The impacts of wildfire aerosols on crop growth in the Pacific Northwest

Quentin Rayane Baret

In the western United States, forest fires occur each year in the summer releasing a significant amount of smoke (aerosols) into the atmosphere. To investigate the impact of aerosols from wildfires on crops in the Pacific Northwest, atmospheric data from summer 2015 are used. Meteorological profiles are obtained from the Weather Research and Forecasting (WRF) model, while aerosol optical depths (AOD) are generated from the Community Multiscale Air Quality (CMAQ) chemical transport model and the BlueSky fire emission modeling framework. These are used as input into the Rapid Radiative Transfer Model (RRTM), which calculates the direct and diffuse solar radiation at the surface. There is a decrease in the total radiation due to a decrease in the direct solar beam with higher aerosol amounts. However, there is also a corresponding increase in the diffuse fraction of the solar radiation reaching the surface due to the scattering characteristics of the aerosols. Moreover the partitioning between the direct and diffuse radiation depends on the time of day. Using the canopy transpiration and photosynthesis (CTP) model, calibrated for spring wheat in Pullman, WA, an increase in net primary production for summer with a constant aerosol loads in June, July and August 2015 is simulated compared to a no-aerosol scenario. The results suggest that there is an initial strong increase in growth due to the rapid increase in diffuse radiation and the associated assimilation by the shaded canopy. At the higher aerosol loads, the decrease in the total radiation begins to be important, and there is less available overall radiation for photosynthesis. Realistic aerosol loads for the entire summer season were used to simulate how the fire season may change relative to the growing season. The results suggest that higher crop production will be realized if wildfires occur earlier in summer. It was estimated that the percent change in net primary production of spring wheat in the study area could increase by 12.0 to 12.6%. By including an increase in temperature due to climate change in addition to the aerosol impacts, the effect on crop growth is still beneficial regionally, but reduced.

The impacts of wildfire aerosols on crop growth in the Pacific Northwest

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