ACSM

The Aerosol Chemical Speciation Monitor (ACSM) is an instrument designed to provide near-real-time, quantitative measurements of the non-refractory chemical composition of submicron aerosol particles (typically <1 μm aerodynamic diameter). ACSM measures mass concentrations of major aerosol species, including organics, sulfate, nitrate, ammonium, and chloride, by sampling ambient air through an aerodynamic lens system that focuses particles into a narrow beam. Particles are flash-vaporized at a heated surface (~600 °C) under high vacuum, and the resulting vapor is ionized via electron impact (typically at 70 eV). The ions are detected by a mass spectrometer, which determines the chemical composition and mass concentration of the aerosol. ARM operates three quadrupole ACSMs and two time-of-flight ACSMs.

Quadrupole ACSM (Q-ACSM) instruments use a quadrupole mass spectrometer for mass-to-charge (m/z) analysis. Quadrupoles filter ions based on stability in an oscillating electric field, allowing only ions with a specific m/z to reach the detector at a given time. The Q-ACSM provides unit mass resolution (integer m/z) and is optimized for routine, continuous monitoring. The typical limit of detection for Q-ACSM is approximately 0.3–0.5 μg/m³ (for 30-minute averaged data).

Time-of-Flight ACSM (TOF-ACSM) instruments, in contrast, use a time-of-flight mass spectrometer. In TOF-MS, ions are accelerated to the same energy and separated based on their time to reach the detector, allowing higher mass resolution and accuracy. TOF-ACSM can distinguish between ions with closely spaced masses and produce full mass spectra at high temporal resolution. The typical limit of detection for TOF-ACSM is approximately 0.05–0.1 μg/m³ (for 5–30-minute averaged data), depending on specific instrument settings and operating conditions.

• Inorganic chemical composition
• Organic Carbon Concentration
• Organic Material Concentration