Secondary aerosol formation through photochemical reactions estimated by using air quality monitoring data in Taipei City from 1994 to 2003

Analyses of diurnal patterns of PM10 in Taipei City have been performed in this study at different daily ozone maximum concentrations (O3,max) from 1994 to 2003. In order to evaluate secondary aerosol formation at different ozone levels, CO was used as a tracer of primary aerosol, and O3,max was used as an index of photochemical activity. Results show that when O3,max exceeds 120 ppb, the highest photochemical formation of secondary aerosol can be found at 15:00 (local time). The produced secondary aerosol is estimated to contribute 30 μg m−3 (43%) of PM10 concentration, and about 77% of the estimated secondary PM10 is composed of PM2.5. The estimated maximum concentration of secondary aerosol occurs 2–3 h later than the maximum ozone concentration. As revealed in an O3 episode, PM10 and PM2.5 vary consistently with O3 at daytime, which suggests that they are mostly secondary aerosols produced from photochemical reactions. Data collected from Taipei aerosol supersite in 2002 indicates that for all O3 levels, summertime PM2.5 is composed of 23%, 20%, 9%, and 7% of organic carbon, sulfate, nitrate, and elemental carbon, respectively. Aerosol number and volume size spectra are dominated by submicron particles either from pollution transport or photochemical reactions. Secondary PM10 concentrations show increasing tendencies for the time between 15:00 and 19:00 from 1994–1996 to 2001–2003. This reveals that the abatement of secondary PM10 becomes more important after pronounced primary PM10 reduction in a metropolis.