The first use for the low-cost sensors is comparative monitoring, in which values of PM_2.5 and PM₁₀ (PMs) are compared with the time event. This analysis provides patterns of the concentration with the change of experimental conditions such as road vs. home, cooking vs. none, day vs. night, construction site vs. residential home. The comparative analysis is suitable for high-school and undergrad students to have some peek into the change of PMs with the condition.

I myself conducted three studies by the time writing this post using the comparative analysis. The first one is to evaluate the PMs inside a closed room and the balcony during a 5-day vacation. The next two are PMs emitted by a firewood cook stove and removal efficiencies of face masks to PMs. The details of these studies posted here. For each study, I compared PMs concentration from a sensor measuring the background or ambient concentration and the other for the experimental conditions. When one sensor is available, I have to move between experimental conditions and the one for the background.

3. Summary of studies on observed value and calibration

Comparative studies are simple and useful for hobbyist and personal uses. When communicating results to the public or for a research level, the accuracy of the low cost devices to the standard method is a must. Table 1 listed the error of PMS7003, SDS011, HPMA115S0 is 10% and ±15µg/m³ which is a good start, but to be sure with local conditions, the so-called "co-location" study is needed. In a co-location study, all devices are placed in proximity and expose to the same ambient condition. The output of sensors is cross-checked with additional to a calibration curve based a reliable device using the reference method.

Available testing on optical sensors measuring PMs including the low-cost class are carried out by the US. EPA and the Air Quality Sensor Performance Evaluation Center (AQ-SPEC). The study done by the US. EPA included mid-range devices in term of price, from $500-$2500. Only DC1100 Pro was included in this study in North Carolina, US. The AQ-SPEC carried extensive testing on commerical devices ranging from $150-$300. Testing bare sensors is not included rather a package with possible customized calibration by the device's maker. The reference device is Federal Equivalent Method (FEM) approval such as GRIMM (EDM 180) or Met One (BAM-1020).

Over 10 reports and journal articles I skimmed through, a consensus summary as follow:

• SDS011, PMS7003, HPMA115S0 overestimated PMs concentration by FEM device
• The R² values is in the region of 0.6-0.8 for the field test