Ammonia and Atmospheric ChemistryAmmonia and Atmospheric Chemistry

Spyros PandisSpyros Pandis

Departments of Chemical Engineering andDepartments of Chemical Engineering andEngineering & Public PolicyEngineering & Public Policy

Carnegie Mellon UniversityCarnegie Mellon University

MAIN AIR QUALITY PROBLEMSMAIN AIR QUALITY PROBLEMS

Ozone Ozone Particulate Matter Particulate Matter (PM(PM2.52.5 and PMand PM1010))Visibility ReductionVisibility ReductionAcid Deposition Acid Deposition (acid rain)(acid rain)Air ToxicsAir Toxics

Ozone HoleOzone HoleGreenhouse effectGreenhouse effect (global change)(global change)

NH3

???

Ammonia and GasAmmonia and Gas--Phase Phase ChemistryChemistry

•• Most species in the atmosphere react with the Most species in the atmosphere react with the hydroxyl radical (OH)hydroxyl radical (OH)

•• These reactions remove the primary species, but initiate a These reactions remove the primary species, but initiate a complicated chain of radical reactions leading to the formation complicated chain of radical reactions leading to the formation of ozone and other secondary pollutantsof ozone and other secondary pollutants

•• The reaction of ammonia with OH is quite slow (2 The reaction of ammonia with OH is quite slow (2 months average lifetime)months average lifetime)

•• Negligible importance for the atmospheric fate of ammoniaNegligible importance for the atmospheric fate of ammonia

•• Ammonia does not contribute directly to the Ammonia does not contribute directly to the atmospheric photochemistryatmospheric photochemistry

•• No role in the formation of ozone or other secondary gasNo role in the formation of ozone or other secondary gas--phase pollutantsphase pollutants

Sulfate34%

Nitrate7%

Ammonium14%

Crustal10%

Black Carbon

2%

Organics33%

Fine PM Composition in NE USFine PM Composition in NE US(Pittsburgh, 2001)(Pittsburgh, 2001)

PM2.5=20.1 µg m-3

SO2 Emissions

PrimaryH2SO4

Gas-PhasePhotochemistry

NH3 Emissions

NH3

NOxEmissions

Gas-PhasePhotochemistry

Combustion Process Emissionsprimary OC - EC

H2SO4

H+, SO42-,

HSO4-,H2SO4

NH4+,OH-

HNO3

NO3-,H+

primary OC - EC

Gaseous Organics Emissions

Gas-PhasePhotochemistry

CondensibleOrganics

SecondaryOC

Dust, Fly Ash Emissions

Dust, fly ashmetals

Sea-SaltEmission

HClemissions

Cl-,

Na+

HCl

H2O

Ca2+,Mg2+,Fe3+, etc.

S(IV)

Sulfuric Acid in the AtmosphereSulfuric Acid in the Atmosphere•• Sulfuric acid in the presence of water vapor Sulfuric acid in the presence of water vapor

has an extremely low vapor pressurehas an extremely low vapor pressure•• As soon as sulfuric acid vapor is formed in As soon as sulfuric acid vapor is formed in

the atmosphere it is transferred to the the atmosphere it is transferred to the particulate phaseparticulate phase

–– Condensation on existing particles, or inCondensation on existing particles, or in--situ formation situ formation of new particles (nucleation)of new particles (nucleation)

•• The preferred form of sulfuric acid in the The preferred form of sulfuric acid in the aerosol phase is ammonium bisulfate aerosol phase is ammonium bisulfate (NH(NH44))22SOSO44

–– Each sulfuric acid molecule is looking for two ammonia Each sulfuric acid molecule is looking for two ammonia molecules (neutralization)molecules (neutralization)

–– If there is not enough ammonia present, sulfuric acid If there is not enough ammonia present, sulfuric acid exists either as Hexists either as H22SOSO44(aq) or NH(aq) or NH44HSOHSO44

The Sulfuric Acid/Ammonia SystemThe Sulfuric Acid/Ammonia System

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0 1 2 3 3.6 5 10

AmmoniaAmmoniumSulfateBisulfateSulfuric acid

Total Available Ammonia (µg m-3)

Con

cent

ratio

n (µ

g m

-3)

2 Ammonia:1 Sulfate

Ammonium Nitrate FormationAmmonium Nitrate Formation

NHNH33 (g) + HNO(g) + HNO33 (g) (g) NHNH44NONO33 (s) (low RH)(s) (low RH)NHNH33 (g) + HNO(g) + HNO33 (g) (g) NHNH44

+ + ((aqaq) + NO) + NO33-- ((aqaq) (high RH)) (high RH)

•• The formation of ammonium nitrate requiresThe formation of ammonium nitrate requires–– Nitric acid Nitric acid (major sources of NOx in the US are transportation (major sources of NOx in the US are transportation

and power plants)and power plants)–– Free ammonia Free ammonia (ammonia not taken up by sulfate)(ammonia not taken up by sulfate)

•• The formation reaction is favored at:The formation reaction is favored at:–– Low temperatures Low temperatures (night, winter, fall, spring)(night, winter, fall, spring)–– High relative humidityHigh relative humidity

Linear and Nonlinear Behavior of Inorganic PM

Sulfate Mass (µg m−3)0 2 4 6 8 10 12

0

2

4

6

8

10

12

14

16

Temp=278 KRH=70%

HNO3T=6 µg m−3

NH3T = 6 µg m−3

NH3T = 0

PM (µ

g m

−3)

Limiting Reactant: Ammonia or Nitric Acid?Limiting Reactant: Ammonia or Nitric Acid?

0 10 20 30 40

0

20

40

60

80

10

30

50

Ammonium Nitrate (µg m-3)

Tota

l Nitr

ic A

cid

(µg

m-3

)

Total Ammonia (µg m-3)

A

B

A:Ammonia limited

B: Nitric acid limited

A Multidisciplinary Research Consortium for A Multidisciplinary Research Consortium for Atmospheric Aerosol Research Atmospheric Aerosol Research Funded by EPA and DOE/NETLFunded by EPA and DOE/NETL

Aerosol Nitrate, Ammonia and Aerosol Nitrate, Ammonia and Inorganic PMInorganic PM2.52.5 ControlControl

Motivation:Motivation: What is the response of fine PMWhat is the response of fine PM2.52.5when the emissions of SOwhen the emissions of SO22 are reduced? What are reduced? What about changes in NOx or NHabout changes in NOx or NH33 emissions?emissions?Hypothesis:Hypothesis: A significant fraction of the sulfate A significant fraction of the sulfate reduced will be replaced by nitrate when SOreduced will be replaced by nitrate when SO22emissions are reduced.emissions are reduced.Approach:Approach: High resolution measurements of High resolution measurements of aerosol sulfate, nitrate, and total (gas+aerosol) aerosol sulfate, nitrate, and total (gas+aerosol) nitrate and ammonium. Use of numerical models.nitrate and ammonium. Use of numerical models.

OnOn--line (detector) vs. offline (detector) vs. off--line (IC)line (IC)

NH3 gas + PM2.5 NH4+, µg/m3

02468

101214

7/21/010:00

7/28/010:00

8/4/010:00

8/11/010:00

8/18/010:00

8/25/010:00

9/1/010:00

9/8/010:00

DetectorIC

Partitioning of PMPartitioning of PM2.52.5 NitrateNitrate(Pittsburgh, 2001)(Pittsburgh, 2001)

0

2

4

6

0 6 12 18 0 6 12 18 0 6 12 18 0Time (hr)

July 19 July 20 July 21

Con

cen

trat

ion

(µg

m-3

)

PM2.5

Total

GFEMN Evaluation: Nitrate PartitioningGFEMN Evaluation: Nitrate Partitioning(July 19, 2001)(July 19, 2001)

0

1

2

3

4

5

MeasuredPredicted

PM2.

5N

itrat

e (µ

g m

-3)

0 4 8 12 16 20 24

Time (hr)

Evaluation of ISORROPIA and GFEMN

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hr)

PM10 NitrateRiverside, CA, June 25, 1987

ObservedPredicted (GFEMN)Predicted (ISORROPIA)

cet

atC

onn

ri o

n .µ g

m-3

Availability of Nitric Acid/NitrateAvailability of Nitric Acid/Nitrate(Pittsburgh 2001(Pittsburgh 2001--02)02)

0

0.5

1

1.5

2

2.5

3

Jul Aug Sept Oct Nov Dec Jan Feb

Vapor

PMCon

cent

ratio

n (µ

g m

-3)

Where does the HNOWhere does the HNO33 go?go?

Time

• The HNO3 declines rapidlyevery afternoon with an averagerate of around 30% per hour.

• Dry removal can explain rates of10-30% per hour.

• Net contribution of horizontaltransport should be small because of the relatively spatial homogeneity in the area.

• During the night the dry deposition becomes very slow because the HNO3goes to the aerosol phase

0

2

4

6

0 6 12 18 0

July 19

Con

cen

trat

ion

(µg

m-3

)

PM2.5

Total

0

2

4

6

0 6 12 18 0

July 19

Con

cen

trat

ion

(µg

m-3

)

PM2.5

Total

Some potential implicationsSome potential implications

•• During the summer there is 1During the summer there is 1--2 2 µµg mg m--3 3 of HNOof HNO33(g) (g) on average (and as much as 10 on average (and as much as 10 µµg mg m--33) that could ) that could be transferred to the aerosol phase.be transferred to the aerosol phase.

•• The lifetime of nitric acid vapor is only a few The lifetime of nitric acid vapor is only a few hourshours

–– Reaction with ammonia will increase its lifetimeReaction with ammonia will increase its lifetime

•• Decreasing sulfate or increasing ammonia will Decreasing sulfate or increasing ammonia will transfer nitric acid to the particulate phase, transfer nitric acid to the particulate phase, increase its lifetime, and its concentration levels.increase its lifetime, and its concentration levels.

•• During the fall and winter months in Pittsburgh During the fall and winter months in Pittsburgh most of the available nitric acid appears to be most of the available nitric acid appears to be already in the aerosol phase.already in the aerosol phase.

Modeling of the Modeling of the HH22SOSO44/HNO/HNO33/NH/NH33/H/H22O SystemO System

•• Processes:Processes: Partitioning (with GFEMN), removal (dry Partitioning (with GFEMN), removal (dry deposition velocities), emission/productiondeposition velocities), emission/production

•• Framework:Framework: Box model Box model (Pandis and Seinfeld, 1991).(Pandis and Seinfeld, 1991).

•• GasGas--phase production fitted to the observations phase production fitted to the observations (reasonable rates).(reasonable rates).

•• July 2001 and January 2002 periods. Use of July 2001 and January 2002 periods. Use of average diurnal concentrations.average diurnal concentrations.

•• Inputs:Inputs: Sulfate concentrations, meteorology.Sulfate concentrations, meteorology.

Evaluation of Box ModelEvaluation of Box ModelC

once

ntr

atio

n (µ

g m

-3)

0 4 8 12 16 20 24

0

1

2

3

4

5

Measured

Predicted

TOTAL NITRICACID

0 4 8 12 16 20 24

0

1

2

3

4

5

Measured

Predicted

TOTAL AMMONIA

Con

cen

trat

ion

(µg

m-3

)

Time (hr) Time (hr)

0 4 8 12 16 20 24

0

1

2

Measured

Predicted

Con

cen

trat

ion

(µg

m-3

)

PM2.5 NITRATE

PITTSBURGHJuly 2001

Time (hr)

Effect of 20% Reduction of the Sulfate Effect of 20% Reduction of the Sulfate Concentration on Nitrate Levels Concentration on Nitrate Levels

(Pittsburgh, July 2001)(Pittsburgh, July 2001)

0 4 8 12 16 20 240

1

2

3

4

5

6

7

0 4 8 12 16 20 240

1

2

3

4

5

6

7

Con

cen

trat

ion

(µg

m-3

)

Time (hr) Time (hr)

TOTAL NITRATE PM2.5 NITRATE

Base caseBase case

Base caseBase case

--20% Sulfate20% Sulfate

--20% Sulfate20% Sulfate

Effect of Sulfate Concentration Effect of Sulfate Concentration Changes on Inorganic PMChanges on Inorganic PM2.52.5

0

2

4

6

8

10

12

14

BaseCase

10% 20% 30% 40% 50%

SulfateAmmoniumNitrate

Inor

gani

c PM

2.5 (µ

g m

-3)

Sulfate Reduction

Reductions of Sulfuric and Nitric Reductions of Sulfuric and Nitric Acid Acid (Pittsburgh, July 2001)(Pittsburgh, July 2001)

0 10 20 30

30

Inor

gani

c PM

2.5 Red

uctio

n (%

)

20

Same Nitric Acid

--50% Nitric Acid50% Nitric Acid

10

0

Sulfate Reduction (%)

Reductions in AmmoniaReductions in Ammonia(Pittsburgh, July 2001)(Pittsburgh, July 2001)

0 10 20 30 40 50

40

Inor

gani

c PM

2.5 Red

uctio

n (%

)

20% Sulfate Reduction20% Sulfate Reduction30

20

10

0

Ammonia Reduction (%)

Reductions in Ammonia Reductions in Ammonia (Pittsburgh, January 2002)(Pittsburgh, January 2002)

0 10 20 30 40 50

50

--20%20%

-50% Sulfate

Same SulfateSame Sulfate

Inor

gani

c PM

2.5 Red

uctio

n (%

)

40

30

20

10

0

Ammonia Reduction (%)

Effect of 50% Emissions Change Effect of 50% Emissions Change on 24on 24--hr Average PMhr Average PM2.52.5 massmass

0

5

10

15

20

25

30

NH3,VOC,NOx

NH3 PM10 VOC,NOx

VOC SO2

Emissions Reduction

Perc

ent r

educ

tion

in m

ean

PM2.

5 m

ass

5 Station MeanRiverside

Ammonia and Global ChangeAmmonia and Global Change•• Increases in ammonia concentrations will Increases in ammonia concentrations will

result in increases of the fine aerosol massresult in increases of the fine aerosol mass•• Visibility reduction Visibility reduction •• Cooling of the planetCooling of the planet

•• The role of ammonia is expected to become The role of ammonia is expected to become more important in the coming years as SOmore important in the coming years as SO2 2 emissions decreaseemissions decrease

•• The lifetime of ammonia in the troposphere is The lifetime of ammonia in the troposphere is only a few days so it does not have enough only a few days so it does not have enough time to make it to the stratosphere time to make it to the stratosphere

•• no effect on stratospheric ozoneno effect on stratospheric ozone

ConclusionsConclusions

•• Continuous measurements of the major inorganic Continuous measurements of the major inorganic ions and the corresponding gasions and the corresponding gas--phase species phase species allow usallow us

–– to evaluate our understanding of the systemto evaluate our understanding of the system–– provide insights about important processesprovide insights about important processes

•• Existing models (GFEMN) reproduce well the Existing models (GFEMN) reproduce well the partitioning of nitric acid in Western Pennsylvania partitioning of nitric acid in Western Pennsylvania both during the summer and the winter both during the summer and the winter

•• Sulfate reductions result in nitrate level increases Sulfate reductions result in nitrate level increases (change of partitioning, lifetime increase). (change of partitioning, lifetime increase).

Conclusions Conclusions (continued)(continued)

•• Ammonia, is controlling the ammonium nitrate Ammonia, is controlling the ammonium nitrate formation both during the summer and the winter.formation both during the summer and the winter.

•• PMPM2.52.5 control efficiencies (for Pittsburgh)control efficiencies (for Pittsburgh)•• July: Sulfate > Ammonia > Nitric AcidJuly: Sulfate > Ammonia > Nitric Acid•• January: Ammonia >= Sulfate >= Nitric AcidJanuary: Ammonia >= Sulfate >= Nitric Acid

•• Ammonia is not involved in the ozone formation in Ammonia is not involved in the ozone formation in the troposphere and its destruction in the the troposphere and its destruction in the stratospherestratosphere

•• Increases in ammonia levels result in cooling of the Increases in ammonia levels result in cooling of the planet, decreases of the acidity of particles and planet, decreases of the acidity of particles and clouds, but increases of the fine particulate matter clouds, but increases of the fine particulate matter concentrations and reductions of visibilityconcentrations and reductions of visibility