Simulation of nitrate aerosol concentrations over East Asia with the model system RAMS-CMAQ

Abstract

The Models-3 Community Multi-scale Air Quality modeling system (CMAQ) coupled with the Regional Atmospheric Modeling System (RAMS) is used to analyse the geographical and the seasonal characteristics of nitrate aerosol (ANO₃) concentration distributions over East Asia. Three-dimensional concentrations in January,March, April, July and October of 2001 are simulated, and for the evaluation of model performances, the simulated values of wind direction, wind speed, temperature, specific humidity, nitric oxide (NO), nitrogen oxide (NO₂), ANO₃ and ammonium aerosol (ANH₄) are compared with the observational data obtained onboard the P-3B aircraft during its four consecutive flights over the South China Sea, the East China Sea, the Yellow Sea and the water areas to the east of Japan conducted on 13, 17, 18 and 21 March 2001. The observed data are 5-min averaged and the model results with a 1-hour temporal resolution are interpolated to the aircraft location and time using trilinear interpolation. Comparison shows that the observed values exhibit strong temporo-spatial variations, and the model reproduces these variations reasonably well. The simulated values of wind direction, wind speed, temperature and specific humidity are generally in good agreement with the observed ones, their correlation coefficients reach 0.66, 0.94, 0.99 and 0.96, respectively. Comparison also shows that the average mixing ratios of modelled and observed NO, NO₂, ANO₃ and ANH₄ agree reasonably well with each other, the correlation coefficient for ANO₃ and ANH₄ are larger than 0.8, but the simulated standard deviations are smaller than the observed one, and the correlation coefficient for NO and NO₂ are 0.48 and 0.44.

Analysis of horizontal distributions of monthly averaged ANO₃ concentrations in the boundary layer indicates that the ANO₃ mixing ratios have noticeable differences among the four seasons. Generally the concentrations are high in the winter, spring and fall and low in the summer, and seasonal variations are typically strongest over China and Japan. Highest concentrations are found over eastern China where emissions of nitrogen oxides and ammonia are high and long-range transport may cause elevated concentrations in remote areas under favourable meteorological conditions, but the seasonal variation in the ANO₃ concentrations is heavily influenced by the changes in precipitation and temperature.