MUMBAI, Nov. 24 -- For the first time this season, the city woke up on Sunday to a thick haze and smog as the air quality steadily worsened in Mumbai. Visibility in several areas, including the coastal road, was low, and the overall air quality index (AQI) stood at 176, with ozone as the primary pollutant. On Saturday, the AQI had peaked at 188 or very close to the "poor" zone. A civic official told HT that several stations had consistently logged a dismal AQI of over 200. "We have two squads in place who will check whether construction norms are being followed," he said. "Meanwhile, instructions have been given at the ward level to implement the Graded Response Action Plan (GRAP) 4 if the air quality remains poor for the next two days." GRAP is an action response formulated by the central government's Commission for Air Quality Management to tackle the issue of air pollution. GRAP 1, 2, 3, and 4 refers to the four stages, with each stage involving progressively stricter measures. GRAP 4 is the most stringent. "The implementation of GRAP 4 means that all the operations at construction sites, at ready-mix concrete plants and at other pollution sources will come to a standstill till the air quality improves," said the official. According to officials, the BMC has already sent stop work notices to 59 construction sites and ready-mix concrete plants. The civic body's comprehensive air pollution guidelines issued last year require a slew of measures to be taken at construction sites. These include installing tin sheet barricades, covering sites with green cloth, ensuring regular sprinkling of water during demolitions and water-fogging during loading and unloading of material, storing debris scientifically and installing air-quality monitoring and dust extraction systems. The city's air pollution started worsening after the first week of November, immediately after the post-monsoon showers subsided. Since then, the AQI has remained above 100 or in the "moderate" grade. According to experts, the major component affecting the overall AQI situation of Mumbai is an early onset of La Nina conditions. "For a coastal city like Mumbai, La Nina usually slows down the overall wind speed which forces the AQI to go down. This phenomenon was earlier seen during 2021-22, which had resulted in a serious dip in Mumbai's air quality," said Gufran Beig, chair professor at the National Institute of Advanced Sciences. La Nina is a climate pattern characterised by the cooling of surface-ocean waters in the equatorial Pacific Ocean. During La Nina, atmospheric circulation changes, resulting in reduced wind speeds that prevent pollutants from dispersing quickly, causing them to linger longer in the air, particularly in coastal cities like Mumbai. Experts stated that when winds start to cool down, they become heavy, and due to the combined effect of this and slow wind speed, the pollutants remain suspended in the lower atmosphere for a prolonged period of time, creating smog and mist-like conditions which mainly comprise suspended particulate matter (SPM), which is very dangerous. "Once the temperature drops, the wind pattern slows, due to which SPM stays trapped in the atmosphere for a prolonged period of time," said Mahesh Palawat, vice-president of meteorology at Skymet Weather Services. "The city's air quality has already started to dip after the monsoon withdrawal, mainly because during the rains, the atmosphere is naturally cleansed and stays free of pollution. That is why the AQI improved after the unseasonal rains. Now that the rain has gone, the AQI is plummeting." (SEE BOX) According to the Central Pollution Control Board, AQI readings are categorised as follows: 0-50: Good, 51-100: Satisfactory, 101-200: Moderate, 201-300: Poor, 301-400: Very Poor, Above 400: Severe....