Changwoo Ahn | George Mason University (original) (raw)

Papers by Changwoo Ahn

Research paper thumbnail of K-12 Participation Is Instrumental In Enhancing Undergraduate Research And Scholarship Experience

Journal of College Teaching & Learning (TLC), 2015

Research paper thumbnail of Dominance by an obligate annual affects the morphological characteristics and biomass production of a planted wetland macrophyte community

Journal of Plant Ecology, 2015

Research paper thumbnail of Aerosol Remote Sensing from OMI Observations: An Overview

Research paper thumbnail of Bacterial community composition of soils in created freshwater mitigation wetlands as affected by soil chemistry and site hydroconnectivity

Tuesday, August 3, 2010 Bacterial community composition of soils in created freshwater mitigation... more Tuesday, August 3, 2010 Bacterial community composition of soils in created freshwater mitigation wetlands as affected by soil chemistry and site hydroconnectivity. Rita M. Peralta, C. Ahn, and Patrick M. Gillevet. George Mason University. Background/Question/Methods ...

Research paper thumbnail of A Ten-Year Record of Aerosol Absorption Properties from OMI Observations

Research paper thumbnail of Assessing controls of nitrogen dynamics in created and natural wetlands in the Virginia Piedmont, USA

ABSTRACT Background/Question/Methods Natural wetlands have been widely noted for their ability to... more ABSTRACT Background/Question/Methods Natural wetlands have been widely noted for their ability to improve the water quality of downstream systems by retaining, transforming, and permanently removing nitrogen (N) from polluted water. While there have been numerous studies on N dynamics in natural riparian and coastal plain wetlands, there is insufficient information on the controls of N cycling in created wetlands; especially in the Virginia piedmont. This study examined the influences of three factors known to control N cycling: hydrologic connectivity (HC) to the surrounding hydrologic system, age, and microtopography (MT). Coupled N fluxes were measured along gradients of the three factors using sedimentation tiles to quantify inputs of particulate N, modified resin cores to quantify ammonification (AMMON) and nitrification (NTR) rates, and denitrification enzyme assays to quantify denitrification (DNT) rates. To investigate the HC gradient, soil and hydrologic variables were entered into a principle component analysis that identified three primary components: (1) bulk density/soil moisture/total organic carbon, referred to as soil condition (SC), (2) standing water/sedimentation/redox (HC), and (3) soil moisture variability (SMVAR). Results/Conclusions The SC component was a significant positive predictor of NTR, N mineralization (N MIN), and DNT. The HC component was a significant positive predictor of AMMON and a significant negative predictor of percent NTR. The SMVAR component was not related to any fluxes. These results indicate that HC controls some N processes, like AMMON, which is the rate-limiting step in inorganic N cycling. Soil conditions, however, such as higher SM, TOC, and associated lower BD, can stimulate other N processes, such as DNT and NTR, which produces the NO3- substrate for DNT. Investigation of age-related factors showed that 3 and 4 year old created wetlands had significantly lower TOC and higher BD than 7 and 10 year old wetlands. Denitrification followed the same trend as TOC (and opposite trend of BD) with 3 and 4 year old wetlands demonstrating significantly lower DNT than 7 and 10 year old wetlands. TOC was also a significant positive predictor of NTR, AMMON, and N MIN. Experimental manipulations of MT demonstrated that NTR and DNT increased with certain aspects of MT. This study highlights the importance of HC, soil maturation, and incorporation of MT in the development of N cycling and coupled N processes in wetlands – wetlands with greater HC, soil organic matter, and MT had greater coupled N cycles – findings that can inform design and management decisions in created wetland systems.

Research paper thumbnail of Scaling considerations of mesocosm wetlands in simulating full-scale wetlands

Research paper thumbnail of Characterization of Spectral Absorption Properties of Aerosols Using Satellite Observations

Research paper thumbnail of An Enhanced TCP Mechanism for Detecting and Differentiating the Loss of Retransmission over Wireless Networks

Abstract— In the current internet, Transmission Control Protocol (TCP) is used as the most popula... more Abstract— In the current internet, Transmission Control Protocol (TCP) is used as the most popular transport layer protocol. However, the end-to-end throughput of TCP degrades significantly when operates in wireless networks. Frequent retransmission timeout is one of the well-known ...

Research paper thumbnail of Relationship between structural and functional vegetation characteristics in three wetlands created in the Piedmont region of Virginia

Research paper thumbnail of Effects of recycled FGD liner material on water quality and macrophytes of constructed wetlands: A mesocosm experiment

Water Research, Feb 28, 2001

Research paper thumbnail of Improvements to the OMI Near-uv Aerosol Algorithm Using A-train CALIOP and AIRS Observations

Research paper thumbnail of Development and Testing of the New Surface LER Climatology for OMI UV Aerosol Retrievals

Research paper thumbnail of Assessing Microbial Community Structure Using LH-PCR in Treatment Microcosm Wetlands

Research paper thumbnail of Net primary productivity of macrophyte communities after seven growing seasons in experimental planted and unplanted marshes

Research paper thumbnail of A Dynamic Model to Predict Responses of Millets (Echinochloa SP.) to Different Hydrologic Conditions for the Illinois Floodplain-River

Millets grow on floodplain mud flats exposed when seasonal floods recede, and the seeds of this p... more Millets grow on floodplain mud flats exposed when seasonal floods recede, and the seeds of this plant are an important food source for waterfowl during their spring and autumn migrations in the Mississippi Flyway. Productivity of millets along the Illinois River has declined because of unnaturally frequent floods that inundate the mud flats and drown the plants during the summer

Research paper thumbnail of Improving OMI Aerosol Retrieval by Integrating Multisensor Measurements

The near-UV aerosol remote sensing technique applied to the Ozone Monitoring Instrument (OMI) mea... more The near-UV aerosol remote sensing technique applied to the Ozone Monitoring Instrument (OMI) measurements successfully retrieves the aerosol extinction and absorption properties over both land and ocean globally. As an ongoing activity of improving the aerosol retrieval, recently, the OMI aerosol algorithm has gone through a major stage of development which includes incorporation of organics in the existing carbonaceous aerosol

Research paper thumbnail of Inferring Aerosol Angstrom Absorption Exponent using satellite observations

Agu Fall Meeting Abstracts, Dec 1, 2013

The Angstrom Absorption Exponent (AAE) is a parameter commonly used to characterize the wavelengt... more The Angstrom Absorption Exponent (AAE) is a parameter commonly used to characterize the wavelength-dependence of aerosol absorption optical depth (AAOD). It is closely related to aerosol composition. Black carbon (BC) containing aerosols yield AAE values near unity whereas Organic carbon (OC) aerosol particles are associated with values larger than 2. Even larger AAE values have been reported for desert dust aerosol particles. Knowledge of spectral AAOD is necessary for the calculation of direct radiative forcing effect of aerosols and for inferring aerosol composition. We have developed a satellite-based method of determining the spectral AAOD of absorbing aerosols. The technique uses multi-spectral measurements of upwelling radiation from scenes where absorbing aerosols are present above clouds. The upwelling reflectance at the cloud top is attenuated by the absorption effects of the overlying aerosol layer. This attenuation effect can be described using an approximations of Beer's Law. The upwelling reflectance at the cloud-top in an aerosol-free atmospheric column is mainly a function of cloud optical depth (COD). In the proposed method of AAE derivation, the first step is determining COD which is retrieved using a previously developed color-ratio based approach. In the second step, the spectral AAOD is derived by an inversion of the measured spectral reflectance. The proposed technique will be discussed and application results making use of OMI multi-spectral measurements in the UV-Vis. will be presented.

Research paper thumbnail of An online aerosol retrieval algorithm using OMI near-UV observations based on the optimal estimation method

Atmospheric Chemistry and Physics Discussions, 2015

Research paper thumbnail of Improvements to the OMI near UV aerosol algorithm using A-train CALIOP and AIRS observations

Atmospheric Measurement Techniques Discussions, 2013

Research paper thumbnail of K-12 Participation Is Instrumental In Enhancing Undergraduate Research And Scholarship Experience

Journal of College Teaching & Learning (TLC), 2015

Research paper thumbnail of Dominance by an obligate annual affects the morphological characteristics and biomass production of a planted wetland macrophyte community

Journal of Plant Ecology, 2015

Research paper thumbnail of Aerosol Remote Sensing from OMI Observations: An Overview

Research paper thumbnail of Bacterial community composition of soils in created freshwater mitigation wetlands as affected by soil chemistry and site hydroconnectivity

Tuesday, August 3, 2010 Bacterial community composition of soils in created freshwater mitigation... more Tuesday, August 3, 2010 Bacterial community composition of soils in created freshwater mitigation wetlands as affected by soil chemistry and site hydroconnectivity. Rita M. Peralta, C. Ahn, and Patrick M. Gillevet. George Mason University. Background/Question/Methods ...

Research paper thumbnail of A Ten-Year Record of Aerosol Absorption Properties from OMI Observations

Research paper thumbnail of Assessing controls of nitrogen dynamics in created and natural wetlands in the Virginia Piedmont, USA

ABSTRACT Background/Question/Methods Natural wetlands have been widely noted for their ability to... more ABSTRACT Background/Question/Methods Natural wetlands have been widely noted for their ability to improve the water quality of downstream systems by retaining, transforming, and permanently removing nitrogen (N) from polluted water. While there have been numerous studies on N dynamics in natural riparian and coastal plain wetlands, there is insufficient information on the controls of N cycling in created wetlands; especially in the Virginia piedmont. This study examined the influences of three factors known to control N cycling: hydrologic connectivity (HC) to the surrounding hydrologic system, age, and microtopography (MT). Coupled N fluxes were measured along gradients of the three factors using sedimentation tiles to quantify inputs of particulate N, modified resin cores to quantify ammonification (AMMON) and nitrification (NTR) rates, and denitrification enzyme assays to quantify denitrification (DNT) rates. To investigate the HC gradient, soil and hydrologic variables were entered into a principle component analysis that identified three primary components: (1) bulk density/soil moisture/total organic carbon, referred to as soil condition (SC), (2) standing water/sedimentation/redox (HC), and (3) soil moisture variability (SMVAR). Results/Conclusions The SC component was a significant positive predictor of NTR, N mineralization (N MIN), and DNT. The HC component was a significant positive predictor of AMMON and a significant negative predictor of percent NTR. The SMVAR component was not related to any fluxes. These results indicate that HC controls some N processes, like AMMON, which is the rate-limiting step in inorganic N cycling. Soil conditions, however, such as higher SM, TOC, and associated lower BD, can stimulate other N processes, such as DNT and NTR, which produces the NO3- substrate for DNT. Investigation of age-related factors showed that 3 and 4 year old created wetlands had significantly lower TOC and higher BD than 7 and 10 year old wetlands. Denitrification followed the same trend as TOC (and opposite trend of BD) with 3 and 4 year old wetlands demonstrating significantly lower DNT than 7 and 10 year old wetlands. TOC was also a significant positive predictor of NTR, AMMON, and N MIN. Experimental manipulations of MT demonstrated that NTR and DNT increased with certain aspects of MT. This study highlights the importance of HC, soil maturation, and incorporation of MT in the development of N cycling and coupled N processes in wetlands – wetlands with greater HC, soil organic matter, and MT had greater coupled N cycles – findings that can inform design and management decisions in created wetland systems.

Research paper thumbnail of Scaling considerations of mesocosm wetlands in simulating full-scale wetlands

Research paper thumbnail of Characterization of Spectral Absorption Properties of Aerosols Using Satellite Observations

Research paper thumbnail of An Enhanced TCP Mechanism for Detecting and Differentiating the Loss of Retransmission over Wireless Networks

Abstract— In the current internet, Transmission Control Protocol (TCP) is used as the most popula... more Abstract— In the current internet, Transmission Control Protocol (TCP) is used as the most popular transport layer protocol. However, the end-to-end throughput of TCP degrades significantly when operates in wireless networks. Frequent retransmission timeout is one of the well-known ...

Research paper thumbnail of Relationship between structural and functional vegetation characteristics in three wetlands created in the Piedmont region of Virginia

Research paper thumbnail of Effects of recycled FGD liner material on water quality and macrophytes of constructed wetlands: A mesocosm experiment

Water Research, Feb 28, 2001

Research paper thumbnail of Improvements to the OMI Near-uv Aerosol Algorithm Using A-train CALIOP and AIRS Observations

Research paper thumbnail of Development and Testing of the New Surface LER Climatology for OMI UV Aerosol Retrievals

Research paper thumbnail of Assessing Microbial Community Structure Using LH-PCR in Treatment Microcosm Wetlands

Research paper thumbnail of Net primary productivity of macrophyte communities after seven growing seasons in experimental planted and unplanted marshes

Research paper thumbnail of A Dynamic Model to Predict Responses of Millets (Echinochloa SP.) to Different Hydrologic Conditions for the Illinois Floodplain-River

Millets grow on floodplain mud flats exposed when seasonal floods recede, and the seeds of this p... more Millets grow on floodplain mud flats exposed when seasonal floods recede, and the seeds of this plant are an important food source for waterfowl during their spring and autumn migrations in the Mississippi Flyway. Productivity of millets along the Illinois River has declined because of unnaturally frequent floods that inundate the mud flats and drown the plants during the summer

Research paper thumbnail of Improving OMI Aerosol Retrieval by Integrating Multisensor Measurements

The near-UV aerosol remote sensing technique applied to the Ozone Monitoring Instrument (OMI) mea... more The near-UV aerosol remote sensing technique applied to the Ozone Monitoring Instrument (OMI) measurements successfully retrieves the aerosol extinction and absorption properties over both land and ocean globally. As an ongoing activity of improving the aerosol retrieval, recently, the OMI aerosol algorithm has gone through a major stage of development which includes incorporation of organics in the existing carbonaceous aerosol

Research paper thumbnail of Inferring Aerosol Angstrom Absorption Exponent using satellite observations

Agu Fall Meeting Abstracts, Dec 1, 2013

The Angstrom Absorption Exponent (AAE) is a parameter commonly used to characterize the wavelengt... more The Angstrom Absorption Exponent (AAE) is a parameter commonly used to characterize the wavelength-dependence of aerosol absorption optical depth (AAOD). It is closely related to aerosol composition. Black carbon (BC) containing aerosols yield AAE values near unity whereas Organic carbon (OC) aerosol particles are associated with values larger than 2. Even larger AAE values have been reported for desert dust aerosol particles. Knowledge of spectral AAOD is necessary for the calculation of direct radiative forcing effect of aerosols and for inferring aerosol composition. We have developed a satellite-based method of determining the spectral AAOD of absorbing aerosols. The technique uses multi-spectral measurements of upwelling radiation from scenes where absorbing aerosols are present above clouds. The upwelling reflectance at the cloud top is attenuated by the absorption effects of the overlying aerosol layer. This attenuation effect can be described using an approximations of Beer's Law. The upwelling reflectance at the cloud-top in an aerosol-free atmospheric column is mainly a function of cloud optical depth (COD). In the proposed method of AAE derivation, the first step is determining COD which is retrieved using a previously developed color-ratio based approach. In the second step, the spectral AAOD is derived by an inversion of the measured spectral reflectance. The proposed technique will be discussed and application results making use of OMI multi-spectral measurements in the UV-Vis. will be presented.

Research paper thumbnail of An online aerosol retrieval algorithm using OMI near-UV observations based on the optimal estimation method

Atmospheric Chemistry and Physics Discussions, 2015

Research paper thumbnail of Improvements to the OMI near UV aerosol algorithm using A-train CALIOP and AIRS observations

Atmospheric Measurement Techniques Discussions, 2013