The Variation of Ozone Concentrations in Urban Districts of Hangzhou and Their Relationship with Meteorological Factors

Based on the observation data of near surface O₃ and meteorological factors in 7 monitoring sites of Hangzhou urban area from 2005 to 2007, the O₃ concentration and their temporal variation characteristics are researched. The peak hourly concentration and its nonattainment rate of Hangzhou are compared with those of other major cities at home and abroad. The variation of O₃ concentration and its nonattainment rate under different weather conditions are studied, and the variation of O₃ concentration with different levels of UV intense is discussed. The results show that the ozone concentration increases significantly year by year, the annual mean ozone concentration and the peak hourly concentration in 2007 are 44 μg.m^-3 and 348 μg.m^-3, respectively, showing a growth of about 20%from 2006. The ozone nonattainment rate in 2007 is 13.2%, about twice as high as that in 2006. The percentages of ozone nonattainment days from June to September are 75% in total nonattainment days. The appearing time of ozone nonattainment is from 10:00 to 18:00, and the most frequent appearing time of highest nonattainment concentration is around 14:00. The higher ozone nonattainment rate occurs in summer, primarily under the wind direction of SSE, lower in spring, still lower in fall, and the low est value occurs in winter, taking on a very obvious seasonal variation trend. The variation of ozone concentration in sites differs during four seasons, showing highest concentration in summer and lowest in winter where concentration are higher, but on the others sites highest value occurs in spring and lowest in winter. The diurnal mean concentration during the rainy season is low er with effects of air mass and meteorological conditions brought by summer monsoon. The diurnal variation ranges of ozone concentration are different in four seasons, showing higher peak value and low er valley in summer and fall. The concentrations of peak and valley are all low er in winter. The diurnal variation range of higher concentration sites is bigger than that of lower concentration sites. The peak hourly concentration in Hangzhou is close to the levels of Texas, US and Hong Kong, China, but the nonattainment range is higher. During the daytime, the highest ozone concentration occurs under the wind direction of SE, and during nighttime under the wind direction of E. The variation of ozone concentration changes with synoptic systems. The ozone concentrations in areas controlled by high pressure passage and high pressure systems are higher, and percentages of nonattainment days are 37.8% and 24.4% respectively. The ozone concentration is higher on the UV intense days. The ozone concentration and intensity of UV radiation are significantly correlated. The results of multiple regression analysis between ozone concentrations and various weather factors show that the temperature, relative humidity, duration of sunshine are major factors that affect ozone concentration. The ozone concentration is remarkable negatively correlated with relative humidity and visibility (P < 0.05), and is significantly positively correlated with the temperature and sunshine (P < 0.05). The cause is that higher temperature, lower relative humidity and longer time of sunshine may accelerate the rate of photochemical reaction, which has a positive impact on O₃ generation, and leads to higher concentration of ozone, otherwise ozone concentration is lower.