Using in-situ CO2, PM1, PM2.5, and PM10 measurements to assess air change rates and indoor aerosol dynamics

A common technique to assess indoor air quality is to estimate a space's air change rate by quantifying the decay of a tracer species, such as metabolic carbon dioxide (CO2). However, CO2 decay does not fully represent the complexities of aerosol dynamics. In this study, low-cost sensors (QuantAQ/Aerodyne Research ARISense v200 and QuantAQ MODULAIR-PM) were used for continuous measurements of size-resolved particulate matter (PM1, PM2.5, PM10) and CO2 in a university classroom across >5 months in 2021. Occupant-generated emissions from classroom activity and cleaning (fogging aerosols) events were used to determine the decay time constant of each pollutant, which varied substantially (e.g., 25-86 min for CO2, 14-66 min for class PM10 and 24-103 min, 25-82 min, and 18-56 min for fogging PM1, PM2.5, and PM10, respectively across 3 months in the Spring). The range of measured CO2 and PM decay rates was comparable, indicating that quantifying metabolic CO2 decay is a viable method to estimate the timescale of indoor aerosol decay, and that both species provide comparable air change rate estimates. However, the effect of deposition on PM decay was evident and uncertainties in using occupant-generated tracer decay to determine air change rates should be considered. These results provide insights into the practicality and limitations of using in-situ CO2 and PM decay measurements to assess ventilation, and of using CO2 decay to estimate aerosol decay. This work also highlights the importance of performing continuous mea-surements over extended periods of time to quantify a range of air change rates.

Automated summary

This study used low-cost sensors to continuously measure size-resolved particulate matter and carbon dioxide in a university classroom. The decay time constant of each pollutant was determined using occupant-generated emissions, and the results showed that quantifying metabolic CO2 decay is a viable method to estimate the timescale of indoor aerosol decay. Both CO2 and particulate matter provided comparable air change rate estimates.