北京市内到郊区氮沉降时空变化特征

对北京市区到远郊梯度带上的4个采样点(北京师范大学(BNU)、奥林匹克森林公园(AS)、减河公园(JH)、延庆上辛庄(YQ))降雨和针叶树穿透雨雨量及其化学成分进行了监测,分析了从市中心到远郊的氮沉降时空变化规律.结果表明:梯度带上各点铵态氮(NH+4-N)与硝态氮(NO-3-N)均为5、6月最大,7月开始迅速减小,雨季降雨的稀释作用是主要原因.在空间上,NH+4-N与NO-3-N沉降量大小为BNUJHASYQ,4个样点对应的可溶性无机氮(铵态氮与硝态氮)总沉降量分别为22.6、13.7、12.1、5.42 kg·hm~(-2)a-1(以N计).无机氮沉降以湿沉降为主,4个样点上湿沉降所占比例分别为BNU 72.44%、AS 65.97%、JH 62.78%、YQ 93.86%;沉降成分以铵态氮为主,所占比例分别为BNU56.41%、AS 59.47%、JH 61.21%、YQ 63.33%,从城区到远郊逐次增大.国内外同类研究综合表明,湿沉降中无机氮浓度均是工业区市区郊区远郊区,北京降雨中无机氮浓度相对较高.大多数研究区的城区与郊外大气氮沉降均是以铵态氮为主,与市区交通及生活源氮氧化物排放较高的实际并不一致,其成因与机制尚不明确.

Spatial and temporal variation of dissolved inorganic nitrogen deposition along an urban to rural transect in Beijing

Four sampling sites were selected to study the characteristics of atmospheric nitrogen deposition along an urban to rural transect in Beijing. Weekly bulk precipitation and canopy throughfall under coniferous trees were collected from each site during May to September, 2014 and analyzed for chemical composition. Spatial and temporal variations and the deposition flux of dissolved inorganic nitrogen along this transect were further investigated. Results showed that concentrations of NH₄⁺-N and NO₃^--N at all sampling sites were higher in May and June, but decreased rapidly during July due to monsoonal rainfall dilution. The wet and total nitrogen deposition fluxes decreased by the order of BNU> JH> AS> YQ. Total deposition flux of dissolved inorganic nitrogen (NH₄⁺-N and NO₃^--N) at BNU, JH, AS and YQ sites were 22.6, 13.7, 12.1, 5.42 kg (N)·hm^-2·a^-1, respectively. It was higher at city center and lower at the suburban sites. Wet deposition dominated the inorganic nitrogen deposition, with the percentage of 72.44%, 65.97%, 62.78%, 93.86% at BNU, AS, JH and YQ, respectively. The principal composition of deposition was NH₄⁺-N, which accounted for 56.41%, 59.47%, 61.21%, 63.33% at BNU, AS, JH, YQ, respectively, and increased gradually from the city to the suburbs. Similar studies worldwide suggested that deposition flux decreased by the order of industrial zone> urban area> suburbs> outer suburbs. Concentrations of NH₄⁺-N and NO₃^--N at sites in Beijing were much higher. Atmospheric nitrogen deposition at most urban and suburban sites from previous studies was dominated by NH₄⁺-N, which was inconsistent to the fact that the urban transport and living sources generated significant nitrogen oxide emissions.