ALBERT

Description: ABSTRACT [1]  Continuous measurements of ozone (O 3 ), carbon monoxide (CO), beryllium-7 ( 7 Be) and lead-210 ( 210 Pb) in aerosols along with relative humidity (RH) were carried out at the Mt. Hehuan site (24.05°N, 121.10°E, 3380 m.a.s.l.) from September 2011 to September 2012 in order to investigate the seasonality of stratospheric influence (SI) and its effect on surface ozone concentration in the subtropical free troposphere over central Taiwan. During the 13-month period, the measured O 3 concentration fluctuated around a mean of 41 ppb and showed a broad springtime maximum and summertime minimum that can be attributed to regional circulation over subtropical Asia. Beryllium-7, CO, RH and the normalized fraction of 7 Be, f( 7 Be, 210 Pb), were used to identify SI days based on several criteria. The results showed that the seasonal patterns of SI days identified by different criteria were very similar, with the absolute values deviating within ~29% from those based on our reference methodology. Of the total analyzed days, fourteen SI days (approximately 4.6%) were found, indicating that SI phenomenon in the subtropical region is much less frequent than at northern mid-latitudes. About two-thirds of the SI days occurred in the winter, whereas none were found in the summer. The seasonality of SI occurrence could be related to the position of the subtropical jet stream (SJS). The proportion of surface O 3 derived from the stratosphere was estimated to be only 1.3% on a yearly basis, increasing to ~3.2% in the winter and 27% during the SI days, demonstrating the importance of downward transport of stratospheric air in affecting the level of surface ozone.