In the year 2014, WHO declared Delhi City as the worst polluted in the world. In December 2015, PM 2.5 levels in Delhi were at 295 microgram/m³ and PM10 levels were at 470 microgram/m³ resulting in Air Quality Index at a severely high 430-435. Air pollution is responsible for 10,000 to 30,000 deaths in Delhi every year. A critical convergence of public concern, policymaker attention, and academic contribution is now taking place to address this issue. Reducing emissions of PM2.5 not only has an immediate effect on air quality, but also mitigates near-term climate change and helps achieve food security. Data gathered over three year (2013-2015) on particulate air pollution PM 2.5 in Delhi reveals a systematic wave-like pattern each year. PM 2.5 rises and falls with rhythmic precision in winter and summer with a minimum range of values in the monsoon when the Delhi air is washed clean by rainfall. These waves of air pollution change are found to be nearly synchronous with a combination of weather factors, specifically ambient air temperature and dew point, modeled appropriately as a ‘weather index’. A near linear relationship is shown to exist between the air pollution (PM 2.5) level and the ‘weather index’ for the year 2015. So far, weather is not ascribed to be one of the primary reasons for unusually high particulate air pollution in Delhi, but merely a factor that influences air pollution. This study attempts to fill this gap Baseline Reference PM 2.5 measurements were obtained from published sources from a air quality monitoring sensor located in an elite, less densely populated locality near Delhi’s wooded Reserve Forest ridge, somewhat secluded from Delhi’s zones of high density traffic, industries and thermal power stations. Air quality measurements at this sensor location are routinely taken at hourly intervals throughout the year. The distance of the Reference Sensor from main sources of air pollution results in longer transport time from source to sensor, enabling the polluted air to be well modulated by weather. These weather-modulated measurements were compared with other reliable published data by the Government of India’s Central Pollution Monitoring Board from sensors at 10 other locations on the ground-level at kerb-sides of roads with high density vehicular traffic; and much closer to the commercialized and industrialized areas of the city. The street-located sensors record near instantaneous PM 2.5 measurements at the very starting place of air pollution so that the transport time interval required for deep modulation by weather may be presumed to be too short. Using the “weather index” property, theoretical values of air pollution, if there were no modulation by weather, were calculated from the Reference sensor values. These theoretical values are compared with aggregated experimental values of air pollution from sensor locations on the kerb-side of the roads so that were not modulated by weather. They are found to be in reasonably close (within 9%) agreement. Thus the validity of an exact relationship between PM 2.5 with the ‘weather factor’ determined by ambient air temperature and dew point is established ; and thereafter validated by measurements for seven different sources of air pollution at 10 locations in three seasons of year 2015: summer, monsoon and winter.

Particulate Air Pollution, Health Standards, Weather Index, Kerb Sensors, Transport Time, Source Apportionment