Particulate matter in the atmosphere alters cloud formation and reduces visibility. When the air we breath contains particles of about 2.5 microns in size, we are prone to respiratory and cardiovascular disorders. When this happens, in an overcrowded city, it becomes a public health issue.
Last fortnight, V Sreekanth, from the CMR Institute of Technology, Bengaluru and colleagues from the Andhra University, Visakhapatnam examined the levels of ambient ﬁne particulate matter over five rapidly developing cities: Chennai, Hyderabad, Mumbai, Kolkata, and Delhi.
The researchers used real-time hourly results from about four years – 2013 to 2016 – to compare ambient PM2.5 levels. They found that, despite implementing pollution mitigation strategies, Delhi recorded the highest PM2.5 of all, across all seasons, at all times of day, followed by Kolkata and Mumbai. Based on daily mean concentration, “Chennai emerged the cleanest and Delhi the dirtiest,” says K Niranjan, Andhra University.
Winter is the most hazardous, with almost all days exceeding the WHO threshold for daily mean. During the monsoon, PM2.5 levels come down.
In Chennai and Hyderabad, the researchers noticed higher levels of PM2.5 from 8 to 9 in the morning and 8 to 10 at night. The researchers say that this could, perhaps, be because of higher levels of traffic at these times. But in Kolkata, there is a peak in PM2.5 around midnight – more than during morning rush hours. However, these diurnal variations also disappear during monsoon.
PM2.5 concentrations reduce during weekends only in Mumbai and Kolkata. In Delhi, Hyderabad and Chennai, the team found that there is no reduction in PM2.5 concentrations even during weekends.
“Generally, there is a trend of increasing PM2.5 as we go from the south to the north of the country”, says B Mahesh, Andhra University.
In addition to the differences in regional emissions of PM2.5, the blocking of air movements by the Himalayan mountain ranges and the variations in the height of the planetary boundary layer, where air mixing can take place, also play a significant role in modulating air pollution over a certain location. These factors could perhaps explain this pattern, says V Sreekanth, CMR Institute of Technology.
The results of the study can help policy makers better strategise region-speciﬁc mitigation eﬀorts.
Urban Climate, 25: 99-108 (2018) DOI: 10.1016/j.uclim.2018.06.001
R Baskar, GJUST, Hisar