Gail P. Box

Black Carbon Measurement using Laser Integrating Plate Method

An intensive study to test and validate the Laser Integrating Plate Method (LIPM) of determining absorption coefficient and black carbon mass was carried out. Measurements by LIPM were compared to Smoke Stain Reflectometer measurements and Mie calculations based on accelerator ion beam analysis (IBA) elemental composition measurements. Results show that the value of mass absorption coefficient ϵ = 10 m2g− 1 previously used for mass determination, and widely accepted for black carbon generated by combustion processes, is an inappropriate choice for the type of carbon measured in Sydney. A value of ϵ = 7 m2g−1 for soot and ambient aerosol particles was found to be more appropriate.

Multi year satellite remote sensing of particulate matter air quality over Sydney, Australia

Particulate matter (PM) air‐quality information is usually derived from ground‐based instruments. These measurements, while valuable, are not well suited to provide air‐quality information over large spatial scales. In this study, using 4 years of satellite aerosol optical thickness (AOT) at 0.55 µm derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASAs Terra and Aqua satellites, we present a multi‐year air analysis of PM air quality over Sydney, Australia. We then compare the satellite data with PM2.5 mass concentration measurements from six ground‐based stations in the area. Our results indicate significant diurnal variations and an overall increase in PM2.5 during Southern Hemisphere spring and summer seasons due to bush fires. The air quality in Sydney, Australia is good throughout the year except during major bushfires when PM2.5 mass loading can increase from normal (<20 µg m−3) to unhealthy conditions (>70 µg m−3). The satellite data also show corresponding AOT changes from less than 0.1 to greater than 1.0 during bushfire events. We conclude that satellite data are an excellent tool for studying PM air quality over large areas, especially when ground measurements are not available. While this is the first multi‐year combined satellite and ground‐based air quality analysis over Sydney, ancillary information from lidars, sun photometers, and size‐resolved chemistry measurements will further enhance our capability to monitor and forecast air quality in and around Sydney.

Inversion of Mie extinction measurements using analytic eigenfunction theory

Abstract We have employed the analytic eigenfunction technique, first developed by McWhirter and Pike, to obtain aerosol columnar size distributions from realistic Mie extinction measurements. The theory was tested out and its range of validity established using synthetic data. After that, the method was applied to several sets of volcanic ash cloud data, and the results compared with those for a constrained linear inversion. The technique is extremely easy to apply once the Mellin transform of the kernel has been obtained.

Experimental validation of the solar aureole technique for determining aerosol size distributions

This paper describes the inversion procedures by including multiple scattering contributions to almucantar radiance in the solar aureole region and discusses the retrieved size distribution results by comparing them with ground truth measurements. The agreement between the two sets of results is good and provides yet another experimental validation to show that the solar aureole technique is a simple, practical, and accurate method for obtaining the columnar size distribution of atmospheric aerosols. Comparisons between the results obtained by the two approximations--single and multiple scattering--show that the size distribution retrievals obtained by the latter are more accurate than those obtained by the former. Retrieved estimates of surface albedo show that even though surface albedo cannot be accurately determined from solar aureole measurements, these measurements can provide reasonable estimates. A recommendation is made for the use of solar aureole measurements from satellites to retrieve the size distribution and concentration of stratospheric and upper tropospheric aerosols.

New method for inferring total ozone and aerosol optical thickness from multispectral extinction measurements using eigenvalue analysis

Eigenvalue analysis provides a way of predicting the measurement value at a particular wavelength from measurements made at other wavelengths. It was applied to MFRSR data to find the aerosol contribution made to the measured total optical thickness at the Chappuis band centered near 0.614 µm, within the measurement accuracy. This was used to calculate the total ozone column. The estimated total ozone columns using this technique are compared with the ozone determined using the method described by (King and Byrne, 1976), both were compared with ADEOS and EP total ozone mapping spectroradiometer (TOMS) measurements. The new method gave an excellent agreement with the satellite measurements. The weekly mean percentage difference between the total ozone and both ADEOS and EP/TOMS was 1% and 2% respectively.

Size-resolved chemical composition of Australian dust aerosol during winter

Environmental context Mineral dust aerosol is both an efficient scatterer of solar radiation, potentially cooling the planet, and a moderate absorber, potentially warming it: the exact balance is both uncertain, and geographically variable. Australian desert soils are noticeably more reddish than most Northern Hemisphere deserts, most probably a result of enhanced iron mineralogy. This paper contains results from a field campaign designed to increase our understanding of the chemistry of Australian mineral dust aerosol, especially in relation to iron and salt. Abstract Australia is the dominant source of mineral dust aerosol in the Southern Hemisphere, yet the physical, chemical and optical properties of this aerosol remain poorly understood. Four sets of size-resolved aerosol samples were collected at a site on the edge of the Lake Eyre Basin (LEB), in the south-east dust transport pathway. Back trajectory analysis shows that three samples were sourced from the LEB (one during a rare winter dust storm), and one from coastal regions to the south. All samples were subjected to both ion beam analysis and ion chromatography. A Fe/Al ratio of 0.9 was found, consistent with results from our other campaigns to sites in the LEB, significantly higher than typical Northern Hemisphere values (~0.45–0.6). This confirms the iron-rich character of central Australian soils. Clear evidence of marine advection in the fourth sample was also found, and evidence of chloride depletion by nitric acid in two samples.

On the Spectral Sensitivity of the Approximate Method for Retrieving Aerosol Size Distributions from Multispectral Solar Extinction Measurements

Abstract This paper describes the retrieval of aerosol size distributions from the measurements of multispectral solar extinction data, by the use of two retrieval methods—a fast approximate method and the numerical nonlinear least-squares (NLLS) method. Comparison of eight sets of results obtained by the two methods, in general, show a good agreement to within 15%. In addition, size distribution results retrieved by the approximate method show that the method is particularly sensitive to the wavelengths used, and differences in the wavelengths used for making observations and for generating the look-up tables can lead to appreciable errors in retrieved results. Thus, one cannot assume a universal data base for use with radiometers with different wavelengths; generation of a separate data base for each set of wavelengths is recommended.

The great Sydney dust event: Size-resolved chemical composition and comparison

In September 2009, a major dust storm crossed eastern Australia, blanketing Sydney on two occasions. We collected size-resolved aerosol samples on both days, and have subjected them to Ion Beam Analysis at ANSTO. In this paper we present these results, and compare them with some of the analysis of similar samples obtained in field trips to the Lake Eyre Basin of central Australia, the primary source region of the dust. In particular, the Fe/Al ratios (~0.9) are similar to LEB values, and higher than northern hemisphere values (~0.55). Salt entrainment indicates a source near dry lakes.

Effects of smoothing and measurement-wavelength range on the accuracy of analytic eigenfunction inversions

A smoothing technique is applied to improve the accuracy of inversions of Mie-extinction measurements with analytic eigenfunction theory. It is shown that a moderate amount of smoothing allows the inclusion of further terms, and thus extra information, in the expansion. The effects of measurement-wavelength range on the accuracy of inversions are also investigated, and it is shown that when large particles are present, measurements in the infrared region are necessary for accurate inversions.

Aerosol Properties Computed from Aircraft-Based Observations during the ACE-Asia Campaign: 1. Aerosol Size Distributions Retrieved from Optical Thickness Measurements

In this article, aerosol size distributions retrieved from aerosol layer optical thickness spectra, derived from the 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14) measurements during the ACE-Asia campaign, are presented. Focusing on distinct aerosol layers (with different particle characteristics) observed in four vertical profiles, we compare the results of two different retrieval methods: constrained linear inversion and a non-linear least squares method. While the former does not use any assumption about the analytical form of the size distribution, the latter was used to retrieve parameters of a bimodal lognormal size distribution. Furthermore, comparison of the retrieved size distributions with those measured in-situ, aboard the same aircraft on which the sunphotometer was flown, was carried out. Results of the two retrieval methods showed good agreement in the radius ranges from ∼0.1 μm to ∼1.2–2.0 μm, close to the range of retrievable size distributions from the AATS-14 measurements. In this radius interval, shapes of retrieved and measured size distributions were similar, in accord with close wavelength dependencies of the corresponding optical thicknesses. Additionally, the effect of a size-resolved refractive index on the retrieved size spectra was investigated in selected cases. Retrieval using a constant refractive index pertaining to particle sizes within the range of retrievable size distributions resulted in a size distribution very close to the one retrieved using a size-resolved refractive index.