Experimental Investigation of the Collection Efficiencies of the AMS Aerodynamic Focusing Lens

Transcription

1 Experimental Investigation of the Collection Efficiencies of the AMS Aerodynamic Focusing Lens Peter S.K. Liu, Terry Deshler and Derek C. Montague John Jayne and Doug Worsnop Xuefeng Zhang, Kenneth A Smith and Deng Rensheng,

2 Limited verification of Fluent model result 1.2 Goal: Extend the measurements CE Fluent model result John's result D va

3 Intrumentations Aerosol generation system Atomizer classifier system to generate particles of known size and concentration Aerosol sizing Scanning Mobility Particle Sizer (SMPS) to verify the sizes of test aerosol. Condensation Particle Counters (CPC) CPC to measure concentration reference

4 Monodisperse Aerosol Generation

5 Monodisperse Aerosol Generation Polydisperse aerosol from atomized solution of salts or oil Singly Charged Doubly Charged Select a narrow size distribution with classifier and confirm the size with an SMPS

6 Concentration Reference Correct concentration reference crucial in CE determination Liu and Deshler (2003)* showed significant losses of <100 nm classified charged particles in CPC3010 Losses in CPC are minimized by neutralizing the classified aerosol

7 Experimental Setup

8 Determining Effective Ionization Efficiency (EIE) Use particles size that are detectable as single particles. The effective ionization efficiency is then determined by comparing the ion count rate with the SMPS and CPC determined particle mass. Shown are AMS measurements of 250 nm particles. AMS detects the same # concentration as CPC 100% CE

9 Determining Effective Ionization Efficiency (EIE) Calculate mass of singly charged particles, CPC(ref), requires: CPC average number concentration Size (average volume diameter from SMPS), density and shape factor of the particles Percent of singly versus multiple charged particles (from SMPS scan or computed) Calculate AMS ion count rate for singly charged particles, AMS(ref) Integrating the average ion count rate from the first peak of the TOF distribution. Obtain EIE: EIE = CPC(ref) AMS(ref )

10 Determine CE for Other Sizes Make measurements at other sizes and determine: AMS ion count rate at size x (AMS(x)) and CPC mass concentration at size x (CPC(x)). Collection Efficiency for size x: CE ( x) = EIE AMS( x) CPC( x)

11 Example of AMS measurement of particles of D m of 48 nm Non-single particle Single particle

12 SMPS scan of, D m of 48nm

13 Number of Particles Required to Get Above Detection Limit and Those Used in CE Experiments 10 4 Concentration of singly charged ammonium nitrate aerosol used in CE experiments relative to AMS detection limit; SNR~2 Coincidence in CPC 1.0 Number Concentration (cm -3 ) CE _A _B _C AMS detection limit SNR~2 CE from Fluent D va (nm) 0.2

14 CE Results From 3 Different Experiments 1.2 AMS Lens Collection Efficiency CE Fluent Result John's CE CE_Aug1904 CE_Aug2404 CE_Sep D va

15 After Coincidence Correction for CPC 1.2 AMS Lens Collection Efficiency CE Fluent Result _A _B _C D va

16 CE comparison with Qi s data & Peter s Model result 1.2 AMS Lens Collection Efficiency CE Fluent Result _A _B _C CE_dehs OrgData AMS Organics D va

17 Issues with Large particles (D va >600nm) Smaller size tail measured for large particles

18 Comparison With SMPS 10 AMS m/z46 signal SMPS scan (U. Buffalo L-curve method) Talukdar and Swihart (2003) AST 37: AMS m/z 46 signal (bits) 6 4 No small size tail with SMPS scan dn/dlogd va D va (nm)

19 Compare Counting vs Integrating mode for >300nm AMS Lens Collection Efficiency Evidence of fracturing for >600nm CE _A_integrating _B_integrating _C_integrating Fluent _A_count _B_count _C_count Counting Mode Integrating Mode D va

20 Difficulties extending results to larger particles using Pb( ) 2 ; Density=4.53g.cm 3 There is no size where there is a 1:1 ratio for the AMS and CPC count. Thus cannot obtain a reference. Bounce at vaporizer

21 Experiment With Spherical Particles to Check If CE Is Dependent on Shape Di ethyl hexyl sebacate (DEHS) dissolved in isopropanol Activated charcoal to dry solvent Results agreed with ammonium nitrate results For particles > 350 nm the CE obtained from count mode significantly >1 Suggests particle fragmentation for liquid particles is worse than for solid ammonium nitrate.

22 AMS#/CPC# >1 for DEHS >350nm (1.66 for 500 nm)

23 Results of Spherical Particle; DEHS Oil 1.2 AMS Lens Collection Efficiency CE Fluent Result _A _B _C CE_dehs D va

24 Summary Measured the AMS CE for and DEHS particles over a wider size range, D va nm, than previously done The (solid) and DEHS (liquid) particles had similar collection efficiencies Compared to Fluent model calculations: Ce measured < Ce modeled for particles < 120 nm Ce measured = Ce modeled for nm Ce measured > Ce modeled for particles nm Evidence to suggest that large and DEHS particles break up upon entering the AMS Experiments for larger vacuum diameter requires more work Measurements with Pb( ) 2 could not provide a reference