A regional scale modeling study of atmospheric transport and transformation of mercury. I. Model development and evaluation

A three-dimensional regional scale air quality model was developed to study the atmospheric transport, transformation and deposition of mercury (Hg) b y formulating and incorporating mercury chemistry, cloud processes, and air -surface exchanges into the framework of Sarmap Air Quality Model (SAQM). T hree mercury species were included: elemental mercury Hg(O), divalent mercu ry Hg(II), and particulate mercury Hg(p). Precipitating clouds, co-existing non-precipitating clouds, and fair weather clouds were considered in model ing the in-cloud transformation processes. A formulation of bi-directional air-surface exchange of elemental mercury was used for emission from, and d ry deposition to, natural surfaces. Preliminary evaluation of the model was conducted by comparing six major weekly output variables, including ambien t Hg concentrations and Hg concentration in precipitation, with correspondi ng measurements at eight monitoring stations in Connecticut for a summer we ek and a winter week. Model predictions of surface-level gaseous Hg concent rations were close to measured levels, agreeing to within 12% on average, a bout half the estimated error in measurements. The predicted Hg concentrati ons in precipitation were 50% higher than measured values on average, sligh tly lower than the estimated 60% error in measurements. The model was shown to be capable of predicting hourly concentrations and deposition fields of the three tig species as well as in-cloud transformation of Hg(O) by each of the three cloud types, and useful in analyzing the effects of various co ntrolling factors on the transport and transformation of Hg species in the atmosphere. (C) 2000 Elsevier Science Ltd. All rights reserved.