# Presentation: 740953

## Measurements of theClimate-forcing Properties of  Atmospheric Aerosols

**John Ogren**
**NOAA Climate Monitoring and Diagnostics Laboratory**

## Aerosol Effects on Climate

- _Direct Effects_
- Aerosols scatter and absorb visible and infrared radiation
- Could cause warming or cooling
- Depends on size distribution and index of refraction of the particles, as well as albedo of the underlying surface

- _Indirect Effects_
- Cloud condensation nuclei (CCN) and ice nuclei (IN) are precursors for all cloud particles in the atmosphere
- CCN and IN influence cloud optical and microphysical properties
- Possible effects include changes in Earth's albedo and changes in hydrological cycle

## Estimates of Global Mean Radiative Forcing

***"The balance of evidence suggests a discernible human influence on global climate (IPCC, 1996)"***

**Confidence level**
**	****High****	****Low****	****Low****	****Low****	****Very****	****Very****	****Very****	****Very****	****	****	****	****	****low****	****low****	****low****	****low**

## Placeholder (4 slides)

**Kiehl and Briegleb, GHG forcing**
**Kiehl and Briegleb, sulfate forcing**
**Kiehl and Briegleb, GHG+sulfate forcing**
**Santer et al., spatial pattern matching**

## CAVEAT

**Aerosol cooling and greenhouse warming cannot offset each other, because ...**** **
**The effects occur at different times and in different parts of the atmosphere, ...**** **
**Leading to a change in the differential heating/cooling that drives the general circulation of the atmosphere.**** **

## Uncertainties Of Parameters Used To Estimate Direct Aerosol Forcing Of Climate

- Source: Penner et al. (1994)

## Scientific Questions

**What are the means, variabilities, and trends of the climate-forcing properties of different types of aerosols?**** **
**What chemical species are responsible for these properties?**** **
**What are the factors that control these properties?**

**Notes:**
Note change in focus since 1991, in response to global model predictions of sulfate distributions and corresponding forcings.
Change from viewing aerosols as indicators of global change (i.e., response) to agents of global change (i.e., forcing).
Define forcing:  an externally imposed perturbation to the radiative balance of the earth.  Implies need to separate anthropogenic and natural contributions.
GMCC sites too far away from anthropogenic aerosol source regions, so established regional sites in/near to source regions.

## Sampling Strategy forCMDL Aerosol Monitoring Program

**We can never achieve adequate global coverage with a ground-based sampling program.**
**Recognizing this, our strategy is to characterize key optical, chemical, and microphysical  properties of a number of aerosol types that are needed for development and validation of global models and satellite data retrieval algorithms.**

## Key Aerosol Types For Evaluating Climate Forcing By Anthropogenic Aerosols

## CMDL Aerosol Network

**South Pole**

**Samoa**

**Mauna Loa**

**Barrow**

**Hungary**

**Sable Is.**

**Bondville**

**So. Great Plains (ARM)**

**Notes:**
Yellow stars have modern equipment with size/RH control and 3 BS/TS nephelometers (to estimate ).
Notice lack of sites directed towards biomass burning or mineral dust aerosols.
DOE/ARM asked CMDL to mentor SGP AOS in Jan’97
DOE/ARM paid for BRW upgrade 10/97.

## Parameters controlling aerosol forcing

- D	daylight fraction
- S0	solar constant
- Tat	atmospheric 	transmission
- Ac	cloud fraction
- Rs	surface albedo

_**Parameters controlling aerosol forcing**_

**Source:  Haywood and Shine (1995)**

- F	average aerosol 	forcing at top of 	atmosphere (TOA)
**	aerosol optical depth

**Notes:**
I dislike using equations too, but sometimes they are the clearest way to illustrate relationships.
This eqn (or close variant) being used in current models of aerosol forcing, e.g., IPCC estimates.
This eqn represents an annual mean forcing at TOA, instantaneous forcings can be much larger.  Sfc. Forcings can also be much larger
CMDL is measuring the relevant aerosol terms in this eqn. Corresponding eqn for GHG much simpler, as radiative properties of all CO2 molecules are identical.  Not so for aerosols.

## Linking up-scatter optical depthto aerosol chemical composition

**Notes:**
All terms are -dependent
Different versions illustrate dependence on vertical dist., RH, chem. comp.
ASD & chemical composition control , , f.
Don’t forget there is an f(RH) for  in previous slide
CTM’s calculate mass concentration.  Other terms are crudely parameterized in models.
CMDL aerosol pgm directed towards characterizing these parameters and understanding the controlling processes.

## Aerosol climate-forcingproperties observed by CMDL

_**Routinely:**_
*********sp******d*****,  *********************0******d***
**wavelength dependence of ***********sp******d*****, *********** ***
**and at regional sites**
***b******d***** (surrogate for **********)**
***M***** (coarse/fine, total and ions)**
************mass and ions)**

_**Intensive campaigns:**_
***f************(RH), f******b ******(RH)***
**vertical profiles of ***********sp******d*****, ***********0******d*****, *****b******d*****, ********-dependence **** **
_**Missing**_
***f************(RH), f (RH,z)***
*********(z)***
***M *****(carbon, dust)**

**Notes:**
Note  depends on zenith angle, can’t measure directly.  So we measure a surrogate instead.  Both are integral properties of angular scattering function.
CMDL doesn’t do aerosol chemistry measurements.  Close coordination with PMEL for this at regional sites & BRW.
No coordinated aerosol sampling at other baseline sites, so can’t evaluate .
Limited lidar measurements of vertical profiles - MLO & SGP only.
We don’t have the resources to measure all of the important terms at all stations, all of the time.  So we must rely on intensive campaigns.

## KEY FEATURES OF CMDLAEROSOL SAMPLING SYSTEM

**Heated inlet, so that all size cuts and optical measurements are performed at a low relative humidity.  The sample is heated just enough to maintain the RH below 40% and the temperature below 40° C.**
**Measurements performed on two size-fractions, D****p**** < 1.0 ********m (fine particles) and 1 < D****p**** < 10 ********m (coarse particles) diameter.**
**Optical and chemical measurements are synchronized so that they can be related quantitatively.**
**Real-time contamination control using N****tot****, wind speed, and wind direction.**
**Highly-automated so that minimal operator attention is required.**

## NOAA Aerosol Sampling Inlet

**10 m agl inlet stack, 20 cm diameter, 50 cm/s flow velocity**
**2.4 m stainless steel tube, 5 cm diameter, 130 cm/s velocity, heated to RH ******** 40%**
**Split into 5 lines, 1.9 cm dia, 30 lpm each**
**10 ********m impactor, then switched 1 ********m impactor**
**Filtered pump exhaust**

- Barrow, Alaska

**Notes:**
Illustrate our experimental approach with photos from Barrow, which was upgraded in Oct’97.
Aerosol radiative properties are strongly dependent on RH, as most swell up when RH increases.
We control RH by gentle heating, to remove that confounding factor from our data set.
Strategy is to measure f(RH) separately.

## NOAA Aerosol Sampling System

**TSI integrating nephelometer (********sp****, ********bsp****, 3 wavelengths)**
**Radiance PSAP (********ap****, 550 nm)**
**Real-time contamination control using TSI CN counter and winds**
**Sample handling in glove box**

**Notes:**
PSAP is calibrated in terms of ap, aethalometer is not.
Leads to difficulty in interpreting old baseline data in terms of forcing.

## NOAA Aerosol Sampling Rack

**Switched impactor for D < 1 ********m and D < 10 ********m size ranges.**
**Large particle (1 < D < 10 ********m) impactor and 8-filter holder for submicrometer particles.**
**Gravimetric and IC analyses.**
**Automated CO****2**** span check system for nephelometer.**

**Notes:**
Aerosol radiative properties are strong functions of particle size, so we do separate measurements on fine & total fractions.  We find, for example, that the coarse (supermicrometer) particles have very little absorption compared to the fine particles.
Integrated chemical and physical measurement systems allow quantitative merging of the two data sets, e.g., for evaluating .

## Arctic Haze has Decreasedat Barrow, Alaska since the mid-1980’s

**Parameter plotted is atmospheric aerosol light scattering coefficient at 550 nm (1/Mm)**

**Notes:**
Trend up to 1991 published by Bodhaine & Dutton.  They attributed likely cause of 1982-1991 decrease to changes in emissions from FSU.
Subsequent flattening out of “trend” illustrates need to continue monitoring to follow up apparent trends.  Could it just be that 1982-1987 were anomalies?
Followup analysis being done as part of NOAA Arctic Research Initiative.
No other obvious trends at baseline stations..
Aerosol properties at baseline sites show clear seasonality, reflecting seasonal changes in sources and sinks.

## Aerosol single-scattering albedoat Bondville, Illinois

- Values are daily averages for =550 nm, RH<40%, D<1 m

## Aerosol Chemical Compositionat Bondville, Illinois (1995)

**Aerosol Chemical Composition**_**at Bondville, Illinois (1995)**_

- Speciation of “other” based on 4-weeks of data

**Notes:**
Initial aerosol forcing estimates were for sulfate, as it is the best understood species.
Sulfate is often the the single most important species, but other species can be of major importance.
Illustrates critical value of gravimetric mass measurement, to put measurements of individual species into context.
Need to do more OC/EC work!

## Sensitivity of aerosol forcing to observed variation in aerosol properties at Bondville 

- IPCC (1995) temperature change predictions used a single value and considered only scattering.

**forcing per unit optical depth (W m****-2****)**

**frequency**

**Notes:**
Assumed a particular shape of the angular distribution of scattered light to estimate upscatter fraction from backscatter fraction.
Values are for low-RH, sub-m particles only.
Yellow curve shows response of DF to observed variations in upscatter fraction alone.
Black curve shows additional variability, and shift in magnitude, when observed absorption is included as well.
This shows average forcing at top of atmosphere.  Instantaneous & surface forcings are much larger.
WSA polluted continental case looks the same, suggesting hope for parameterizations.

## 3-D Isentropic Airmass Trajectories

**Sector boundaries guided by SO****2**** emission field**
**Cases were included only if the trajectory stayed in a single sector for five days prior to arriving within 200 km of Sable Island**
**Data averaged for 2 hours, centered on trajectory arrival time**

**Marine**

**Polluted Continental**

**Clean Continental**

## Wavelength-dependence of aerosol scattering is controlled by coarse/fine mix

-   Daily averages from Sable Is., 8/92-6/97, 550/700 nm wavelengths

- Ångström exponent *(å)*

*f*

**Notes:**
Y-axis shows a wavelength dependence, X-axis shows a size dependence.
Clean-marine cases at lower left, polluted continental at upper right.
Super-m sea-salt particles control sp most of the time at WSA
This approach may aid interpretation of long-term record from SMO & BRW.
Ångström exponent is used by some as an index of the slope of the aerosol size distribution (e.g., by satellite retrievals).  These data suggest it is more appropriate to view it as the relative abundance of coarse vs. fine-mode particles.

## Aerosol Extensive Properties atSable Island by Trajectory Sector

**Light scattering (Mm****-1****)**

**Light absorption (Mm****-1****)**

## Aerosol Intensive Properties atSable Island by Trajectory Sector

**Aerosol Intensive Properties at**_**Sable Island by Trajectory Sector**_

**single-scattering albedo**

**hemispheric backscatter**

## Hygroscopic growth at Sable Island

- fsp(RH)

- fb(RH)

-  marine
-  polluted

-  polluted

- relative humidity

- relative humidity

**1. Chemical composition controls hygroscopic growth.**

**2. Backscatter fraction decreases with increasing RH.**

**Notes:**
Intensive experiment Sept’96
f(RH) also depends on size dist.
Our measured ASD would give greater f(RH) for polluted case
NoteF depends on product of left & right panes
Important to consider co-variance of different terms in eqn. 1.

## Aerosol Forcing Efficiency (W m-2)at Sable Island by Trajectory Sector

**Aerosol Forcing Efficiency (W m****-2****)**_**at Sable Island by Trajectory Sector**_

**absorption effect (D < 1 ********m)**

**size dependence**

## Calculation of aerosol opticaldepth from surface observations

**Calculation of aerosol optical**_**depth from surface observations**_

- measured with shadowband radiometer

- calculated from***sp*, *f(RH)*, lidar

*f****(RH)* = 1.5  0.2

- Measurements from Southern Great Plains site.  Lidar and radiometer data provided by DOE/ARM program.

**Notes:**
Need lidar to scale surface measurements
Need f(RH) to connect nephelometer and lidar measurements of aerosol scattering at the surface
Error bars show sensitivity to assumed range of f(RH) 1.3-1.7
So you can use surface observations to calculate .  Next step is to evaluate when surface measurements of other terms in forcing equation, such as  and , can be used to evaluate F.

## Surface aerosol properties in southeast USA may represent the column

- altitude (m agl)

- Results are for RH < 40%, D < 1 m.

**Notes:**
Measurements from NOAA P-3 with identical instruments at surface sites.
Piggybacked onto atmos.chem measurement program
Note huge decrease in sp above 2 km
Much less vertical variation in aerosol radiative properties.

## NOAA/CMDL Measurements of Single-scattering Albedo

**Notes:**
Synthesis of measurements from baseline stations, regional sites, and aircraft campaigns.
Submicrometer particles, low RH.  Values for total, ambient aerosol would be closer to 1.0
Boxes show upper/lower quartiles, bars show medians, lines extend to 5&95th percentiles
This variability induces large variability in forcing (eqn. 1)
Note similarity between BND and polluted continental at WSA
P3 measurements, especially above PBL, are very noisy due to limited sampling duration.

## Aerosol RadiativeProperties in Mexico City

**single-scattering albedo**

**light scattering coefficient (Mm****-1****)**

**Black bars denote night-time.  Observations were at a wavelength of 550 nm and relative humidity < 40%.**

## CMDL Aerosol Monitoring Highlights

**Observations of aerosol climate forcing properties reveal pronounced differences for different aerosol types, with variabilities that are large compared with the values used in current models.**
**More species than sulfate make significant contributions to aerosol radiative forcing.**
**Aerosol hygroscopic growth depends strongly on chemical composition.**
**Surface measurements can be representative of the vertical column.**

## Conclusions

******F/******** is a useful concept for combining different measurements and for comparing results from different cases or sites**
**Consideration of scattering only suggests that some models may be underestimating the sulfate forcing by about 30%**
**Aerosol radiative properties exhibit considerable variability, in both upscatter fraction and single-scattering albedo, which is not included in current models**