Satellite-Based Application For Air Quality Monitoring and Management at National Scale (SAANS)

What is the status in Delhi NCR?

Delhi national capital region (NCR) is amongst the most polluted regions in the world. PM2.5 build-up during the dry season (Oct-Jun) week-by-week has been tracked by high-resolution (1 km) satellite data. To examine the spatial gradient of PM2.5 concentration across the NCR (Figure 1), we focus on two regions - A and B in the upwind and downwind flanks of the NCR respectively, depending on the dominant wind direction. We estimate the long-term (15 years) dry season averaged PM2.5 concentration over NCR to be 171±23 µg/m3. In the same time period, the long term PM2.5 concentrations in the upwind box A and downwind box B are estimated to be 161.2±19 µg/m3 and 202.1±24.4 µg/m3, which implies that the PM2.5 concentration in the downwind region of Delhi NCR is 1.25 times the PM2.5 concentration in the upwind region. The higher concentration in the downwind region may be attributed to the ‘megacity outflow’ of pollution from Delhi NCT (marked in bold in Fig.1), which affects the downwind districts in Delhi NCR. Though this spatial gradient is persistent throughout the post-monsoon (October-November), winter (December-February) and summer (March-May) seasons, its magnitude varies across seasons. Mean (±1 standard deviation) ground-level PM2.5 concentration (290±54 µg/m3) during the post monsoon season is close to five times the 24-hour Indian standard. Pollution level (210±35 µg/m3) remains more than 3 times higher than the 24-hour Indian standard in the winter season. During the summer season, though PM2.5 mean ground-level PM2.5 concentrations decrease substantially over the NCR (90±13 µg/m3), they remain substantially higher than the 24-hour Indian standard. It may be noted that WHO 24-hour guideline for PM2.5 is much lower (25 µg/m3) and therefore health risks would exist even if the Indian standard is met.

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This Figure is Published in Chowdhury et al., 2019

Figure 1. Spatial distribution of PM2.5 over Delhi NCR during the dry season for the period 2001-2015. Upwind and downwind sides are demarcated by ‘A’ and ‘B’

Two major pollution episodes are identified in Delhi NCR (Figure 2). Mean ambient PM2.5 concentration remains >300 µg/m3 (five times the Indian 24-hour standard) for several weeks around the two peak pollution episodes. The first peak is attributed to pollution transport from upwind areas affected by open biomass burning, coupled with stable atmospheric conditions, while the second is attributed to enhanced local emissions and perhaps secondary aerosol formation under favourable meteorological conditions.

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This Figure is Published in Chowdhury et al., 2019

Figure 2: Weekly PM2.5 concentration (in µg/m3) over Delhi NCR and NCT. The blue and orange lines depict mean (shaded areas represent ±1 standard deviation)