Field Calibration of a Low-cost Particulate Matter (PM) Sensor at a Suburban Site in the North-Western Indo-Gangetic Plain (NW-IGP)

Recent years have witnessed an upsurge in the usage of portable low-cost particulate matter (PM) sensors that have the potential to augment sparse measurement networks, but need to be validated under field conditions. Here, we report the field calibration of Laser Egg (LE), a low-cost PM sensor working on the principle of laser based light scattering, against Class III US EPA Federal Equivalent Method PM₁₀ and PM_2.5 β-attenuation analyzers. The calibration was performed at a representative site in the north-western Indo-Gangetic Plain (NW-IGP) over a three month period (April 27, 2016 - July 25, 2016) with ambient mass loadings ranging from < 1-838 µg m^-3 and < 1-228 µg m^-3 for PM₁₀ and PM_2.5 respectively.

Best agreement between LE and reference PM_2.5 mass concentration was observed when ambient relative humidity (RH) was below 40% with slopes of 0.76-0.93. At RH > 80%, LE overestimated PM_2.5 because it uses wet aerosol diameter to estimate the mass as opposed to dry mass measured by the reference analyzer. Large scatter in PM_2.5 normalized discrepancies ((LE PM_2.5 - Reference PM_2.5)/Reference PM_2.5)observed at similar ambient RH at Mohali, presumably caused by variability in aerosol physicochemical properties prevents corrections to LE readings using power-fit growth function and RH measurements.

When coarse fraction of PM₁₀ was > 50%, the inlet fan in LE acted as a plate impactor and blocked particles with large aerodynamic diameter causing underestimation of PM₁₀ mass concentration (slope: 0.31-0.76). The accuracy of PM₁₀ measurements from LE was highest (slope: 0.97-1.25) when coarse fraction of PM₁₀ was < 50% and RH > 80% as the errors introduced by inlet losses and hygroscopic growth canceled out.

Customizing LE with a fan-less inlet and a short diffusion dryer can enable accurate high-density PM monitoring in the near future.