Detection of peroxyacetyl nitrate at phytotoxic level and its effects on vegetation in Taiwan

Symptoms of toxicity of peroxyacetyl nitrate (PAN), namely bronzing and silvering, were initially discovered on lettuce plants in the Taipei area in 1989. The cause and effect relationship was established by the detection of large ambient concentrations of PAN and by reproduction of the symptoms upon exposure to PAN. In the following years, several other plants including spinach, Solanum nigrum, Nicotiana excelsior, and Galinsoga parvifiora were also found with symptoms of a similar nature. Symptoms were artificially reproduced in these plants on exposure to PAN proving that they were also sensitive to PAN. These PAN-type symptoms were prominently different from those caused by ozone or other general pollutants described in the literature. Based on these symptoms, PAN is believed to occur in all three major urban areas of Taiwan, namely Taipei in the north, Taichung in the center, and Kaohsiung in the south. Hydrocarbons emitted from numerous motorcycles are believed to be the principal precursor of this photochemical pollutant. Ambient measurements of PAN in the Taipei area showed that on calm sunny days, PAN typically exceeded 4–12 ppb, and native Sword-leaf lettuce began to show the symptoms the next morning. From July 1992 to April 1993 there were at least 34 d with PAN at significantly phytotoxic concentrations that affected lettuce and other plants. The concentration of PAN was highly correlated with that of ozone according to the regression equation Y(ozone/ppbv) = 16.9X(PAN/ppbv) + 13.5 in warm seasons, and Y(ozone/ppbv) = 3.OX(PAN/ppbv) + 11.1 in cooler seasons, indicating the co-occurrence but distinct ratios of these pollutants.