Time series ozone precursor data for the interpretation of surface ozone levels in
Central Taiwan
Kuang-Lin Yang, Jia-Lin Wang*
Department of Chemistry, National Central University, Chungli 320, Taiwan
cwang@cc.ncu.edu.tw
Abstract
Although air quality in Taiwan in general has been steadily improved over the
last decade, surface ozone levels appear to show the opposite trend. Seasonal ozone
episodes not only occur in urban areas, but also frequently affect rural areas of Taiwan.
While urban air pollution has been the focus of great interest in the past worldwide,
high ozone in rural areas constitutes even greater challenges in understanding the
causes. In light of the deteriorating trend of ozone, central Taiwan was selected as the
first area to be systematically investigated by the deployment of a network of 3
photochemical assessment monitoring stations (PAMS). The network uses automated
gas chromatographs performing year round hourly measurement of 56 ozone
precursors with either high abundance or significant photochemical reactivity.
Measurements made by PAMS in central Taiwan in the past 3 years have unveiled
numerous key mechanism of ozone formation in rural areas. Methodology of
disentangling the PAMS dataset of enormous size for revealing indicative
photochemical evidence has also been developed along with the operation of PAMS.
Compound pairs of various lifetime differences releasing from vehicular sources
were successfully identified from the PAMS data to serve as air indicator of air
parcels. It has been found that these compound ratios are in excellent agreement
with ozone hourly data, exhibiting coherent oscillations with ozone cycles with
maximum occurring around noon when the air parcel is more aged, and minimum
occurring in the morning and evening hours. Wind data also indicate that the more
aged air parcels thus containing higher ozone are being transported from distant areas.
For instance, at Chushun (竹山) station high ozone (also high VOC ratio) always
correlates with northwestlies and higher wind speeds in noon time.
Comparison of seasonal variation of ozone between summer and fall also sheds
light to the understanding of seasonal high ozone phenomenon in rural environments.
More elevated ozone in fall in central Taiwan is the combined effect of the average
lower inversion height and more complete photochemical reactions, which enhance
the VOC and NOx mixing ratios by a factor of 2-5, and the degree of aging by 1.8
between summer and fall values.
Unlike the extremely synchronous relationship with compound pairs of traffic
origin, peak ozone levels show little correlation with biogenic emissions represented
by isoprene whose abundance is driven by sunlight and thus peaks at noon. Together
with the decline in isoprene concentration in fall by nearly 2 fold suggests the higher
ozone in rural areas in fall is likely to be more controlled by the vehicular emissions
rather than biogenic in the upwind areas compounded by the concentration
enhancement by the lowering of inversion heights.