How Physical and Chemical Properties Data 

Reflects Aerosol Formation Processes and their 

Evolution 

Xiaolu Zhang, Rodney Weber

Georgia Tech

Outline:

1. Instrumentation based on analysis of water-soluble 

gas/particle components.

1. Example application: Secondary organic aerosol (SOA) 

formation in LA vs Atlanta. 

Funded by NSF and EPA
Instruments to Quantify Particle/Gas Components 

Filter Collection with Off-line WaterExtraction and Analysis

Particle Phase Species Automated On-line Analysis

Gas Phase Species

Mist Chamber: gas scrubber

(or use; continuous flow wet 

denuders)
Inorganic and Organic Ions WSOCg and WSOCp

WSOCg, WSOCp

Mist 

Chamber

Soluble gases

Ion Chromatograph

Particle Into Liquid 

Sampler (PILS)

PM2.5 (?) 

Cations: Na+, NH4

+, K+, Mg2+, Ca2+, Amines...

Anions: Cl-, NO2

-

, NO3

-

, SO4

2-

, Organic Acids...

Cations: NH3

Anions: Cl, HONO, HNO3, SO2, Organic Acids...

Mist 

Chamber

Soluble gases

Total Organic Carbon Analyzer

Particle Into Liquid 

Sampler (PILS)

PM2.5 (?) 

ANALTYICAL METHOD: Approaches to Measure a Range 

of Gas and Aerosol Phase Properties

(Weber et al AS&T 2001; Orsini et al, Atm Env. 2003) (Sullivan et al; GRL 2004, JGR 2006)

5 to 15min 3s to 6min
WSOCp & Soluble Light Absorption Spectra

ANALTYICAL METHOD: Approaches to Measure a Range 

of Gas and Aerosol Phase Properties

Total Organic Carbon Analyzer

Particle Into Liquid 

Sampler (PILS)

PM2.5 (?) 

XAD-8 

Column

Penetrates Hydrophilic WSOC

(lower MW..., highly soluble)

Hydrophobic 

WSOC Retained

(high MW, 

aromatics, HULIS, 

less soluble ...)

Actuated Valve

Total Organic Carbon Analyzer

Particle Into Liquid 

Sampler (PILS)

PM2.5 (?) 

Compact UV-VIS

Spectrophotometer

(200 to 800nm)

1-m path length

LWCC (0.25 mL)

“Brown Carbon”

Abs’n Å ~ 2 to 7 (λ−Å)

Abs365

WSOCp & Hydrophilic/phobic fractions

Mass Abs’n Eff: Abs/WSOCp

(Sullivan et al JGR 2006) (Hecobian et al, ACP 2010)
South and Southeastern US are Interesting Aerosol Regions

-3.9 W/m2

Large role played by BVOCs in Eastern US

http://www.epa.gov/climatecha

nge/science/

recenttc_tempanom.html

Aerosol Optical Thickness(AOT): JJA - DJF

(Goldstein et al., 

PNAS, 2009)

Annual Temperature 

anomalies 1901-2005

Predicted Biogenic-VOC Emissions
levoglucosan

WSOCp

Temp

PM2.5 mass

WSOCp

Lev < 50 ng/m3

2007 FRM Filter Data: WSOC High In Summer When Hot

24h-filters from 15 FRM sites throughout SE, 60 

filters/site, 900 filters: 

Measured: non-volatile WSOCp, Light Abs, Carbohydrates, 

sulfate...

WSOCp Highest in Summer: highly correlated 

with PM2.5 mass (r2= 0.70) and highly 

correlated with T (biogenic VOCs?)

Non-biomass burning

(Zhang et al, ACP 2010)
How Secondary Organic Aerosol Mass is Created

- Conceptual Ideas -

BVOC &

AVOC

SVOC

OH, O3

NO3

SVOC Water (haze, 

cloud)

Organic 

Fraction

stable

BVOC &

AVOC

Possible aqSOA

Route(important?)

“Classic” (Pankow) Partitioning

KP = CP / MO

Cg

= 760 R T fOM

106 MWOM ξ pi

o

Partitions to aerosol organic mass (Mo)

and depends on SVOC vapor pressure 

(pi

°)

Partitions to liquid water (LWC) and 

depends on SVOC solubility (Henry’s 

Law Constant, KH), then undergoes 

heterogeneous oxidation to lower pi

°

OH, O3

NO3

pi° KH
Estimated VOC Emissions in LA and Atlanta

4.8 times

1.7 times

Biogenic VOC Anthropogenic VOCs

• BVOCs are much higher in the SE and within Atlanta, does that impact SOA 

formation within Atlanta ? Compare Atlanta to LA

Predicted Biogenic-VOC Emissions
SOA Formation in the SE is Subtle

In Atlanta (and other sites in eastern US) no clear diurnal trend to SOA

WSOCp

WSOCp
Atlanta: Urban vs Rural

(Atlanta vs Yorkville)

Jst and Yrk generally track on times scales of synoptic meteorological processes

Jst consistently higher on a higher frequency scale due to anthrop. Influence.

YRK

Atlanta

August 2008
Urban/Rural WSOCp Diurnal Tends

Summer 2008

Time of day

~ 70 km apart 

Atlanta

Yorkville (NW of Atl)

Atlanta signal riding on large regional background

Some evidence for a daytime increase (photochemical SOA)

WSOCp
Changes relative to background  Secondary process that day

LA: CalTech

Atl: GaTech

∆WSOCg tracks other ∆(Oxyg-VOCs)

∆WSOCp tracks other ∆SOA

(eg, gas and particle organic acids)

∆WSOCg and ∆WSOCp

completely different trends 

wrt each other and LA. 

Comparison of Diurnal Trends: Daytime Increases
∆Abs

∆Abs

∆WSOCp = 0.77 x 10-6

∆Abs

∆WSOCp

= 0.14 x 10-6

LA: Colored SOA

Atl (fresh) SOA is not.

LA 5 to 6x higher

∆WSOC

∆Abs

LA-CalTech

Atl-GaTech

Comparison of Diurnal Trends: Abs365
∆Abs

∆Abs

∆WSOCp = 0.77 x 10-6

∆Abs

∆WSOCp

= 0.14 x 10-6

LA: Colored SOA

Atl (fresh) SOA is not.

LA 5 to 6x higher

∆WSOC

∆Abs

LA-CalTech

Atl-GaTech

Comparison of Diurnal Trends: Abs365

FRM Data From One Site

But over time in Southeast, Abs 

and WSOCp are related 
Many more differences...  differences in the SOA formation process??

LA: CalTech

Atl: GaTech

Comparison of Diurnal Trends: Oxalic Acid

Oxalic acid and WSOCp (SOA) track

Oxalic acid formed later in day

But over time (FRM Data) WSOCp, 

Abs365, Oxalate become correlated 

(PMF analysis)
Different Process?

Partitioning to Organic Mass?

OC

VOC

Oxidant

(OH, O3...)

SVOC Organic 

Aerosol

Fp = 

WSOCp

WSOCp + WSOCg

(WSOCg)

(WSOCP) ~ SOA

KP = CP / MO

Cg

= 760 R T fOM

106 MWOM ξ pi

o

“Classic” (Pankow) Partitioning Theory

Partitioning related to OC ? Not Partitioning to OC ?

Atl-GaTech LA-CalTech
Fp partitioning to liquid water

0.40

0.35

0.30

0.25

0.20

0.15

0.10

Fp

30 40 50 60 70 80 90

Relative Humidity (%) Peak at 60 – 70% RH is due to Fp

peak in afternoon, no relationship to 

RH.

BVOC & AVOC

SVOC Water (haze, 

cloud)

OH, O3 stable

NO3

KH

Atl-GaTech

LA-CalTech

Tend same as liquid 

water vs RH

Different Process?

Partitioning to Aerosol Water ?

Atlanta SOA formation process very different from LA and generally 

consistent with aqSOA formation route
Sorooshian et al, GRL 2010 

Ensemble of Caltech aircraft data

Convective Clouds

Sunglint

Aircraft Measurements also Indicate AqSOA Occurs Aloft

Peltier et al., ACP 2008; NSF C-130 INTEX-B

Organic acids produced in haze/clouds WSOCp formed in (associated w/) clouds
Pasadena Aerosol Characterization Observatory 

(PACO) 5/21 to 8/6 2009 CalTech

5/21 - 6/14

High RH Rainy

6/19 - 7/19

Marine layers am

Sunny Photochem pm

7/13 - 8/6

Hot Dry

Hersey et al, 

ACPD, 2011

AMS data

• Huang et al, JGR 2006 (S. China)

• Lin et al, JAS 2010 (Rural S. China)

• Sorooshian et al, JGR, 2006 (midwest)

• Yao et al, Atm Env 2003 (Beijing)

Other studies show organic 

acids & WSOC correlated with 

droplet mode and sulfate

Size Distributions of Organic Aerosol and Correlations with SO4

= also 

Provide Evidence For Prevalence of aqSOA

0.1 1μm

OA

SO4=

NO3

-

NH4

+
Summary Thoughts

1. Understand processes to assess responses to changing climate 

– Eg, role of T on BVOCs and their link to SOA: BVOC ≠ “natural” BSOA.

2. Current models predicting SOA ignore aqSOA route, most interested in 

getting SOA mass, yet process affects size distribution  CCN, Scattering 

3. aqSOA has interesting cloud-SOA-cloud interactions

– WSOCp mass fraction affects water uptake; haze and CCN activation

– WSOCp, to some extent, is produced in haze and clouds

– WSOCp aqueous chemical aging affects MW, light abs, surfactant properties ...

(aerosol undergoes many cycles of wet/dry..., day/nite and cloud cycling)

Measurement Opportunities/Needs

1. Utilize existing network sites by developing novel analysis techniques (use FRMfilters for: BC by filter darkness, WSOCp, Absn Spectra, levog ... from teflon filters)

2. To understand partitioning process, measure gas and particle.

3. Size-resolved composition informs process: condensation/droplet modes

4. Aerosol phase oxidants critical for aqSOA (interest in health studies)