Seasonal cycles of ozone and oxidized nitrogen species in northeast Asia 1. Impact of regional climatology and photochemistry observed during RISOTTO 1999-2000

Measurements of oxidized nitrogen species, including peroxyacetyl nitrate (PAN), nitrogen oxides (NO_x), nitric acid (HNO₃), and nonmethane hydrocarbons (NMHCs), were made along with ozone (O₃) and carbon monoxide (CO) at Rishiri Island, a remote island in northern Japan, as part of the Rishiri Island Study of Oxidants and Transport for Tropospheric Ozone (RISOTTO). Full seasonal observations of O₃, CO, NMHCs, and PAN, together with data sets of NO_x, and HNO₃ for 6 months reveal short-term and seasonal characteristics of chemistry of the air masses in northeast Asia. Temporal variations of O₃, PAN, and HNO ₃ typically show day-to-day variations in winter and diurnal variations in summer, dominated by long-range transport and local photochemistry, respectively. Seasonal variations of O₃ and PAN show a spring maximum and a summer minimum, which are consistent with previous field observations made in Europe and North America. Air mass segregation based on back trajectory calculation suggests that PAN, which is photochemically produced in continental source regions, is most effectively transported to remote sites in spring owing to low temperatures in this season, while HNO₃ is not effectively transported due to its high deposition velocity. It is concluded that transport of polluted air masses from continental source regions considerably enhances the April maximum in O₃ and PAN observed at remote sites in northeast Asia. Back trajectory analysis also indicates that the seasonal cycles of PAN in the Eurasian continental background air masses are maximum in spring, minimum in summer, and show a secondary maximum in fall in contrast to NO_x and HNO₃ which have a summer maximum and a winter minimum.