Assessing the Role of Brewer Spectrophotometer in Determining Aerosol Optical Properties in the UK and Tropics.

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Abstract

Aerosol effects are one of the major uncertainties in assessing global climate change, ecosystem processes and human health. This is because they critically change the balance between the radiation entering and leaving the atmosphere, as well as influencing cloud formation and having direct effects on biological systems e.g. through the respiratory system. It is the direct radiative effects of aerosol that are the focus of this work. The Aerosol Optical Depth (AOD) is a measure of the extinction of radiation by aerosol throughout the depth of the atmosphere. It is wavelength dependent and is traditionally measured at a number of visible wavelengths, but there is little AOD data available at UV wavelengths especially in the UVB. The Brewer spectrophotometer makes direct sun measurements in the UV spectral range, which can in principle be used to calculate AOD at those wavelengths using a form of Beer’s law. This work explores the capabilities of the Brewer for UV AOD measurements and applies the results to data from the Tropics and temperate mid-latitudes. Instrument specific weighting functions were tested for their ability to improve the AOD retrieval, but while they changed the partitioning of absorption between ozone and SO2, they had negligible effect on the resulting AOD. After correcting the existing Brewer software for AOD retrieval, data from Manchester UK were compared with independent measurements of AOD, and measurements from the Manchester instrument were also evaluated against a standard Brewer on location in Spain. The inter Brewer comparisons were consistent with differences of the order 6-10%, while comparison with independent methods was qualitatively consistent, but absolute differences were of the order 10-30%. This might partially be attributed to wavelength mismatches between the different methods, and assumptions in the various methods of calculating the AOD.Following the validation exercise the AOD was retrieved from Brewer instruments, using standard weighting functions, in Manchester from 2000- 2008 and Reading from 2003-2008. Based on this work in the UK, the method was then applied to data from Malaysia (1998-2007), where the climate is totally different to that of the UK. The AOD data obtained from the UK and from the Tropics have been compared. Kuala Lumpur, Malaysia gave the highest average AOD values probably due to it being a developing city with high pollution levels indicating a human impact on climate change. Brewer AOD measurements obtained at a narrow range of wavelengths were then used to calculate Angstrom parameters by applying Volz Method. The results, often generating a negative alpha, were deemed unreliable at the UK sites. This was largely attributed to the high solar zenith angle and low signal to noise of the direct sun measurement, exacerbated by a limited number of clear sky measurements available for the work. However, calculation of Angstrom parameters was more successful in Malaysia due to a low solar zenith angle, high intensity, and greater frequency of truly could free sky. Results indicated that aerosol particles in the capital Kuala Lumpur show a clear domination by fine mode. This domination is probably caused by urban pollution, mainly from road traffic, industrial and anthropogenic activities, which is consistent with a large capital city undergoing rapid development.Thus the capabilities and limitations of the Brewer spectrophotometer to provide both AOD data at UV wavelengths, and from those further aerosol properties, has been tested and demonstrated in two contrasting climatic regions. Air mass, limiting the UV signal, and stray-light within the instrument are two of the factors that limit the success of the Brewer for these measurements, which proved more reliable when the sun was high in the sky as in the Tropics.

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Aerosol optical depth

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