Theoretical implication of reversals of the ozone weekend effect systematically observed in Japan
Abstract. Systematic changes of the ozone weekend effect are found over broad areas of Japan. These changes are characterized by (1) spatial reversals from a weekend increase in the vicinity of huge precursor source areas to a weekend decrease in the surrounding rural areas, and (2) temporal reversals from a weekend increase under relatively unsuitable meteorological conditions for ozone formation to a weekend decrease under relatively suitable conditions. We developed a simple numerical advection–reaction model to explain the relationship between the duration of advection and the supplied solar energy, which causes the daily maximum ozone concentration to be lower near the precursor source. Ozone isopleth diagrams for individual advection durations (equivalent to the distance from the source) for a wide range of initial precursor conditions show that both VOC-limited and NOx-limited regimes exist for each advection duration, but the area of NOx-limited regime becomes dominant as the advection duration increases because of the increased exposure of the air mass to solar energy. For given initial VOC and NOx concentrations, the area remote from the source becomes a NOx-limited regime even if the precursor source area is in the VOC-limited regime. The rate of reduction of weekend emissions of NOx is larger than that of VOC, causing a weekend increase in ozone inside an area of VOC-limited regime near the source, but a weekend decrease in remote areas with a NOx-limited regime. The boundary between these two ozone formation regimes depends on meteorological conditions: when sunlight intensity and temperature are relatively low, the change from a VOC-limited to a NOx-limited regime occurs at a point more remote from the source than when they are relatively high, which causes a prevailing ozone weekend increase over a wide geographical area on days with lower ozone potential. Therefore, observations of ozone weekend changes can be interpreted in light of the theoretical implications of our model; they can be used for determination of ozone formation regimes, which change in different locations and under different meteorological conditions.