Uncertainties in AMS mass concentrations
Roya BahreiniAMS Clinic 2012
Especially an important consideration when presenting AMS data to those outside the users’
community!
Calculating mass concentrations
QCERIEIEmass
speciesNOspecies
1111
3
Bahreini et al., JGR, 2009
Parameter Uncertainty Procedure Reference
IENO3 10% Routine NH4NO3 cals
RIEspecies Jimenez et al., 2003; Alfarra et al., 2004; Canagaratna et al., 2007
NH4+ 10% Routine NH4NO3 cals
SO42- 15% Previous lab+
computational resultsOA 20% Previous lab+
computational resultsCE 30% Previous lab+ field
resultsMatthew et al., 2008, Middlebrook
et al., 2012Q <0.5% Routine cals
TE 10% Lens transmission
Error propagation
John R. Taylor. (1997) An Introduction to Error Analysis: The study of uncertainties in physical measurements. (Second Edition) Sausalito, CA: University Science books.
Overall uncertainty
Q, IE, CE uncertainties
Q, IE, CE, TE uncertainties
NH4+ 33% 34%
NO3- 33% 34%
SO42- 35% 36%
OA 37% 38%
• Total AMS mass: Overall uncertainty of 20-35%
• Note: uncertainty in the ratios may be better since some of the parameters cancel out (for example, NH4/SO4 has a better uncertainty since uncertaitny in IENO3, CE, and Q cancel out.
Accuracy (uncertainty) vs. Precision
• Precision: • Noise in the data • detection limit
High AccuracyLow Precision
Low AccuracyHigh Precision
Mass Closure• Mass Ratio =(AMS mass+BC mass)/(volume×r)
– AMS mass: 30% uncertainty– BC mass: 15% uncertainty– Volume: 30-45% uncertainty from UHSAS– mass weighted density (r): 7% uncertainty (10%
uncertainty of rOA )
8.1/25.1/75.1/)( 2434
2434
BCOASONONHBCOASONONH
r
Mass Ratio: Overall uncertainty of 45%
Mass closure:Mass Ratio
Middlebrook et al., AST, 2012
2s circle of combined uncertainties
x
Aerosol Acidity Effect
NH4+
,predict=18×(2×SO42-/96 +NO3
-/62 +Cl-/35)
“Apparent CE”(AMS w/ CE=1)
• Acidity , CE
CEdry = max (0.45, 1.0 − 0.73 ×NH4/NH4,predict) [Eq. 4]
Aerosol Nitrate Effect
ANMF=80/62×NO3-/(SO4
2-+NO3-+Cl-+NH4
++OA)
ANMF , CE
CEdry = max(0.45, 0.0833 + 0.9167×ANMF) [ Eq. 6]
“Apparent CE”(AMS w/ CE=1)