A global analysis of air quality found that Indian cities, while recording particulate matter emissions (PM2.5) that are among the highest in the world, do relatively better on nitrogen dioxide (NO2) emissions.
The report, Air Quality and Health in Cities, released by US-based Health Effects Institute on Wednesday, analyses pollution and global health effects for more than 7,000 cities around the world, focusing on two of the most harmful pollutants - fine particulate matter (PM2.5) and nitrogen dioxide (NO2).
The report, using data from 2010 to 2019, found that global patterns for exposures to the two key air pollutants were “strikingly different.” While exposures to PM2.5 pollution tend to be higher in cities located in low- and middle-income countries, exposure to NO2 is high across cities in high-income as well as low- and middle-income countries.
Delhi and Kolkata were ranked first and second in the list of top 10 most polluted cities when PM2.5 levels were compared, with Delhi and Kolkata reporting an average annual exposure of (relative to population) of 110 µg/m3 and 84 µg/m3 respectively. µg/m3 refers to microgram per cubic metre.
However no Indian city appeared in the list of top 10 – or even top 20 - polluted cities when N02 levels were compared. This list saw Shanghai at the top with an average annual exposure of 41 µg/m3. Average NO2 levels for Delhi, Kolkata and Mumbai, according to the report, ranged from 20-30 µg/m3.
NO2 comes mainly from the burning of fuels in older vehicles, power plants, industrial facilities and residential cooking and heating. As city residents tend to live closer to busy roads with dense traffic, they are often exposed to higher NO2 pollution than residents of rural areas.
In 2019, 86% of the more than 7,000 cities analysed in the report exceeded the WHO’s 10 µg/m3 guideline for NO2, impacting about 2.6 billion people. “While PM2.5 pollution tends to get more attention on known hotspots around the world, less data has been available for NO2 at this global scale,” the report notes.
An expert, who was not associated with the study, told The Hindu that this paradoxical situation in India was likely due to the relatively lower adoption of high-efficiency engine vehicles. “Complete combustion of fuel results in higher NOx (nitrogen oxides) where incomplete combustion sees other kinds of emissions,” said Sachchida Nand Tripathi, Professor, IIT-Kanpur and an expert on air pollution in India. Other cities with high NO2 population levels included Moscow, Beijing, Paris, Istanbul and Seoul.
Due to their highly reactive nature, nitrogen oxides also contributed to the formation of other pollutants, including ozone and particulate matter. NO2 also has a shorter lifetime compared with PM2.5 and other air pollutants. As a result, NO2 levels show very high variability in space and time — levels can vary significantly even across a few kilometres within the city. In comparison, PM2.5 levels tend to show less spatial variation at the fine scale.
In 2019, the global average NO2 exposure was 15.5 µg/m3, but exposure levels varied considerably across cities (ranging between 0 and 68.9 µg/m3).
Ground monitoring of air quality remains limited in many regions of the world, particularly in low- and middle-income countries, the report adds, obscuring the true degree of NO2 pollution in countries such as India.
For this report, the scientific team systematically estimated annual average concentrations of PM2.5 and NO2 across the entire globe, which was divided into grid cells, each covering 0.0083° × 0.0083°of longitude and latitude (approximately 1 × 1 kilometers at the equator). They then used information from all grid cells within a city boundary to estimate the PM2.5 concentration in a specific city.
To estimate the annual average PM2.5 and NO2 exposures (that is, the concentrations that a population in a specific city is more likely to be exposed to in one year), the team linked the concentrations in each grid cell with the number of people living within each block to produce a population-weighted annual average concentration. “Because these population-weighted air pollutant concentrations represent annual averages across an entire city, they include, but do not represent, the considerably higher concentrations that may be observed day to day,” the report added.