Simplification of an Inorganic Salt Thermodynamic Equilibrium Model and Its Application

In order to simulate the spatial distribution and temporal variation of volatile nitrate aerosols, it is necessary to introduce a thermodynamic equilibrium model and calculate the concentration under the state of equilibrium from the viewpoint of chemical thermodynamics. However, it costs much time to carry out a large scale and long term simulation because the thermodynamic equilibrium model contains a lot of species and reactions. The inorganic salt thermodynamic equilibrium model ISORROPIA is simplified using look up table method. A look up table for concentrations of 5 species (sulfate, nitrate, ammonium, nitric acid and ammonia) under the state of equilibrium is constructed according to various conditions of effecting factors (temperature, relative humidity, NH₃+NH₄⁺, HNO₃+NO₃^- and HH₂SO₄+SO₄^2-). The regional climate chemistry modeling system (RegCCMS) is an on line coupled modeling system which contains the regional climate model (RegCM3) and the tropospheric atmosphere chemistry model (TACM). The look up table scheme and the thermodynamic equilibrium model are introduced into TACM to treat nitrate, sulfate and ammonium. These two schemes are respectively adopted in RegCCMS to investigate spatial distribution of nitrate aerosol over China in January and July of 2003. Nitrate aerosol is mainly distributed in Sichuan, Yunnan, Guizhou, Hebei, Henan, Shandong in January and Henan, Hebei, Shandong, Jiangsu in July, which is roughly consistent with the distribution of the industry development. Influenced by temperature on thermodynamic equilibrium, wet scavenging of precipitation and the monsoon climate, the nitrate concentration is high in winter, and is very low in summer. The results of monthly average surface concentration show that look up table method has a good agreement with the thermodynamic equilibrium model in both quantity and distribution. The effect of look up table method is better on land than on ocean. Look up table method can save about 80% of the computing time compared with the thermodynamic equilibrium model, greatly improving the computational efficiency. The surface concentrations every 6 hours in several cities got from the look up table method and the thermodynamic equilibrium model are compared. The correlation coefficients of these two schemes in Chongqing, Xi'an and Xiamen are 0.99, 0.99, 0.97 in January and 0.75, 0.81, 0.86 in July. The results show that look up table method is able to simulate the characteristic of temporal variation. Due to higher temperature and humidity, differences between these two schemes are more obvious in July than those in January. As a whole, the look up table method can deal with the thermodynamic equilibrium processes successfully and reduce time for calculation significantly, therefore it has broad application prospect in research of climate effects of aerosols.