Collection Efficiency: A summary of what we know so far.

Transcription

1 Collection Efficiency: A summary of what we know so far. AMS Users Highlighted work Eben Cross, Tim Onasch Ann Middlebrook, Roya Bahreini Brendan Matthews

2 Collection Efficiency Definition

3 Why is it important? AMS Mass Quantification mass species 1 IE NO3 1 RIE species 1 CE 1 Q

4 Collection Efficiency: EL Liu et al. (2007) Transmission can vary between lenses CE (NRPM 1 )= Measured NRPM1/ Actual NRPM1 NOTE: Difference between PM1 and PM2.5 is not included in CE Important when comparing AMS NRPM1 with PM2.5 instrumentation

5 Transmission Measurements FIG. A1 AMS collection efficiency measured using laboratory-generated particles. The solid, open, and shaded circles are the results for NH 4 NO 3, NaNO 3, and KNO 3, respectively. The values in parentheses in the legend indicate the vaporizer temperature. The solid line indicates the theoretical curve from a computational fluid dynamics calculation (FLUENT model: D. R. Worsnop, personal communication). Takegawa, N. et al., 'Performance of an Aerodyne Aerosol Mass Spectrometer (AMS) during Intensive Campaigns in China in the Summer of 2006', Aerosol Science and Technology, 43:3, , 2009.

6 Collection Efficiency: Es Huffman et al. E S ~1 Most particles (even aspherical soot particles) are collected with greater that 95% efficiency

7 Collection Efficiency: Eb Bounce Loss is dominant Eb<EL<Es f(composition,phase, particle size ) Typical Methods of Estimating Eb Vaporization 1) Known input size (Dva= ), Compare CPC vs AMS count OR CPC vs AMS Bounce 2) Light Scattering Module

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9 Null Undetected mass detected mass Eb= (total- Null)/total

10 From Lab and Field Work Eb affected by 1) nitrate content 2) acidity/neutralization 3) Relative humidity in the sampling line 4) organic liquid content MUCH (ALL?) OF THIS IS DRIVEN BY PHASE DEPENDENCE: Solids Bounce, Liquids Don t Bounce Impactor 101, same is true for ELPI (Virtanen et al. Nature 2010)

11 LAB Eb: Phase Dependance (NH 4)2 SO 4 /NH 4 NO 3 mixed particles CE increases with increasing NO3 content CE=1 for pure NO3 CE=0.3 for pure SO4 Eb (Solid) Eb(Mixed Phase) Eb(Liquid) Matthews et al.

12 FIELD Eb: Phase Dependance Quinn et al. Eb increases with acidity, CE=1 for H2SO4

13 Tim Onasch s presentation in 2005 Quinn et al Martin et al

14 LAB Eb: Phase Dependence OM/Total =13% OM/Total =32% CE increases with increasing RH CE Increases with liquid Coating Matthews et al.

15 Ambient composition How does CE vary with varying ambient composition?

16 Ambient CE: LS Measurements CE=0.5 reproduces NRPM1 and speciated mass to 20-30% at most sites Mexico City LS-AMS Measurements particles (> 250 dva) Cross et al., ACP (2010) Null Fraction= 51% Eb ~.49 CE=0.35 CE=0.5 CE=0.65 Roya s supp

17 Ambient CE: CE=1 observations - Acidic plumes (H2SO4) - NH4NO3-dominated plumes (These are typically infrequent plumes) - Water condensation (Frequent/Infrequent- depends on how inlet line is setup and RH)

18 Field data in support of CE parameterizations Middlebrook, Bahreini, Jimenez, and Canagaratna AMS 2 nd Clinic 2009

19 Field Data TEXAQS: Acidic, Anthopogenic SOA ARCPAC: Acidic, Aged BBOA MESA: High NO3 Instrumentation: AMS UHSAS (PM1 mass)- ultra-high sensitivity aerosol spectrometer PILS SP2 (BC mass) Estimates of CE ~ Ratio of (PM1 mass (UHSAS)- BC mass (SP2))/ AMS mass

20 Acidic No NO3 Nitrate Effect: Ratio as f(nh4no3 Mass Fraction) High NO3 (not acidic) Consistent w/ EUCAARI FIELD LAB CE increases with nitrate content.

21 Acidity Effect: Ratio as f(nh4/so4 Acidic Events molar ratio) High NO3 CE increases for acidic particles.

22 Organic Effect: Ratio as f(organic Mass Fraction) Data were filtered for ammonium sulfate particles: NH4/SO4 molar ratio > 1.5 and ammonium nitrate mass fraction < 0.4. CE does not change significantly w/ org content Consistent with Virtanen et al., Nature 2010 in press: ambient SOA is solid (glass)

23 Ratios for All sites Combined All Field Studies: Ratio =.5 +/-.15 CE~ Ratio Assumption requires extreme caution- multiple, carefully operated instruments Uncertainty in CE of 0.5 is 30%

24 Propagating uncertainty in AMS mass/fine Mass ratio Bahreini et al. mass species 1 IE NO3 1 RIE species 1 CE 1 Q Houston Data IENO3 : (10% uncertainty) RIEspecies: Ionization efficiency of species relative to nitrate NH4 (10% uncertainty) SO4(15% uncertainty) ORG (20% uncertainty) CE: 25% Q: measured during routine calibrations (<0.5% uncertainty) Uncertainties propagated: The AMS total mass (30%), fine volume (30%), and aerosol density (7%) Histogram of observed data is within modelled uncertainty

25 Sources of CE variability 1) CE is not fixed in time - Composition/phase may vary with time 2) CE is not rigorously species independent - Partial internally mixed particles - SO4/NO3/OOA - Externally mixed particles - HOA, BBOA

26 Ambient CE: LS Measurements OOA BBOA NO3 HOA Mexico City LS-AMS Ensemble Measurements Mexico City LS-AMS Single Particle Measurements CE=0.35 CE=0.65 CE=0.5 Variability not clearly associated w/ org composition

27 Queen s Data Different HOA and OOA CEs due to differential H2O condensation Loading (µg/m 3 ) ACSM HR-ToF-AMS HOA CE=0.5 Solid Little H2O condensation 0 0 Loading (µg/m 3 ) ACSM HR-ToF-AMS 17/07/ /07/ /07/ /07/ /07/ /07/ /07/ /07/ /08/2009 dat OOA CE=1 Liquid: H2O condensation

28 CE of BBOA Aged BBOA OOA Aged BBOA CE ~0.5 BBOA properties pretty variable- more work needs to be done to characterize range - some fresh BBOA may have CE>0.5

29 Conclusions CE=0.5 reproduces OA (+/- 0.15) - Larger than pure (NH4)2SO4 CE ( 0.3) effects of other species (Org, NO3) Lab particles are poor models for ambient p., But trends are useful In AMS ambient particles are solid/ mix of solid & liquid phase/glass phase (This is the phase in ambient, or this is what becomes to them when they go through the lens)

30 Recommendations I: Drying Remove H2O Condensation Recommended: dry to between 30-40% RH Alternative: humidity to > 80% RH All the time! This doesn t work for non-hygroscopic organics Worse: don't do either, then your CE can change all over the place as f(rh) Not easy to model or post-correct

31 Recommendations II 1) CE should be estimated directly from AMS chemical composition Use CE=0.5 (Uncertainty is +/-0.15 = +/- 30%) Check for RH, NO3, Acidity effects Use parameterization based on AMS chemical composition to correct for these effects if they exist (Empirical corrections are being derived to do this from multiple datasets) - It is VERY difficult to derive this for each dataset, can be affected by issues with instruments you compare with (see later) 2) Evaluate AMS mass concentrations vs other instrumentation (multiple instruments best) - Each instrument will have different issue to account for in comparison (PM2.5 vs PM1, ) - How well was the other instrument operated? - Report the level of agreement or disagreement. DO NOT SCALE! - Note that SMPSs often disagree by % in volume (IAC Wiedensohler), UHSAS needs factor of x2 correction, etc.

32 Discussion on How to Deal with CE / Eb

33 Resources for Nafion Dryer s/pd-series-gasdryers/?ind=science&prod=497 the info is in the product manual (pdf) link and the method for efficient drying is the reflux method (#2)