Beat Schmid

The Reno Aerosol Optics Study: An Evaluation of Aerosol Absorption Measurement Methods

The Reno Aerosol Optics Study (RAOS) was designed and conducted to compare the performance of many existing and new instruments for the in situ measurement of aerosol optical properties with a focus on the determination of aerosol light absorption. For this study, simple test aerosols of black and white particles were generated and combined in external mixtures under low relative humidity conditions and delivered to each measurement system. The aerosol mixing and delivery system was constantly monitored using particle counters and nephelometers to ensure that the same aerosol number concentration and amount reached the different instruments. The aerosol light-scattering measurements of four different nephelometers were compared, while the measurements of seven light-absorption instruments (5 filter based, 2 photoacoustic) were evaluated. Four methods for determining the aerosol light-extinction coefficient (3 cavity ring-down instruments and 1 folded-path optical extinction cell) were also included in the comparisons. An emphasis was placed on determining the representativeness of the filter-based light absorption methods, since these are used widely and because major corrections to the raw attenuation measurements are known to be required. The extinction measurement from the optical extinction cell was compared with the scattering measurement from a high-sensitivity integrating nephelometer on fine, nonabsorbing ammonium sulfate aerosols, and the two were found to agree closely (within 1% for blue and green wavelengths and 2% for red). The wavelength dependence of light absorption for small kerosene and diesel soot particles was found to be very near λ− 1, the theoretical small-particle limit. Larger, irregularly shaped graphite particles showed widely variable wavelength dependencies over several graphite runs. The light-absorption efficiency at a wavelength of 530 nm for pure kerosene soot with a number size distribution peak near 0.3 μ m diameter was found to be 7.5 ± 1.2 m2 g− 1. The two most fundamental independent absorption methods used in this study were photoacoustic absorption and the difference between suspended-state light extinction and scattering, and these showed excellent agreement (typically within a few percent) on mixed black/white aerosols, with the photoacoustic measurement generally slightly lower. Excellent agreement was also observed between some filter-based light-absorption measurements and the RAOS reference absorption method. For atmospherically relevant levels of the aerosol light-absorption coefficient (< 25 Mm− 1), the particle soot absorption photometer (PSAP) absorption measurement at mid-visible wavelengths agreed with the reference absorption measurement to within ∼ 11% for experiment tests on externally mixed kerosene soot and ammonium sulfate. At higher absorption levels (characterized by lower single-scattering albedo aerosol tests), this agreement worsened considerably, most likely due to an inadequate filter loading correction used for the PSAP. The PSAP manufacturers filter loading correction appears to do an adequate job of correcting the PSAP absorption measurement at aerosol single-scattering albedos above 0.80–0.85, which represents most atmospheric aerosols, but it does a progressively worse job at lower single-scattering albedos. A new filter-based light-absorption photometer was also evaluated in RAOS, the multiangle absorption photometer (MAAP), which uses a two-stream radiative transfer model to determine the filter and aerosol scattering effects for a better calculation of the absorption coefficient. The MAAP absorption measurements agreed with the reference absorption measurements closely (linear regression slope of ∼ 0.99) for all experimental tests on externally mixed kerosene soot and ammonium sulfate.

Comparison of methods for deriving aerosol asymmetry parameter

Received 21 December 2004; revised 19 March 2005; accepted 7 June 2005; published 21 January 2006. [1] Values for Mie-equivalent aerosol asymmetry parameter (g) were derived using a variety of methods from the large suite of measurements (in situ and remote from surface and aircraft) made in Oklahoma during the 2003 aerosol Intensive Operations Period (IOP). Median values derived for dry asymmetry parameter at 550 nm ranged between 0.55 and 0.63 over all instruments and for all derivation methods, with the exception of one instrument which did not measure over the full size range of optically important aerosol. Median values for the ‘‘wet’’ asymmetry parameter (i.e., asymmetry parameter at humidity conditions closer to ambient) were between 0.59 and 0.72. Values for g derived for surface and airborne in situ measurements were highly correlated, but in situ and remote sensing measurements both at the surface and aloft did not agree as well because of vertical inhomogeneity of the aerosol. Radiative forcing calculations suggest that a 10% decrease in g would result in a 19% reduction in top of atmosphere radiative forcing for the conditions observed during the IOP. Comparison of the different methods for deriving g suggests that in computing the asymmetry parameter, aerosol size is the most important parameter to measure; composition is less important except for how it influences the hygroscopic growth (i.e., size) of particles.

Comparison of Sun photometer calibration by use of the Langley technique and the standard lamp

Asix-channel Sun photometer has been calibrated by means of two different methods: Langley plots and standard irradiance lamps. A 4-month calibration campaign was carried out at a high mountain site, Jungfraujoch (3580 m above sea level), in the Swiss Alps. Calibration constants V(0)(λ) determined on clear and stable days by means of a refined Langley-plot technique scatter by less than 0.25% (rms) for wavelengths outside of strong gaseous absorption bands. Inside the 0.94-µm water-vapor absorption band, the V(0)(λ) values retrieved by means of modified Langley plots scatter by 1.0% (rms). Repeated calibrations of the Sun photometer by means of irradiance standard lamps were performed at the World Radiation Center in Davos. The comparison of both methods ranges from perfect agreement to a deviation of 4.9% for the different channels. A discussion of the errors introduced by both methods shows that the Langley-plot calibration, when performed under very clear atmospheric conditions, is superior. However, by means of the standard-lamp calibrations a temporal degradation of the instruments response up to 4.6% per year was found, implying that a single calibration campaign as done here is not sufficient. Thus we recommend the use of a combination of both methods for maintaining an accurate calibration.

The ARM program's water vapor intensive observation periods - Overview, initial accomplishments, and future challenges

A series of water vapor intensive observation periods (WVIOPs) were conducted at the Atmospheric Radiation Measurement (ARM) site in Oklahoma between 1996 and 2000. The goals of these WVIOPs are to characterize the accuracy of the operational water vapor observations and to develop techniques to improve the accuracy of these measurements. The initial focus of these experiments was on the lower atmosphere, for which the goal is an absolute accuracy of better than 2% in total column water vapor, corresponding to ~1 W m−2 of infrared radiation at the surface. To complement the operational water vapor instruments during the WVIOPs, additional instrumentation including a scanning Raman lidar, microwave radiometers, chilled-mirror hygrometers, a differential absorption lidar, and ground-based solar radiometers were deployed at the ARM site. The unique datasets from the 1996, 1997, and 1999 experiments have led to many results, including the discovery and characterization of a large (> 25%) sonde-to-sonde variab...

Influence of Humidity On the Aerosol Scattering Coefficient and Its Effect on the Upwelling Radiance During ACE-2

Aerosol scattering coefficients (σsp) have been measured over the ocean at different relative humidities (RH) as a function of altitude in the region surrounding the Canary Islands during the Second Aerosol Characterization Experiment (ACE-2) in June and July 1997. The data were collected by the University of Washington passive humidigraph (UWPH) mounted on the Pelican research aircraft. Concurrently, particle size distributions, absorption coefficients and aerosol optical depth were measured throughout 17 flights. A parameterization of σsp as a function of RH was utilized to assess the impact of aerosol hydration on the upwelling radiance (normalized to the solar constant and cosine of zenith angle). The top of the atmosphere radiance signal was simulated at wavelengths corresponding to visible and near-infrared bands of the EOS-AM )“Terra” (detectors, MODIS and MISR. The UWPH measured σsp at 2 RHs, one below and the other above ambient conditions. Ambient σsp was obtained by interpolation of these 2 measurements. The data were stratified in terms of 3 types of aerosols: Saharan dust, clean marine (marine boundary layer background) and polluted marine aerosols (i.e., 2- or 1-day old polluted aerosols advected from Europe). An empirical relation for the dependence of σsp on RH, defined by σsp(RH)=k. (1−RH/100)−γ, was used with the hygroscopic exponent γ derived from the data. The following γ values were obtained for the 3 aerosol types: γ(dust)=0.23±0.05, γ(clean marine)= 0.69±0.06 and γ(polluted marine)=0.57±0.06. Based on the measured γs, the above equation was utilized to derive aerosol models with different hygroscopicities. The satellite simulation signal code 6S was used to compute the upwelling radiance corresponding to each of those aerosol models at several ambient humidities. For the pre-launch estimated precision of the sensors and the assumed viewing geometry of the instrument, the simulations suggest that the spectral and angular dependence of the reflectance measured by MISR is not sufficient to distinguish aerosol models with various different combinations of values for dry composition, γ and ambient RH. A similar behavior is observed for MODIS at visible wavelengths. However, the 2100 nm band of MODIS appears to be able to differentiate between at least same aerosol models with different aerosol hygroscopicity given the MODIS calibration error requirements. This result suggests the possibility of retrieval of aerosol hygroscopicity by MODIS.

Coordinated Airborne, Spaceborne, and Ground-Based Measurements of Massive, Thick Aerosol Layers During the Dry Season in Southern Africa

During the dry season airborne campaign of the Southern African Regional Science Initiative (SAFARI 2000), coordinated observations were made of massive thick aerosol layers. These layers were often dominated by aerosols from biomass burning. We report on airborne Sun photometer measurements of aerosol optical depth (λ = 0.354-1.557 μm), columnar water vapor, and vertical profiles of aerosol extinction and water vapor density that were obtained aboard the University of Washingtons Convair-580 research aircraft. We compare these with ground-based AERONET Sun/sky radiometer results, with ground based lidar data (MPL-Net), and with measurements from a downward pointing lidar aboard the high-flying NASA ER-2 aircraft. Finally, we show comparisons between aerosol optical depths from the Sun photometer and those retrieved over land and over water using four spaceborne sensors (TOMS, MODIS, MISR, and ATSR-2).

Comparison of Columnar Water-Vapor Measurements from Solar Transmittance Methods

In the fall of 1997 the Atmospheric Radiation Measurement program conducted a study of water-vapor-abundance-measurement at its southern Great Plains site. The large number of instruments included four solar radiometers to measure the columnar water vapor (CWV) by measuring solar transmittance in the 0.94-mum water-vapor absorption band. At first, no attempt was made to standardize our procedures to the same radiative transfer model and its underlying water-vapor spectroscopy. In the second round of comparison we used the same line-by-line code (which includes recently corrected H(2)O spectroscopy) to retrieve CWV from all four solar radiometers, thus decreasing the mean CWV by 8-13%. The remaining spread of 8% is an indication of the other-than-model uncertainties involved in the retrieval.

Direct aerosol forcing: Calculation from observables and sensitivities to inputs

[1] Understanding sources of uncertainty in aerosol direct radiative forcing (DRF), the difference in a given radiative flux component with and without aerosol, is essential to quantifying changes in Earth’s radiation budget. We examine the uncertainty in DRF owing to measurement uncertainty in the quantities on which it depends: aerosol optical depth, single scattering albedo, asymmetry parameter, solar geometry, and surface albedo. Direct radiative forcing at the top of the atmosphere and at the surface is calculated at three locations representing distinct aerosol types and radiative environments. Sensitivities, the changes in DRF in response to unit changes in individual aerosol or surface properties, are also calculated for these conditions. The uncertainty in DRF associated with a given property is computed as the product of the sensitivity and typical measurement uncertainty in the respective property. Sensitivity and uncertainty values permit estimation of total uncertainty in calculated DRF and identification of properties that most limit accuracy in estimating forcing. Absolute total uncertainties in modeled local diurnally averaged forcing range from 0.2 to 3.1 W m � 2 for the ranges of properties examined here. Relative total uncertainties range from � 20 to 80% with larger values at higher latitudes, where fluxes are low. The largest contributor to total uncertainty in DRF is single scattering albedo; however, decreasing measurement uncertainties for any property would increase accuracy in DRF. Comparison of two radiative transfer models suggests the contribution of modeling error is small compared to the total uncertainty although comparable to uncertainty arising from some individual properties.

In situ aerosol-size distributions and clear-column radiative closure during ACE-2

As part of the second Aerosol Characterization Experiment (ACE-2) during June and July of 1997, aerosol-size distributions were measured on board the CIRPAS Pelican aircraft through the use of a DMA and 2 OPCs. During the campaign, the boundary-layer aerosol typically possessed characteristics representative of a background marine aerosol or a continentally influenced aerosol, while the free-tropospheric aerosol was characterized by the presence or absence of a Saharan dust layer. A range of radiative closure comparisons were made using the data obtained during vertical profiles flown on 4 missions. Of particular interest here are the comparisons made between the optical properties as determined through the use of measured aerosol-size distributions and those measured directly by an airborne 14-wavelength sunphotometer and 3 nephelometers. Variations in the relative humidity associated with each of the direct measurements required consideration of the hygroscopic properties of the aerosol for size-distribution-based calculations. Simultaneous comparison with such a wide range of directly-measured optical parameters not only offers evidence of the validity of the physicochemical description of the aerosol when closure is achieved, but also provides insight into potential sources of error when some or all of the comparisons result in disagreement. Agreement between the derived and directly-measured optical properties varied for different measurements and for different cases. Averaged over the 4 case studies, the derived extinction coefficient at 525 nm exceeded that measured by the sunphotometer by 2.5% in the clean boundary layer, but underestimated measurements by 13% during pollution events. For measurements within the free troposphere, the mean derived extinction coefficient was 3.3% and 17% less than that measured by the sunphotometer during dusty and non-dusty conditions, respectively. Likewise, averaged discrepancies between the derived and measured scattering coefficient were −9.6%, +4.7%, +17%, and −41% for measurements within the clean boundary layer, polluted boundary layer, free troposphere with a dust layer, and free troposphere without a dust layer, respectively. Each of these quantities, as well as the majority of the >100 individual comparisons from which they were averaged, were within estimated uncertainties.

Comparison of aerosol single scattering albedos derived by diverse techniques in two North Atlantic experiments

Aerosol single scattering albedo v (the ratio of scattering to extinction) is important in determining aerosol climatic effects, in explaining relationships between calculated and measured radiative fluxes, and in retrieving aerosol optical depths from satellite radiances. Recently, two experiments in the North Atlantic region, the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) and the Second Aerosol Characterization Experiment (ACE-2), determined aerosol v by a variety of techniques. The techniques included fitting of calculated to measured radiative fluxes; retrievals of v from skylight radiances; best fits of complex refractive index to profiles of backscatter, extinction, and size distribution; and in situ measurements of scattering and absorption at the surface and aloft. Both TARFOX and ACE-2 found a fairly wide range of values for v at midvisible wavelengths (;550 nm), with 0.85 # vmidvis # 0.99 for the marine aerosol impacted by continental pollution. Frequency distributions of v could usually be approximated by lognormals in vmax 2 v, with some occurrence of bimodality, suggesting the influence of different aerosol sources or processing. In both TARFOX and ACE-2, closure tests between measured and calculated radiative fluxes yielded best-fit values of vmidvis of 0.90 6 0.04 for the polluted boundary layer. Although these results have the virtue of describing the column aerosol unperturbed by sampling, they are subject to questions about representativeness and other uncertainties (e.g., thermal offsets, unknown gas absorption). The other techniques gave larger values for vmidvis for the polluted boundary layer, with a typical result of vmidvis 5 0.95 6 0.04. Current uncertainties in v are large in terms of climate effects. More tests are needed of the consistency among different methods and of humidification effects on v.