汾渭平原至黄土高原不同海拔高度地区近地面臭氧浓度差异

利用2017~2019年中国生态环境监测总站逐小时地面臭氧(O₃)和二氧化氮(NO₂)数据, 结合再分析气象数据集,分析了从汾渭平原至黄土高原三个不同海拔高度的典型城市郊区(西安:500m、榆林:1100m和鄂尔多斯:1300m)O₃浓度的季-月-日变化特征,以及导致三地O₃浓度差异可能的化学和气象成因.结果表明:与其他季节比较,夏季三地的O₃浓度都较高且差值较小,其中西安昼间O₃的净增量最大、夜间净减量也最大且前体物NO₂浓度最高,说明西安夏季白天O₃光化学反应最强烈、夜间NO滴定O₃效应也最强,榆林其次、鄂尔多斯最弱;冬季三地的O₃浓度都较低且差异较大,其中西安最低、鄂尔多斯最高,可能是由于冬季白天光化学反应都弱、夜间NO滴定O₃效应差异和高海拔地区背景O₃浓度高共同导致的,反映了三地O₃浓度水平差异不仅受不同NO_x水平下局地化学作用影响,还由区域背景值决定.分析还发现,高海拔的鄂尔多斯和榆林二地O₃浓度在上午升高的速率快于西安,与二地边界层向上发展的速率一致,可能是由于此时的夹卷效应将高海拔自由对流层的高背景O₃向下湍流输送所致.在每个季节雨天夜间,三地的O₃浓度均高于其阴、晴天,但是这一差异在西安较弱,而在榆林和鄂尔多斯较强,这进一步意指高海拔地区近地面O₃在雨天夜间更强烈地受到高浓度背景O₃的影响,一方面是通过降水的拖曳作用,另一方面是因为雨天夜间NO的滴定作用减弱.本研究通过长期观测资料分析,推测了不同海拔高度对近地面O₃的影响机制,还需在更多地区进行分析和利用模式开展验证.

Analysis on the difference of near-surface ozone concentration between the Fenwei Plain and the Loess Plateau at different altitudes

Using the hourly dataset of ground-level ozone (O₃) and nitrogen dioxide (NO₂) collected by the General Ecological and Environmental Monitoring Station of China from 2017 to 2019, combined with reanalysis of meteorological datasets, we analyze the O₃ concentrations characteristics in seasonal, monthly and diurnal scales in typical suburban areas (Xi'an, 500m; Yulin, 1100m and Ordos, 1300m above sea level) at three different altitudes from the Fenwei Plain to the Loess Plateau. The potential chemical and meteorological causes of the differences in O₃ concentrations at the three locations are also conducted. The results show that the O₃ concentrations are high at all three sites with less differences of concentrations in summer. In Xi'an, the net O₃ increase in daytime, the net decrease in nighttime and the NO₂ concentrations (one of the precursors of O₃) are the highest among the three sites in summer, indicating that O₃ photochemical production in daytime and the NO titration O₃ effect in nighttime are the strongest in Xi'an, followed by Yulin and Ordos. In winter, O₃ concentrations in all three sites are low and the lowest in Xi'an and the highest in Erdos, with large differences. This may be due to the combination effect of weak photochemical reactions in daytime, differences in NO titration O₃ effects at night and background O₃ concentrations at different altitudes, reflecting that the O₃ concentration levels at the three sites are not only dominant by local chemistry at different NO_x levels, but also determined by regional background values. The study also reveals that O₃ concentrations in Ordos and Yulin (at the higher altitudes) increase faster in the morning than in Xi'an, consistent with the faster upward increase of the boundary layer height in two sites in the morning, which could be caused by the entrainment effect, downward transport high background O₃ from free troposphere to the surface by turbulence at this period. During the rainy night in each season, O₃ concentrations in all three sites are higher than those on cloudy and sunny days, with a small difference in Xi'an and significant differences in Yulin and Ordos, which further implies that O₃ concentrations at sites with high altitudes could be influenced strongly by high background O₃ concentrations, partly through the dragging effect of precipitation, and partly because of the weakened titration of NO at night on rainy days. In this study, the hypothesis of the altitude impact on surface O₃ has been proposed through the analysis of long-term observations, which also needs validate by model simulations in widely regions.