Use of Remote Sensing Data in Environmental Organic Chemistry

ABSTRACT: Remote sensing data can be a useful tool for environmental organic chemistry. It was hypothesized that urban influences, in this case from Baltimore, MD, would cause a net deposition of polycyclic aromatic hydrocarbons (PAHs) into Chesapeake Bay. Data from the Advanced Very High Resolution Radiometer (AVHRR) was used to help calculate fluxes of PAHs into and out of the Chesapeake Bay. It was found that fluoranthene was generally deposited into the Bay while pyrene and benzo[a]fluorene were generally volatilized out of the Bay.

• Objectives
• Introduction
• Material and Methods
• Discussion
• Flux Data
• Conclusion
• Future Work

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  Objectives

  • To use sea surface temperature data in order to better estimate fluxes of polycyclic aromatic hydrocarbons into and out of Chesapeake Bay
  • To test the hypothesis that urban areas such as Baltimore will cause a net deposition to the Bay
  • To find other uses of remote sensing data in environmental organic chemistry

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  Introduction

  The use remote sensing data can be a useful tool for environmental organic chemistry. The use of sea surface temperature (SST) data can be used to better estimate fluxes of PAHs into and out of water bodies. PAHs are formed during the incomplete combustion of fossil fuel and are also a significant component of petroleum. They are even formed during the charbroiling of meats. The reason we are interested in them is that they pose a cancer risk (1). For more information about PAHs click here. These are several PAHs that we will discuss:

  Several common PAHs

  The atmosphere plays an integral role in the delivery and removal of organic chemicals to and from bodies of water. The processes of wet deposition, dry deposition, and gas transfer across the air-water interface constitute major mechanisms in the exchange of toxic chemicals. The atmosphere is thought to account for a substantial portion of the overall input to water bodies; this being of special consideration to those with larger surface areas for chemical deposition and volatilization (2).

  Our hypothesis is that urban areas will enhance the net input of PAHs to nearby water bodies. One problem in calculating fluxes into or out of a water body is to use accurate measurements of temperature at the air-water interface. Usually temperature measurements are taken at one point in time and space. However, water temperatures can vary significantly spatial and temporally. By using SST data derived from satellite imagery, we hope to better estimate this flux.

  Material and Methods

  As part of the AEOLOS VI (Atmospheric Exchange Over Lakes and OceanS) Project funded by the EPA, air samples were taken on the roof of the Maryland Science Center, while water samples were taken in the Inner Harbor Area of Baltimore, MD. Two air samplers are shown on the roof with the city of Baltimore in the background:

  
  Roof of the Maryland Science Center, Baltimore, MD.

  Click here for a map of the Chesapeake Bay area.

  From 22 July 1997 to 25 July 1997 integrated 4 hour air samples were collected using a General Metalworks high volume sampler (Hi-vol) and analyzed for PAHs, polychlorinated biphenyls (PCBs), and pesticides. Total suspended particulate matter (TSP) was collected on a secondary Hi-vol sampler. From 25 July 1997 to 28 July 1997, we switched to 12 hour sampling. The particulate phase was collected on a pre-combusted quartz fiber filter (GFF), while the gas phase was collected on a soxhlet extracted polyurethane foam plug (PUF). The samples were soxhlet extracted for 24 hours using petroleum ether for the PUFs and dichloromethane for the QFFs.

  Water samples were collected every 12 hours from 22 July 1997 to 28 July 1997. A peristaltic pump was used to pump water through a filter head, followed by a XAD-2 column. The XAD-2 column was soxhlet extracted using acetone.

  PAHs were analyzed on a gas chromatograph mass spectrophotometer running in the single ion monitoring mode.

  SST temperature data was converted to flux values. The image below shows SST data. By clicking on the picture, you will see how the scale changes from temperature to flux values.

  

  SST data from the AVHRR was provided by the Institute of Marine and Coastal Sciences. For a discussion of the AVHRR click here.

  Discussion

  In order to estimate the rate, magnitude, and direction of gas transfer across the air-water interface, it is useful to incorporate models due to the difficulties encountered in direct measurement. The Whitman 2-Film model:

  Whitman Two-Film Model (Schwarzenbach et al., 1993)

  assumes that at the air-water interface there are two thin stagnant films, one on the air side and one on the water side (3). The turbulent mixing in the bulk water does not account for the movement through these films, where the transfer of chemicals is limited by slow molecular diffusion. The direction of transfer is determined essentially by the concentration gradient between the air and water sides of the interface. The concentration gradient does not wholly determine the magnitude of the flux. It is driven by the mass transfer coefficients of both the water and the air sides, which, in conjunction with the concentration gradient is the magnitude of the driving force toward equilibrium. The sum of these mass transfer coefficients is constrained primarily by the saturation capacity of water for the chemical, provides an overall mass transfer coefficient which is responsible for the rate of the total flux. The net flux is related to the concentration gradient between the air and water film and the mass transfer coefficient.

  The flux (ng m-2 d-1) is given by the equation:

  F = Kol (Cw - Ca RT/H)

  where Kol (m d-1) is the sum of the air and water-side mass transfer coefficients which establishes the rate of transfer across the interface, Cw (ng m-3) is the dissolved phase water concentration of the chemical of interest, Ca RT/H (ng m-3) is the concentration of the chemical of interest in air expressed as a water concentration in equilibrium with the air with R as the ideal gas constant (8.31 Pa m3 mol-1 K-1), T (K) as the absolute temperature at the air-water interface, and H (Pa m3 mol-1) as the temperature-corrected Henry's Law Constant. It is feasible to assume that hydrophobic organic compounds (HOCs) behave as ideal gases, because it is unlikely that the concentrations are high enough as to permit one molecule of the contaminant to actually "see" another molecule of the contaminant and subsequently interact. A more detailed description can be found elsewhere (4).

  By using sea SST data we are better able to estimate these fluxes (a positive flux is a net volatilization, while a negative flux is a net deposition to Chesapeake Bay) (if you want to zoom out on an image, click on the image):

  

  For another image on the same day, but different time click here,or for the zoomed out version here.

  

  For another image on the same day, but different time click here,or for the zoomed out version here.

  

  For another image on the same day, but different time click here, or for the zoomed out version here.

  Click here to see a flyby where the satellite missed our site entirely.

  The first image has very little data due to clouds. Remember that clouds are cold and will give misleading information. If you compare all the data, you see that fluoranthene is generally being deposited into Chesapeake Bay, while pyrene and benzo[a]fluorene are volatilized out of the Bay. The greatest flux of fluoranthene occurred on 26 July 1997. The flux into Chesapeake Bay may have been due to the wind direction. On this day the wind was coming from the west, directly from Baltimore. On the other two days, the winds were coming from the southeast. The clean air from the southeast cause an increase in volatilization from Chesapeake Bay. Another parameter that would greatly aid the calculations of these fluxes is the use of wind vectors from remote sensing. Since we currently only measure wind direction and velocity from one point, the errors in these flux calculations could be significant. By combining SST data and wind vector data one may be able to predict the fluxes to Chesapeake Bay.

  There are several things that one has to keep in mind when looking at these images. The air concentrations are highly variable with changes in time and location. Since the concentrations of PAHs are on the order of 1 ng m-3, we have to collect integrated samples that may have been taken for as long as 12 hours. The other problem is that of resources. It takes a significant amount of time to process one sample (on the order of 4 hours per sample). This means we are limited as to the number of locations and the number of samples we can take. Special thanks to Holly Bamford from Chesapeake Biological Laboratory for providing me with water data.

  Conclusions

  The use of remote sensing data can be a valuable aid in environmental organic chemistry. SST was used to calculate fluxes of three PAHs into and out of Chesapeake Bay. It was found that fluoranthene was generally being deposited to Chesapeake Bay, while pyrene and benzo[a]fluorene were volatilizing from the Bay. The greatest flux (net deposition) for fluoranthene occurred on 26 July 1997 when the winds were from the west (directly from Baltimore). The greatest fluxes (net volatilization)for pyrene and benzo[a]fluorene occurred when the winds were from the southeast. This project has shown one example of the many uses of remote sensing. Future work will incorporate many other facets of remote sensing.

  Future Work:

  This page will be updated as soon as more data is available. Future work will try to correlate wind vectors with water concentration data. Since water concentrations do not vary as much as air concentrations, it would be reasonable to assume water concentrations may partially drive air concentrations. The other driving force would be from the land. PAHs may volatilize from the land, and based on the direction of the winds, one may be able to model this parameter.

  Another future project utilizing remote sensing data would be to model the uptake of PAHs and other organic contaminants by phytoplankton in order to develop a mass balance model. Remote sensing data would provide concentrations of phytoplankton, SST data, wind vectors, and surface currents. All of these parameters would be essential in order to construct a mass balance model.

  Any comment or questions, email me here.

  References:

  (1) http://atsdr1.atsdr.cdc.gov:8080/tfacts69.html

  (2) Hornbuckle, K.C., J. Jeremiason, C. Sweet and S.J. Eisenreich. 1994. Seasonal variations in air-water exchange of polychlorinated biphenyls in Lake Superior. Environmental Science and Technology 28:1491-1501.

  (3) Schwarzenbach, R., P. Gschwend and D. Imboden, eds. 1993. Environmental Organic Chemistry. Wiley and Sons, New York, NY, USA, pp 217-219.

  (4) Achman, D.R., K.C. Hornbuckle and S.J. Eisenreich. 1993. Volatilization of polychlorinated biphenyls from Green Bay, Lake Michigan. Enviromnental Science and Technology 27:75-87.