春季南海东沙岛硝酸盐干沉降通量、形成机制及其来源

Dry deposition fluxes,formation mechanisms and sources of nitrate in total suspended particles in springtime on Dongsha Island,South China Sea

硝酸盐是大气中主要的酸性离子,也是大气活性氮沉降主要的组成部分。中国目前对大气硝酸盐形成机制及其来源的研究主要集中在陆地区域,对海洋气溶胶的硝酸盐的形成机制及其来源的研究较少。本研究于2013年春季(3—5月),在中国南海东沙岛共计采集了86个总悬浮颗粒(TSP)气溶胶样本,分析了TSP样品中NO_(3)^(-)浓度、δ^(15)N-NO_(3)^(-)和δ^(18)O-NO_(3)^(-)值。结果表明:东沙岛春季TSP中NO_(3)^(-)浓度呈现逐月下降的趋势。基于NO_(3)^(-)浓度估算东沙岛春季NO_(3)^(-)-N干沉降通量范围为(1.0±0.5)—(3.4±1.7)mg·d^(-1)·m^(-2)(以N计,余同)。δ^(18)O-NO_(3)^(-)值在3月和4月(76.1‰±3.8‰和79.1‰±5.6‰)明显高于5月(67.0‰±7.5‰),表明东沙岛3月和4月硝酸盐的形成路径不同于5月。贝叶斯同位素混合模型计算结果显示:N_(2)O_(5)参与的路径(N_(2)O_(5)+H_(2)O/Cl-和NO_(3)^(+)VOCs)在3月和4月生成了37.2%和43.3%的硝酸盐,5月NO_(2)+·OH路径是硝酸盐的主要形成路径,形成了80.2%的硝酸盐。δ^(15)N-NO_(3)^(-)值和气团后向轨迹表明,东沙岛春季不同月份TSP中NO_(3)^(-)的来源不同,3月和4月以陆地源为主,5月主要受海洋源影响。

Background,aim,and scope Nitrate,the primary acidic ion in the atmosphere,is the main component of deposited atmospheric reactive nitrogen.The concentrations of nitrate and its precursors have shown increasing trends,causing a number of climatic effects and altering ecological circumstances,including a reduction in surface radiation and influences on the regional climate.The sources and formation mechanisms of atmospheric nitrate have been widely studied inland,but marine aerosols have rarely been focused on.No studies on dry deposition fluxes of the aerosol nitrate,nitrogen or oxygen isotope composition of nitrate in spring near Dongsha Island have been reported.The aims of this study were to quantify the dry deposition nitrate fluxes,the associated formation mechanisms and the aerosol nitrate sources near Dongsha Island,South China Sea.Materials and methods A total of 86 total suspended particulate(TSP)aerosols were collected at Dongsha Island in the South China Sea in spring 2013(Mar.—May),and the NO_(3)^(-),δ^(15)N-NO_(3)^(-)andδ^(18)O-NO_(3)^(-)concentrations in the TSP were analyzed.To analyze the possible origin of spring aerosol air masses at Dongsha Island,the backward trajectories of the air masses were analyzed over 3 d(72 h)using the HYSPLIT(Hybrid Single Particle Lagrangian Integrated Trajectory)model for retracing.This study calculated the relative contributions of different pathways to NO_(3)^(-)in the TSP by a Bayesian isotope mixing model.Results The NO_(3)^(-)concentrations were significantly lower near the island than those in urban China's Mainland and coastal regions(Mar.,4.0μg·m^(-3);Apr.,2.9μg·m^(-3);and May,1.2μg·m^(-3)).The NO_(3)^(-)concentration in the TSP of Dongsha Island showed a monthly decreasing trend from Mar.to May,which was related to the air mass and rainfall sources at Dongsha Island during springtime.The dry deposition flux of NO_(3)^(-)-N on Dongsha Island was 1.0—3.5 mg·d^(−1)·m^(-2),as estimated by the concentrations of NO_(3)^(-).Theδ^(18)O-NO_(3)^(-)values in Mar.and Apr.(76.1‰±3.8‰and 79.1‰±5.6‰)were higher than those in May(67.0‰±7.5‰),indicating that the nitrate formation pathways in Mar.and Apr.were different from those in May.The contribution of the NO_(2)+·OH pathway to NO_(3)^(-)in the TSP of Dongsha Island,China,in Mar.and Apr.(56.7%—62.8%)was significantly lower than that of the NO_(2)+·OH pathway in May(80.2%).In contrast,the pathways of NO_(3)^(+)VOCs and N_(2)O_(5)+H_(2)O/Cl−contributed more to the TSP NO_(3)^(-)at Dongsha Island in Mar.and Apr.(37.2%—43.3%)than in May(19.8%).We further estimated the 15N fractionation factors between NOx and particulate nitrate.The 15N fractionation factors formed by the NO_(2)+·OH,N_(2)O_(5)+H_(2)O,and NO_(3)^(+)VOCs pathways were−2.0‰—0‰,24.6‰—26.6‰,and^(−1)8.4‰—−17.8‰,respectively.Theδ^(15)N-NO_(3)^(-)values in Mar.,Apr.and May were−2.1‰±0.9‰,−1.4‰±1.4‰and^(−1).1‰±1.1‰,respectively,at Dongsha Island during springtime.Discussion The Bayesian isotope mixing model results showed that the pathways of N_(2)O_(5)+H_(2)O and N_(2)O_(5)+Cl−contributed 10.8%and 14.8%of the nitrate formation in Mar.and Apr.,respectively.The NO_(2)+·OH pathway contributed 80.2%of the TSP nitrate formation pathways in May.The contribution of the NO_(2)+·OH pathway to NO_(3)^(-)was influenced by the temperature and sunshine hours.The monthly differences in marine aerosol nitrate formation pathways indicated the influences of meteorological parameters,such as solar irradiation and temperature,on nitrate formation.Although the concentrations of NO_(3)^(-)in Mar.and Apr.were obviously higher than those in May,theδ^(15)N-NO_(3)^(-)values in Mar.and Apr.were close to those in May,indicating that both NOx sources and 15N fractionation mediated the aerosolδ^(15)N-NO_(3)^(-)values.Theδ^(15)N-NO_(3)^(-)values in the TSP of Dongsha Island in spring showed a monthly increasing trend;this trend was related to its sources and fractionation behaviors during the formation process.Conclusions Spring is the transition season in the study area;as the months change,the sources and formation pathways of NO_(3)^(-)vary.During spring,the NO_(2)+·OH pathway dominated nitrate production on Dongsha Island(56.7%—80.2%).In Mar.and Apr.,the N_(2)O_(5)+H_(2)O and NO_(3)^(+)VOCs pathways were nonnegligible pathways for aerosol nitrate formation at Dongsha Island.Terrestrial and anthropogenic emissions were the main sources of aerosol nitrate on Dongsha Island in Mar.and Apr.,while natural sources dominated aerosol nitrate production in May.Recommendations and perspectives This study,for the first time,quantified aerosol nitrate formation pathways and explored nitrate sources at Dongsha Island,South China Sea,during springtime.The understanding of the dry deposition fluxes of nitrate in marine aerosols was also enhanced in this study.