As we continue to battle air pollution, one lesser known contributor to it, apart from industrial and vehicular pollution, is the presence of aerosol in the atmosphere, which has reached concerning levels. This new-found threat deserves attention

The issue of air pollution has always been visualised in the classical sense with an obvious reference made to industrial and vehicular pollution. But of late, the subject of aerosol pollution has been garnering quite a lot of attention. While most air quality bulletins revolve around usual suspects such as particulate matter and other air-borne pollutants, the subject of aerosol build-up has managed to stay out of attention of weather as well as environmental experts.

In fact, recently, air pollution monitoring stations were able to study and observe the dust storm that hit Delhi, which resulted in a severe dip in air quality levels. But aerosol build-up is neither being studiously documented nor is the subject being viewed as a significant threat.

Mankind has made a significant leap in atmospheric monitoring and modelling and thankfully the same has been able to detect the growing problem of aerosol build-up in the atmosphere. Recent images put out by the Copernicus Atmospheric Monitoring Service (CAMS) paint a very intriguing story, which introduces us to the emerging threat posed by aerosol pollution. CAMS, which is operated by the European Commission, puts out daily forecasts of aerosol build-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 or modelled data. 

The data provided by satellites is spatially continuous but not temporally continuous. Moreover, failed instruments or clouds obstructing the view of the satellite affect the accuracy of satellite observations.

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.

India still lags behind in developing this expertise to track and analyse the aerosol data in its atmosphere. Currently, there are only 167 continuous monitoring stations in our country and most of them are located in the northern part. This is one of the primary reasons for the lack of information on this type of 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 buildup over India is now reaching concerning levels. According to images retrieved from CAMS, the aerosol database indicates significant build-up of aerosols across the country, more so over the national capital. While along the northwestern part of the country the depth of dust aerosol seems to be significantly high, in the southeastern coast, the aerosol concentration seems to be primarily driven by sulfates.

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 one goes 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 can be a game-changer. Satellite derived air pollution data has already found an application in national-level health studies and can potentially be used in local assessments.

Sulfate aerosol levels in particular have been increasing rapidly. For instance, images taken by the CAMS showed sulfate aerosol maps. A careful study of the same will make it apparent that the 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 can also have harmful human health implications. Given this, it is crucial for India to start developing state-of-the-art weather monitoring stations that are specially equipped to deal with aerosol pollution.

India must focus on using latest technology to control the increasing problem of aerosol pollution. It is essential for policy planners and climate scientists to take immediate action so that one is able to control the problem while it is still manageable.

The very first is to cover the geographic extent of the country and ensure that enough monitoring stations are relaying sufficient and credible data pertaining to the presence of aerosol so that proper planning and action are visualised to effectively neutralise the growing problem.

(The writer is an environmental  journalist)