Supercritical Fluid Cartridges for Calibration of Air Pollution Sensors

Supercritical Fluid Cartridges for Calibration of Air Pollution Sensors Project Summary The advent over the past decade of community monitoring of air pollutants using inexpensive sensors has resulted in massive amounts of data potentially useful for improving community health, but the quality of data is often unknown or poor due to lack of adequate sensor calibration, both prior to and after deployment in the field. This is largely due to lack of a highly portable method to provide zero and span gases of known concentrations. Here, we will develop a new calibration method based on the use of small carbon dioxide (CO2) cartridges of the type commonly used for air and paint guns, carbonating beverages and inflating tires. CO2 cartridges have the advantage of producing ~4 times more gas volume than a pressurized cylinder of the same volume and at the same pressure. When the cartridge is heated above the critical temperature of 31.1oC, the CO2 becomes a supercritical fluid having a uniform density, therefore making them suitable for delivering constant concentrations of pollutant gases at trace (ppb and ppm) levels and potentially particles of known concentrations and size distributions as well. Another advantage of supercritical fluids is that, unlike gases, they have solvent power and can dissolve semi-volatile compounds such as the Hazardous Air Pollutants (aka Air Toxics) that often have insufficient vapor pressure to be made available in compressed gas cylinders. A highly novel application of supercritical CO2 cartridges is the possibility of delivering known concentrations of polystyrene latex (PSL) nanospheres and microspheres of very narrow size distributions, having mean diameters ranging from tens of nanometers to several microns, for calibrating particle sensors (e.g., the Plantower sensors of Purple Air networks) to have uniform responses to atmospheric particles. Here we will evaluate the feasibility of the use of CO2 cartridges spiked with known concentrations of air pollutants to provide calibration source gases of highly volatile gases such as the EPA Criteria pollutants CO, NO2 and SO2, the semi-volatile Air Toxics benzene, toluene, ethylbenzene and xylenes (collectively known as BTEX) and PSLs for particle counters. We also will develop a dynamic dilution device for attaching CO2 cartridges, safely controlling their temperature, and providing dilutions into scrubbed ambient air to provide calibrated gas concentrations varying by up to three orders of magnitude. If successful, this project would provide a new and relatively inexpensive means of calibrating both research-grade instruments and low-cost sensors. For example, based on the $16.98 price for a box of ten 16-g cartridges, the cost of materials for manufacturing a single cartridge is less than $2, considering that the ppb levels of analytes costs a few cents at most. A single cartridge of this size (1.85 cm diameter x 8.3 cm length) would provide 8.9 L of gas at room temperature, enough to provide 17 five-minute calibrations at a flow rate of 100 cm3/min or 35 calibrations at 50 cm3/min. Alternatively, a 65-g cartridge (3.2 cm x 17.5 cm) would provide ~4 times that number of calibrations, enough for more than 4 months of remote operation for flow rates of 50 cm3/min.