Assessing rural vs. urban PM2.5 disparities in the Indo-Gangetic Plain: A low-cost network approach

The Indo-Gangetic Plain (IGP) experiences severe air pollution, with typical annual-average fine particulate matter (PM2.5) concentrations in the range 75 - 150 µg m^-3, leading to average life expectancy for nearly a billion people to be shortened by approximately two years. Air pollution policies often emphasize city-by-city management, yet air pollution in the IGP is a regional challenge, and in-situ observations of rural air quality are limited. Here, we present initial results from our ongoing study to characterize rural vs. urban air pollution gradients in North India, based on a stratified sampling scheme designed to cover a broad range of conditions from villages to towns to megacities and evaluation of sensors performance during the 12 months period including collocation and field deployment.

Our experimental approach employs a carefully maintained network of lower-cost PM_2.5 sensors (Purple Air [PA] II, each encompassing two Plantower PMS5003 nephelometers), calibrated on the basis of multiple beta-attenuation reference PM_2.5 monitors (MetOne BAM-1022) and hourly-average measurements. During two mass colocation events in urban Bangalore (South India) and rural Hamirpur (IGP), we observed that PA sensors produced highly reproducible results with low unit-to-unit variability. The PA sensors estimated BAM-reported PM_2.5 with R² > 0.8 and normalized root-mean-squared error (NRMSE) of 20-30%, for concentration ranging from 5 to 150 μg m^-3.

We deployed PA sensors in a clustered pattern to characterize within-settlement variability along a gradient from remote rural to highly urban conditions in eastern Uttar Pradesh. We observed mass concentration of PM_2.5 ranged from 8.5 to 560 with an average of 82.7 μg m^-3 in Varanasi city (population ~1.2 million) whereas Mai village (population ~5200) observed a concentration range from 9.1 to 512.7 μg m^-3 with an average of 105.2 μg m^-3. Overall, we found that PM_2.5 concentrations in our pilot sample of villages and small towns were often similar to or higher than the nearest large city. These initial results suggest that additional observations in rural settings may provide valuable new information about the sources, dynamics, and consequences of PM_2.5 across this region.