Theoretical predictions of the atmospheric equilibrium involving $HNO_3, NH_3 and NH_4NO_3$ were compared with atmospheric measurements of particulate nitrate$(NO_3^-)$, $HNO_3$ and $NH_3$ concentration by triple filter pack sampler and Andersen air sampler in Seoul from 8th to 11th Oct 1991. The measured $HNO_3-NH_3$ concentration product K was higher than equilibrium costant Kc calculated from thermodynamic data of $HN_4NO_{3(s)} -HNO_{3(g)} -NH_{3(g)}$. The cause of K is greater than Kc was the result of air pollution, partcicularly anthropogenic $NH_3$.