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Biogenic emission measurements from tropical ecosystems are scarce due to logistical challenges and are usually confined to a specific accessible location. The understanding of regional emission distributions in Amazonia has been greatly extended with the FY06 analysis of selected flights from the 2004 Chemistry and Production of Smoke (CAPOS) airborne experiment conducted in collaboration with Paulo Artaxo of the University Sao Paulo and Robert Yokelson of the University of Montana. The flight path covered various landscapes.
Areas indicated by numbers in the left panel above are based on different landcover information:
- mixed forest/plantation
- primary tropical forest
- soybean plantations
- mixed forest/plantation
- water
- urban
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Scatterplot comparing isoprene fluxes estimated with the NCAR MEGAN model and from NCAR aircraft measurements. |
The left panel above depicts the NCAR Model of the Exchange of Gases between the Atmosphere and Nature (MEGAN) model distribution of isoprene emission factors at standard conditions (30 C and 1000 PAR [Photosynthetically Active Radiation]). On top of the emission map, the racetrack pattern is mapped in gray. The right panel above shows the surface flux based on airborne variance measurements. The Volatile Organic Compound (VOC) variability was calculated for a 60-second running average. The qualitative agreement between model and measurements is reasonable. Both indicate that landcover change can greatly perturb isoprene emissions which will likely result in significant changes in oxidants, particles and other important atmospheric constituents. MEGAN isoprene emissions are compared with measurements in the scatterplot figure which shows a tight linear correlation (R2=0.99) between the model and measurements for isoprene which demonstrates that MEGAN is able to represent the variability associated with landcover change in this region. However, the MEGAN isoprene emission estimates are ~40% lower (slope: 0.6) than our observations. This is within the model uncertainty, but indicates that the MEGAN estimates may be a lower, rather than an upper, limit at least for this Amazonian region.
These observations are in stark contrast to most modeling studies, which assume the opposite (i.e., up to 50% reduced isoprene emissions compared to MEGAN). Isoprene emissions incorporated in these large scale chemical-transport models are therefore most likely too low and yet still greatly overestimate PBL isoprene concentrations. |
