E-Paper
What does Mumbai being more polluted than Delhi tell us about pollution in India?
A look at why favourable geographical and climatic conditions are no guarantee that pollution will not become a problem
Mumbai’s Air Quality Index (AQI) has been worse than Delhi’s on 12 days this year (till February 17). If not unprecedented – pollution data going back in the past is not exactly comparable because of changes in number of monitoring stations – this is certainly rare. The main learning is that favourable geographical and climatic conditions are no guarantee that pollution will not become a problem. Here are four charts that explain this in detail.
Mumbai was more polluted than Delhi on 12 days this year
Mumbai’s average AQI from January 1 to February 17 was 201 . While this lower than the average of 278 for Delhi, Mumbai’s AQI was higher than that of Delhi on 12 days in this period. Historical data shows that Mumbai has been more polluted than Delhi less often in the past. To be sure, such historical comparison may not be very accurate. This is because the number of stations monitoring Mumbai’s air reached their current level in 2021 (one station from 2016 to 2019 and nine in 2020, compared to 16-20 operating since 2021). Delhi reached its current coverage two years earlier in 2019. Therefore, it might be better to make comparisons since 2021, when both cities reached their current level of coverage.
See Chart 1
Is this turning of tables a result of change from historic pollution levels in Delhi and Mumbai?
Even comparisons since 2021 show that Mumbai’s air has been the worst this year in three years almost consistently since mid-January. To be sure, even Delhi’s air was worse than in its own past averages in first half of January. It is after that period that Delhi’s air was cleaner than its own averages consistently before it deteriorated last week. This means that Mumbai crossing Delhi’s AQI levels is the result of both an increase in the former’s pollution level and a decrease in the latter’s pollution levels.
See Chart 2:
Did rain and wind speed play a role in Mumbai being more polluted?
Although unreliable, rain and wind can are natural air purifiers. Have they played a role in Mumbai being more polluted this year? Not completely. In the period of the year up to February 17, it rained on four days in Mumbai in 2021, two days in 2022, and hasn’t at all this year. However, all four days of rain in 2021 were in the first 10 days of January, a period in which Mumbai’s air this year (average AQI of 193) was far better than in 2021 (average AQI of 252). This means that lack of rain didn’t affect Mumbai’s air early in 2023. In 2022, it rained on January 9 and January 23, too far apart to keep the air clean continuously that January. The lack of rain might have hurt Delhi in first half of January. It rained in Delhi only on January 30 this year. In 2021 and 2022, it rained on seven and ten days in the period up to February 17, five of them in early January in both years.
Chart 3
Wind speed data, similarly, shows that AQI can be blamed on the weather only partially in Mumbai. In the last week, an anti-cyclone developed over the Arabian sea. An anti-cyclone is a region of high pressure, which causes calm weather conditions, and can increase pollution. In the six days ending February 17, winds were on average 14% slower in Mumbai than during 2021-2022. The winds slowed down from February 15, around the time of the development of the anti-cyclone. Slow winds might also have affected AQI in second half of January. However, wind cannot be blamed for the AQI in early February because it had not slowed down then. On the other hand, Delhi’s AQI had more relation to wind speed. When it did not, other factors could explain Delhi’s air, as the next section shows.
Chart 4
Even temperature does not explain the increase in Mumbai pollution levels
Is Mumbai being more polluted a result of it being colder than usual? Low temperatures, after all, trap pollutants close to the ground. Delhi had significant help from both maximum and minimum temperature in second half of January and in February, when it also had help from wind sometimes. For Mumbai, temperatures appear to have played a role in the second half of January when winds were slower. Both maximum and minimum in the city was below normal during this period. However, even temperatures don’t explain (both maximum and minimum are higher than the 2021-2022 average this month) why AQI in the city is worse than in the past in early February.
See Chart 5:
This leaves only one explanation: an increase in pollutants, which is exactly what anecdotal accounts of construction happening in the city suggest. While Delhi’s pollution levels are often ascribed to its landlocked location, Mumbai’s sea winds are expected to be natural buffers against high pollution levels. The latest experience shows that such buffers are no guarantee against pollution and urban planners must be proactive in curbing pollutants at source.
