Concentration variations of total reactive nitrogen and total nitrate during transport to Fukue Island and to Cape Hedo, Japan in the marine boundary layer

Abstract

We conducted continuous measurements of NOy and total nitrate (T.NO3 = HNO3 + NO3−(p)) at Fukue Island, Nagasaki, and Cape Hedo, Okinawa, Japan. We compared variations of NOy or T.NO3 concentrations measured at two remote sites when the air masses were originated from the same regions of the Asian Continent. Long-range transport events from the Asian Continent were extracted by CO concentration peaks at Fukue and Hedo and backward trajectory analyses. We compared the transport time difference at two sites from the Asian continent with the ratios of NOy at Hedo to that at Fukue (=R(NOy)) to find the predominant factor in NOy removal. R(NOy) were less than unity and decreased with the transport time difference. The ratios of NOy/CO at Hedo to those at Fukue (R(NOy/CO)) was assumed to be affected by the deposition process. R(NOy/CO) showed negative correlation with transport time difference as the same the case of R(NOy). This indicates that NOy was mainly removed by deposition rather than dilution. Air mass ages were classified by the ratio of T.NO3 to NOy at Fukue: T.NO3/NOy < 0.2 was fresh air, 0.2–0.4 was middle-aged, and greater than 0.4 was aged. The ratios of T.NO3/NOy at Hedo to those at Fukue (R(T.NO3/NOy)) in the fresh air mass were greater than unity. In contrast, R(T.NO3/NOy) values in the aged air mass were unity and were less correlated with transport time difference. This indicates that T.NO3 generation proceeded in fresh air mass. On the other hand, T.NO3 generation rate was comparable to removal rate in the aged air mass. According to the relationship between R(NOy/CO) and transport time difference, the removal rate constant and NOy lifetime were (1.24 ± 0.33) × 10−5 s−1 and 18–31 h, respectively.