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The Estimation of Stratospheric Aerosol Optical Depth from Measurements of Stellar Extinction

Date

2023-01-03

Journal Title

Journal ISSN

Volume Title

Publisher

ORCID

0000-0002-1393-7535

Type

Thesis

Degree Level

Masters

Abstract

Stratospheric aerosol optical depth (SAOD) data is obtained for the period 1955-present based on measurements of starlight intensity by ground-based astronomical equipment. These measurements of intensity at different stellar zenith angles allow for the calculation of optical depth, a quantity measuring how much light is extinguished by the atmosphere for a given time and location. This data presents an exciting opportunity to supplement measurements of SAOD in periods where satellite data is either sparse or non-existent. The use of astronomical observations to reconstruct SAOD was pioneered by Richard Stothers (Stothers, 2001), who published hemispheric annual means for SAOD using astronomy data from 1961-1979. This reconstruction is still a component of SAOD reconstructions used in climate modeling today. This period, referred to here as the Stothers period, fills a gap between SAOD reconstructions using ice cores and satellite instruments (1979-present). Stothers claimed that the astronomy data is the most accurate source of SAOD information in this period. However, this claim has yet to be substantiated as no quantified values of uncertainty were presented and only minimal comparisons to other methods were made. Therefore, it is not currently clear how accurately the astronomy data represents the true SAOD in the Stothers period. To establish uncertainty in this period, a comprehensive literature search for was done for astronomical optical depth measurements in the modern period. Six observing sites with long term time series data in the modern period were chosen. These sites include Canary Islands, Flagstaff, Washington Camp, and San Pedro Martir in the Northern Hemisphere, as well as Cerro Tololo and La Silla Tololo in the Southern Hemisphere. The Stothers method was applied to measurements at these sites in addition to several other observing sites with shorter coverage to deduce SAOD from astronomical data in both the pre-satellite and modern periods. The GloSSAC stratospheric aerosol reconstruction, based primarily on satellite data is taken to be the true value of SAOD in the modern period and the error in the astronomy data is obtained by considering the standard deviation in difference between the measurements. Using this method, a value for the typical error of ±0.0176 with a mean percent difference of ∼13% was found for annual means of SAOD obtained from astronomical measurements in the Northern Hemisphere and ±0.0169 with a mean percent difference of ∼16% in the Southern Hemisphere, leading to a global average error of ±0.0173 with a mean percent difference of ∼14.5%. This error estimate can be used to impose constraints on values of SAOD obtained from astronomical data in the pre-satellite era and will inform more robust quantification of the effect of volcanic eruptions on 20th century global mean temperature variability.

Description

Keywords

Aerosol, Extinction, Optical Depth, Astronomy, Climate, Atmospheric Physics,

Citation

Degree

Master of Science (M.Sc.)

Department

Physics and Engineering Physics

Program

Physics

Advisor

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DOI

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