China national air quality monitoring network has become the core data source for air quality assessment and management in China. However, during network construction, the significant change in numbers of monitoring sites with time is easily ignored, which brings uncertainty to air quality assessments. This study aims to analyze the impact of change in numbers of stations on national and regional air quality assessments in China during 2013-18. The results indicate that the change in numbers of stations has different impacts on fine particulate matter (PM2.5) and ozone concentration assessments. The increasing number of sites makes the estimated national and regional PM2.5 concentration slightly lower by 0.6-2.2 µg m-3 and 1.4-6.0 µg m-3 respectively from 2013 to 2018. The main reason is that over time, the monitoring network expands from the urban centers to the suburban areas with low population densities and pollutant emissions. For ozone, the increasing number of stations affects the long-term trends of the estimated concentration, especially the national trends, which changed from a slight upward trend to a downward trend in 2014-15. Besides, the impact of the increasing number of sites on ozone assessment exhibits a seasonal difference at the 0.05 significance level in that the added sites make the estimated concentration higher in winter and lower in summer. These results suggest that the change in numbers of monitoring sites is an important uncertainty factor in national and regional air quality assessments, that needs to be considered in long-term concentration assessment, trend analysis, and trend driving force analysis.
中国国家环境空气质量监测网已成为中国空气质量评估和管理的核心数据来源. 在监测网的早期建设过程中, 监测站点数量随时间推移发生了显著变化, 这种变化会给空气质量长期趋势评估带来不确定性. 为了了解这种不确定性, 本研究基于2013年监测网站点(SON)和各年实际已有监测网站点(DON), 分别计算了2013--18年间全国和区域细颗粒物(PM2.5)浓度和臭氧浓度日最大值(O3_Max)在不同尺度下的评估值. 建设过程中新增站点对全国和区域PM2.5浓度均值估计的影响相对较小, 使得基于DON估计的全国和区域年均分别比基于SON的估计值低0.6--2.2µ m3和1.4--6.0µ m3, 其主要原因是监测网络在建设过程中逐渐从城市中心扩展到人口密度和污染物排放量相对较低的郊区. 但是, 建设过程中监测站点数量的变化对臭氧日最大值的估计影响较大, 会影响其长期趋势评估, 特别是使得2014--15年全国臭氧日最大值的年均浓度趋势从轻微上升转变为下降趋势. 此外, 监测站数量的增加对臭氧浓度评估的影响还呈现出显著(0.05的显著性水平)的季节性差异, 使基于DON估计的臭氧浓度值在冬季高于基于SON的臭氧估计值, 而在夏季低于基于SON的臭氧估计值. 这些结果表明, 监测站点数量的变化是国家和区域空气质量评估中的一个重要不确定性因素, 需要在空气质量长期趋势评估及其驱动力分析等研究中加以考虑.
Keywords: PM2.5; monitoring network; newly added sites; ozone.
© Institute of Atmospheric Physics/Chinese Academy of Sciences, and Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2022.