# Presentation: 285250

## Secondary Organic Aerosol

**John H. Seinfeld**
*California Institute of Technology*

## Slide 2

##    
The acidity difference between “acid” and 
“nonacid” seeds is greater in MgSO4 case
 than in (NH4)2SO4 case, by about 4 pH units.

***The acidity difference between “acid” and ***
***“******nonacid” seeds is greater in MgSO******4****** case***
*** ******than in (NH******4******)******2******SO******4****** case, by about 4 pH units.***

|  | “nonacid”   seed | “acid” 
seed 
(+ H2SO4) | pH difference between “nonacid” and “acid” seeds |
| --- | --- | --- | --- |
| MgSO4 | 6.5  | -0.3 | ~ 6.8 |
| (NH4)2SO4 | 4.6 | 2.4 | ~ 2.2 |

**Acidity of Seed Aerosols**(pH values at RH = 55%)

## SOA Yield vs. [α-pinene] in “nonacid” vs. “acid” MgSO4 seed (pH ~ 6.8)

**SOA Yield vs. [****α****-pinene] in ****“nonacid”**** ****vs. ****“acid”**** MgSO****4**** seed (**_****__**pH ~ 6.8**_**)**

## Slide 5

## Presence of Oligomers in SOA: MgSO4-H2SO4 seed, “+” MS mode

_**55 ppb **__**α**__**-pinene**_
- Small oligomers;
**Large oligomers (MW 450 – 900): even more abundant!**

## Ion trap MS (+) of SOA formed on “nonacid” (left) vs. “acid” (right) MgSO4 seeds

_**55 ppb **__**α**__**-pinene**_

## Ion trap MS (+ ion mode) of SOA from the ozonolysis of 50 ppb α-pinene in the absence of seed particles 

- 956.8

- 851.0

- 994.9

- 312.9

- 1124.8

- 864.9

- 300.9

- 1182.7

- 1409.2

- 1313.5

- 255.0

- 1475.7

- 1518.1

**Relative abundance**

**Background < 5% max.**

_**Broad implication in the atmosphere:**_
-     **Organic acids produced from gas phase oxidation may readily catalyze oligomer formation in SOA.  External acids, such as H****2****SO****4****, are not required for oligomer formation.**

## MS/MS of Some Oligomers Reveal They Originate from Low-MW Ozonolysis Products

**- H****2****O**

**MS/MS of 367 ion in SOA from ****α****-pinene ozonolysis**

**- H****2****O**

**- H****2****O**

**305**

**323**

**185**

**199**

_**Aldol Addition:**_** two pinonic acid monomers**

## Glyoxal

- Seed: (NH4)2SO4 (0.03M) + H2SO4 (0.1 M)
- 50% RH, 20°C

## Aerosol Growth vs. Glyoxal Concentration

- No dependence on seed acidity
- Strong dependence on RH (water content of inorganic seed)
- Dry seed (ammonium sulfate, RH=13%): no growth

## Oligomers in SOA from hydrocarbon ozonolysis: chamber studies

**Oligomers (MW up to 1600) are present in SOA from the ozonolysis of ****α****-pinene and six cycloalkenes, regardless of the presence, acidity, or water content of initial seed particles.**
**Of particular interest, similar oligomers (in terms of MW and structure) form in SOA with or without seed particles.  Oligomers result from reactions of first-generation oxidation products.**
**Oligomers are estimated to comprise at least 10% of the total SOA mass in the cycloalkene ozonolysis systems and well over 50% in the ****α****-pinene ozonolysis system.**
**Particle phase acidity has a direct impact on the composition and yield of SOA.  As the particle acidity increases, larger oligomers form at a faster rate.**
**Within the range of initial ****α****-pinene mixing ratio studied (12ppb to 135 ppb), SOA yield increases by 40 – 10% on the acidic seed (H****2****SO****4**** added) over the neutral seed.  **

## Research Plan

**Comprehensive laboratory studies of SOA formation for a variety of parent VOCs and aerosol conditions (e.g. acidity), including molecular speciation by mass spectrometry and hygroscopicity characterization.**
**Development of the next-generation models of SOA formation, based on laboratory data, appropriate for inclusion in global models, and assessment using a GCM.**