Interactions between climate and atmospheric chemistry in the US

Loretta J. Mickley, Harvard University

Collaborators:

Rynda Hudman, Daniel Jacob, Eric Leibensperger, Jennifer Logan, Havala Pye, Dominick Spracklen, Amos Tai, Shiliang Wu, Moeko Yoshitomi

Funding for this work: NASA, EPA, EPRI

Smog over Pittsburgh, ranked #1 city for particulate pollution in 2008 by American Lung Association

• Effects of climate change on air quality

• Effects of short-lived species on climate

Part 1: Effects of climate change on air quality.

Millions of people in U.S. already live in areas of high pollution. How will a changing climate affect pollution?

Calculated with new 0.075 ppm standard

Number of people living in areas that exceed the national ambient air quality standards (NAAQS) in 2008.

EPA’s Technical Support Document for the proposed finding on CO2 as a pollutant.Cites the threat of climate change to air quality.

Public hearings last week on EPA proposed finding in Detroit + NYC.

O3

O2 h

O3

OH HO2

h, H2O

Deposition

NO

H2O2

CO, VOC

NO2

h

STRATOSPHERE

TROPOSPHERE

8-18 km

Nitrogen oxide radicals; NOx = NO + NO2

combustion, soil, lightningMethane wetlands, livestock, natural gasNonmethane volatile organic compounds (VOCs) vegetation, combustion, industryCO (carbon monoxide) combustion, VOC oxidation

Troposphericozone

precursors

Stagnation promotes ozone production

Chemistry of tropospheric ozone: oxidation of CO, VOCs, and methane in the presence of NOx

Weather plays a large role in ozone air quality.

The total derivative d[O3]/dT is the sum of partial derivatives (O3/xi)(xi/T).

x = ensemble of ozone forcing variables that are temperature-related.

Lin et al., 2001

Probabilityof ozone exceedancevs. daily max. temperature

Curves include effects of• Biogenic emissions• Stagnation• Clear skies

Northeast

SoutheastLos Angeles

Temperature (K)

Pro

babi

lity

Day

s

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

1988, hottest on record

Low pressure systems (aka cyclones) cross southern Canada and sweep out ozone pollution from Eastern US.

Stalled high pressure system associated with:

• increased biogenic emissions

• clear skies

• weak winds

• high temperatures.

cold front EPA ozone levels

3 days laterCold front pushes smog poleward and aloft on a warm conveyor belt.

cold front

Hazardous levels of ozone

L

L

Sample storm tracks,summer 1979-1981

Correlation between cyclone number each summer in red and green boxes and number of US ozone episodes

Strong anti-correlation of cyclone number and number of ozone episodes in eastern US:

Leibensperger et al., 2008

Cyclone passage through southern Canada/Great Lakes region strongly affects frequency and duration of U.S. ozone episodes.

27 year record

Fewer cyclones per summer in green box leads to more ozone episodes in US.

1950-2000 observed trend in cyclone frequency matches that in climate model with increasing greenhouse gases.

Trend in cyclones appears due in part to weakened meridional temperature gradients, reduction of baroclinicity over midlatitudes.

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

1950-2006 trend in JJA cyclones in S. Canada

0.14 yr-1

0.16 yr-1

NCEP/NCAR obs

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

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

Cyclones: less frequent cyclones + cold fronts mean more persistent pollution episodes

Emissions: reduced emissions means fewer episodes.

NE ozone episodes

cyclones

1980-2006 trends

Decline in emissions of ozone precursors from US mobile sources.

Parrish 2006.

Ozone pollution days in the Northeast US

cyclonesemissions dt

sexceedanced

dt

sexceedanced

dt

sexceedanced

)()()(

5.1)(

84.0)(

63.0)15.0)(2.4()(

)(

)()(

emissions

cyclones

dt

sexceedanced

dt

exeedancesd

dt

cyclonesd

cyclonesd

sexceedanced

dt

sexceedanceddays yr-1

days yr-1

days yr-1 Trend in pollution days due to decline in emissions

We find that if 1980-2006 cyclone frequency had remained constant, we would have had zero episodes over Northeast.

Trend in pollution days due to decline in cyclone frequency

If emissions had remained constant, decline in mid-latitude cyclone number over Canada would have meant more persistent stagnation episodes, more ozone.

Climate response

Particulate matter (PM, aerosols) sources and processes

SO2H2SO4

NH3

VOCs

NOx

RCO…

HNO3

nucleation coagulation

condensation

carbonaceouscombustion

particlessoil dustsea salt

..

...

.oxidation cycling

ultra-fine(<0.01 m)

fine(0.01-1 m)

cloud(1-100 m)

combustionbiospherevolcanoes

agriculturebiosphere

coarse(1-10m) scavenging

precursor gases

wildfires

Observed correlations of total PM2.5 with meteorology

• Precipitation

• Stagnation

• Temperature

Positive correlation with temperature occurs due to:

• Increased oxidation of SO2

• Greater biogenic emissions

Results from EPA AQS database: 1000+ sites sampled every 1-6 days from 1998 to 2007.

Observed correlations provide means to test model simulations.

Temperature

Precipitation

Stagnation

Tai et al., ms.

met fields

met fields

chemistry fields

What do models project for future air quality?We have developed GCAP (Global Change and Air Pollution).

Regional climate model

Regional chemistry model

met fields

Chemistry model driven by GCM meteorology to study influence of climate on air quality.

Mickley et al., 2004

GEOS-Chem

Chemical transport model

chemistry, emissions

2050s

1990s

2000-2050 decrease in cyclone frequency leads to increased stagnation.

CO tracer Northeast, Jul-Aug

AIR QUALITY

GISS GCM

Physics of the atmosphere

Qflux ocean, well-mixed GHGs

2000-2050 change in max daily 8-hour average JJA ozone

Climate penalty for air quality:Harvard model shows 2-12 ppb increase in surface ozone in East

2000-2050 change in max daily 8-hour average JJA ozone

Multi-model comparisonWeaver et al., 2009

Most models agree that surface ozone will increase over the Northeast.Disagreement occurs elsewhere due to differences in chemistry and cloud cover change.

How will US surface ozone change in a changing climate?

ppb

Calculated response in surface PM to +2.5 oC temperature change applied uniformly for July.

Dawson et al., 2007

Uncertainty in response of surface PM to changing meteorology is large. We can use present-day observations to test models.

(μg/m3)

Observed correlation between surface temperature and surface PM concentrations.

Tai et al, ms. in progress

Positive correlation with T due to:

• Increased oxidation of SO2

• Greater biogenic emissions

Part 2: Effects of short-lived species on climate.

Case study of US aerosols and regional climate change.

Radiative forcing:

• Easily calculated metric of climate change

• Suggests the relative magnitude of surface temperature response to a given perturbation.

Present-day radiative forcing due to aerosols over the eastern US is comparable in magnitude, but opposite in

sign, to global forcing due to CO2.

Globally averaged radiative forcing due to CO2 is +1.7 Wm-2. warming

Due to short lifetime, forcing due to aerosols is not uniform across globe.

Over the US, radiative forcing due to sulfate aerosols is -2 Wm-2. cooling

IPCC, 2007; Liao et al. , 2004

Comparison to observed sulfate concentrations shows good agreement.

Sequence shows increasing sulfate from 1950-1980, followed by a decline in recent years.

1950 1960

1970 1980

1990 2001

Leibensperger et al., ms

Calculated trend in surface sulfate concentrations, 1950- 2001.

Trend in aerosols over United States suggests cleaner skies, possible warming?

Recent US Climate Change report suggests more global than regional response, but the report looks at an ensemble of short-lived species all over the globe.

– U.S. Climate Change Science Program, Synthesis and Assessment Product 3.2

Harvard’s work to date suggests more regional than global response at least for US aerosols.Decline in the aerosol burden over the eastern US will lead to regional warming, in a way that the US Climate Change report would not have recognized.

Is the climate response to changing aerosols regional or global?

Calculated present-day aerosol optical depths

What is the influence of changing aerosol on regional climate? In pilot study, we zero out aerosol optical depths over US.

For pilot study, 2 scenarios were simulated: Control: aerosol optical depths fixed at 1990s levels. Sensitivity: U.S. aerosol optical depths set to zero (providing a radiative forcing of about +2 W m-2 locally over the US); elsewhere, same as in control simulation.

Each scenario includes an ensemble of 3 simulations.Caveats: No transport, only direct effect considered in this pilot study.

GISS GCM

Removal of anthropogenic aerosols over US increases annual mean surface temperatures by 0.5 o C.

Summertime temperatures increase as much as 1.5 oC.

Mean 2010-2050 temperature difference: No-US-aerosol case – Control

White areas signify no significant difference.Results from an ensemble of 3 for each case.

Annual mean surface temperature change in Control.

Warming due to 2010-2050 trend in greenhouse gases.

Additional warming/ cooling due to zeroing of US aerosols

Mickley et al., ms. 2009

oC oC

The regional surface temperature response to aerosol removal persists for many decades in the model.

Te

mp

era

ture

(oC

)

No-US-aerosols case

Control, with US aerosols

Annual mean temperature trends over Eastern US

Mickley et al., ms

Bottom line: Efforts to clear the air of anthropogenic aerosol over the US may exacerbate regional warming.

GEOS-Chem chemistry transport model

GISS GCM III climate model

Calculation of cloud droplet number concentrations

aerosolconcentrations

aerosol indirect effect

We use historical/projected emissions of SO2, NOx, BC, and OC to quantify the climatic role of US aerosols in the past and future.

1950-2050 Control simulation (EDGAR/Tami Bond historical emissions and A1B; includes rising U.S. aerosol sources until 1980 and subsequent decline)

Sensitivity simulations:• 1950-2050 No US aerosols. Quantifies the past effect of U.S. anthropogenic sources on regional climate.

• 2010-2050 Constant US emissions Quantifies the warming effect from the projected decrease in U.S. emissions.

Ongoing study: Perform realistic simulation of changing aerosol optical depths over the US, together with sensitivity studies.

Climate response to aerosol trends over the US

Implications for policymakers

• Policymakers need to consider “climate change penalty,” i.e., the additional emission controls necessary to meet a given air quality target.

• Efforts to clear the air of anthropogenic aerosol over the US may exacerbate regional warming.

Directions for future research

Understand causes in interannual variability of air quality.

Investigate model sensitivity of pollutants to meteorology, and compare to observations.

Understand chemistry of biogenic species, e.g. isoprene

Improve emission inventories for recent past/future, especially for NH3, black carbon, organic carbon, mercury

Understand secondary organic aerosols: sources, chemistry.

Improve modeling of fine scale features, investigate how best to downscale meteorology from global climate models, test effects of land use change.

Understand aerosol-cloud interactions, characterize aerosol composition

Extra slides

Multiple linear regression to fit 1998-2008 deseasonalized EPA/AQS data for PM2.5:

9

10 n termsinteractio

kkk xy xk Meteorological Parameter

(NCEP/NCAR Reanalysis 1 & NOAA CPC)

x1 Surface temperature (K)

x2 Relative humidity (10%)

x3 Precipitation (cm/d)

x4 Cloud cover (10%)

x5Geopotential height at 850 hPa (100 m)

x6 dSLP/dt (hPa/d)

x7 Wind speed (m/s)

x8E-W wind direction indicator (cosθ)

x9N-S wind direction indicator (sinθ)

Observed Correlations of PM2.5 with Meteorology

Observed +ve correlation with sulfate is larger by 10x stagnation and air mass origins

Strong +ve correlation with nitrate in the west and north agricultural NH3 and soil NOx emissions

Strong +ve correlation with OC VOC emissions and fires

2000-2050 climate change increases JJA surface ozone:

1-5 ppb on average across US, 5-10 ppb during heat waves in Midwest

Daily max 8h-avg ozone averaged in JJA (ppb)

Wu et al., 2007

We define the climate change penalty as the effort required to meet air quality standards under future climate change.

Effect of climate change alone

Increase of summer max-8h-avg ozone

Cumulative probability (%)

Max

. 8-

hr-

avg

ozo

ne

2000s conditions2050s climate

2050s emissions2050s climate & emis

99th percentile

Midwest

We define the climate change penalty as the effort required to meet air quality goals in the future atmosphere.

present-day NOx emissions + climate

40% cut in NOx + present-day climate

40% cut in NOx + 2050s climate

50% cut in NOx + 2050s climate 2000–2050 climate

change implies an additional 25% effort in NOx emission controls to achieve the same ozone air quality.

Wu et al., 2007

climate change penalty

}

Midwest surface ozone

Weaver et al., 2009

Models tend to agree on 2000-2050 changes in surface temperature over the United States.

Most models agree that surface ozone will increase over the Northeast.

Disagreement occurs elsewhere due to differences in chemistry and cloud cover change.

2000-2050 JJA surface temperature change

2000-2050 change in max daily 8-hour average ozone

Change in annual mean surface inorganic aerosol from 2000-2050 climate change (no change in emissions)

Increase in Northeast due to increased temperature and accelerated oxidation rates

Decrease in Southeast due mainly to increased precipitation.

Calculation of future aerosol levels is challenging because of uncertainty in future rainfall over mid-latitudes.

Also, mix of aerosol species is expected to change, so sensitivity to climate will also change.

Pye et al., 2009sulfate nitrate ammonium

Present-day annual average

Projected increase in wildfires could affect air quality in the US.

We predict future wildfires using observed relationships between meteorology and area burned for different ecosystems.

2000-2050 changes in fire season surface ozone.

Perturbation due to climate change only

Spracklen et al., 2009Hudman et al, ms.

1980 20001990

R2=52%

Are

a b

urn

ed /

106

Ha

0.5

0.25

observations

model

May-Oct area burned in Pacific Northwest

1990

Projected increase in wildfires could affect air quality in the US.

2000-2050 change in JJA surface organic aerosol due to increased wildfires

g m-3

We have developed a fire prediction tool based on observed relationships between meteorology and area burned.

Applying these relationships to GCM meteorology, we predict area burned and future emissions of wildfire pollutants.

Changes in JJA surface ozone concentrations

Perturbation due to climate change only

Spracklen et al., 2009Hudman et al, ms.

Observations of a possible relationship between trends in aerosol optical depths and surface temperature.

Smoothed monthly mean AOD for sites in Europe

Ruckstuhl et al., 2008

Annual mean fluxes, temperatures

Anomaly of clear sky shortwave downward radiation at surface

Surface temperature anomalies averaged over all sites, excluding 2003.

Pinatubo

+0.42 C /decade