A43E-01 13:40h
Particulate Organic Matter Downwind of the NE U.S. and its Role in Light Extinction and the Relative Humidity Dependence of Light Extinction during ICARTT
Measurements during the Tropospheric Aerosol Radiative Forcing Experiment (TARFOX) and the New England Air Quality Study (NEAQS 2002) revealed large mass fractions (18 to 80 percent) of particulate organic matter (POM) in the submicron aerosol downwind of the northeastern United States. This same result was observed during ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) which took place in the Gulf of Maine in July and August of 2004. A priority onboard the NOAA RV Ronald H. Brown during ICARTT was the characterization of the POM, its role in light extinction, and its effect on the relative humidity dependence of light extinction. Measurements were made of non-refractory organic matter (Aerodyne aerosol mass spectrometer), total organic carbon (TOC) (Sunset Labs thermo-optical carbon analyzer), water soluble organic carbon (WSOC) (Sievers Total Organic Carbon Analyzer), light scattering (nephelometer), absorption (particle soot absorption photometer), and extinction (Cavity Ring Down Extinction Cell), and the relative humidity dependence of light scattering and extinction, f(RH) (humidified nephelometer and CRD extinction cell). The ratio of WSOC to TOC covered a wide range from near 0 to near 1 with a mean value of 0.63 +/- 0.15. Based on a multiple linear regression, experiment-wide mass scattering efficiencies for non-sea salt sulfate and OC were 7.2 and 11 m2 g-1, respectively. In general, lower values of f(RH) were observed when organics dominated the submicron aerosol mass while higher values were observed when sulfate dominated the mass. These results will be discussed in the context of the prevailing meteorology and origin of the sampled air masses.
A43E-02 13:55h
Organic Aerosol Growth Mechanisms and their Climate Forcing Implications
Chemical reactions leave signatures in aerosol particle size distributions of organic functional group composition, from which we measured conversion rates of 13-24% per day. This rate varied little for both surface and volume-limited reactions on African mineral dust with primary organic, Asian black carbon, and American secondary organic aerosols, but all measured rates were a factor of three lower than the 60% per day typically used in climate models. The particle types observed reflected a range of altitudes from 85 to 2000 m above sea level, of natural and anthropogenic sources, and of reaction mechanisms all with very similar rates for converting from volatile or hydrophobic to condensed and hydrophilic organic compounds. These longer-lived, less-oxidized, hydrophobic organic particles will increase carbonaceous aerosol burdens in climate models by 70%.
A43E-03 14:10h
Acid-Catalyzed Organic Reactions Change the Optical Properties of Atmospheric Sulfuric Acid Aerosols
Unlike most environments present at Earth$'$s surface atmospheric aerosols can be favorable to organic reactions. We present series of laboratory experiments showing that the absorption index of sulfuric acid solutions exposed to gas-phase carbonyl compounds known to be present in the atmosphere (acetone, acetaldehyde, methyl ethyl ketone) increases dramatically in the near UV and visible range (190-1100 nm), where aerosols have a direct impact on Earth's radiative balance. Our results show that the absorption index of stratospheric sulfuric aerosols exposed to 100 pptV of acetaldehyde (1 pptV = 10$^{-12}$ v/v) would increase by four orders of magnitude over their lifetime. Rough estimates based on previous radiative calculations suggest that this reaction could result in an increase the radiative forcing of sulfate aerosols of the order of 0.01 W m$^{-2}$.
A43E-04 14:25h
Atmospheric Measurement of Optical Extinction by Aerosol Using the Cavity Ring-down Technique
Accurate measurement of the optical properties of aerosol is important for quantifying their influence on climate. Important parameters include extinction, functional dependence of light-extinction on relative humidity f(RH), angular scattering distribution, and single scattering albedo as a function of wavelength. We have developed a field system for the direct measurement of optical extinction by aerosol based on Cavity Ring-Down (CRD) technique. The system measures extinction and f(RH) at four wavelengths with excellent sensitivity and time response. Complementary measurement of scattering or absorption is required to determine the single scattering albedo. We will present results for optical extinction by atmospheric aerosols at 355 nm, 532 nm, 683 nm, and 1064 nm along with comparison to extinction determined by the sum of light scattering (nephelometer) and absorption (particle soot absorption photometer) measured in the marine boundary layer in the Gulf of Maine and vicinity during ICARTT/NEAQS (International Consortium for Atmospheric Research on Transport and Transformation) onboard the {\it NOAA RV Ronald H. Brown}.
A43E-05 14:40h
In-Situ Measurements of Aerosol Optical Properties using new Cavity Ring-Down -- Results From Two Recent Field Missions
Carbonaceous species (BC and OC) are responsible for most of the absorption associated with aerosol particles. The amount of radiant energy an aerosol absorbs has profound effects on climate and air quality. It is ironic that aerosol absorption coefficient is one of the most difficult aerosol properties to measure. A new cavity ring-down (CRD) instrument, called Cadenza (NASA-ARC), measures the aerosol extinction coefficient for 675 nm and 1550 nm light, and simultaneously measures the scattering coefficient at 675 nm. Absorption coefficient is obtained from the difference of measured extinction and scattering within the instrument. A study was recently conducted in the Caldecott Tunnel, a heavily-used tunnel located north of San Francisco, CA. The aerosol sampled in this study was characterized by fresh automobile and diesel exhaust. Measurements from Cadenza and from an aethalometer are presented. The aethalometer is a filter-based photometer and the near infrared channel is calibrated to produce a measure of BC mass loading. Cadenza also measured during a recent airborne campaign EVE (Extended Wavelength Modis Validation), based in Marina, California. The timing of this experiment is chosen to coincide with the maximum transport of Asian dust to the US West coast. Results and comparisons of Cadenza with other types of instrumentation will be presented.
A43E-06 14:55h
Variability of Physical and Optical Properties of Particles Emitted from Combustion of Wildland Fuels
Emissions from wildland fires make an important contribution to aerosol radiative forcing. The estimation of this forcing is limited not only by uncertain emission factors and activity levels but also by uncertain optical properties of the emitted particles. To better characterize the optical properties of particle emissions from wildland fuels, in-situ measurements of aerosol light absorption (532 and 1047 nm photoacoustic instrument), scattering (TSI, Radiance and DRI nephelometers), and extinction coefficients (cavity enhanced detection instrument) were taken on emissions from burning several wildland fuels under laboratory controlled conditions. Fuels used include Ponderosa pine needles, White pine needles, Ponderosa pine wood, Sagebrush, Poplar wood, grasses, and Tundra cores. Substantial variability was found in particle single scattering albedo as a function of fuel type, combustion conditions, and combustion stage. Ponderosa pine wood combustion produces the lowest single scattering albedo of 0.3 - 0.4 during the flaming phase, approximating the reported values for pure elemental carbon (EC); during the smoldering phase a single scattering albedo of $>$ 0.97 is measured. Fine particulate mass was measured by gravimetery and elemental carbon/organic carbon (OC) were determined by four different thermal/optical protocols from concurrent time-integrated filter samples. The observed single scattering albedo generally decreases with increasing EC/OC ratios for all analysis protocols, with the mass absorption efficiency of EC ranging from 3.7 to 14.2 m$^{2}$/g. An aerosol optical model using measured size distributions and commonly modeled refractive indexes and mixing schemes for OC and EC demonstrates agreements with the measured particle optical properties at various degrees. The natural and artificial variability in the measurement of wildland fire smoke lead to biases in the current radiative forcing estimates.
A43E-07 15:10h
Toward Resolution on the Optics of Light-Absorbing Carbon
Compendia of the absorption cross-section of atmospheric particles yield a wide range of values for light-absorbing carbon; similar anthologies drawn from the combustion literature support a wide range of refractive indices. While these values have been tabulated in previous reviews, an acceptable resolution of the reported variability has been missing. Here, we present the results of an exhaustive review of reported optical properties of LAC: inferred refractive indices and measured absorption efficiencies. In all cases, we have returned to the original source of the data, and we have frequently re-examined the measurements that led to the reported values in light of current theoretical understanding. Instead of simply rejecting values measured with differing techniques, we attempt to use each study to bound the relevant properties of LAC and thereby achieve consensus between studies. Our revised tabulation shows that values of refractive index and absorptive properties of light-absorbing carbon are not as different as is commonly believed, and that many of the apparent discrepancies result from variations in interpreting measurements. We identify the origins of two common values in the atmospheric science community: the refractive-index values recommended by OPAC, and the 10 m2/g value widely cited for the mass absorption efficiency of pure LAC. Neither value is taken from material representative of atmospheric LAC, and we provide new recommendations and discuss the implications for aerosol models. Next, we discuss the parameters needed to represent absorbing aerosol in climate models. In particular, internal versus external mixing has been cited as a cause of large uncertainty in such modeling. While the number of mixing states is effectively infinite, we identify a limited number of boundaries that could be used to make such a representation manageable. Finally, we examine estimates of direct climate forcing by carbonaceous aerosols. We believe there is still high uncertainty in these estimates that is not bounded by presently-published model results. Paradoxically, we report that some of the variability in published estimates of climate forcing results from basic assumptions, and that model results agree much better when adjusted for these considerations.
A43E-08 15:25h
Climate Response of Direct Radiative Forcing of Anthropogenic Black Carbon}
The equilibrium climate effect of direct radiative forcing of anthropogenic black carbon (BC) is examined by 100-year simulations in the Goddard Institute for Space Studies General Circulation Model II-prime with a mixed-layer ocean model. Anthropogenic BC is predicted to raise globally and annually averaged equilibrium surface air temperature by 0.20 K if BC is assumed to be externally mixed. The predicted increase is significantly greater in the Northern Hemisphere (0.29 K) than in the Southern Hemisphere (0.11 K). If BC is assumed to be internally mixed with the present-day level of sulfate aerosol, the predicted annual mean surface temperature increase rises to 0.37 K globally, 0.54 K for the Northern Hemisphere, and 0.20 K for the Southern Hemisphere. In the tropics and midlatitudes, the largest temperature increase is predicted to occur in the upper troposphere. Direct radiative forcing of anthropogenic BC is also predicted to lead to a change of precipitation patterns in the tropics; precipitation is predicted to increase between 0 and 20$\deg$N and decrease between 0 and 20$\deg$S. If BC is assumed to be internally mixed with sulfate instead of externally mixed, the change in precipitation pattern is enhanced. The change in precipitation pattern is not predicted to alter the global burden of BC significantly because that occurs predominantly in regions removed from BC sources.