Quality assurance and maintenance

Zero checks were conducted weekly, usually before or after collocation (described below). These checks used a 20-L Tedlar gas sampling bag filled with zero air (~ 80% N₂, ~20% O₂) connected to the gas sampling inlet and an extended surface area 1 μm pore size filter connected to the PM sampling manifold. Times and instrument readings after stabilization (after 5–15 min) were documented on the field sampling datasheet.

Collocations in which MPAL was parked next to a monitoring site operated by the Michigan Department of Environment, Energy, and Great Lakes (EGLE) were performed weekly during field campaigns. To avoid self-pollution from the vehicle’s exhaust, the vehicle was positioned facing upwind, or the vehicle’s engine was turned off if battery power was sufficient to support the day’s sampling. Because EGLE maintained data as hourly averages, collocations were scheduled at the start of the hour and lasted for at least 45 min.

Instrument and system maintenance followed manufacturer’s protocols and our standard operating protocols (SOPs). Seasonally or after ~ 50 days of sampling, we calibrated the NO_x, SO₂, and O₃ instruments using a dynamic diluter and certified zero and span gases, and the CO instrument (Picarro G2401) was checked with zero and span gases at the EGLE instrument shop in Lansing, Michigan. The TSI instruments were sent back for calibration after a year’s operation. Generally, the Magee aethalometer does not require calibration, though flows were regularly checked, and the instrument was collocated with other aethalometers for extended periods (several days). For the Horiba PX375, we checked the X-ray intensity and energy calibration every 6 months. The Picarro G2401 and G2204 were checked using CH₄ and H₂S standards. In addition, CH₄ is measured independently by these instruments, and the zero check and typical atmospheric levels (2.0 ppm) represent additional quality checks.