13. Aerosol pollution

Daniel J. Jacob, Atmospheric Chemistry, Harvard University, Spring 2017

AIR POLLUTION IN THE US TODAY:Ozone and fine particulate matter (PM2.5) are the major pollutants

http://epa.gov/airtrends

US population exposed to air pollutants in excess of national ambient air quality standards (NAAQS), 2015

PM2.5

PM10

SO2

Lead

Ozone

CO

NO2

108M

31M31M

0

1M

5M

11M

0

70 ppb (8-h average)

12 µg m-3 (annual), 35 µg m-3 (24-h)

• “Particulate matter” (PM) is the same thing as “particles” or “aerosols”

• “PMx” refers to the mass concentration of particles smaller than x μm diameter

TYPICAL AEROSOL SIZE DISTRIBUTION

by volume

PM2.5

PM10

PM2.5 excludes coarse component of the volume size distribution (dust, sea salt, pollen)

Wiley Interdisciplinary Reviews

obstacle

impaction

diffusion

interception

Airflow

Aerosol deposition in respiratory system

Deposition fraction of inhaled particles

Life Expectancy vs annual PM2.51978-82

71

72

73

74

75

76

77

78

79

80

81

5 10 15 20 25 30 35PM2.5 (µg/m3)

Life

Exp

ecta

ncy

Steubenville, OH

Topeka

Boston

EPA standard

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

Life Expectancy vs annual PM2.51997-2001

71

72

73

74

75

76

77

78

79

80

81

5 10 15 20 25 30 35PM2.5 (µg/m3)

Life

Exp

ecta

ncy

Topeka

Boston

Steubenville

EPA standard

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

Life Expectancy vs annual PM2.51980-2000

71

72

73

74

75

76

77

78

79

80

81

5 10 15 20 25 30PM2.5 (µg/m3)

Life

Exp

ecta

ncy

Topeka

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

Annual mean PM2.5 observed from satellite

http://www.nasa.gov/topics/earthUS air quality standard

FINE AEROSOL COMPOSITION IN NORTH AMERICAAnnual mean PM2.5 concentrations (2013)

Current air quality standard is 12 µg m-3

Two dominant components:• Sulfate-nitrate-ammonium (SNA)• Organic

U.S. SO2 EMISSIONS

Industrial

Volcanoes

BiomassburningOceans

Sulfur emissions,Tg a-1

78 8.3

GLOBAL UNITED STATES

Main source is coal combustion

10

SO2 columns over Midwest observed from space, 2005-2010

Ben De Foy,Saint Louis U.

Global SO2 columns observed from space

Andreas.Richter@iup.physik.uni-bremen.de

Norilsk, Siberia: the most polluted city in Russia

Sulfur dioxide observed from space: emissions from coal combustion, metal smelters

Sulfate observations show that SO2 is oxidized near its source

Sulfate deposition, kg ha-1 a-1 Sulfate aerosol, µg m-3

Leibensperger et al. [2012]

Gas-phase oxidation by OH is too slow for this

SO2(aq)/HSO3-/SO3

2- partitioning vs. pH

typical cloud pH range

Acid-catalyzed aqueous-phase oxidation of SO2 in clouds

2 2 2

2 2 3 1

2 2 2 2

23 2 2 4 2

1

( )

( ) ( )

( ) 2

SO g SO H O

SO H O HSO H

H O g H O aq

HSO H O aq H SO H O H

− +

− + − +

→ •←→• +←

→←

+ + → + +

H,SO2

H,H2O2

2-4

H,SO2 H,H2O2 SO2 H2O2

K

K

K

k

d[SO ] =kK K K p pdt

Rate does not slow down as H+ increases

OBSERVED TITRATION OF SO2 BY H2O2 IN CLOUD

First aircraft observations by Daum et al. [1984]

Formation of sulfate-nitrate-ammonium aerosol

SO2

NH3 NOx HNO3

NH3(aq), NH4+

H2SO4(aq), HSO4

-, SO42-

HNO3(aq), NO3-

High RH (aqueous) Low RH

NH4+,SO4

2-, NO3-

dry salts

EMISSIONS

oxidation

oxidation

• Uptake of NH3 and HNO3 by sulfate aerosol follows thermodynamics• This uptake affects the mass and phase of the aerosol

Ammonia emissions

Global ammonia emissions 2005-2008 (kg N ha-1 a-1)

Agriculture is 75% of global source

IASI 2007-2012 satellite observations of ammonia

Paulot et al. [2014]

Van Damme et al. [2014]

Thermodynamics of sulfate-nitrate-ammonium aerosol formation

22 4 4 4( ) 2H SO aq HSO H SO H− + − +→ →+ +← ←

H2SO4 condenses to aqueous solution; dissociation to HSO4-, SO4

2- governed by pH

2

3 4( ) H ONH g NH OH+ −→ +←

NH3 condenses into acid aerosol until neutralization

3 3( )HNO g NO H− +→ +←

HNO3 condenses only if excess NH3 is available

4 4 4 3 4 2 4 2 4 4 3( ), ( ) ( ) , ( ) ( ), ( )NH HSO s NH H SO NH SO s NH NO s

At low RH, equivalent equilibria apply to the solid salts

Three different regimes for aerosol formation[S(VI] = [H2SO4(aq)] + [HSO4

-] + [SO42-]

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

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

Equivalents per m3 of air

[S(VI)] > [N(-III)]• Aerosol is acidic• N(-III) all in aerosol• No N(V) in aerosol

S(VI

)

N(-I

II)

[S(VI)]+[N(V)] > [N(-III)] > [S(VI)]• Aerosol is neutralized• N(-III) all in aerosol• N(V) partly in aerosol

gasaerosol

S(VI

)

N(-I

II)

N(V

)

N(V

)

gasaerosol

[S(VI)]+[N(V)] < [N(-III)]

S(VI

)

N(-I

II)

gasaerosol

N(V

)

• Aerosol is neutralized• N(-III) partly in aerosol• N(V all in aerosol

SULFATE-NITRATE-AMMONIUM AEROSOLS IN U.S. (2006)

sulfate nitrate ammonium

Zhang et al. [2012]

LONG-TERM TREND IN US SO2 EMISSIONS

Decline of sulfate aerosol in the US

1990Observed sulfate concentrations (circles), GEOS-Chem (background)

Leibensperger et al. [2012]

2010

µg m-3

Scrubbers on coal power plants, transition to natural gas

Decline of US sulfur doxide (SO2) emissions seen by OMI

OMI SO2

Fioletov et al. [2011]

SO2 is the principal source of particulate pollution, haze, acid rain

2005-2007 2008-2010

GOME and SCIAMACHY SO2 over China

• Large increase in SO2 loading observed from 2000 to 2007 Turnover in 2007

Volcanic eruption

New SO2 pollution frontier: IndiaOMI reveals rapid growth in SO2 emissions from coal use

Lu et al. [2013]

[Zhang et al., 2007]

NO3

Organic

Northern hemisphere aerosol componentsOrganic 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

fuel/industry open fires

OH

vegetation fuel/industry open fires

600

isopreneterpenesoxygenates…

50

alkenesaromaticsoxygenates…

alkanesalkenesaromatics…

VOC emissions Primary organic aerosol (POA)

VOC

200 20 100

Global sources in Tg C year-1

Secondary organic aerosol (SOA)multiple steps

Fine particulate matter (PM2.5) in the Southeast US

Kim et al. [2015]

annualstandard

PM2.5 in Aug-Sep 2013: observed (circles), GEOS-Chem model (background)

Sources of sulfate and organic aerosol in SoutheastSurface networks (circles), GEOS-Chem (background)

Aug-Sep 2013

Kim et al. [2015]

(12%)

(29%)

Two models for formation of secondary organic aerosol

A. Classical model for reversible uptake by pre-existing organic aerosol

GAS ORGANIC PHASE

VOCoxidation

semi-volatilegas

semi-volatileaerosol

B. Alternate model for irreversible uptake by aqueous aerosol

GAS AQUEOUS PHASE

VOCoxidation

water-solublegas

dissolvedgas oxidation

complexationoligomerization

nonvolatilespecies

Aqueous-phase mechanism for organic aerosol from isoprene:the short version

Gas-phaseaerosol

precursors

isoprene OH

Aqueous aerosol

glyoxal

epoxide (IEPOX)

Marais et al. [2016]

Aqueous-phase formation of organic aerosol from glyoxal

GAS AQUEOUS PHASE

Oligomers

OHOrganic acids

KH* ~ 105 M atm-1

glyoxal

olig

omer

izat

ion

tetrol

Aqueous-phase formation of organic aerosol from epoxides

H+, SO4=

10%

Isoprene epoxide (IEPOX)

H+, H2O90%

Aqueous aerosolIEPOX SOA

Marais et al., 2016

Acid-catalyzed ring cleavage to produce non-volatile species

Observations show correlation of IEPOX SOA with sulfate

Centerville, AL SEAC4RS

Correlations with sulfate in SEAC4RS and at Centerville, Alabama research site

Sulfate ↑Aerosol volume ↑pH ↓ IEPOX SOA ↑↑

Marais et al. [2016]

SO2 emission ↑

Suggests that SO2 emission controls decrease organic aerosol as co-benefit

Long-term trends of organic and sulfate aerosol in Southeast

Marais et al. [2017]