Pollution

Tracking aerosol built-up in the country through satellite images

While air quality bulletins recently highlighted north India’s dust problems, satellite reanalysis data show the problem of toxic Sulfate aerosol building up in southern India

 
By Tanushree Ganguly
Last Updated: Wednesday 15 May 2019
Photo: Getty Images

While most air quality bulletins last week revolved around the dust storm hitting Delhi resulting in severe air quality levels, images put out by the Copernicus Atmospheric Monitoring Service (CAMS) paint a very different story.

CAMS, operated by the European Commission, puts out daily forecasts of aerosol built up across the globe. It is important to understand that this data is not directly observed satellite data, but re-analysis data — or in other words simulated/modeled data.  

The data provided by satellites is spatially continuous, but not temporally continuous. Also satellite observations are affected by failed instruments or clouds obstructing the view of the satellite.

Aerosol re-analysis helps eliminate these inconsistencies by generating a consistent, spatially and temporally continuous dataset. Re-analysis takes into account past and present satellite observations, meteorology and global emissions.

This service also puts out forecasts of aerosol speciated products, including dust aerosol, sulfate aerosol, biomass-burning aerosol and sea-salt aerosol. Again, these forecasts are based on re-analysis data.

Currently, there are only 167 continuous monitoring stations in India and most of these stations are located in the northern part of the country — one of the primary reasons behind the lack of information on air pollution in other parts of the country.

In the absence of ground-based observations of particulate matter (PM) concentrations, scientists have often resorted to near-real time satellite data to arrive at ground-level particulate concentration from satellite observed aerosol optical depth (AOD).

Aerosol optical depth is a measure of aerosols like urban haze, smoke particles, desert dust and sea salt distributed within a column of air from the earth’s surface to the top of the atmosphere. Post application of empirical correction factors, a scaling factor is used to convert the AOD to PM concentration.  

Aerosol build up across India

Images retrieved from CAMS aerosol database indicate significant built up of aerosol across India, and not just Delhi. While along the northwestern part of the country the depth of dust aerosol seems to be significantly high, along the southeastern coast the aerosol concentration seems to be primarily driven by sulfate aerosols.

While the correlation between AOD and PM2.5 levels depends upon background particulate concentration, meteorology-depth of the boundary layer being most important and local emission strength, in most cases high aerosol optical depths correlate with high particulate levels.

If we go by the number of real-time stations in the country, continuous monitoring of pollution is still in the nascent stage. In the absence of real time data, taking responsive pollution control measures becomes almost impossible.

Under such circumstances harnessing satellite-derived air pollution data could be a game changer. Satellite derived air pollution data has already found application in national-level health studies and could potentially be used in local assessments.

For instance, if one looks at the sulfate aerosol maps carefully, it becomes evident that states of Odisha and Andhra Pradesh are exposed to a cloud of sulfate aerosol. Sulfate aerosols are a result of chemical transformation of Sulfur dioxide (SO2) — exceeding levels of which are rarely reported.

Given that SO2 is toxic for humans, exposure to sulfate aerosols could also have human health implications. 

May 9, 2019

1) Total aerosol

2) Dust aerosol

3) Sulfate aerosol

May 10, 2019

1) Total aerosol

2) Total aerosol

3) Sulfate aerosol

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