Friday, 27 May 2011

Satellite Measurements of Trace Species

Post written by Keith Tereszchuk.

One of the objectives of BORTAS will be to compare the in-situ aircraft measurements made during the flight campaign with remote sensing data provided by satellites. Space-borne observation is used extensively in many facets of monitoring of the Earth’s surface and atmosphere including weather forecasting, air quality measurements, ozone levels (UV Index), volcano emissions, ocean currents, sea/lake roughness (wave height), desertification assessment and numerous other such environmental management studies.  These are just a few examples of the importance of satellite monitoring and how the information they provide us directly impacts our day-to-day lives. The identification and characterization of biomass burning plumes and their effect on atmospheric chemistry is yet another area of study conducted using satellite remote sensing.


BORTAS will be using data from the Atmospheric Chemistry Experiment (ACE) on-board the Canadian satellite SCISAT-1, which uses a high-resolution Fourier transform spectrometer (ACE-FTS) for remote sensing of the limb (see picture) of the Earth's atmosphere down to 3-km above the Earth’s surface (just above the tropospheric boundary layer). The ACE-FTS has wide spectral coverage in the infrared region of the electromagnetic spectrum scanning a contiguous region from 750 to 4400 cm-1 and currently offers data retrievals for 38 molecular species as well as their isotopologues, over a dozen of which are known biomass burning marker species, e.g. CO, HCN, HCOOH, H2CO, C2H6, C2H2, CH3OH, HNO3, CH4.

View of the limb of the Earth at sunset

Unlike nadir instruments, which look directly down towards the surface of the Earth, ACE peers through the limb of the atmosphere recording sequential absorption spectra using the sun as an emission source. Each sunrise and sunset of the satellite is called an occultation and they provide concentration profile information of each molecular species with respect to altitude.  Much like ogres and onions, ACE can be used to separate the atmosphere into distinct layers with a spatial resolution of 1 km. On average, 20 measurements are made on a daily basis.

The Atmosphere: It has layers

ACE occultation of the Earth’s Atmosphere

               ACE provides widespread global coverage and during the BORTAS campaign, it will make a total of five measurements that will be within the 500 nautical mile range of the FAAM aircraft which will be based in Halifax.  These measurements will be used to compliment the data recorded during the aircraft flights to further understand the chemical evolution of molecular species emitted by biomass burning. 

Positions of the predicted ACE occultations, shown by circular ACE logos. The red circles show the distance from Halifax where the aircraft will be based.

              Ideally we would like to be able to identify the sources of the plumes that will be measured during the campaign to study molecular evolution within them, and satellites can help us do that too. We will be using data from the MODIS Terra instrument, which is a spectroradiometer that records surface temperatures of the Earth including thermal anomalies such as actively burning fires, to identify potential source regions. When the location of fires are known, we can determine the plume sources by calculating air-mass trajectories using a program called HYSPLIT, which is a Lagrangian particle dispersion/trajectory model based on satellite climatologies. These climatologies provide information on air-mass flow at a particular time at any point in the atmosphere across the entire globe.  Using the ACE profiles of the biomass burning marker species, we can determine the altitude that corresponds to the highest concentrations of these molecules and use this altitude as the injection point for backtracking the air-mass flow to its source using HYSPLIT. In addition, we can calculate forward trajectories from identified sources. If these trajectories coincide both spatially and temporally, we can confidently confirm the source of the plume measured.

HYSPLIT trajectories. Backtracking from an ACE measurement made over Hudson Bay and four forward trajectories made from known biomass burning events over northern Saskatchewan and the Northwest Territories (July 2008).

              Bringing together all the aforementioned information, we will be able to characterize biomass burning plumes to further our understanding of the overall impact of biomass burning on atmospheric chemistry.

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