西宁近郊大气氮干湿沉降研究

Dry and Bulk Nitrogen Deposition in Suburbs of Xining City

本研究在青海省西宁市城郊二十里铺莫家泉湾气象站开展了完整的2年(2014和2015年)大气无机氮干沉降和混合沉降(湿沉降加部分干沉降)监测.干沉降估算通过被动采样器采集的NH_3和NO_2浓度和Geos-Chem全球化学模式模拟的气体干沉降速率相乘获得;混合沉降的测定采用雨量器.结果表明,气态NH_3和NO_2年均浓度分别为8.8μg·m-3和19.6μg·m-3,且2015年NH_3月均浓度显著高于2014年.NH_3浓度的季节变化呈现春夏高、秋冬低的特征,而NO_2浓度季节变化幅度较小.降水中NH_4^+-N和NO-3-N年均浓度为2.2 mg·L^(-1)和1.8 mg·L^(-1),秋季降水NH_4^+-N浓度比其他季节低55%,而NO-3-N浓度在秋冬季比春夏季高约26%.气态NH_3和NO_2的干沉降量分别为9.0 kg·(hm^2·a)^(-1)和2.8 kg·(hm^2·a)^(-1),降水中NH_4^+-N和NO-3-N的混合沉降量分别为7.6 kg·(hm^2·a)^(-1)和6.2 kg·(hm^2·a)^(-1).还原态氮(NH_3和NH_4^+-N)在氮沉降中占主导地位.大气氮素总沉降(干沉降加混合沉降)量为25.6 kg·(hm^2·a)^(-1),为城郊农田提供重要的环境养分;但这一氮素输入量超过了陆地生态系统氮沉降临界负荷[10~20 kg·(hm^2·a)^(-1)],意味着研究区周边林地(如北山)和水体生态系统面临"氮饱和"的环境风险.

This study conducted a full two-year（2014 and 2015） measurements of dry and bulk deposition of atmospheric inorganic nitrogen（N） at the suburb of Xining city,Qinghai province.Dry N deposition fluxes were calculated by multiplying the atmospheric concentrations of NH_3 and NO_2measured using passive samplers with the modeled dry deposition velocities provided by the GEOS-Chem global chemical transport model,while bulk N deposition fluxes were measured using precipitation gauge.Annual mean concentrations of gaseous NH_3 and NO_2averaged 8.8 and 19.6 μg·m-3,respectively,with significantly higher values in 2015 than in 2014.Seasonal mean NH_3 concentrations were higher in spring and summer than in autumn and winter,but the concentrations of NO_2 changed little from season to season,with a small peak in autumn.Annual mean concentrations of NH_4~＋-N and NO_3^--N in precipitation averaged 2.2and 1.8 mg·L^-1,respectively.Concentrations of NH_4~＋-N in autumn were ~ 55% lower than those in other seasons,whereas those of NO_3^--N in autumn or winter were ~ 26% higher than those in spring and summer.Dry deposition of NH_3 and NO_2was 9.0 and 2.8kg·（hm^2·a）^-1,respectively.Bulk deposition of NH_4~＋-N and NO_3^--N in precipitation was 7.6 and 6.2 kg·（hm^2·a）^-1,respectively.Reduced N（gaseous NH_3 and NH_4~＋-N in precipitation） was the dominant form of N deposition.The total dry and wet N deposition was25.6 kg·（hm^2·a）^-1,which represented significant nutrient input from the environment to the suburban farmland,but this amount of deposited N exceeded the critical loads [10-20 kg·（hm^2·a）^-1]of terrestrial ecosystems,suggesting a risk of＂N saturation＂in the local natural environment.