Chun Zhao, L. Ruby Leung, Richard EasterPacific Northwest National Laboratory, Richland, WA, USA

Jenny HandColorado State University, Fort Collins, CO, USA

Jeremy AviseCalifornia Air Resources Board, CA, USA

13th WRF Users' Workshop Boulder, Colorado, June 28, 2012

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Characterization of speciated aerosol direct radiative forcing over California

Aerosol impact over California

California is one of the most polluted regions in the world,

with air quality that is likely affecting well-being of people.

Air pollution control reduced aerosol concentrations, which

has potential to cause an increase in solar radiation and

weaken the aerosol effect of regional climate change

mitigation.

Understanding the seasonal variation and speciation of

aerosol and its direct radiative forcing over California is

important to provide further information as guidance for future

emission control strategies.

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WRF-Chem MADE/SORGAM aerosol scheme (Modal) coupled with

GOCART dust emission scheme [Ginoux et al., 2001, Zhao et

al., 2010].

Aerosol SW and LW direct radiative effects coupled with RRTMG radiation schemes [Zhao et al., 2011].

Morrison microphysics scheme and Grell convective scheme

12km horizontal resolution; simulation for 2005; driven by NARR reanalysis

ARCTAS-CARB anthropogenic emission inventory for June 2008; GFEDv3 biomass burning emission inventory for 2005

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Aerosols in the model are assumed internally mixed.

A methodology is developed to diagnose the optical depth and

direct radiative forcing of individual aerosol species:

AOD[species i] = AOD[all-species] – AOD[without species i]

Forcing[species i] = Forcing[all-species] – Forcing[without species i]

Optical properties and direct radiative forcing for OM, EC, dust,

sulfate, and a single group lumping all other aerosol species

are estimated.

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Spatial correlation coefficient and mean bias: 2m T: 0.90-0.94 with mean bias < 2 oC2m RH: 0.7~0.9 with mean bias < ~0.1Rain: 0.5-0.8 with mean bias < ~10 mm/monthSolar radiation: 0.98

EPA (urban): triangles (100)

IMPROVE (rural): circles (40)

High total PM2.5 over the

Central Valley (CV) and the

Los Angeles metropolitan

regions (LA)Seasonality with highest in

winter and lowest in

summer Model captures the spatial

distribution and seasonality;

and high PM2.5

concentrations over

southeastern California (due

to dust)

Low emission?Resolution?

Low emission?Resolution?SOA?

Doubled EC

Photochemistry Gas-aerosol partition

High bias nearby the source regionResolution?

High bias nearby the coastal areasEmission scheme? Measurements?

2008 for 2005?

Improve speciation profiles

High AOD and

AAOD over the CV,

LA, and the deserts

Higher AOD/AAOD

over the CV and the

LA than over the

deserts

More distinct

seasonality of

AOD/AAOD over

the CV and the LA

than over the

deserts

ECx2 simulation

Differences between the diagnosed and simulated AOD and AAOD:

non-linear interactions among the internal-mixed aerosol species

AOD: winter maximum and fall minimum, determined by that of

anthropogenic aerosols. Sulfate AOD is largest. The AOD for EC and

OM is small, but may have low biases.

AAOD: summer maximum and winter minimum, determined by EC and

dust. Aerosol species other than EC and dust also enhance the

absorption and account for 15-20% of AAOD.

Non-linear interactions

TOA: most aerosols-

negative radiative

forcing, except EC

Atmosphere: EC and

dust are biggest

contributors (~90%) to

warming; Internal

mixing enhances

warming.

Surface: all aerosols

have a cooling effect.

EC is the largest

contributor in summer,

while sulfate is in

winter.

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Summary WRF-Chem captures the observed seasonal meteorological

conditions over California.

WRF-Chem reproduces the observed spatial and seasonal distribution of most aerosol species, except underestimating the surface concentrations of OM and EC, potentially due to uncertainties or/and interannual variabilities of the anthropogenic emissions of OM and EC and the outdated SOA mechanism. A sensitivity simulation with anthropogenic EC emission doubled significantly reduces the model low bias of EC.

The seasonality of aerosol surface concentration is mainly determined by vertical turbulent mixing, ventilation, and photochemical activity, with distinct characteristics for individual aerosol species and between urban and rural areas.

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Summary Anthropogenic aerosols dominate the aerosol optical depth (AOD).

The ratio of AOD to AAOD shows distinct seasonality with a winter

maximum and a summer minimum.

On statewide average over California, aerosol reduces the seasonal-

average surface radiation fluxes by about 3 W m-2 with a maximum

of 10 W m-2 in summer. In the atmosphere, aerosol introduces a

warming effect of about 2 W m-2 with a maximum of 10 W m-2 also in

summer, with EC and dust as the main contributors (about 90%). At

the TOA, the overall aerosol direct radiative effect is cooling with a

maximum of -3.5 W m-2. EC contributes exclusively to the TOA

warming of up to about 0.7 W m-2.

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Extra

Impact of BC emission control (1980’s~2000’s)

Clear Sky All Sky

Solar Radiation at Surface

Impact of BC emission control (1980’s~2000’s)

Surface Temperature Atmospheric Heating

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