Climate change and aerosols: How much do aerosols mask global

warming?Joyce Penner

Clean Air Leadership Talk: Haagen-Smit AwardsAir Resources Board, Sacramento, CA

June 21, 2017

Aerosols scatter and absorb radiation

Absorption can decrease the total cloud amount (semi-direct effect)

The first indirect effect of aerosols is due to changes in cloud droplet concentrations (Twomey effect)

The second indirect effect is due to changes in precipitation efficiency.

Addition of aerosol forcing results in temperature response unlike the pattern of sulfate aerosols forcing (Taylor and Penner, 1994)

Novakov and Penner, 1993

Recognition that anthropogenic source types include organic and black carbon, and smoke,

nitrates, ammonium, dust

Penner et al., 3rd IPCC assessment report, 2001

3-D model study of carbonaceous aerosols aerosols (Liousse, Penner et al., 1996)

Direct forcing from biomass and fossil fuel carbon

Penner et al. (1998)

Biomass burning BC at high altitude causes cooling of surface, whereas fossil fuel causes warming

Penner et al. (2001)

The first indirect effect of aerosols is due to changes in cloud droplet concentrations (Twomey effect)

Indirect forcing requires developing a parameterization

LWP at ARM sites is sampled frequently so that the changes in slope may allow aerosol effects to be

quantified

Dong, 2002

Changes in optical depth at ARM sites are consistent with the measured surface aerosol concentrations

causing changes to droplet number

Penner et al., 2004

Correlation between Na and Nc (particles/cm2) maybe used to estimate the first indirect effect

of -0.5 to -1.5 Wm-2:

Nakajima, Penner et al., 2001

The second indirect effect (deeper clouds and larger coverage) is due to changes in precipitation efficiency.

Correlation between LWP and Na is relatively flat which may indicate that the 2nd indirect effect is small:

Nakajima, Penner et al., 2001

Aerosol forcing determines climate sensitivity and future temperatures

Penner et al., 2010

SOA

By decreases in short-lived species we can lower future temperatures

Penner et al, 2010

SOA

Issue for the future: Effects of climateon biosphere Penner et al, 2010

SOA production by biosphere may lead to additional cooling

Organic aerosol comprises a large fraction of submicron aerosol (Zhang et al., 2007)

Sulfate: red; nitrate: blue; ammonium: orange; organics: green

On average 45% of total mass is organics, most is SOAThe fraction may be even higher in the future 22

Global chemical transport model (IMPACT)

• Explicit gas phase chemistry• Basic photochemistry of O3, OH, NOx and VOCs (Ito et al.,

2007).• Epoxide formation from isoprene (Paulot et al., 2009).• HOx regeneration through isoprene oxidation proposed by

Peeters et al. (2009) but with the recycling rate reducedby a factor of 10 (Karl et al. 2009).

• SOA formed from gas-particle partitioning of semi-volatiles (Pankow 1994). For example,

23sv_SOAlv_SOA

1 day (Paulsen et al., 2006)

Gas-particle partitioning

Lin, Penner et al., 2014

SOA formation mechanisms

24

SOA formed from the cloud processing of glyoxal and methylglyoxal

Lin, Penner et al., 2014

SOA formation mechanisms

25

SOA formed from the reactive uptake of glyoxal, methylglyoxaland epoxide onto sulfate aerosol

γ: reactive uptake parameter.A: surface area of aqueous sulfate aerosolsMajor products: oligomers and organosulfate

Lin, Penner et al., 2014

Present-day SOA and POA column burden

26

• SOA source: 60.4 Tg/yr Biogenic SOA: 93%

• SOA burden: 1.0 Tg SOA formed from gas-particle

partitioning: 60% SOA formed from glyoxal and

methylglyoxal: 16% SOA formed from epoxide: 24%

• POA source: 63 Tg/yr• Fossil fuel/biofuel POA:

25%• POA burden: 0.77 Tg

Lin, Penner et al., 2014

Change in the SOA/POA column burden between PD and PI

Global POA burden increase by 0.51 Tg (196%) Global SOA burden increase by 0.4 Tg (68%)

o SOA formed from gas-particle partitioning: 45%o SOA formed from glyoxal and methylglyoxal: 25%o SOA formed from epoxide: 30%

Biogenic SOA source increases by 80%

Lin, Penner et al., 2016

Effect of 2100 climate and CO2 change on SOA +25%

Isoprene emissions

(µg/m3)

SOA surfaceconcentration

Lin, Penner, Zhou, 2106(µgC/m2/h)

Most of SOA is associated with accumulation mode sulfate

Radiative forcing: Reduced by about

½ when include prognostic size of

SOA.

Future cooling potential is -0.07

Wm-2

SOA on nucleation mode sulfate:2.1% SOA on Aitken mode sulfate: 14%

SOA on accumulation mode sulfate: 50% SOA on fossil fuel carbon: 18%

SOA on biomass burning: 15% Other SOA: 1%

Zhu and Penner, in preparation2017

So where do we stand in estimating current forcing? IPCC (2013) direct effects of anthropogenic aerosols:

-0.35 W/m2

-0.4 W/m2

Results do NOT account for size-distributed SOA! IPCC, 2013

Indirect effects IPCC (2013)

Aerosol radiative forcing is still the largest uncertainty

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Need to follow a strategy to examine importance of second indirect effect• Adopt GEWEX strategy to improve clouds in GCM’s adding

aerosol/cloud interactions:

Will this strategy allow us to quantify the 2nd indirect effect?

Zhou, Penner et al., 2017

Summary• Aerosol direct and indirect forcings remain the

largest uncertainty in current forcing for climate but we are making progress in improved representations, especially for direct forcing

• The indirect forcing uncertainty needs to be narrowed in order to use historical temperature records to estimate climate sensitivity and future temperatures

• With better observations and increased computer power we can reach the goal of decreasing uncertainty to an acceptable amount

Climate change and aerosols: How much do aerosols mask global warming?Aerosols scatter and absorb radiationAbsorption can decrease the total cloud amount (semi-direct effect)The first indirect effect of aerosols is due to changes in cloud droplet concentrations (Twomey effect)The second indirect effect is due to changes in precipitation efficiency.Addition of aerosol forcing results in temperature response unlike the pattern of sulfate aerosols forcing (Taylor and Penner, 1994)Slide Number 7Recognition that anthropogenic source types include organic and black carbon, and smoke, nitrates, ammonium, dust3-D model study of carbonaceous aerosols aerosols (Liousse, Penner et al., 1996)Direct forcing from biomass and fossil fuel carbonBiomass burning BC at high altitude causes cooling of surface, whereas fossil fuel causes warmingThe first indirect effect of aerosols is due to changes in cloud droplet concentrations (Twomey effect)Indirect forcing requires developing a parameterizationLWP at ARM sites is sampled frequently so that the changes in slope may allow aerosol effects to be quantified Changes in optical depth at ARM sites are consistent with the measured surface aerosol concentrations causing changes to droplet numberCorrelation between Na and Nc (particles/cm2) may�be used to estimate the first indirect effect �of -0.5 to -1.5 Wm-2:The second indirect effect (deeper clouds and larger coverage) is due to changes in precipitation efficiency.Correlation between LWP and Na is relatively flat which may indicate that the 2nd indirect effect is small:�Aerosol forcing determines climate sensitivity and future temperaturesSlide Number 20Slide Number 21Organic aerosol comprises a large fraction of submicron aerosol (Zhang et al., 2007)Global chemical transport model (IMPACT) SOA formation mechanismsSOA formation mechanismsPresent-day SOA and POA column burdenChange in the SOA/POA column burden between PD and PIEffect of 2100 climate and CO2 change on SOA +25%Most of SOA is associated with accumulation mode sulfate��Radiative forcing: Reduced by about ½ when include prognostic size of SOA.��Future cooling potential is -0.07 Wm-2So where do we stand in estimating current forcing? �IPCC (2013) direct effects of anthropogenic aerosols:Indirect effects IPCC (2013)Aerosol radiative forcing is still the largest uncertaintyNeed to follow a strategy to examine importance of second indirect effectSummary