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Interactions Between Air Quality and Climate Change over the Eastern United States: An Investigation of

Climate Change in Our Own Backyard

Loretta J. MickleyHarvard University

Wildfires in Quebec the same day.Haze over Boston on May 31, 2010

Daniel J. Jacob, Eric M. Leibensperger, Amos P.K.A. Tai, Shiliang Wu

EPA Science Forum, March 14, 2012

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Our work focuses on interactions of short-lived gases and particles in the atmosphere and climate change.

Lifetimes in atmospheric chemistry

Centuries: SF6, some CFCs

Decades: many greenhouse gases: CO2, N2O, . . .

9-10 years: CH4 (methane, precursor to ozone and greenhouse gas)

Days-weeks: O3 (ozone), particulate matter (PM)

Seconds: OH, NO

Pollution over Hong Kong

Air pollution over Hong Kong reached dangerous levels one of every eight days in 2009

Air pollution in the United States: Ozone and fine particulate matter (PM2.5) are the two main pollutants

75 ppb (8-h average)15 mg m-3 (1-y av.)

Ozone PM2.5

Counties violating the EPA standards

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Millions of people living in areas in violation of the standards.

The greenhouse effect keeps the Earth warm.

IPCC, 2007

Greenhouse Effect• Radiation from the earth’s

surface is absorbed and re-emitted by clouds and greenhouse gases: CO2. . .

• This process warms the earth.

CO2

CO2

CO2

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Change in CO2 since mid-1800s: 280 ppm to 390 ppm.

Observed trends in surface temperature, 1880-2011.

NASA Goddard Institute for Space Studies

Global mean temperature increase is ~0.5 oC since 1950s.

Boston/ Logan Airport

Over Boston, we see lots of year-to-year variability, but a significant trend towards warming.

Record annual mean temperature

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Climate models show that observed global warming trends cannot be explained without human influence.

IPCC, 2007

Models with human activity.

Models with just natural processes

Observed trend

Observed trend

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Basic working of climate models

All climate models depend on basic physics to describe motions and thermodynamics of the atmosphere:

E.g., vertical structure of pressure is described by hydrostatic equation

( ) ( ) a adPP z P z dz gdz gdz

Climate models also depend on parameterizations for many processes.

E.g., microphysics of cloud droplet formation, vegetation processes.

Tilt of earth, geography, greenhouse gas content

Weather + Climate

InputPhysics + Parameterized processes

Climate model Output

Simulations of future climate depend on the path of socio-economic development.

Different scenarios follow different socio-economic paths for developed and developing countries.

IPCC 2007

Global mean surface temperature anomalies

A2 = heavy fossil fuelB1 = alternative fuelsA1B = mix of fossil + alternative fuels

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What do climate models predict for future (2100) climate?

Christensen et al., 2007

Annual mean Winter Summer

Annual mean Winter Summer

Temperature increases everywhere, especially at high latitudes.

Precipitation will likely increase over high latitudes but decrease over low latitudes.

In between, the trend is not clear!

Much uncertainty in future precipitation.

Precipitation changes are more complicated.

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How do particles affect regional climate?

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Particles affect solar radiation directly…and also indirectly by modifying cloud properties.

Pollution off U.S. east coast Black carbonCalifornia fire plumes

Aircraft contrails and cirrus over EuropeLight-colored particles reflect sunlight and cool the earth’s surface.

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Comparison to observed sulfate concentrations shows good agreement.

Sequence shows increasing sulfate from 1950-1980, followed by a decline in recent years. Most of aerosol has already cleared by 2010.

1950 1960

1970 1980

1990 2001

Leibensperger et al., 2011

Calculated trend in surface sulfate concentrations, 1950- 2001.

Clearing trend in particles over United States since 1980s suggests possible recent warming.

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Observed US surface temperature trend

GISTEMP [2010]

Is the U.S. “warming hole” a signature of cooling due to particles?

Change in temperatures over 60 year period, 1930-1990

No trend between 1930 and 1980

Warming trend after 1980

Contiguous USo C

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We can examine the influence of particles on regional climate using a climate model.

Most of the warming from reducing particle sources has already been realized.

Observations

Model without US particles

Standard model

• US manmade particles can explain the “warming hole.”

• Warming since 1990s can be attributed to reductions in particle sources.

Leibensperger et al., 2012

Eastern US

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How does climate change affect smog episodes?

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EPA

Roles of ozone (O3) in the atmosphere. O2

O3

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Weather plays a large role in ozone air quality.

1988, hottest on record

Days

Number of summer days with ozone exceedances, mean over sites in Northeast

Lin et al., 2001

A very hot summer can mean more ozone exceedances, even if emissions of ozone precursors are declining.

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In the Northeast, cold fronts sweep out pollution.

During heat wave:• Clear skies• Faster chemical reactions• Greater biogenic emissions• Little ventilation of pollutants

Leibensperger et al., 2008

Can climate change affect the frequency of cold fronts?Fewer cold fronts could mean more persistent smog episodes.

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The number of cold fronts coming through the Northeast has already showed a signal of decline.

Trend in low-pressure systems and associated cold fronts is linked to rapid warming at high latitudes.

What does this trend mean for ozone pollution in US?

Emissions of ozone precursors have declined during this period.

Mickley et al., 2004; Leibensperger et al., 2008

Trend in summer low-pressure systems in S. Canada

0.14 /year

0.16 /year

observations

with increasing GHGs

with constant GHGs

model

model

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Trend in emissions and trend in cold fronts have competing effects on surface ozone.

Emissions

Ozone EpisodesCold fronts per summer

Ozone Episodes

More heat waves, less ventilation, more persistent smog.

Smog over Baltimore in 2002. 20

Observed trends of ozone pollution and cold fronts in Northeast US

Ozone exceedance days in Northeast dropped from 30 in 1980 to 10 in 2006, but would have dropped to ≈ zero in the absence of cold-front trend.

Leibensperger et al. [2008]

Number of ozone episode days (O3> 80 ppb) + Number of cold fronts, 1980-2006

Cold fronts each summer

Ozone episodes

Ozone episodes, constant climate

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Change in max daily 8-hour average JJA surface ozone in 2050s, relative to present-day.

Most models agree that surface ozone will increase over the Northeast in a warming climate.

Climate penalty for air quality:

Harvard model shows 3-7 ppb increase in surface ozone in Midwest US in 2050s climate.

During heatwaves, increases in ozone reach 10 ppb in future climate.

ppb Wu et al., 2008 22

Take home messages:

1. Reductions in particles (PM2.5) over the Eastern US has likely contributed to rapid warming in recent years.

2. Smog episodes in the East are sensitive to many meteorological variables, especially the frequency of cold fronts.

3. The climate penalty: climate change will likely worsen ozone air quality over the East.

Contiguous USo C

Rapid warming after 1980

Acknowledgments:

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O3

O2 hn

O3

Deposition

STRATOSPHERE

TROPOSPHERE

8-18 km

Lifecycle of tropospheric ozone: production is via oxidation of CO, VOCs, and methane in the presence of NOx.

NOx

•Nonmethane volatile organic compounds (VOCs)•NOx = NO + NO2

Human activityFires Biosphere

emissions

Many processes affected by climate

NOxNOx

NOxVOCs

NOx

NOxVOCs

VOCsVOCs

VOCsCO

CO CH4

CH4

Soup of chemical reactions

Ozone is produced in the atmosphere in sunlight.

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Life cycle of particulate matter (PM, aerosols)

nucleation coagulation

condensation

wildfirescombustion

soil dustsea salt

. . ... .

cycling

ultra-fine(<0.01 mm)

fine(0.01-1 mm)

cloud(1-100 mm)

combustionvolcanoes

agriculturebiosphere

coarse(1-10 mm) scavenging

precursor gases

Climate change affects many processes, including gas-particle partitioning.

Soup of chemical reactions

NOxNOx

NOx

NOxNOx

VOCs

VOCsVOCs

VOCsVOCs

SO2

NH3

SO2

Current standard (75 ppb)

EPA-recommended range for revision of standard (60-70 ppb)

Exceedances of the ozone air quality standard

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