In the SIM 25-2009 working paper, "Photochemistry of Air Pollution in Delhi, India: A Monitoring Based Analysis", the monitoring data from the Indian Tax Office (ITO) station in Delhi, India, (operated by the Central Pollution Control Board) is analyzed to explain the observed trends in the criteria pollutants, PM2.5, NOx, CO, and Ozone.The analysis of the measurements shows that
- the diurnal variation of the mixing layer height is very pronounced in Delhi, which effects the night time concentrations. This is very important for the cities like Delhi, where the diesel operated trucks are allowed to pass through the city only at night, and thus enhancing the night time ambient concentrations. However, since the population exposed to these higher concentrations of PM (mostly diesel soot) and other pollutants is lower during the night time, the impacts of the night time emissions are generally less observed.
- while the passenger travel in the city has grown over the last decade, the importance of the freight transport (via trucks) in the night should not be neglected, since the high concentrations observed during the night tend to linger during the rush hours (mixed with the passenger travel) and beyond (through ~11 AM) and hence increasing the exposure times and related health concerns along the major corridor.
- In the city, the night time concentration of particulates is a growing concern; the night-time concentrations particularly in the winter and spring seasons are approximately twice the day time levels. All the heavy duty trucks are diesel based with an average age of at least 5 years. Also, the lower mixing heights and poor ventilation in the night, prevents proper dispersion of these pollutants, causing this buildup of pollutants. At the sunrise, the buildup of the pollutants is felt among the rush hour commuters.
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