Chapter 10: Aerosol chemistry

Life cycle of atmospheric aerosol

SO₂

coal

H₂ SO₄

sulfuric acid

VOCs

combustion

Industry

vegetation

oxygenated

VOCs

NO_x

combustion

HNO₃

nitric acid

atmospheric oxidation

agriculture

vehicles

NH₃

ammonia

molecular clusters

(nucleation)

gas condensation

organic particles

black carbon

condensation from gases

coagulation

combustion

soil dust

sea salt

wind

direct emission

cloud cycling

deposition

1 nm 10 nm 100nm 1 μm 10 μm

Atmospheric transport

Typical aerosol size distribution

Liquid particles are spherical; solid ones are not but their size is expressed as equivalent diameter

nucleation

fast

condensation

slow

further growth

fast

coagulation

primary emission

PM_2.5

Fine particulate matter (PM_2.5) is a major air pollution killer

US EPA [2020], IHME [2020]

CO

NO₂

137M

31

38M

0

1M

3M

16M

0

70 ppb (8-h average)

12 µg m^-3 (annual), 35 µg m^-3 (24-h)

PM_2.5: particulate matter smaller than 2.5 μm in diameter

Wiley Interdisciplinary Reviews

Airflow

Why PM_2.5?

Particles <2.5 μm can penetrate deep into the lung

Air pollution metrics: PM_2.5 and PM₁₀

coarse

fine

Direct emission (primary particles):

dust, sea salt, pollen

​

Condensation from gas phase

(secondary particles):

sulfate, nitrate, ammonium, organics

Life expectancy vs annual PM_2.5�1978-82

Steubenville, OH

Boston

Pope, Ezzati, Dockery. NEJM 2009; 360:376

Six-city study by Harvard School of Public Health

Life expectancy vs annual PM_2.5�1997-2001

Boston

Steubenville

Pope, Ezzati, Dockery. NEJM 2009; 360:376

Boston in 1978-1982

1-year increase from improving air quality

2-year increase from other factors

1978-1982

regression line

Annual mean PM_2.5 in US, 2000-2016

Wu et al. [2020]

EPA air quality standard (9 μg m^-3)

Annual mean PM_2.5, 2020-2022

Randall Martin, Washington U.

Annual mean PM_2.5 composition at US sites

Brasseur and Jacob [2017]

WHO standard (most stringent) = 5 μg/m³ annual average PM_2.5

World Air Quality, 2024

Annual mean PM_2.5 �inferred from satellite observations of aerosol optical depth

http://www.nasa.gov/topics/earth

US air quality standard

[Zhang et al., 2007]

NO₃

Organic

Northern hemisphere aerosol components

Global PM_2.5 composition

SO₄

NH₄

[IPCC, 2013]

Southern Africa

South America

Southeast Asia

South Asia

Oceania

(Rural)

(Urban)

Tropics and southern hemisphere aerosol components

Sulfate originates from atmospheric oxidation of SO₂

Global SO₂ emissions

Main anthropogenic source: coal combustion

Coal is dead organic matter, contains ~1% S

SO₂ emission

atmospheric oxidation

H₂SO₄

sulfuric acid

condensation to particles

SO₄^2- (sulfate)

Coincidence of SO₂ emissions, �sulfate aerosol concentrations, sulfate wet deposition

SO₂ in US is mainly from coal combustion

and is rapidly oxidized to sulfate…

but oxidation by OH is too slow (~1 week)

There must be another oxidant!

Deposition

Aqueous-phase SO₂ oxidation enabled by clouds (1980s)

SO₂

sulfurous acid

bisulfite

sulfite

Non-radical oxidants can be important in the aqueous phase because of cage effect:

oxidant

reductant

Water molecules form a cage forcing many collisions

Michael Hoffmann

SO₂(aq)/HSO₃^-/SO₃^2- partitioning vs. pH in clouds

typical cloud pH range

pK₁ = 1.8

pK₂ = 7.0

Most of the dissolved SO₂ in cloud is present as HSO₃^-

Aqueous-phase oxidation of SO₂ by ozone

Reaction shuts itself down as H⁺ increases

Acid-catalyzed aqueous-phase oxidation of SO₂ by H₂O₂

Rate does not slow down as H⁺ increases

OBSERVED TITRATION OF SO₂ BY H₂O₂ IN CLOUD

First aircraft observations by Daum et al. [1984]

Peter Daum

In most cases, H₂O₂ is in excess so all SO₂ is oxidized

What was Jacob doing during that time?

20120415

Newport Beach

Sampling ‘acid fog’ in Los Angeles to understand how sulfate is produced

Problem is that LA was in the VOC-limited regime so H₂O₂ was very low

My first paper…

Long-term trends in US SO₂ emissions

EPA National Emission Inventory

Scrubbers on coal power plants, transition to natural gas

Decline of sulfate aerosol in the US has tracked SO₂ emissions

1990

Observed sulfate concentrations (circles), GEOS-Chem (background)

Leibensperger

et al. [2012]

2010

µg m^-3

Chemistry is linear, meaning that SO₂ oxidants are in excess

SO₂

emission

oxidant

sulfate

2013-2018 PM_2.5 decline in China as SO₂ emissions declined

Annual mean data from Chinese national network

Zhai et al. [2019]

New SO₂ pollution frontier: India

Satellites reveal rapid growth in SO₂ emissions from coal use

Lu et al. [2013]

Global formation of sulfate aerosol: major processes

H₂SO₄(g)

H₂SO₄∙H₂O∙X

Sulfate aerosol

S(VI) ≡ H₂SO₄(aq)+HSO₄^- + SO₄^2-

SO₂

(CH₃)₂S

dimethylsulfide

(DMS)

Marine

biosphere (15)

Coal combustion (45)

Oil refining (10)

Smelters (5)

Volcanoes (5)

Open fires (1)

OH

multisteps

1 day

OH

1 week

Global emissions in Tg S a^-1

deposition

SO₂(aq)

aerosol,

clouds

heterogeneous

oxidation

nucleation

condensation

new particles

coagulation

H₂O, X (ternary)

X ≡ NH₃, organics…

1 minute

S oxidation state

-II

+IV

+VI

1 day

[Zhang et al., 2007]

NO₃

Organic

Northern hemisphere aerosol components

Global PM_2.5 composition

SO₄

NH₄

[IPCC, 2013]

Southern Africa

South America

Southeast Asia

South Asia

Oceania

(Rural)

(Urban)

Tropics and southern hemisphere aerosol components

Ammonia in the atmosphere

Global ammonia emissions (kg N ha^-1 a^-1)

Agriculture (manure, fertilizer) is 75% of global source

IASI satellite observations of ammonia

Paulot et al. [2014]

Van Damme et al. [2014]

Ammonia is a weak base:

It partitions into the aerosol if the aerosol is sufficiently acidic

Simple thermodynamic rules for SNA aerosol formation

H₂SO₄ all goes to aerosol; dissociation to HSO₄^-, SO₄^2- governed by pH

NH₃ condenses into acid sulfate aerosol until titration; no further uptake

HNO₃ condenses only if excess NH₃ is available

neutralized conditions acid conditions

(NH₃(aq) negligible in both cases)

acid conditions neutralized conditions

(HNO₃(aq) negligible in both cases)

Formation of sulfate-nitrate-ammonium (SNA) aerosol

SO₂

NH₃

NO_x

HNO₃

NH₃(aq) NH₄⁺

H₂SO₄(aq) HSO₄ SO₄^2-

HNO₃(aq) NO₃^-

High RH (aqueous aerosol)

Low RH (dry aerosol)

EMISSION

oxidation

oxidation

• Sulfuric acid produced from SO₂ oxidation is ~100% incorporated into the aerosol
• Ammonium and nitrate are incorporated as determined by acid-base titration

H₂SO₄(g)

NH₄HSO₄

(NH₄)₂SO₄

NH₄NO₃

SO₂: coal combustion

NH₃: agriculture

NO_x: fuel combustion

H₂O

-

Three regimes for sulfate-nitrate-ammonium aerosol formation

Total sulfate [S(VI] = [H₂SO₄(g)] + [H₂SO₄(aq)] + [HSO₄^-] + [SO₄^2-]

Total ammonia [N(-III)] = [NH₃(g)] + [NH₃(aq)] + [NH₄⁺]

Total nitrate [N(V)] = [HNO₃(g)] + [HNO₃(aq)] + [NO₃^-]

moles per m³ of air

2[S(VI)] > [N(-III)]

PM_2.5 controlled

by sulfate

2S(VI)

N(-III)

gas

aerosol

N(V)

REGIME 1

SNA ratio < 0

SNA ratio =

Control SO₂ emission

Control SO₂ and NO_x emission

Control NH₃ emission

Winter PM_2.5 composition in Beijing: nitrate has replaced sulfate, �and nitrate hasn’t decreased despite 30% decrease in NO_x emissions

Organic

Sulfate

Nitrate

Ammonium

Chloride

Elemental carbon

H. Li et al., 2019; Zhai et al., 2021

Decreasing NH₃ emission will be necessary

Acid rain

Natural pH of rain

• Equilibrium with natural CO₂ (280 ppmv) results in a rain pH of 5.7:

• This pH can be modified by natural acids (H₂SO₄, HNO₃, RCOOH…) and bases (NH₃, CaCO₃) 🢫 natural rain has a pH in range 5-7

“Acid rain” refers to rain with pH < 5 🢫 damage to ecosystems

Electroneutrality equation:

We know that pH < 7 so [H⁺] >> [OH^-], [HCO₃^-] >> [CO₃^2-]

Mean pH of precipitation, 1990: acid rain across eastern US

​

National Acid Deposition Program

Ionic composition of precipitation (late 1980s)

Acid rain is caused by H₂SO₄ and HNO₃ originating from SO₂ and NO_x…

and can be neutralized by NH₃ and soil dust (e.g., CaCO₃)

Since 1990, SO₂ emissions have decreased by 90% and NO_x emissions by 60%; pH is now above 5 everywhere

Mean pH of precipitation, 1990

​

National Acid Deposition Program

[Zhang et al., 2007]

NO₃

Organic

Northern hemisphere aerosol components

Organic Aerosol is Ubiquitous in the Atmosphere

SO₄

NH₄

[IPCC, 2013]

Southern Africa

South America

Southeast Asia

South Asia

Oceania

(Rural)

(Urban)

Tropics and southern hemisphere aerosol components

Primary and secondary organic aerosol (POA and SOA)

fuel combustion

and industry

primary organic aerosol

(direct emission)

open fires

VOCs

vegetation

VOC

atmospheric oxidation

low-volatility products

VOCs

VOCs

condensation

secondary

organic aerosol

large functionalized molecules

VOC atmospheric oxidation cascade

VOC

RO₂

NO₂

O₃

organic

peroxy

radical

NO

hν

carbonyl

R’O₂

hν

OH + products

organic aerosol

ROOH

organic

peroxide

OH

HO₂

OH, hν

OH

products

EARTH SURFACE

biosphere

combustion

industry

deposition

Increasing functionality & cleavage

• sources of organic aerosol
• sources/sinks of oxidants (ozone, OH)

Successive VOC oxidation steps produce gradually smaller (more volatile)

but more multifunctional (less volatile) products

VOC functionalization during oxidation can be complicated

OH can also add to double bonds of unsaturated VOCs, producing hydroxyorganics

RO can also decompose or isomerize to produce a range of aldehydes, ketones, dicarbonyls…

NO can also add to produce organic nitrates

Oxidation product goes on to react with OH, adding functionality and making more ozone

RO₂ can also

isomerize, or react with HO₂ or RO₂,

producing peroxides, epoxides,

alcohols,

carboxylic acids…

The case of isoprene (examples of first few steps)

OH

HO₂

OH

O₂

.OO

OH

HOO

NO

NO

O

Isoprene

Isoprene hydroperoxide (ISOPOOH)

Isoprene peroxy radical

(4 isomers)

Methylvinylketone

Methylglyoxal

+ HCHO

+ HO₂

OH

+ HCHO + HO₂

Fine particulate matter (PM_2.5) in the Southeast US

Kim et al. [2015]

annual

standard

Summer 2013 SEAC⁴RS campaign : observed (circles), GEOS-Chem model (background)

Sources of organic aerosol in the Southeast

Organic aerosol concentrations from surface networks (circles), GEOS-Chem model (background)

SEAC⁴RS campaign (Aug-Sep 2013)

biogenic

Two models for formation of secondary organic aerosol

Classical model for reversible uptake by pre-existing organic aerosol

GAS

ORGANIC PHASE

VOC

oxidation

semi-volatile

gas

semi-volatile

aerosol

Alternative model for irreversible uptake by aqueous aerosol

GAS

AQUEOUS PHASE

VOC

oxidation

water-soluble

gas

dissolved

gas

oxidation

complexation

oligomerization

nonvolatile

species

Classical SOA modeling as gas-aerosol equilibrium�of semivolatile products of VOC oxidation

VOC oxidation generates semi-volatile organic gases SOG:

…which then partition between the gas and aerosol phase to produce SOA:

• [SOA_i] is the concentration in the aerosol phase [g g^-1 of aerosol]
• p_SOGi is partial pressure, p_SOGi^* is vapor pressure, n_i^* is volatility [g m^-3 of air]
• n_i(a) and n_i(g) are SOA_i and SOG_i mass concentrations [g m^-3 of air]
• M_o is the total mass concentration of organic aerosol [g m^-3 of air]

VOC + oxidant → α₁SOG₁ + α₂SOG₂ + …

SOG_i

SOA_i

Organic aerosol phase

mass concentration M_o

M_o ≡ POA + ΣSOA_i

Equilibrium constant (similar to Henry’s law constant)

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 n_i* :

Donahue et al. [2006]

M_o

Bar = sum concentration of organics in a given volatility class;

​

In green: concentration in aerosol phase

​

high volatility

low volatility

Neil Donahue

Volatility basis set: effect of dilution

Donahue et al. [2006]

M_o

M_o

Measuring particle emission at exhaust overestimates atmospheric contribution

Volatility basis set: effect of chemical aging

Donahue et al. [2006]

As organics go through successive oxidation steps, products become more oxygenated (less volatile) but also smaller (more volatile)

Nonradical reactions

(NH₄⁺, SO₄^-2, HSO₄^-

Oxidants

(·OH, O₃, HO₂,

NO, NO₃)

​

Emitted VOCs

​

AQUEOUS PHASE

Aldehydes

Epoxides

Glyoxal

Methylglyoxal

Oxygenated VOCs

​

Hydration, oligomerization, ionization

Radicals

(OH, HO₂,

SO₄^-, HSO₄)

• Organosulfates
• Light-absorbing

• Organic acids
• CO₂
• Organosulfates

Organic mass

> Henry’s Law

GAS PHASE

Henry’s Law

​

Pathways for aqueous-phase SOA formation (Faye McNeill, 2020)

Faye McNeill

Aqueous-phase mechanism for organic aerosol from isoprene:

the short version

Gas-phase

aerosol precursors

isoprene

OH

Aqueous aerosol

glyoxal

epoxide (IEPOX)

Marais et al. [2016]

Eloise Marais

Aqueous-phase formation of organic aerosol from glyoxal

GAS

AQUEOUS PHASE

Oligomers

OH

Organic acids

glyoxal

oxidation

Oligomerization

by H₂O ablation

oligomerization

tetrol

hydration

Observations show correlation of IEPOX SOA with sulfate

Centerville, AL

SEAC⁴RS

aircraft

Correlations with sulfate in SEAC⁴RS (observed and GEOS-Chem)

Sulfate ↑

Aerosol volume ↑

pH ↓

IEPOX SOA ↑↑

Marais et al. [2016]

SO₂ emission ↑

Suggests that SO₂ emission controls decrease biogenic SOA as co-benefit

Van Krevelen diagram for chemical aging of organic material

Heald et al. [2010]

RCH₃ → RCH₂(OH)

RCH₃ → RCO(OH)

RCH₃ → RCHO

Measuring H:C and O:C molar ratios gives insight into added functionalities

Colette Heald

Van Krevelen diagram: application to organic aerosol

Heald et al. [2010]

-1 slope suggests that aging is by adding of carboxylic functionalities, consistent with aqueous mechanism

aging

Two models for formation of secondary organic aerosol

Classical model for reversible uptake by pre-existing organic aerosol

GAS

ORGANIC PHASE

VOC

oxidation

semi-volatile

gas

semi-volatile

aerosol

Alternative model for irreversible uptake by aqueous aerosol

GAS

AQUEOUS PHASE

VOC

oxidation

water-soluble

gas

dissolved

gas

oxidation

complexation

oligomerization

nonvolatile

species

Even if VOC is biogenic, aerosol formation is contingent on pre-existing aerosol

Decline of organic PM_2.5 in the southeast US in summer

Marais et al. [2017]

…even though most is from biogenic isoprene