Regional Chemical Transport Modelling with a Forest Canopy Parameterization

Abstract

The incorporation of forest shading processes into a regional chemical transport model (Makar et al., Nat Commun 2017) greatly reduced the predicted July O₃ mean biases and root mean square errors, as well as reducing the magnitude of predicted PM_2.5 mean bias. However, the parameterization resulted in a degradation of NO₂ performance. A sensitivity study of the regional model’s canopy parameterization reduced this NO₂ degradation, but suggests that the parameterization has a strong scale dependence. Grid squares with relatively low population densities influence North American ozone biases by a factor of two. Simulations at higher resolution may be required in order to simultaneously improve O₃, PM_2.5 and NO₂.

Questions and Answers

Questioner: Heinke Schlüzen

Question: Did you apply the canopy parameterization for meteorology fields (wind and temperature values within a forest as well)?

Answer: No—the intent was to construct an “add-on” parameterization which could be used for both on and off-line models, with a focus solely on turbulent transport. The parameterization could be used within the weather forecast portion of an on-line model, but that has not been attempted here.

Questioner: Hosein Foroutan

Question: What caused the increase in ozone over the ocean (especially Pacific) when you applied your parameterization?

Answer: Advection (plus chemistry). The reduction in light levels and reduced vertical transport within the forested canopy in coastal regions results in the build-up of ozone precursors there compared to the no-canopy base case. When the winds are blowing from land to ocean, this results in these precursors to ozone formation, which are higher in the canopy model than the base case, being blown offshore. This in turn results in slightly higher off-shore ozone levels in some areas such as the west coast, the Gulf of Mexico near New Orleans, and the shoreline areas of Hudson Bay. Similar effects associated with transport were also observed in some interior areas; for example, just in-land of northern Los Angeles, ozone levels increased for the canopy simulation, in non-forested regions just downwind of forests. Again, this was due to precursors not reacting due to lower light levels and reduced turbulence being transported downwind by the larger scale transport winds.

Questioner: Peter Viaene

Question: I saw that the biogenic emissions are in your model. One of the reasons the vegetation emits biogenics is to protect itself from high O₃ concentrations. Are your biogenic emissions modulated with O₃ concentrations?

Answer: The model emissions of biogenic hydrocarbons are dependent on temperature, and, in the case of isoprene, the levels of photosynthetically active radiation reaching the foliage—as has been shown in many measurement studies. In that sense, the emission rate of biogenic hydrocarbons in this model and others are completely independent of the concentration of O₃. Both model biogenic hydrocarbon emission rates and ozone formation reactions are temperature dependent and hence may correlate, but are not necessarily causally linked.