ABSTRACT. Data on light absorption by atmospheric particles are scarce relative to the need for global characterization. Most of the existing data come from methods that measure the change in light transmission through a lter on which particles are collected. We present a calibration of a recently developed lter-based instrument for continuous measurement of light absorption (model PSAP, Radiance Research, Seattle, WA) that has been incorporated in several measurement programs. This calibration uses a reference absorption determined as the difference between light extinction and light scattering by unaltered (suspended) particles. In addition, we perform the same calibration for two other common lter-based methods: an Integrating Plate and the Hybrid Integrating Plate System. For each method, we assess the responses to both particulate light scattering and particulate light absorption. We nd that each of the instruments exhibits a signi cant response to nonabsorbing aerosols and overestimates absorption at 550 nm by suspended particles by about 20-30%. We also present correction factors for the use of the PSAP.

Methods for reducing and quantifying the uncertainties in aerosol optical properties measured with the TSI 3563 integrating nephelometer are presented. For nearly all applications, the recommended calibration gases are air and CO,. By routinely characterizing the instrumental response to these gases, a diagnostic record of instrument performance can be created. This record can be used to improve measurement accuracy and quantify uncertainties due to instrumental noise and calibration drift. When measuring scattering by particles, size segregation upstream of the nephelometer a t about 1 p m aerodynamic diameter greatly increases the information content of the data for two reasons: one stemming from the independence of coarse and fine particles in the atmosphere, and the second stemming from the size dependence of the nephelometer response. For many applications (e.g., extinction budget studies) it is important to correct nephelometer data for the effects of angular nonidealities. Correction factors appropriate to a broad range of sampling conditions are given herein and are shown to be constrained by the wavelength dependence of light scattering, as measured by the nephelometer. Finally, the nephelometer measurement is nondestructive, such that the sampled aerosol can be further analyzed downstream. Data from two nephelometers operated in series are used to evaluate this procedure. A small loss of super-pm particles (5-10%) is found, while the sub-pm data demonstrates measurement reproducibility within k 1%.

Abstract. Aerosols affect the Earth's energy budget directly by scattering and absorbing radiation and indirectly by acting as cloud condensation nuclei and, thereby, affecting cloud properties. However, large uncertainties exist in current estimates of aerosol forcing because of incomplete knowledge concerning the distribution and the physical and chemical properties of aerosols as well as aerosol-cloud interactions. In recent years, a great deal of effort has gone into improving measurements and datasets. It is thus feasible to shift the estimates of aerosol forcing from largely model-based to increasingly measurement-based. Our goal is to assess current observational capabilities and identify uncertainties in the aerosol direct forcing through comparisons of different methods with independent sources of uncertainties. Here we assess the aerosol optical depth (τ ), direct radiative effect (DRE) by natural and anthropogenic aerosols, and direct climate forcing (DCF) by anthropogenic aerosols, focusing on satellite and ground-based measurements supplemented by global chemical transport modelCorrespondence to: H. Yu (hyu@climate.gsfc.nasa.gov) (CTM) simulations. The multi-spectral MODIS measures global distributions of aerosol optical depth (τ ) on a daily scale, with a high accuracy of ±0.03±0.05τ over ocean. The annual average τ is about 0.14 over global ocean, of which about 21%±7% is contributed by human activities, as estimated by MODIS fine-mode fraction. The multiangle MISR derives an annual average AOD of 0.23 over global land with an uncertainty of ∼20% or ±0.05. These high-accuracy aerosol products and broadband flux measurements from CERES make it feasible to obtain observational constraints for the aerosol direct effect, especially over global the ocean. A number of measurement-based approaches estimate the clear-sky DRE (on solar radiation) at the top-of-atmosphere (TOA) to be about −5.5±0.2 Wm −2 (median ± standard error from various methods) over the global ocean. Accounting for thin cirrus contamination of the satellite derived aerosol field will reduce the TOA DRE to −5.0 Wm −2 . Because of a lack of measurements of aerosol absorption and difficulty in characterizing land surface reflection, estimates of DRE over land and at the ocean surface are currently realized through a combination of satellite Published by Copernicus GmbH on behalf of the European Geosciences Union. 614H. Yu et al.: Measurement-based aerosol direct forcing retrievals, surface measurements, and model simulations, and are less constrained. Over the oceans the surface DRE is estimated to be −8.8±0.7 Wm −2 . Over land, an integration of satellite retrievals and model simulations derives a DRE of −4.9±0.7 Wm −2 and −11.8±1.9 Wm −2 at the TOA and surface, respectively. CTM simulations derive a wide range of DRE estimates that on average are smaller than the measurement-based DRE by about 30-40%, even after accounting for thin cirrus and cloud contamination.A number of issues remain. Current estimates of the aerosol direct effect ...

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[1] During Transport and Chemical Evolution over the Pacific (TRACE-P) and Asian Aerosol Characterization Experiment (ACE-Asia) we measured the dry size distribution of Asian aerosols, their state of mixing, and the optical properties of dust, black carbon (BC) and other aerosol constituents in combustion and/or dust plumes. Optical particle sizing in association with thermal heating extracted volatile components and resolved sizes for dust and refractory soot that usually dominated light absorption. BC was internally mixed with volatile aerosol in $85% of accumulation mode particles and constituted $5-15% of their mass. These optically effective sizes constrained the soot and dust size distributions and the imaginary part of the dust refractive index, k, to 0.0006 ± 0.0001. This implies a single-scatter albedo, v (550 nm), for dust ranging from 0.99+ for D p < 1 mm to $0.90 at D p = 10 mm and a size-integrated campaign average near 0.97 ± 0.01. The typical mass scattering efficiency for the dust was $0.3 m 2 g À1 , and the mass absorption efficiency (MAE) was 0.009 m 2 g À1 . Less dust south of 25°N and stronger biomass burning signatures resulted in lower values for v of $0.82 in plumes aloft. Chemically inferred elemental carbon was moderately correlated with BC light absorption (R 2 = 0.40), while refractory soot volume between 0.1 and 0.5 mm was highly correlated (R 2 = 0.79) with absorption. However, both approaches yield an MAE for BC mixtures of $7 ± 2 m 2 g À1 and higher than calculated MAE values for BC of 5 m 2 g À1 . The increase in the mass fraction of soot and BC in pollution aerosol in the presence of elevated dust appears to be due to uptake of the volatile components onto the coarse dust. This predictably lowered v for the accumulation mode from 0.84 in typical pollution to $0.74 in high-dust events. A chemical transport model revealed good agreement between model and observed BC absorption for most of SE Asia and in biomass plumes but underestimated BC for combustion sources north of 25°N by a factor of $3.