Interpretation of ground-level ozone episodes with atmospheric stability index measurement.

Purpose: This paper presents a novel approach to interpret ground-level O(3) with the measured atmospheric stability index (ASI).

Methods: O(3) concentrations were monitored by automatic analysers at three types of stations: traffic site, residential site and regional background site in 2005, and the ASI was simultaneously measured by observing radon and its short-lived decay products.

Results: The observed results showed a clear annual variation of O(3) concentrations with a maximum in spring, relatively high at the regional background site over 120 ppb, and lower at the residential and traffic sites at about 70 ppb. ASI gives information about the dilution properties of the lower boundary layer and allows to highlight the relevant role of the dilution factor in determining atmospheric pollution events. We demonstrated the analysis of O(3) night peak episodes with vertical wind and ASI.

Conclusions: With the advantage of ASI and vertical wind profiles, it was possible to isolate particular photochemical pollution phenomena of O(3) peaks from the free troposphere reservoir or formed by local reactions. This shows that the index constitutes a powerful and valuable tool for describing O(3) night-peak episodes at background station.