Technical Note: Use of a beam width probe in an Aerosol Mass Spectrometer to monitor particle collection efficiency in the field

Abstract

Abstract. Two Aerodyne Aerosol Mass Spectrometers (Q-AMS) were deployed in Mexico City, during the Mexico City Metropolitan Area field study (MCMA-2003) from 29 March-4 May 2003 to investigate particle concentrations, sources, and processes. We report the use of a particle beam width probe (BWP) in the field to quantify potential losses of particles due to beam broadening inside the AMS caused by particle shape (nonsphericity) and particle size. Data from this probe show that no significant mass of particles was lost due to excessive beam broadening; i.e. the shape-and size-related collection efficiency (E s ) of the AMS during this campaign was approximately one. Comparison of the BWP data from MCMA-2003 with other campaigns shows that the same conclusion holds for several other urban, rural and remotes sites. This means that the aerodynamic lens in the AMS is capable of efficiently focusing ambient particles into a well defined beam and onto the AMS vaporizer for particles sampled in a wide variety of environments. All the species measured by the AMS during MCMA-2003 have similar attenuation profiles which suggests that the particles that dominate the mass concentration were internally mixed most of the time. Only for the smaller particles (especially below 300 nm), organic and inorganic species show different attenuation versus particle size which is likely due to partial external mixing of these components. Changes observed in the focusing of the particle beam in time can be attributed, in part, to changes in particle shape (i.e. due to relative huCorrespondence to: J. L. Jimenez (jose.jimenez@colorado.edu) midity) and size of the particles sampled. However, the relationships between composition, atmospheric conditions, and particle shape and size appear to be very complex and are not yet completely understood.

Primary and secondary contributions to aerosol light scattering and absorption in Mexico City during the MILAGRO 2006 campaign

“…Previous measurements of total PM 2.5 in Mexico City, 2003, have been shown to agree well with the sum of aerosol species from the AMS plus refractory BC and soil , and most of the aerosol mass in MCMA is likely in internally mixed particles by the afternoon (Johnson et al, 2005;Salcedo et al, 2007). In the current study, total aerosol scattering at 532 nm correlates well with total PM 1 aerosol mass as obtained from the AMS plus refractory BC obtained from the photoacoustic measurements of light absorption and use of a mass absorption efficiency factor of 8.8 m 2 /gram, as shown in Fig.…”

The formation, properties and impact of secondary organic aerosol: current and emerging issues

“…followed by post-analysis. This creates the inherent risk of positive and negative artifacts due to adsorption, evaporation, and chemical reactions during the multi-step collection and analysis procedure (e.g., Turpin et al, 2000;Schauer et al, 2003;Subramanian et al, 2004;Dzepina et al, 2007). This type of sampling also severely limits both spatial and temporal sampling densities.…”

Development and Characterization of a Fast-Stepping/Scanning Thermodenuder for Chemically-Resolved Aerosol Volatility Measurements

“…Decreased collection efficiency due to shape-dependent losses (E s ) has been shown to be unimportant in multiple field campaigns (i.e., E s ∼1) (Huffman et al 2005;Salcedo et al 2007). The collection efficiency due to bounce (E b ) dominates total CE for particle sizes that the aerodynamic lens allows to pass efficiently, causing CE to remain remarkably constant at approximately 50% during most ambient campaigns (Alfarra et al 2004;Canagaratna et al 2007;Drewnick et al 2004;Zhang et al 2004), although it has known deviations from this value for some extremes of phase and composition, e.g., for very acidic particles (Quinn et al 2006).…”