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
32
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