Diurnal peroxy radical chemistry at a remote coastal site over the sea of Japan

Abstract

The Rishiri Fall Experiment (RISFEX) was performed in September 2003 at a remote coastal site (45.07°N, 141.12°E, and 35 m asl) on Rishiri Island in the Sea of Japan. Peroxy radicals were measured from 15–21 September using a peroxy radical chemical amplifier (PERCA) instrument. Simultaneously measured were O3, NO, NO2, CO, nonmethane hydrocarbons (NMHCs), biogenic volatile organic compounds (BVOCs), carbonyl compounds, aromatics, volatile organoiodines, organobromines, black carbon, aerosols and photolysis frequencies of j(O1D) and j(NO2) as well as meteorological parameters. The data set covers the measurements under relative background conditions and polluted episodes over the period. The midday 30‐min averages of peroxy radicals ranged from 10 to 40 pptv with the highest observed on the event when the air masses were transported over the polluted continent in northern China before reaching the site. The significant radical signals were observed repeatedly in the early morning and late afternoon when j(O1D) was greatly reduced, resulting in the diurnal cycles of peroxy radicals much broader than those expected from the photolysis of O3 alone. A box model based on RACM (Regional Atmospheric Chemistry Mechanism) was used to calculate OH, HO2 and RO2 concentrations from measured stable species and parameters. The calculated peroxy radical concentrations agree well with measured ones during 0900–1530 Japan Standard Time (JST) (JST = UT + 9 hours) from 19 to 21 September. However, in the early morning and later afternoon when monoterpenes were the dominant VOC, the model overpredicts peroxy radicals by up to 85%. The including of uptake of radicals to aerosols into the model cannot account for the discrepancy. An underprediction by the model up to 45% was observed during noon hours on 18 September when isoprene concentration was high. It appears that unknown terpenes are present, which might produce extra radicals beyond the predicted by the model. The measured‐to‐modeled ratio of peroxy radical concentrations is near unity for [NO] < 120 pptv, but rises up to 15 for [NO] > 120 pptv, suggesting the presence of an unknown radical production process which is related with high NO concentration.