Ranked the world’s most polluted capital for seven years in a row, can Delhi’s air quality get any worse? Apparently, it already is. A study published in a Nature journal on Wednesday says air pollution in the capital is more severe than currently estimated because ultrafine particulate pollutants (PM1) are being undercounted by up to 20%.
The study, authored by Ying Chen of UK’s University of Birmingham, found that the underestimation of PM1 in Delhi — due to “hygroscopic growth” of particles — was the highest among megacities around the world.
Hygroscopic growth is a phenomenon where the size of a particle increases due to its ability to absorb water. Since air quality sensors identify pollutants by size, an increase in the size of PM1 particles leads to an underestimation of their levels.
A major implication of the study is that Delhi’s severe pollution episodes during winter are deadlier for the health of city residents than previously estimated.
The study, published in Nature’s open access publication, NPJ Clean Air, said the underestimation of PM1 concentrations in Delhi’s air varies with humidity and pollution levels.
The study notes: “The greatest underestimation in estimated concentrations happen during winter morning rush hours, when humidity is highest and pollution is most severe.” Under these conditions — winter mornings — PM1 levels were found to be underestimated by as much as 20%.
During spring (February to March), the underestimation was found to be 8.6% in the morning rush hours while it was negligible during summer and the monsoon months, the study said.
It said this was the first time such a study was done in India although the underestimation of pollution levels due to hygroscopic growth of particles has been recognised and its impacts reported in Europe, America, Africa and China.
One of major factors for the high hygroscopic growth in ultrafine particulates in Delhi’s air was high levels of chlorides. “Anthropogenic chloride largely enhances the particle growth under humid conditions in New Delhi contributing 40-50% of aerosol liquid water in spring and winter; this could be the reason for the uniquely high bias in humid conditions compared to other cities worldwide,” the study noted.
The main sources of chlorides in the air are open biomass burning and residential emissions. The study suggests that control of these emissions would not only improve air quality but also help reduce bias in particle observations and hence facilitate better understanding of air pollution.
PM1 is not currently among the eight pollutants that go into the calculation of the Air Quality Index but is considered one of the major pollutants in the region. It’s the smallest of the three categories of particulate matter (PM10 and PM2.5 are the others) and the deadliest. These particles can penetrate directly through the lungs into the bloodstream and spread to the organs.