Ozone reservoir layers in a coastal environment-A case study in southern Taiwan

The air layer between the nocturnal boundary layer and the top of the daily mixing layer in an ozone-polluted area is known to serve as an ozone reservoir since the ozone that is produced in the mixing layer on the preceding day is effectively preserved throughout the night in the air layer. Ozone reservoir layers existing at night can enhance the accumulation of surface ozone on the following day. However, our knowledge of the characteristics of ozone reservoir layers and their effects on the daily ozone accumulations is limited. In this work, ozone reservoir layers were experimentally investigated at a coastal, near-mountain site in southern Taiwan, 30 km away from the coastline. Tethered ozone soundings were performed to obtain vertical profiles of ozone and meteorological variables during a four-day ozone episode in November 2006. Observation-based methods are adopted to evaluate the effects of the ozone reservoir layers on the surface ozone accumulation during the four-day ozone episode. Ozone reservoir layers were found to develop every evening with a depth of 1200-1400 m. Ozone concentrations within the reservoir layers reached over 140 parts per billion (ppb). From each evening to midnight, the size of the ozone reservoir layer and the ozone concentration inside dramatically changed. As a result, a concentrated, elevated ozone reservoir layer formed with a depth of 400 m at 800-1200 m every midnight. For the rest of each night, the elevated ozone reservoir layer gradually descended until it reached 500-900 m in the next morning. The observed ozone reservoir layer is formed by the invasion of a cool, marine air mass into a relatively warm, ozone-rich mixing layer in the evening. The descending is related to nocturnal coastal subsidence as well. The ozone concentration at the study site was maximal at 15:00-17:00 LT daily because of the addition of the daily produced ozone on the preceding day. The rate of increase of surface ozone concentration due to the downward mixing of the ozone in the ozone reservoir layers can be as high as 12-24 ppb/h in the late morning. The contribution of the ozone carried over from the preceding day can be 60-85 ppb, which contributes over 50% to the daily ozone pollution as compared with ozone produced on the study day.