Source apportionment of submicron organic aerosols at an urban site by linear unmixing of aerosol mass spectra

V. A. Lanz1, M. R. Alfarra2, U. Baltensperger2, B. Buchmann1, C. Hueglin1, and A. S. H. Prévôt2

[1] Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Air Pollution and Environmental Technology, CH-8600 Duebendorf, Switzerland

[2] PSI, Paul Scherrer Institute, Laboratory for Atmospheric Chemistry, CH-5232 Villigen PSI, Switzerland

submitted to ACPD

Materials Science &Technology

urban background site (Zurich - Kaserne) three-week measurement period in summer 2005

N

www.mapsearch.ch

Measurement campaign

Positive matrix factorization (PMF)

X = G F

- non-negativity of G and F - algorithm accounts for uncertainty in X (weighted least squares)

X{n x m} =

G{n x p}

F{p x m}

species j: 1…m

sam

ples

in t

ime

i: 1…

nColumns (scores)

Rows (loadingsor factors)

number of p must be specified

Goodness of fit (R2) of regressed scores vs. measured organics and rotational freedom as a function of the number of factors chosen in PMF.

Number of factors p ?

Most important step in PMF analyses: interpretation of F (and G), i.e.

the mathematical solutions (G and F matrices) must be linked to sources and aerosol

components

rows of F - factors: e.g check for spectral similarity with ~ AMS reference spectra

columns of G - scores: e.g. look at correlation with other species (time series)

(compare modelled emission/ratios with literature)

Interpretation of F factors

spectral similarity to reference spectra

R2: correlation of all m/z‘sR2

m/z>44: correlation of m/z > 44

m/z 29

m/z 44m/z 18

m/z 43

factor [norm. int]

Ref. MS [norm. int]

similarity measure for AMS spectra ?

300...1 i/ i

iinorm.i msmsms

Interpretation of F factors

example 1:

1st factor is very similar to fulvic acid, (aged urban aerosol, …)

(R2=0.96; Rm/z>442=0.83)

interpretation as OOA (oxygenated organic aerosol)

example 2:

3rd factor is very close to fuel, (lubricant oil, diesel, …)

(R2=0.99; Rm/z>442=0.99):

interpretation as HOA (hydrocarbon-like organic aerosol)

correlation with other species

-secondary processes:

atm. oxidants (O3 + NO2)

temperature

particle-phase nitrate/sulfate

-primary processes:

carbon monoxide (CO)

nitrogen oxides (NOx)

elemental carbon (EC)

Interpretation of G - scores

emission ratio

[ng m-3/ppbv]

Zürich-Kaserne

Pittburgh

(Zhang et al., 2005)

New England

(de Gouw et al., 2005)

Tokyo

(Takegawa et al., 2005)

diesel trucks/light-duty vehicles (calculated

from Kirchstetter et al., 1999)

POA/CO 10.4 4.3 9.4 11 -

HOA/NOx 15.9 42 - - 11/16

AMS (PMF) AMS (2-fact. approach)

total OA, gas-phase tracer

AMS (2-fact. approach)

tunnel studies

modelled emission ratios

Main results

PM1 (Zürich-Kaserne, Summer 2005)

(avg. OM 6.58 g m-3 )

10% wood burning

13% in Zürich, 2002

(14C analysis, Szidat et al., 2006)

charbroiling; 11%

wood burning ; 10%

fossil fuel; 7%

minor source (cooking); 6%

OOA, type II (volatile); 22%

OOA, type I (aged); 44%

POA 34%SOA 66%

SOA and POA estimation

Limitations of PMF

PMF (and other multivariate receptor models) cannot resolve sources/components when

(a) profiles are too similar

(b) when sources/components show similar temporal variation (e.g. Zurich winter data set)

→ CMB approach (collinearity and multicollinearity of profiles might be a problem)

→ Hybrid model combining CMB and PMF features ?

Outlook: CMB - type approach (Zurich winter)

Temporal variation of three mass tracers (m/z 44, m/z 57 and m/z 60) during the Zurich winter 2006 campaign

Outlook: CMB - type approach (Zurich winter)

Sources (F) known: X F+ = GF F+ ˜ G F+ the pseudo-inverse of F

X(w) {F(s) }+ = G(w) F(s) {F(s) }+ ˜ G(w) w: winter, s: summer

G(w) 95%-c.i.= X(w)

95%-c.i. {F(s) }+ 95%-c.i. propagating uncertainty

measurement uncertainty

model uncertainty

0%

10%

20%

30%

40%

50%

60%

OOA HOA wood burning aerosol

calculated G factors

average contribution(incl. uncertainty estimates)

Outlook: CMB - type approach (Zurich winter) – preliminary !

Calculated factors and its interpretation

Spectra of all PMF factors (interpreted as the denoted source profiles) calculated by 6-factorial PMF. Only for the minor source the full mass range up to 300 m/z is shown because it is the only source or component with significant features in the high mass region on the linear scale.

Measured AMS-sulphate and modelled aerosol from wood burning on the Swiss national holiday (1 August)