Abstract: To determine the emission standard for an area,the National Standard of China( GB /T13201-91)recommends the A- value method developed from single-box-model.In this paper,anew approach has been developed based on the combined distribution of wind direction and atmospher-ic stability instead of circle assumpption for control area. This new single box model has been appliedto calculate the annual SO2 emisson standard for Hangzhou,Ningbo,Wenzhou and Quzhou in Zhe-jiang province. The results for Wenzhou and Quzhou are different from those derived from the A-value method.