Delhi is grappling with severe air pollution once again, as the average Air Quality Index (AQI) has surged past 310, categorizing it as 'very poor.' In some areas, the AQI has even reached 405, with PM2.5 concentrations currently 17.6 times above the World Health Organization's annual air quality guideline. To combat this crisis, the second stage of the Graded Response Action Plan (GRAP) has been enacted, prohibiting the use of coal and firewood, including in tandoors at restaurants and open eateries as well as diesel generators, except for emergencies.
Each winter, Delhi experiences a significant rise in pollution levels. But why is the capital particularly vulnerable during this season?
The Science Behind Winter Pollution
During winter, the colder air is denser compared to the warmer summer air, which leads to a phenomenon known as temperature inversion. In summer, the warm air rises, allowing pollutants to disperse. Conversely, in winter, the cooler air is trapped under a layer of warm air, creating an atmospheric lid. This inversion hampers vertical air mixing, causing pollutants to accumulate near the surface, particularly at night. Consequently, air quality tends to worsen during these hours, which is why experts recommend avoiding outdoor exercise in the mornings when pollution levels peak.
The stagnant winter air retains a variety of pollutants, leading to the formation of smog-a mixture of smoke and fog. This smog significantly reduces visibility and poses serious health risks. Delhi's geography exacerbates the situation; as a landlocked city with low wind speeds during winter, the polluted air lingers longer in the area.
Contributing Factors to Pollution
Delhi's AQI deteriorates due to multiple sources of pollution, including vehicular emissions, dust, industrial activities, and seasonal contributors like firecrackers and stubble burning. According to the System of Air Quality and Weather Forecasting and Research (SAFAR), stubble burning now accounts for 19-22% of Delhi's pollution levels. Farmers often resort to this method to clear leftover straw from paddy fields after harvest due to its low cost and ease, despite the harmful environmental impact.
Moreover, the use of firecrackers during Diwali significantly contributes to air pollution. Although there are restrictions on their sale and use, the impact remains substantial, releasing harmful metal particles and toxins into the air. The Centre for Science and Environment in India notes that air pollution from firecrackers during Diwali can lead to a 30-40% increase in respiratory issues among the population.
Geographic Influences
The wind patterns in Delhi also play a crucial role in its air quality. During the monsoon, easterly winds bring moisture and rainfall from the Bay of Bengal. However, once the monsoon season ends, north-westerly winds prevail, transporting particulate matter-primarily from stubble burning in Punjab-toward Delhi and further down the Gangetic basin. Although these winds also bring dust from Rajasthan and even smoke from Pakistan and Afghanistan, the high temperatures during summer keep pollutants from settling low.
The Indo-Gangetic plain, where Delhi is located, is surrounded by geographical barriers, making it susceptible to pollution. This region is akin to a long valley enclosed by plateaus and mountains, which trap air and pollutants. Delhi's unique “tabletop” structure, bordered by the Aravalli hills, makes it particularly prone to pollution peaks, especially between October and mid-December when weather conditions and vehicle emissions align to create critical pollution levels.
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