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Sudip Pradhan, Birendra Bajracharya & Utsav Maden
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With transportation heavily restricted and industries largely shut down because of the COVID–19 pandemic, we are experiencing improvements in the air quality around us. The sky is bluer and the mountains are visible more often. This present reality is a temporary respite from a situation across the Hindu Kush Himalayan (HKH) region otherwise marked by a rapid increase in air pollution over the past two decades. Many cities in and near the HKH have recorded annual average PM2.5 concentrations almost 10 times higher than the World Health Organization guideline.
While both weather conditions and human activities contribute to the flow of air pollutants, a drastic reduction in the latter provides us with a unique situation to observe and compare air pollution patterns. Satellite data provide a picture of air pollution over large areas and facilitate monitoring of different air quality parameters, which is especially valuable in a region mired by data-sharing reservations and limited monitoring stations.
To promote access to these satellite data, we have developed the Air Quality Explorer for the HKH application using freely available satellite data and the Google Earth Engine platform. The application allows visualization of three atmospheric parameters – nitrogen dioxide (NO2), sulphur dioxide (SO2), and Aerosol Optical Depth (AOD) – for any chosen period and location. NO2 is an important trace gas resulting from wildfires, lightning, and human activities like fossil fuel combustion and biomass burning. The majority of SO2 emissions are also anthropogenic in origin. Higher values of AOD indicate presence of dense aerosols, which result from pollution from factories, fires, dust storms, sea salt, volcanic ash, and smog.
The application makes use of the latest NO2 and SO2 data from the Tropospheric Monitoring Instrument (TROPOMI) sensor on board the Sentinel-5 Precursor (S5P) mission satellite and AOD data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra and Aqua satellites.
Users can view and compare before/after maps for NO₂, SO₂, or AOD for a particular area for two user-specified time periods. They can also navigate to a city or location of interest by using the search function.
The “View Statistics” section provides a chart of weekly or monthly mean values of all three data parameters for a user-specified geographic area and time period. Users can either choose from a list of cities provided in the application or draw a rectangle or polygon area on the map to generate such charts. It also allows users to generate maps depicting the averages for the three parameters for the past 30-day period.
We used the application to examine atmospheric NO2 density in four cities: Delhi, Dhaka, Kathmandu, and Lahore. We compared observations for January (before COVID-19 lockdown measures were implemented) and April 2020 (during the lockdown period). April 2020 showed significantly diminished atmospheric NO2 in all the cities.
The charts for weekly average NO2 density in these cities also align with the generated maps. However, a clear decline is not observable for SO2 and AOD. This further underscores the fact that other factors such as meteorological conditions and open burning also contribute to the levels of NO2, SO2, and AOD in the atmosphere, and these need to be studied further.
Satellite data makes it possible to quantify and visualize changes in our surroundings both from spatial and temporal perspectives. This Air Quality Explorer for the HKH application demonstrates the use of big data and generates results on-the-fly using Google Earth Engine. Although the application was developed with the COVID-19 pandemic in mind, it can be customized for wider application.
The display viewport defaults to the HKH region. However, this can be customized to work for any part of the world and is therefore useful to anyone interested in air quality research within and beyond the region. We have a vision on what is possible, how clean the air can be. As we recover from the COVID-19 crisis, we can use this tool to monitor how we are doing on our road towards clean air, and also to help us devise innovative solutions and make good decisions.
The Regional Database System Initiative (RDS) at ICIMOD developed the application. The Initiative’s primary objective is to promote availability and accessibility of scientific data to support evidence-based decision making in the HKH region. Besides developing and maintaining different databases and data sharing platforms in-house, the Initiative also implements the latest innovative technologies in data analysis and visualization. This application is one such example. For more on the RDS, visit https://rds.icimod.org.
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