Theodore L. Anderson

Calibration and intercomparison of filter-based measurements of visible light absorption by aerosols

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 filter on which particles are collected. We present a calibration of a recently developed filter-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 filter-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 find that each of the instruments exhibits a significant response to nonabsorbing aerosols and overestimates absorption at...

Determining Aerosol Radiative Properties Using the TSI 3563 Integrating Nephelometer

ABSTRACT 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 CO2. 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 at about 1 μm 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...

Transport of Asian air pollution to North America

Using observations from the Cheeka Peak Observatory in northwestern Washington State during March-April, 1997, we show that Asian anthropogenic emissions significantly impact the concentrations of a large number of atmospheric species in the air arriving to North America during spring. Isentropic back-trajectories can be used to identify possible times when this impact will be felt, however trajectories alone are not sufficient to indicate the presence of Asian pollutants. Detailed chemical and meteorological data from one of these periods (March 29th, 1997) indicates that the surface emissions were lifted into the free troposphere over Asia and then transported to North America in ∼6 days.

Mesoscale Variations of Tropospheric Aerosols

Abstract Tropospheric aerosols are calculated to cause global-scale changes in the earths heat balance, but these forcings are space/time integrals over highly variable quantities. Accurate quantification of these forcings will require an unprecedented synergy among satellite, airborne, and surface-based observations, as well as models. This study considers one aspect of achieving this synergy—the need to treat aerosol variability in a consistent and realistic way. This need creates a requirement to rationalize the differences in spatiotemporal resolution and coverage among the various observational and modeling approaches. It is shown, based on aerosol optical data from diverse regions, that mesoscale variability (specifically, for horizontal scales of 40–400 km and temporal scales of 2–48 h) is a common and perhaps universal feature of lower-tropospheric aerosol light extinction. Such variation is below the traditional synoptic or “airmass” scale (where the aerosol is often assumed to be essentially ho...

General circulation model assessment of the sensitivity of direct climate forcing by anthropogenic sulfate aerosols to aerosol size and chemistry

Climate response to atmospheric changes brought about by human activity may depend strongly on the geographical and temporal pattern of radiative forcing [Taylor and Penner, 1994]. In the case of aerosols stemming from anthropogenic sulfur emissions, geographical and temporal variations are certainly caused by variations in local mass concentration [Charlson et el., 1991; Kiehl and Briegleb, 1993], but could also arise from variations in the optical properties of sulfate aerosols. Since optical properties (including their relative humidity (RH) variation) depend fundamentally on aerosol size and chemical form and since size and chemical form are features of the aerosol which are not likely to be modeled on the global scale in the near future, geographical and temporal variations in optical properties could represent a stumbling block to accurate climate change forecasts. While extensive measurements of aerosol optical properties are needed to fully assess this problem, a preliminary assessment can be gained by considering the sensitivity of climate forcing to realistic variations in sulfate aerosol size and chemical form. Within a plausible set of assumptions (sulfate aerosol resides in the accumulation mode size range and only interacts with water vapor and ammonia vapor), we show that this sensitivity is fairly small (±20%). This low sensitivity derives from a number of compensating factors linking the three optical parameters identified by Charlson et al. [1991]. By implication, these optical parameters, low RH scattering efficiency, the ratio of hemispheric backscatter to total scatter, and the RH dependence of scattering efficiency, should not be treated independently in either theoretical or experimental investigations of direct climate forcing. A suggested logical focus for such investigations is the backscatter efficiency at high RH. If borne out by future research, low sensitivity to sulfate aerosol size and chemistry would mean that direct sulfate climate forcing can be incorporated in global climate models with only a knowledge of sulfate mass concentration. We emphasize, therefore, the need to study the extent to which our assumptions break down, in particular, the fraction of anthropogenic sulfate that forms on coarse mode particles (i.e., those with diameters >1 μm) and the extent and effects of sulfate interactions with other accumulation mode components. Finally, we find that a significant fraction of direct aerosol forcing occurs in cloud-covered regions, according to a simple bulk parameterization.

An "A-Train" Strategy for Quantifying Direct Climate Forcing by Anthropogenic Aerosols

Abstract This document outlines a practical strategy for achieving an observationally based quantification of direct climate forcing by anthropogenic aerosols. The strategy involves a four-step program for shifting the current assumption-laden estimates to an increasingly empirical basis using satellite observations coordinated with suborbital remote and in situ measurements and with chemical transport models. Conceptually, the problem is framed as a need for complete global mapping of four parameters: clear-sky aerosol optical depth δ, radiative efficiency per unit optical depth E, fine-mode fraction of optical depth ff, and the anthropogenic fraction of the fine mode faf. The first three parameters can be retrieved from satellites, but correlative, suborbital measurements are required for quantifying the aerosol properties that control E, for validating the retrieval of ff, and for partitioning fine-mode δ between natural and anthropogenic components. The satellite focus is on the “A-Train,” a constella...

On the climate forcing consequences of the albedo continuum between cloudy and clear air

It has been long understood that the partly cloudy atmosphere manifests a continuum of states between the end members ‘clear’ and ‘cloud.’ Nevertheless, many research methods are premised on a dichotomy of states—for example, those that use ‘cloud cover’ or ‘cloud-clearing.’ Here we consider the consequences of this practice for studies of aerosolclimate effects. Aerosols affect the Earth’s energy budget primarily by affecting albedo; therefore, we explore the nature of albedo variability in the partly cloudy marine boundary layer on scales down to a few tens of metres. We employ two diagnostic tools: a cloud resolving model and an albedo proxy derived from high altitude lidars. We show that a continuum of albedo values results from indeterminate and variable combinations of hydrated aerosol and wispy (including subvisible) clouds. Two consequences arise. First, cloud-clearing schemes employed by different observational methods are mutually inconsistent and are sensitive to concentrations of unactivated aerosol particles. Second, aerosol radiative forcing (the sensitivity of overall albedo to changes in aerosol concentration) is inaccurately calculated as the average of clear and overcast conditions. Together, these results imply that dividing the aerosol forcing problem into ‘direct’ and ‘indirect’ components may lead to substantial errors.

Observations of ozone and related species in the northeast Pacific during the PHOBEA campaigns: 1. Ground-based observations at Cheeka Peak

As part of the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) project, we made observations of CO, O3, NOx, peroxyacetyl nitrate (PAN), nonmethane hydrocarbons (NMHC), Rn, aerosol scattering, aerosol absorption, and aerosol number density during the springs of 1997 and 1998 at the Cheeka Peak Observatory (CPO) on the western tip of Washington State. The data have been segregated to quantify the mixing ratio of these species in the Pacific marine atmosphere. However, even in these marine air masses, there are occasionally substantial enhancements of NOx and aerosols, but not CO, which we attribute to diesel exhaust from ship traffic to and from major ports in the region. The marine air masses were further classified into four categories based on 10-day back isentropic trajectories; high, mid, and low latitude and those which had crossed over the Asian industrial region. Mean marine mixing ratios in 1998 were significantly higher than the 1997 values for CO (1997 mean equal to 151, 1998 mean equal to 170 ppbv), ethane (1771, 1968 parts per trillion by volume (pptv)), and ethyne (306, 452 pptv). Also, segregation of the 1998 data by air mass origin produced smaller differences in the mixing ratios for most species when comparing different source regions. We attribute both of these results to elevated emissions associated with unusually large areas of biomass burning which took place in Indonesia and Siberia dunng late 1997 and 1998. The relative enhancements of CO, ethane, ethyne, and propane we observed at CPO are consistent with enhanced biomass burning and industrial sources in the spring of 1998, relative to the spring of 1997.

An intercomparison of lidar-derived aerosol optical properties with airborne measurements near Tokyo during ACE-Asia

[1] During the ACE-Asia intensive observation period (IOP), an intercomparison experiment with ground-based lidars and aircraft observations was conducted near Tokyo. On 23 April 2001, four Mie backscatter lidars were simultaneously operated in the Tokyo region, while the National Center for Atmospheric Research C-130 aircraft flew a steppedascent profile between the surface and 6 km over Sagami Bay southwest of Tokyo. The C-130 observation package included a tracking Sun photometer and in situ packages measuring aerosol optical properties, aerosol size distribution, aerosol ionic composition, and SO2 concentration. The three polarization lidars suggested that the observed modest concentrations of Asian dust in the free troposphere extended up to an altitude of 8 km. We found a good agreement in the backscattering coefficient at 532 nm among lidars and in situ 180� backscatter nephelometer observations. The intercomparison indicated that the aerosol layer between 1.6 and 3.5 km was a remarkably stable and homogenous in mesoscale. We also found reasonable agreement between the aerosol extinction coefficients (sa � 0.03 km � 1 ) derived from the airborne tracking Sun photometer, in situ optical instruments, and those estimated from the lidars above the planetary boundary layer (PBL). We also found considerable vertical variation of the aerosol depolarization ratio (da) and a negative correlation between da and the backscattering coefficient (da) below 3.5 km. Airborne measurements of size-dependent optical parameters (e.g., the fine mode fraction of scattering) and of aerosol ionic compositions suggests that the mixing ratio of the accumulation-mode and coarse-mode (dust) aerosols was primarily responsible for the observed variation of da. Aerosol observations during the intercomparison period captured the following three types of layers in the atmosphere: a PBL (surface to 1.2–1.5 km) where fine (mainly sulfate) particles with a low da (<10%) dominated; an intermediate layer (between the top of the PBL and 3.5 km) where fine particles and dust particles were moderately externally mixed, giving moderate da; and an upper layer (above � 3.5 km) where dust dominated, giving a high da (30%). A substantial dust layer between 4.5 and 6.5 km was observed just west of Japan by the airborne instruments and found to have a lidar ratio of 50.4 ± 9.4 sr. This agrees well with nighttime Raman lidar measurements made later on this same dust layer as it passed over Tokyo, which found a lidar ratio of 46.5 ± 10.5 sr. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles

In situ measurement of the aerosol extinction-to-backscatter ratio at a polluted continental site

The extinction-to-backscatter ratio S is a crucial parameter for quantitative interpretation of lidar data, yet empirical knowledge of S for tropospheric aerosols is extremely limited. Here we review that knowledge and extend it using a recently developed in situ technique that employs a 180° backscatter nephelometer. This technique allows robust quantification of measurement uncertainties and permits correlations with other aerosol and meteorological properties to be explored. During 4 weeks of nearly continuous measurements in central Illinois, S was found to vary over a wide range, confirming previous indications that geographical location by itself is not necessarily a good predictor. The data suggest a modest dependence of S on relative humidity, but this explains only a small portion of the variation. Most variation was associated with changes between two dominant air mass types: rapid transport from the northwest and regional stagnation. The latter category displayed much higher aerosol concentrations and a systematically higher and more tightly constrained range of S. Averages and standard deviations were 64±4 sr for the stagnant category and 40±9 sr for the rapid transport category. Considering the 95% confidence precision uncertainty of the measurements, the difference between these averages is at least 13 sr and could be as large as 35 sr. The wavelength dependence of light scattering, as measured by a conventional nephelometer, is shown to have some discriminatory power with respect to S.