On hazy winter mornings in Delhi, the air people breathe at street level may not tell the full story. A new peer-reviewed study published in Nature npj Clean Air reports that PM2.5 concentrations were up to 60% higher at around 100 metres above ground compared to surface levels during severe haze episodes. The findings suggest that pollution can accumulate not only near roads and traffic corridors, but also higher up at an altitude where many residents and office workers in high-rise buildings spend much of their time. Using a custom-built drone equipped with low-cost particulate matter sensors, researchers conducted multiple flights over South Delhi in March 2021. On one hazy morning, PM2.5 levels reached around 160 micrograms per cubic metre at 100 metres, compared to roughly 100 micrograms per cubic metre at ground level. Ajit Ahlawat, Assistant Professor at Delft University of Technology and lead author of the study said about the finding, “This indicates that ground-based monitoring alone substantially underestimates actual exposure to pollution in the lower urban boundary layer, especially during haze episodes.” Traditional air quality monitors typically measure pollutants around five to ten metres above the ground. “This misses vertical gradients in shallow nocturnal boundary layers that trap pollutants aloft,” Ahlawat said, referring to the different heights in the layer of air close to the ground that forms at night which can go up to 300 metres or higher. He added that hygroscopic particle growth, where particles absorb moisture and grow, and secondary aerosol formation often occur…This article was originally published on Mongabay
