Aerosol light absorption measurement techniques : a comparison of methods from field data and laboratory experimentation

Abstract

Popular science
Currently, there is a global drive to better understand the influences behind our changing climate. With each passing decade, the world has observed dramatic changes in weather patterns, seasonal temperatures, and our surrounding natural environments. Within the heavily-studied climate sciences, the influence of atmospheric aerosols remains the most uncertain. As there is no instrument that offers reliable measurements, simplicity in use, and reasonable cost as a single entity, there remains a necessity for advancement in monitoring and measurement techniques. The aim of this investigation is to consider the current measurement techniques for aerosol absorption and compare instrument performance in a variety of conditions. Utilizing data from scientific flights in the California-based CalNex campaign (April-May 2010) and an experiment in the NOAA laboratories of Boulder, Colorado (April 1st, 2012- May 2nd, 2012), many conditions of aerosol absorption measurements are examined.

Within this investigation, instruments for both short term field campaigns and long term monitoring are considered. Filter-based instruments are the most commonly used method for the measurements of absorption of radiation by atmospheric aerosol in a long term global network. With these, potential biases in measurement are scrutinized due to changes in conditions during sampling, high organic aerosol loading, and the application of potentially overestimating empirical corrections. The short term techniques for aerosol measurements are often considered to be references for the filter-based measurements. Here, photoacoustic techniques and the difference method, where absorption is derived from the difference in measurements of extinction and scattering, are compared with the filter-based instruments.

The aim of this investigation is not to decide which of these instruments or methodologies for the quantification of absorption by atmospheric aerosols should be deemed “the best method”, but rather to provide real-world examples of their applications, capabilities, and limitations. Insights obtained from this study may provide further direction in the drive to reduce uncertainty in aerosol measurements, and furthermore, future climate predictions for all atmospheric components collectively.

Abstract

Atmospheric aerosols are of great importance climatically due in part to their ability to scatter and absorb solar radiation. However, their influence in our changing climate holds the greatest uncertainty of all atmospheric components. With this in mind, there remains a necessity for advancement in monitoring and measurement techniques. The aim of this investigation is to consider the current measurement techniques for aerosol absorption and compare instrument performance in a variety of conditions. Filter-based measurements and various referencing techniques for establishing values of the aerosol absorption coefficient (σap) are examined. The report discusses the various instrumentation, mathematical techniques, and uncertainties associated with each through applications employed during a field campaign, the California Air Quality and Climate Nexus Campaign (CalNex; April-May 2010) and a laboratory experiment at the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado (April 1st, 2012- May 2, 2012). The filter-based instruments, particle soot absorption photometer (PSAP) and the continuous light absorption photometer (CLAP), are compared in performance to the in-situ techniques of the photoacoustic spectrometer (PAS) and absorption obtained via the difference method (σap = extinction (σep) – scattering (σsp)). A potential bias of filter-based instruments during high organic aerosol (OA) loading was investigated during in-flight sampling over the Los Angeles metro region, yielding only one bias occurrence for the analysis on a daytime flight. Possible discrepancies in utilization of the difference method as a reference for filter-based σap are investigated in laboratory data, as a 5% inconsistency between σep and σsp during white aerosol runs yields questionable reference σap results. Potential benefits of the novel Corrected Two-Stream (CTS) correction method for filter-based instruments in various conditions are also investigated for both datasets, with obvious advantages during the changing conditions of a grey to white aerosol laboratory run. A comparison of the CLAP and PSAP filter-based method during laboratory experimentation was also examined, demonstrating the highly-correlated measurements of the two instruments. The aim of this investigation is not to decide which of these instruments or methodologies for the quantification of σap should be deemed “the best method”, but rather to provide real-world examples of their applications, capabilities, and limitations.

• author: Walsh, Christine
• supervisor: Thomas Holst; John Ogren
• organization: Dept of Physical Geography and Ecosystem Science
• course: NGEM01 20121
• year: 2012
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: filter-based correction, difference method, in-situ techniques, filter-based instruments, absorption, physical geography, aerosol
• publication/series: Student thesis series INES
• report number: 258
• language: English
• additional info: External supervisor John Ogren, The National Oceanic and Atmospheric Administration Global Monitoring Division of Boulder, Colorado; Master degree thesis, 30 credits in Atmospheric Sciences & Biogeochemical Cycles. Department of Physical Geography and Ecosystems Science, Lund University.
• id: 3046333
• date added to LUP: 2012-09-11 12:11:06
• date last changed: 2014-06-18 11:27:54