Characterization of Organic Aerosol by Volatility and Elemental Ratios Using a New Moderate-cost Detection Method

Abstract:

The chemical composition of atmospheric particulate matter “aerosols” has strong influence on its health and environmental impacts. Unfortunately, routine air quality monitoring generally provides only measurements of particle mass with limited chemical information. We present here the development of a new aerosol chemical composition monitor (“Chemspot”) to fill this gap. We measure aerosol mass, volatility, and elemental ratios (O:C, S:C) in a way that maximizes reliability and autonomous operation at moderate cost. Particles are sampled through a condensational growth tube, then impacted into a ~1 mm spot in a custom passivated quartz cell using a low-velocity, low pressure drop nozzle. Temperature-stepped thermal desorption provides volatility resolution as the vaporized sample is passed through a novel detector train. Gas Chromatographic (GC) detectors including a Flame Ionization Detector (FID) and Flame Photometric Detector (FPD) are combined with a CO₂ monitor to measure aerosol organic carbon, oxygen-to-carbon ratios, and total inorganic and organic sulfur. The use of common and well-understood GC detectors allows us to easily automate the calibration of this monitor using only a small number of gas-phase standards such as CO₂ and CH₄. We demonstrate here the efficient collection of particles (>95%) between 10 nm and 1000 nm in diameter. Subsequent rapid thermal desorption at rates of 20 ^oC/s is shown to provide separation by volatility with a resolution of less than two orders of magnitude in vapor pressure. Initial performance analysis of the monitor along with volatility-resolved carbon, oxygen, and sulfur concentrations in ambient aerosol will be presented.