Springtime aerosol layers in the free troposphere over Australia: Mildura Aerosol Tropospheric Experiment (MATE 98)

Abstract

Abstract. A field campaign focused primarily on free tropospheric aerosol measurements over Mildura, Australia, at 34øS (Mildura Aerosol Tropospheric Experiment (MATE 98)) was conducted in the austral spring of 1998 to test for the current presence of a seasonal aerosol layering activity observed in the 1970s and to obtain additional characteristics that would lead to a better understanding of the phenomenon. Ground-based lidar as well as balloon-borne optical particle counters and backscattersondes with ozone sensors were employed. The results indicate that large horizontal scale layers are present and show that their structure is highly correlated with excess ozone. Particle concentrations in the layers were sufficient to measurably affect aerosol optical depth. The probable source is distant biomass burning regions, but a detailed understanding of associated smoke transport and evolution as observed over Mildura is incomplete. At the time of these measurements it was believed that the enhancements were related to biomass burning, but conclusive supporting evidence for this assumption was lacking. For example, since reliable relative humidity measurements were not made, it could be argued that the layers were simply caused by particle growth in layers of higher relative humidity that might be expected with springtime moisture. In addition, for biomass burning aerosols a good correlation with certain other trace species, such as ozone and/or carbon monoxide, might be expected, but this type of supporting information was unavailable. Also, it was unknown if the layers were localized or of large scale, as would be consistent with distant biomass burning sources.Only a two-point slope of the size distribution was measured in the 1970s, but a more complete size distribution was required to assess the physical and optical properties of the aerosol for relating it to the suspected source regions and estimating its radiative impact. For example, it could not be determined if the steep slope of the size distribution indicated that the aerosols were to be associated with the tail of a much higher concentration condensation nuclei mode or rather were part of a relatively narrow accumulation mode. Since the effective particle size for scattering could not be determined reliably from the available size information, it was not clear to us that the layers had sufficient optical cross sections to appreciably affect the radiative properties of the atmosphere. MATE 98In the austral spring of 1998 the Mildura Aerosol Tropospheric Experiment (MATE 98) was conducted for the purpose of reexamining the layers over Mildura in more detail and with the expectation that substantial evidence could be found linking them to biomass burning sources. The initial goal was to observe the presence of the layers and possibly identify changes in activity and/or magnitude. A more sophisticated OPC was employed to obtain extended size distributions for optical model calculations. Some optical properties of the 17,833

The Climatology of Australian Aerosol

“…Aerosol sources contributing to the noted rise during spring are heterogeneous, with suggested contributions from fine aeolian dust , a combination of biomass burning smoke, fine dust, and maritime aerosol from long-range transport (Radhi et al, 2010), and exogenous biomass burning aerosol originating in southern Africa or even South America (Rosen et al, 2000).…”

“…The implication is that the mechanisms responsible for aerosol generation and transport have much in common at both regional and trans-continental scales. A further mechanism underlying this correlation is the intercontinental transport of aerosol plumes from southern Africa and possibly South America considered by Rosen et al (2000). Although they measured the plumes from only one site, such material is likely to be well dispersed follow- ing long-range transport, and was shown to contribute ∼ 0.02 to the mid-visible aerosol optical depth over Mildura.…”

Retrieval of aerosol backscatter, extinction, and lidar ratio from Raman lidar with optimal estimation

“…Further, the use of correlated variables will emphasise degenerate states of the forward model, which can slow the retrieval's convergence. This can be averted by retrieving the lidar ratio B = α (p) /β (p) instead (Shcherbakov, 2007;R. Hogan, personal communication, 2012), which is independent of N, All elements of x should be positive or there will be a degeneracy in the impact of backscatter and optical depth on the elastic channel, which can impede the retrieval.…”

“…The suitability of this scale height can be assessed by investigating the covariance of some measure of aerosol scattering. A convenient option is backscatter sondes (NDACC, 1989(NDACC, -2000, which measure the light backscattered from a xenon flashlamp approximately every 30 m during a balloon ascent (Rosen and Kjome, 1991;Rosen et al, 2000). Profiles over 10 years of observations at three sites have been used in Fig.…”

Optical, physical and chemical characteristics of Australian continental aerosols: results from a field experiment

“…(Inadequate cloud screening is unlikely to be a factor.) Higher values in AOD along with a wide range of α values during spring months is suggestive of a contribution from a different aerosol type such as regional biomass burning, and/or long range (intercontinental) transport (Rosen, et al, 2000;Gloudemans et al, 2006), and/or the possible influence of marine biogenic emission when the air mass is advected from the ocean. Mt Isa, the largest point source of SO 2 in the Southern Hemisphere, is 550 km due north: while direct advection would be uncommon, some contribution is likely.…”