Acid wet-deposition modeling sensitivity to WRF-CMAQ planetary boundary layer schemes and exceedance of critical loads over an industrializing coastal valley in northwestern British Columbia, Canada

Abstract

Computational tools used to implement the critical-load approach of atmospheric deposition impact often do not elucidate modeling uncertainty making it difficult for environmental policy-makers to know how much confidence to put in its results, also hampering aspects that may need improving. This study evaluated acid deposition modeling for various parameterizations of the planetary boundary-layer (PBL) over the Terrace-Kitimat valley, a physiographically complex region of northwestern British Columbia. Of five schemes, simulations with the Mellor-Yamada-Nakanishi-Niino Level 3 and Mellor-Yamada-Janjic PBL schemes best captured weekly wet deposition fluxes of acidifying ions (SO42−, NO3−, NH4+) within a factor of 2 of observations at an industrial fence line station. Alongside the Yonsei University PBL scheme, these two schemes slightly overestimated the chemical species at a station that was distant from major anthropogenic precursor sources in the valley, hence useful for worst-scenario projections of atmospheric deposition on the natural environment. Forest soils in the vicinity of a large aluminum smelter in Kitimat was estimated to be in exceedance of critical load of acidity by 30.1–53.5 kg S ha−1 yr−1. Exceedance of critical load of nutrient nitrogen restricted to the Terrace area (≤ 7 km2), ranged between none and 0.71 kg ha−1 yr−1. This work provides guidance for using PBL schemes in the Weather Research and Forecasting model that is coupled to a deposition model when calculating critical-load exceedance over temperate, rugged, coastal geographies.