Dissertation Abstract
The analysis of in situ and retrieved aerosol properties measured during three airborne field campaigns
Corr, Chelsea A 2014
Natural Resources and Earth Systems Science Ph.D. Program, University of New Hampshire (United States), 202 pp.
Aerosols directly influence climate, visibility, and photochemistry by scattering and absorbing short-wave solar radiation; how efficiently a particle scatters and/or absorbs radiation depend on its chemical and physical properties. Because certain aerosols also act as nuclei for cloud droplets (CCN) and a smaller population of particles facilitate ice crystal formation (IN), aerosols can also alter cloud-radiation interactions which have subsequent impacts on climate. Thus aerosol properties determine the magnitude and sign of both the direct and indirect impacts of aerosols on radiation-dependent Earth System processes.
My dissertation fills some gaps in our understanding of the role of aerosol properties on aerosol absorption and cloud formation. First, the impact of aerosol oxidation on aerosol spectral (wavelengths: 350-500nm) absorption was examined for two biomass burning plumes intercepted by the NASA DC-8 aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission in Spring and Summer 2008. Aerosol single scattering albedos (SSAs) retrieved using measured actinic flux were used to calculate absorption Angstrom exponents (AAEs). Higher AAE values observed for the older plume indicated aerosol absorption was enhanced in the ultraviolet relative to the visible portion of the short-wave spectrum in the older plume compared to the fresher plume. These differences were largely attributed to the greater oxidation of the organic aerosol in the older plume which can arise from the aging of primary organic aerosol and/or the formation of spectrally-absorbing secondary organic aerosol.
The validity of the actinic flux retrievals used above was also evaluated in this work using data collected in Houston, TX as part of the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) mission in September 2013. SSA retrieved using actinic flux were compared to those values retrieved using ratios of direct and diffuse irradiance at 332, 368, 415, and 500nm, as well as SSAs reported by the NASA Aerosol Robotic Network (AERONET) at 440nm, and those measured aboard the NASA P-3B aircraft at 450 and 550nm. Actinic flux SSAs were consistently lower than all other SSAs. However, AAE values calculated from actinic flux SSAs compared well to AERONET and column-averaged in situ AAEs, suggesting actinic flux retrievals can correctly resolve the spectral dependence of aerosol absorption.
Recent work has suggested that mineral dust is the most important IN, found in both anvil and synoptically formed cirrus clouds over North America. The role of deep convective systems on mineral dust transport to the upper troposphere (> 9 km) where these clouds form was investigated using bulk aerosol Ca2+ concentrations and total coarse (diameter 1-5 micro-m) aerosol volume (Vc) measured aboard the NASA DC-8 during the NCAR Deep Convective Clouds and Chemistry Experiment (DC-3) mission in May/June 2012. High outflow/inflow ratios of both Ca2+ and Vc for twelve thunderstorms intercepted over CO and OK suggested a significant fraction of these efficient IN was transported through these systems. Further, inflow total aerosol number concentrations calculated for IN-relevant diameters (0.5-5 micro-m) generally exceeded anvil cirrus ice particle concentrations, supporting the presence of interstitial dust in storm outflow.
My dissertation fills some gaps in our understanding of the role of aerosol properties on aerosol absorption and cloud formation. First, the impact of aerosol oxidation on aerosol spectral (wavelengths: 350-500nm) absorption was examined for two biomass burning plumes intercepted by the NASA DC-8 aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission in Spring and Summer 2008. Aerosol single scattering albedos (SSAs) retrieved using measured actinic flux were used to calculate absorption Angstrom exponents (AAEs). Higher AAE values observed for the older plume indicated aerosol absorption was enhanced in the ultraviolet relative to the visible portion of the short-wave spectrum in the older plume compared to the fresher plume. These differences were largely attributed to the greater oxidation of the organic aerosol in the older plume which can arise from the aging of primary organic aerosol and/or the formation of spectrally-absorbing secondary organic aerosol.
The validity of the actinic flux retrievals used above was also evaluated in this work using data collected in Houston, TX as part of the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) mission in September 2013. SSA retrieved using actinic flux were compared to those values retrieved using ratios of direct and diffuse irradiance at 332, 368, 415, and 500nm, as well as SSAs reported by the NASA Aerosol Robotic Network (AERONET) at 440nm, and those measured aboard the NASA P-3B aircraft at 450 and 550nm. Actinic flux SSAs were consistently lower than all other SSAs. However, AAE values calculated from actinic flux SSAs compared well to AERONET and column-averaged in situ AAEs, suggesting actinic flux retrievals can correctly resolve the spectral dependence of aerosol absorption.
Recent work has suggested that mineral dust is the most important IN, found in both anvil and synoptically formed cirrus clouds over North America. The role of deep convective systems on mineral dust transport to the upper troposphere (> 9 km) where these clouds form was investigated using bulk aerosol Ca2+ concentrations and total coarse (diameter 1-5 micro-m) aerosol volume (Vc) measured aboard the NASA DC-8 during the NCAR Deep Convective Clouds and Chemistry Experiment (DC-3) mission in May/June 2012. High outflow/inflow ratios of both Ca2+ and Vc for twelve thunderstorms intercepted over CO and OK suggested a significant fraction of these efficient IN was transported through these systems. Further, inflow total aerosol number concentrations calculated for IN-relevant diameters (0.5-5 micro-m) generally exceeded anvil cirrus ice particle concentrations, supporting the presence of interstitial dust in storm outflow.