1961~2019年黄河流域大气自净能力指数变化特征分析

利用2015~2019年环境空气质量监测数据和黄河流域73个站点1961~2019年的气象观测资料,对11个站点的大气自净能力指数的适用性及其与环境空气质量之间的关联性进行了验证,并分析了黄河流域大气自净能力指数的时空变化特征及影响因子.结果表明,1961~2019年黄河流域大气自净能力指数整体呈下降趋势,下降速率为每10a下降0.18t/(d⋅km^(2)),平均值为4.44t/(d⋅km^(2)),且在1969年达到最大值,为5.32t/(d⋅km^(2)),2011年达到最小值,为3.81t/(d⋅km^(2));黄河流域73个站点中有64个站点大气自净能力指数呈下降趋势,50个站点呈显著和极显著下降趋势.从年内变化看出,黄河流域大气自净能力指数最高值出现在4月,为5.30t/(d⋅km^(2)),最低值出现在1月,为3.48t/(d⋅km^(2)).在年空间分布上,黄河流域大气自净能力指数分布以青海西南部,山东、四川大部分地区,内蒙古、宁夏以及甘肃少部分地区,陕西西安,山西的五寨和右玉等大气自净能力较好,大气自净能力指数在4.69~7.18t/(d⋅km^(2)),其余地区大气自净能力相对较差.影响因子中,混合层高度与大气自净能力指数呈极显著正相关,相关系数为0.63(n=4307);小风日数与大气自净能力指数呈极显著负相关,相关系数为-0.78(n=4307),日平均风速≥2.5m/s的日数及日平均风速≥5.5m/s的日数与大气自净能力指数均呈极显著正相关,相关系数分别为0.78和0.55(n=4307);降水日数与大气自净能力指数呈极显著正相关,相关系数为0.18(n=4307),中雨及以上日数与大气自净能力指数呈显著正相关,相关系数为0.03(n=4307),黄河流域小雨对大气自净能力的影响明显高于中雨.逐步回归分析显示,大气自净能力指数主要受日平均风速≥2.5m/s的日数、降水日数和混合层高度的综合影响,其中,日平均风速≥2.5m/s的日数对大气自净能力指数的贡献率最大,其贡献为正.

Trends in Annual and Seasonal Pan Evaporation in the Lower Yellow River Basin from 1961 to 2010

The annual and seasonal trends in pan evaporation in the lower Yellow River Basin based on quality-controlled data from 10 meteorological stations in 1961-2010 are analyzed.The causes for the changes in annual and seasonal pan evaporation are also discussed.The results suggest that,despite the 1.15 C increasing in annual mean surface air temperature over the past 50 years(0.23 C per decade),the annual pan evaporation has steadily declined by an average rate of-7.65 mm per year.By comparison,this change is greater than those previously reported in China.Significant decreasing trends in annual pan evaporation have been observed at almost all stations.As a whole,seasonal pan evaporation decreased significantly,especially in summer,whereas seasonal temperature increased significantly,except in summer.Thus,the pan evaporation paradox exists in the lower Yellow River Basin.The trend analysis of other meteorological factors indicates significant decrease in sunshine duration and wind speed,but no significant variations in precipitation and relative humidity at annual and seasonal time scales.By examining the relationship between precipitation and pan evaporation,it did not show a concurrent decrease in pan evaporation and increase in precipitation.The partial correlation analysis discovered that the primary cause of decrease in annual and seasonal pan evaporation is the decrease in wind speed.A further examination using a stepwise regression shows that decrease in wind speed and sunshine duration,and increase in mean temperature are likely to be the main meteorological factors affecting the annual and seasonal pan evaporation in the lower Yellow River Basin over the past 50 years.