Abstract: According to the main chemical reaction by which the HNO3 is formed from NOx in the atmosphere,the NO2 diffusion equation involving the chemical reaction is solved to obtain the distribution of HNO3 vapor.The HNO3 vapor is scavenged and precipitated to the ground by rain at the same time of its diffusion.Then,the diffusion equation of HNO3 vapor is solved incorporating the washout of rain to obtain the distribution of precipitated HNO3 on the ground in the 24 hours,by which we can estimate the level and domain size of the pollution of HNO3 approximately.