Estimating Mass Concentration Using a Low-cost Portable Particle Counter Based on Full-year Observations: Issues to Obtain Reliable Atmospheric PM2.5 Data

Expanding the use of a recently introduced low-cost particle monitor (DC1700 Dylos Air Quality Monitor) for sensing atmospheric PM2.5 requires comparison with data obtained using a certified method for PM2.5 based on appropriate atmospheric observations. Full-year measurements of atmospheric aerosols were taken in Nagoya, Japan during March 2017-March 2018 using the DC1700 to measure the particle number concentrations of >0.5 and >2.5 μm diameter particles and to measure the PM2.5 mass concentration (Mdry, PM2.5) using an automated β attenuation mass monitor (PM712). The number-size distribution was measured using an optical particle counter (KC01D). The dried mass concentration of 0.5-2.5 μm particles (Mdry, 0.5-2.5) was estimated from the ambient relative humidity and the DC1700 number concentration. The values of Mdry, 0.5-2.5 were invariably less than those of Mdry, PM2.5. The coefficient of determination and slope of Mdry, 0.5-2.5 to Mdry, PM2.5 for the year were, respectively, 0.68 and 0.40. Slope values changed seasonally from 0.24 in July and August 2017 to 0.55 in May and April 2017. Light absorbing particles, smaller-fine particles, and the estimation method of Mdry, 0.5-2.5 were inferred as causes of the difference between Mdry, 0.5-2.5 and Mdry, PM2.5. Especially, we estimated a large contribution (ca. 54% underestimation of Mdry, 0.5-2.5 into Mdry, PM2.5) of particles smaller than the minimum detection diameter of DC1700. The seasonal variation of Mdry, 0.5-2.5/Mdry, PM2.5 was related to the volume fraction of particles smaller than 0.5 μm. Good correlation of Mdry, 0.5-2.5 to Mdry, PM2.5 suggests that data obtained using DC1700 with a correction factor are useful as a rough proxy of atmospheric PM2.5 within a season. However, precise estimation of PM2.5 from the DC1700 number concentrations should include appropriate corrections of the size distribution, not only hygroscopicity.