What is ground-level ozone pollution Delhi is dealing with?

The Center for Science and Environment (CSE) has issued an alert on the rising ozone pollution and multi-pollutant crisis in Delhi and the National Capital Region (NCR) during the summer months. The CSE warns that if left unchecked, this could become a serious public health crisis for years to come.

In a recent analysis by the Center for Science and Environment (CSE), it was found that ground-level ozone excesses occur almost daily during the summer season.

The report, released on the eve of World Environment Day 2023, indicates that although the region is still grappling with a multi-pollutant crisis, ozone levels this summer are relatively lower compared to previous years. If left unaddressed, this problem has the potential to become a serious public health crisis in the future.

The report emphasizes that ground-level ozone has been a year-round concern for the past five years, with particularly high levels seen during the months of April and May.

The CSE suggests that India should learn from advanced economies that have brought particulate pollution under control but now face a growing nitrogen oxide (NOx) and ozone crisis.

Delhi-NCR's highest ozone exceedance locations

What is ground-level ozone?

Ground level ozone is a difficult pollutant to track and mitigate because it is not emitted directly from any source. It is formed from complex interactions between nitrogen oxides, volatile organic compounds, and carbon monoxide emitted from vehicles, power plants, factories, and other sources of combustion. Sunlight triggers cyclical reactions that generate tropospheric ozone. This pollutant not only accumulates in cities but also travels long distances, affecting local and regional areas and impacting public health, crop production, and food security.

Why ozone needs special attention?

Ozone has serious health consequences, particularly for people with respiratory conditions, asthma, chronic obstructive pulmonary disease, children with premature lungs, and older adults. It can inflame and damage the airways, increase susceptibility to infection, worsen asthma, emphysema, and chronic bronchitis, and lead to increased hospitalizations.

According to the 2020 State of Global Air report, India has one of the highest age-standardized death rates attributable to tropospheric ozone. The report also highlights that India has seen a significant increase in daily seasonal maximum ground-level ozone concentrations for 8 hours between 2010 and 2017, indicating a worrying trend.

Lowest ozone pollution locations in Delhi-NCR

Challenges in assessing ozone pollution

The CSE analysis is based on real-time granular data publicly available from the official online portal of the Central Pollution Control Board (CPCB), which captures data from 58 official monitoring stations in Delhi-NCR. The analysis shows that tropospheric ozone exceedances were reported on 87 days between March 1 and May 30. Although the spatial dispersion of tropospheric ozone has decreased compared to previous years, the duration of the exceedances has increased.

The CPCB has set a limit of 200 µg/m3 for tropospheric ozone, which means that no data is available beyond this level. As a result, it is not possible to determine actual concentration levels beyond this cap. To assess daily ozone pollution levels, the analysis follows the United States Environmental Protection Agency (USEPA) approach, which calculates eight-hour averages for each day and considers the maximum value between them.

The method helps to capture the daily level of ozone pollution effectively. Similarly, the analysis considers the highest value recorded among all stations in a city or region, following the USEPA approach for assessing the regional or city-wide air quality index (AQI).

It is worth noting that ozone data available on the CPCB portal never exceeds 200 μg/m3, but data from the Delhi Pollution Control Board may show higher levels for the same period. This data limitation poses a challenge in understanding the nature of the spike in the city.

Worst affected areas in Delhi-NCR

The worst affected areas in Delhi-NCR include Nehru Nagar in South Delhi, followed by Sri Aurobindo Marg, Dr KS Shooting Range and Mandir Marg. Ghaziabad, Gurugram and Greater Noida are also severely affected while Faridabad has the fewest cases of ground level excess ozone. Locations with the lowest ground level ozone pollution include Punjabi Bagh, Ashok Vihar, Siri Fort, Chandni Chowk, North Campus and Pusa in Delhi.

Some areas, such as Mandir Marg in New Delhi and Gwal Pahari in Gurugram, have seen a worsening trend with an increase in the number of days of leave. However, there are also locations that have shown improvements such as Siri Fort, JLN Stadium, Narela, Sonia Vihar, Bawana, Ashok Vihar and Dwarka Sector 8 in Delhi; Sector 16A in Faridabad; and Indirapuram and Loni in Ghaziabad.

The analysis also reveals that ground-level ozone hotspots are located in areas with low levels of nitrogen dioxide (NO2), carbon monoxide (CO) and particulate matter (PM2.5). The spatial distribution of ozone at ground level is inversely related to the levels of NO2, CO, and PM2.5, except for some stations that report high levels of both NO2 and ozone at ground level.

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