Sea Salt a reason

• During the onset of winter, in areas close to the sea such as Colaba, the temperature is always 1 to 2 degree lower than in other areas of the city.
  
  
• But the low pressure in the Arabian Sea is causing the temperature to increase slightly, to the point where the air is becoming just humid enough to hold more particulate matter in place.
  
  
• The sea salt in water vapour tends to react with other pollutants like PM 2.5, and sulphur oxides from vehicles that are already present in the air, to create secondary pollutants.
  
  
• When wind speeds are slow, these secondary pollutants add to the pollution load.
  
  

No breeze

• The sea breeze over Mumbai usually keeps the air clean. But wind speed has been low or stagnant for three days. Thus, emissions from road and metro construction activities and vehicular movement are getting trapped in the air, raising the AQI.
  
  

Urban Canyon Effect

• Levels of traffic-related air pollution can be substantially elevated along a road surrounded by tall buildings. This is called the Urban Canyon Effect. The canyon effect occurs when tall buildings flank both sides of city streets, restricting air movement. Eventually, this increases the concentration of pollutants in the ambient air.