Investigation of Changing Seasonal Cycles of O3 over Japan from 1980 to 2015

Ambient air pollutions in megacities have adverse influences on human health. Japan located in Far Eastern Asia, has the largest megacities all over the world. The concentrations of primary air pollutions from 1970 to 2012 have improved owing to the Japanese air environmental quality standard countermeasure (Wakamatsu et al., 2012). However, surface O₃ have been gradually increasing at both ambient air monitoring stations and roadside air monitoring sites since 1978. Moreover, O₃ contributes to the chemical formation between NO and NO₂.

For better understandings of the O₃ behaviour, a seasonal changing investigation is necessary. However, previous studies mainly focused on long-term trends of whole Japan and the regional and seasonal changes are still stayed unclear. In order to achieve these changing, we compared the seasonal cycles of O₃ in between the 1980s (1980-1989) and the 2010s (2010-2015) at both ambient sites and roadside sites by using grand-level air monitoring stations in Japan. Considering the continuity of observation sites, hourly monitoring data at 644 ambient sites and 7 roadsides with long-term continuous records from 1980 to 2015 were utilized. The first step is dividing the data into 7 regions (Tohoku, Kanto, Chubu, Kansai, Chugoku, Shikoku and Kyushu, as shown in Fig 1 (b)). The next step is calculating the monthly average at each monitoring site, followed by averaging in each region. For quantifying the seasonal cycles of O₃, the sine function fitting described by Parrish et al. (2013) was applied.

As demonstrated in figure 1 (a), seasonal O₃ concentration in the 2010s increased compared to the 1980s in all regions. In addition, double peaks of O₃ mixing ratio appeared in spring and autumn. The fitting results indicated that spring maxima at Tohoku, Shikoku and Kyushu located in high and low latitude regions appeared earlier than the maxima at middle-latitude. Tanimoto et al. (2005) pointed out that the early spring peaks at high and low latitude depended on the O₃ transport patterns of Asian continental outflow. The comparison of the seasonal cycles between the 1980s and 2010s indicates that the shift of seasonal cycles to later in the year for all 7 regions. Specifically, seasonal cycles in the Kanto and Kyushu regions had a more than 2 days/year delay; similar change in the seasonal cycle has also occurred in South Korea (Jung et al.,2018). Nevertheless, background sites demonstrated that the seasonal peak in current cycles was earlier than in past cycles (Parrish et al.,2013).

As spatial distribution of seasonal shifting illustrated in Figure 1 (b), most monitoring sites are located near the coastal line. Focusing on the relationship between seasonal shifts and monitoring location, the stations with more than 1.0 days delay tended to be seen in southern and southwest parts of Japan. However, significant relationships were still unclear. In the future work, we will conduct the air quality modelling from 1980 to 2015 and investigate the mechanism of seasonal cycle delaying.

Acknowledgement

This research was supported by the Environment Research and Technology Development Fund (S-12) of the Ministry of the Environment, Japan.