aerosols in Southern India are observed in May and in Central and Northern India these are observed in June.
The study, which was based on four different sets of TERRA-MODIS images, found that levels of aerosol thickness in northern India remain much higher as compared to Southern and Central India for most of the year. This can be associated with several factors existing in the Northern Indian Plains which include higher density of population and urban areas, extensive agricultural activities, wide spread industrial areas as well as huge volume of heavy vehicle traffic along the Grand Trunk road. The study also found that the Himalayas obstruct the aerosols carrying winds from flowing further north causing an increase in the levels of aerosol thickness along the Himalayan arc.
The variations in the total aerosol covered area and its spatial pattern establish a close relationship with the spatial patterns of surface temperatures and rainfall. Increasing surface temperatures have a delayed effect on the increase in the area covered by aerosols and their thickness because surface takes some time to heat up and become dry enough to let the dust particles blow high with the wind.
On the other side, rainfall has an immediate effect on reducing the aerosols because rain drops fall from above and these either dissolve or bring down the particulate matter suspended in the air. This explains why no aerosols are observed in South Western India in June because monsoon rains start in the beginning of June. The monsoon rains move progressively into the inner parts of Southern India and eventually turn toward Central and Northern India which decreases total area under aerosols; as well as the levels of its thickness also decrease from June to August.
Apart from the other natural sources, winds blowing over intensively cultivated extensive agricultural areas, especially in drier climatic conditions, add a significant amount of dust particles in the atmosphere. For instance, studies have revealed that Patiala in Punjab recorded the highest coarse particles levels over the past few days. The proportion of anthropogenic aerosols constitute only about 10% of the total aerosols in the atmosphere but these, generally, have higher concentration in the air in urban and industrial areas. The major sources of these are a range of urban–industrial activities like vehicular traffic flows, automobile exhausts, incomplete combustion of fossil fuels in power generation plants, construction works, etc.
Despite great progress, many questions remain about the competing impacts of aerosols. Measuring particles within clouds remains challenging. Establishing a spatio-temporal relation between the spatial patterns of surface temperatures and rainfall with the total aerosol covered area and its spatial pattern is imperative to more accurately predict the longer-term effects of climate change.
Since aerosols comprise a broad collection of particles with different properties, studying the overall effect is anything but simple. However, researchers are working on higher resolution images, incorporating other ancillary data as well as exploring further methodologies in expectation of having new insights in near future.