Exploring the discrepancies between SMPS and AMS measurements in secondary organic aerosol formation experiments

Inconsistencies have often been observed between the measurements obtained from the Aerosol Mass Spectrometer (AMS) and the Scanning Mobility Particle Sizer (SMPS) during laboratory experiments focused on secondary organic aerosol (SOA) formation. We show that these discrepancies are due to the use of a size-independent collection efficiency (CE) especially in experiments in which new particle formation takes place. A new methodology for the estimation of a composition and size-dependent CE is proposed. This approach significantly improves the agreement between the measurements from the two instruments with the average deviation of the two types of measurements remaining below 10% for both total aerosol and SOA concentration. We show that in these experiments, ammonium sulfate particles both pure and thinly coated with SOA have a lower CE than the pure SOA particles or particles thickly coated with SOA.