1. Organic Aerosol is Ubiquitous in the Atmosphere
Northern hemisphere aerosol components
NO3
SO4
NH4
Organic
[Zhang et al., 2007]
Tropics and southern hemisphere aerosol components
(Urban)
(Rural)
Southern Africa
South America
Southeast Asia
South Asia
Oceania
[IPCC, 2013]

2. Primary and secondary organic aerosol (POA and SOA)
atmospheric oxidation
condensation
secondary
organic aerosol
VOC
low-volatility products
VOCs
VOCs
VOCs
primary organic aerosol
(direct emission)
open fires
fuel combustion
and industry
vegetation

3. Fine particulate matter (PM2.5) in the Southeast US
PM2.5 in Aug-Sep 2013: observed (circles), GEOS-Chem model (background)
annual
standard
Kim et al. [2015]

4. Sources of sulfate and organic aerosol in Southeast
Surface networks (circles), GEOS-Chem (background)
Aug-Sep 2013
(29%)
(12%)
Kim et al. [2015]

5. Biogenic VOCs as secondary organic aerosol (SOA) precursors
Isoprene (C5H8): mostly from deciduous trees, ~3% SOA yield per C atom
Terpenes (C10H16): mostly from evergreen trees, ~10% SOA yields
α-pinene
β-pinene
Limonene
Myrcene
Sesquiterpenes (C15H24): little understood, highly reactive, high SOA yields
Caryophyllene

6. Two models for formation of secondary organic aerosol
Classical model for reversible uptake by pre-existing organic aerosol
GAS
ORGANIC PHASE
semi-volatile
gas
semi-volatile
aerosol
VOC
oxidation
Alternative model for irreversible uptake by aqueous aerosol
GAS
AQUEOUS PHASE
dissolved
gas
water-soluble
gas
nonvolatile
species
VOC
oxidation
oxidation
complexation
oligomerization

7. Standard SOA modeling as gas-aerosol equilibrium of semivolatile products of VOC oxidation
VOC oxidation generates semi-volatile organic gases SOG:
VOC + oxidant → 1SOG1 + 2SOG2 + …
…which then partition between the gas and aerosol phase to produce SOA:
Ci(a) and Ci(g) are the SOA and SOG concentrations for species i;
Ci* is the species volatility (a.k.a. vapor pressure)
Mo is the mass concentration of pre-existing organic aerosol

8. Volatility basis set (VBS) approach for organic aerosol modeling
No distinction made between primary and secondary organic aerosol:
classify instead all organics by their measurable volatility
Organic species i partitions between aerosol and gas depending on volatility Ci* :
Bar = sum concentration of organics in a given volatility class;
In green: concentration in aerosol phase Mo
Donahue et al. [2006]

9. Volatility basis set: effect of dilutionMo Mo
Donahue et al. [2006]

10. Volatility basis set: effect of chemical agingΩ Ω
As organics go through successive oxidation steps, products become more oxygenated (less volatile) but also smaller (more volatile)
Donahue et al. [2006]

11. Aqueous-phase mechanism for organic aerosol from isoprene:
the short version OH
isoprene
glyoxal
epoxide (IEPOX)
Gas-phase
aerosol precursors
Aqueous aerosol
Include: NASA, Schlumberger, SEAS, NRF, SOAS, SEAC4RS, OMI.
Include: co-authors
Marais et al. [2016]

12. Aqueous-phase mechanism for organic aerosol from isoprene:
GEOS-Chem model simulation over the eastern US
Mean yield is 3.2%; epoxide and glyoxal are the main contributors
0.1%
0.3%
0.22%
1.9%
Marais et al. [2016]
0.94%

13. Aqueous-phase formation of organic aerosol from glyoxal
GAS
AQUEOUS PHASE
Oligomers
oligomerization
oligomerization
glyoxal
tetrol
oxidation OH
KH* ~ 105 M atm-1
Organic acids
Updated references

14. Aqueous-phase formation of organic aerosol from epoxides
Acid-catalyzed ring cleavage to produce non-volatile species
H+, SO4=
10%
90%
H+, H2O
Isoprene epoxide (IEPOX)
Updated references
Aqueous aerosol
IEPOX SOA
Marais et al., 2016

15. Observations show correlation of IEPOX SOA with sulfate
Correlations with sulfate in SEAC4RS and at Centerville, Alabama research site
Centerville, AL
SEAC4RS
Acknowledge that I’m not the first to show this!
Aerosol volume 
Aerosol acidity 
Sulfate 
IEPOX SOA 
Suggests that SO2 emission controls decrease organic aerosol as co-benefit
Marais et al. [2016]

16. Long-term trends of organic and sulfate aerosol in Southeast
Marais et al. [2017]

17. Van Krevelen diagram for chemical aging of organic material
Heald et al. [2010]

18. Van Krevelen diagram: application to organic aerosol
-1 slope suggests aging by adding of –COOH functionalities
Heald et al. [2010]