It has been reported earlier that individual PAH can undergo chemical transformation when exposed to gases such as NO2 and O3. In this paper the possibility of artifact formation due to such transformations during sampling in ambient air is discussed. Addition of 1 ppm NO2 or 120 ppb HNO3 (g) during ambient air samling caused degradation of reactive PAH on particles and formation of the corresponding monoitro-PAH. The degradation due to NO2 exposure increased int he presence of acid on the particles. Addition of 100 ppb HNO2 (g) had not detectable effect. The results indicate that NO2 concentrations within the range normally formed in urban air may cause degradation of some reactive PAH. In the case of O3 addition (200 ppb), degradation was obtained in one of the experiments only