南昌市大氣NHx干沉降及其氮同位素晝夜變化特征

尤子昇, 孔露靜, 張永運(yùn), 王小嫚, 張浩然, 劉 敏, 羅 笠

南昌市大氣NH干沉降及其氮同位素晝夜變化特征

尤子昇1, 孔露靜1, 張永運(yùn)1, 王小嫚1, 張浩然1, 劉 敏2, 羅 笠3*

(1. 東華理工大學(xué) 水資源與環(huán)境工程學(xué)院, 江西 南昌 330013; 2. 江西省環(huán)境監(jiān)測(cè)中心站, 江西 南昌 330013; 3. 海南大學(xué) 南海海洋資源利用國家重點(diǎn)實(shí)驗(yàn)室, 海南 海口 570228)

NH(NH3和NH4+)是大氣中主要的堿性物質(zhì), NH干沉降是大氣中NH移除的重要途徑之一。本研究于2019年8月11日至31日在江西省南昌市東華理工大學(xué)分晝夜采集了大氣NH干沉降樣本(包括顆粒銨(NH4+(p))和氣態(tài)氨(NH3(g))), 測(cè)定了干沉降樣本的NH4+離子濃度、δ15N-NH4+和pH, 基于干沉降樣品中NH濃度估算了大氣中NH干沉降通量。結(jié)果顯示, 白天NH干沉降通量(0.2～25.9 μg/(m2·h), 均值14.2±10.0 μg/(m2·h))低于夜晚(0.9～50.2 μg/(m2·h), 均值23.1±10.4 μg/(m2·h)), 與晝夜溫度變化趨勢(shì)相反, 表明NH干沉降通量晝夜差異與NH3在不同溫度下的溶解度有關(guān)。此外, NH干沉降通量與pH值的晝夜變化一致, 說明干沉降樣本中NH4+濃度對(duì)溶液的pH有影響。干沉降樣本中, δ15N-NH4+值在白天(?14.62‰～17.54‰, 均值3.56‰±8.49‰)高于夜晚(?20.07‰ ～ ?0.25‰, 均值?8.97‰±5.80‰), 這種差異可能與氣態(tài)NH3和離子態(tài)NH4+之間的氮同位素分餾有關(guān), 也可能與晝夜氨氣排放源不同有關(guān)?；诓煌欧旁处?5N-NH3特征值和氮同位素分餾效應(yīng), 南昌市夏季白天大氣中NH主要來源于化石燃料燃燒, 晚上主要受控于農(nóng)業(yè)肥料揮發(fā)與動(dòng)物排泄。

NH干沉降通量; 氮同位素; 南昌市

0 引 言

氨氣(NH3)是大氣中主要的堿性氣體, 能與大氣中的酸性氣體(SO和NO)生成二次氣溶膠顆粒銨鹽((NH4)2SO4和NH4NO3; Kirkby et al., 2011)。大量研究顯示, 銨根離子(NH4+)是二次氣溶膠中主要的堿性離子和細(xì)顆粒的重要成分(Li et al., 2018), 能影響空氣質(zhì)量和降低大氣輻射平衡(郭新彪和魏紅英, 2013; 符傳博和丹利, 2018)。在我國一些地區(qū), 人為活動(dòng)排放大量的NH3已成為灰霾污染的主要誘因(Wang et al., 2016)。大氣中氣態(tài)氨(NH3(g))和顆粒銨(NH4+(p))的移除主要通過直接沉降到地面(干沉降)和通過降雨淋洗及雨雪沉降或霧滴的方式沉降到地面(濕沉降)。研究顯示, 大氣氮沉降已經(jīng)誘發(fā)了一系列的生態(tài)環(huán)境效應(yīng), 如水體的富營養(yǎng)化(Fenn et al., 2003)、破壞生物多樣性(Bobbink et al., 2010)和增加土壤酸化(Yang et al., 2015)等。在一些地區(qū), 大氣氮的干沉降量大致與濕沉降相等, 甚至比濕沉降量大(Xu et al., 2015)。NH(NH3和NH4+)是大氣氮沉降的重要組成部分, 準(zhǔn)確評(píng)估 NH干沉降通量有助于全面了解大氣氮的沉降。

大氣中NH3來源可以分為人為源(化石燃料燃燒、汽車尾氣排放、農(nóng)業(yè)生產(chǎn)、養(yǎng)殖等)和自然源(土壤微生物活動(dòng)、海洋釋放等)兩種(Sutton et al., 2000; Jickells et al., 2003; Huang et al., 2013; Kang et al., 2016)。前人大量的研究顯示, 不同來源的NH3, 其δ15N-NH3值存在明顯差異。例如, 肥料揮發(fā)和人畜糞便的δ15N-NH3值分別為(?40.40±7.80)‰(Felix et al., 2013, 2014; Chang et al., 2016; Savard et al., 2017)和(?22.50±11.41)‰(Heaton, 1987; Schulz et al., 2001; Hristov et al., 2009; Freyer, 2010; Felix et al., 2014; Chang et al., 2016; Savard et al., 2017), 煤燃燒和汽車尾氣的δ15N-NH3值分別為(?17.22±10.41)‰ (Freyer, 2010; Felix et al., 2013; Savard et al., 2017)和(?11.62±5.34)‰(Felix et al., 2013, 2014; Chang et al., 2016; Savard et al., 2017)。NH3是NH4+的前體物質(zhì), 因此早期研究通常直接利用雨水中的δ15N-NH4+值來示蹤降雨中NH3的來源(Freyer, 1978; Xie et al., 2008; Zhang et al., 2008; Huang et al., 2012; Xiao et al., 2012; Liu et al., 2017; Ti et al., 2018; Zheng et al., 2018)。已有數(shù)據(jù)顯示, 氣溶膠δ15N-NH4+(p)值明顯高于δ15N-NH3(g)值(Moore, 1977; Heaton, 1987; Kundu et al., 2010; Kawashima and Kurahashi, 2011; Proemse et al., 2012; Lin et al., 2016; Park et al., 2017; Savard et al., 2017; Pan et al., 2018; Kawashima, 2019)。例如, Kirshenbaum et al. (1947)和Heaton et al. (1997)實(shí)驗(yàn)室箱式控制實(shí)驗(yàn)?zāi)M的NH4+(p)和NH3(g)的氮同位素值分別相差34.00‰和33.00‰; Kawashima (2019)對(duì)日本秋田市2009～2010年連續(xù)一年大氣NH3(g)和NH4+(p)的氮同位素的實(shí)測(cè)發(fā)現(xiàn)δ15N-NH4+(p)值比δ15N-NH3(g)值高33.3‰。這些研究表明, 在利用δ15N來示蹤NH來源時(shí)必須考慮氮同位素在NH3(g)和NH4+(p)之間的分餾系數(shù)。例如, Pan et al. (2018)在城市觀測(cè)研究時(shí)發(fā)現(xiàn), 在不考慮氮同位素分餾的情況下, 可能不能準(zhǔn)確示蹤NH3的來源; Wu et al. (2020)對(duì)西安市δ15N-NH3和δ15N-NH4+進(jìn)行同步觀測(cè), 結(jié)合NH3(g)和NH4+(p)之間的氮同位素分餾系數(shù), 認(rèn)為非農(nóng)業(yè)來源在霧霾污染依然嚴(yán)重的西安市對(duì)大氣NH3的貢獻(xiàn)占主導(dǎo)地位, 并提出控制非農(nóng)業(yè)NH3排放是緩解東亞半干旱地區(qū)空氣污染問題的有效途徑。

上述研究主要基于穩(wěn)定氮同位素示蹤氣態(tài)NH3、降雨和氣溶膠中NH4+的來源, 對(duì)于NH干沉降的通量及來源的研究未見報(bào)道。在過去的30年, 中國已經(jīng)成為氨排放量最大、氨污染最嚴(yán)重的國家之一, 然而NH3減排非常有限。這可能與研究深度不夠、對(duì)NH3排放源存在爭議有關(guān)。本研究通過南昌市的夏季大氣NH干沉降分析, 量化南昌市大氣NH干沉降通量和NH的來源, 為南昌市制定氨減排政策提供了科學(xué)參考。

1 材料與方法

1.1 樣品采集和背景觀測(cè)

本研究于2019年8月11日至8月31日分白天(7: 30～20: 00)和晚上(20: 00～次日7: 30)在江西省南昌市東華理工大學(xué)地學(xué)樓6樓樓頂(距離地面高約20 m,經(jīng)度: 115.8°E, 緯度: 28.7°N)共采集NH干沉降樣本41個(gè)。干沉降樣本采集使用降塵缸法: 在集塵箱(55.5 cm×44.4 cm)中加入550 mL的超純水, 收集NH(NH3和NH4+)干沉降。若下雨, 則蓋住集塵箱, 雨停則打開蓋子繼續(xù)收集。收集的干沉降樣本用50 mL的醫(yī)用針筒和0.22 μm的濾頭對(duì)采集的干沉降水樣進(jìn)行過濾, 記錄水樣體積, 冰凍保存。采樣期間的氣象參數(shù)(溫度、相對(duì)濕度、風(fēng)速和風(fēng)向)和大氣污染物濃度(SO2、NO2、O3、CO和PM2.5)如圖1所示。大氣溫度顯示明顯晝夜差異, 中午溫度最高, 凌晨溫度最低(圖1a); 相反, 相對(duì)濕度在白天低, 晚上高(圖1a)。整個(gè)觀測(cè)期間, 風(fēng)速較小(圖1b)。南昌市地處江西中部偏北, 一二三類產(chǎn)業(yè)共同發(fā)展, 第一產(chǎn)業(yè)中農(nóng)林漁牧并存, 化肥的應(yīng)用與動(dòng)物的排泄可能是南昌市氨氣的重要來源, 第二產(chǎn)業(yè)的輕重工業(yè)也可能是氨氣排放的重要來源。

1.2 數(shù)據(jù)分析

干沉降樣本中NH濃度用納氏試劑分光光度法(HJ 535-2009)測(cè)定, NH干沉降通量估算公式為:

式中,為沉降通量(μg/(m2·h)),為樣品濃度(μg/mL),為樣品體積(mL),為集塵箱底面積(m2),為采樣時(shí)長(h)。

δ15N-NH4+值(δ15N-NH4+(‰)=(樣品/標(biāo)準(zhǔn)?1)×1000,樣品和標(biāo)準(zhǔn)分別是樣品和標(biāo)準(zhǔn)中15N/14N比值)分析方法詳見Zhang et al. (2007)。簡單而言, 利用化學(xué)試劑將樣本中NH4+離子轉(zhuǎn)化為氣態(tài)N2O, 然后用Gasbench Ⅱ-Delta V Advantage穩(wěn)定同位素比值質(zhì)譜儀分析δ15N-N2O值。三個(gè)國際銨氮同位素標(biāo)準(zhǔn)USGS25(?30.4‰), IAEA-N-1(0.4‰)和IAEA-N-2 (20.3‰)用于樣本δ15N-NH4+值的校正, δ15N-NH4+值的總體分析精度優(yōu)于±0.9‰。

(a)和(b)數(shù)據(jù)來源于www.weatherandclimate.info; (c)、(d)和(e)數(shù)據(jù)來源于江西省環(huán)境監(jiān)測(cè)中心。

2 結(jié)果與討論

2.1 NHx干沉降通量

如圖2所示, NH干沉降通量變化較大, 范圍為0.2～50.2 μg/(m2·h), 均值為12.4±11.0 μg/(m2·h)。本研究估算的南昌市夏季NH干沉降通量與邢臺(tái)農(nóng)業(yè)地區(qū)夏季NH干沉降通量相似, 但高于其他地區(qū)夏季NH干沉降通量(表1), NH干沉降通量在地理位置上的差異可能與采樣區(qū)NH3排放強(qiáng)度有關(guān)(Li et al., 2020)。農(nóng)業(yè)地區(qū)的夏季和城市地區(qū)的冬季, 這種差異更加明顯。夏季是農(nóng)業(yè)施肥的季節(jié), 農(nóng)業(yè)源釋放的NH3已被證實(shí)是區(qū)域大氣中NH3的重要來源(Pan et al., 2012), 而在冬季, 化石燃料燃燒則是城市大氣中NH3的重要來源(Pan et al., 2016)。

此外, 南昌市夏季NH干沉降通量高于邯鄲等城市夏季NH干沉降通量(表1), 一方面與不同城市NH3排放強(qiáng)度有關(guān), 另一方面與觀測(cè)期間降雨強(qiáng)度有關(guān)(Li et al., 2020)。本研究期間, 南昌市降雨強(qiáng)度小(連續(xù)15天為晴天), 降雨對(duì)大氣中NH移除作用小, 因此NH干沉降通量高。

NH干沉降通量在白天(0.2～25.9 μg/(m2·h), 均值14.2±10.0 μg/(m2·h))明顯低于夜晚(0.9～50.2 μg/(m2·h), 均值23.1±10.4 μg/(m2·h))(圖3a)。由于前人未對(duì)NH干沉降通量的晝夜變化進(jìn)行相關(guān)研究, 缺少數(shù)據(jù)對(duì)比。本研究只能基于有限的觀測(cè)數(shù)據(jù)對(duì)NH干沉降通量晝夜變化趨勢(shì)進(jìn)行分析。NH干沉降通量與采樣區(qū)晝夜溫差(圖3b)呈現(xiàn)相反的趨勢(shì), 根據(jù)NH3在水中的溶解度與溫度的關(guān)系(亨利定理), 氣溫越高, 干沉降樣本中溶解的NH3濃度就越低, 從而降低了白天干沉降樣本NH4+濃度, 導(dǎo)致白天NH干沉降通量低。干沉降樣本pH值白天(5.75±1.14)稍低于晚上(6.40±0.46)(圖3c), 結(jié)合NH3水溶液呈弱堿性的特征, 表明晚上更多的NH3溶解在干沉降樣本溶液中。前人對(duì)氣溶膠相對(duì)酸度研究顯示, 氣溶膠萃取液中NH4+濃度越高, 則計(jì)算的氣溶膠相對(duì)酸度就越低(Luo et al., 2016), 表明樣本溶液中NH4+濃度能直接影響溶液的pH值。

圖2 NHx干沉降通量晝夜變化

表1 不同地區(qū)NHx沉降通量

2.2 δ15N-NH4+值

本研究觀測(cè)期間, δ15N-NH4+變化范圍為?20.07‰～17.54‰(圖4), 整個(gè)觀測(cè)期平均值為(?2.51±9.60)‰。本研究NH干沉降樣本中δ15N-NH4+值與北京市氣溶膠中δ15N-NH4+值范圍相似, 高于貴陽和成都降雨中δ15N-NH4+值, 低于西安市、太湖地區(qū)以及海洋源氣溶膠中δ15N-NH4+值(表2)。NH干沉降樣本中δ15N-NH4+值也存在明顯的晝夜差異(圖4): 白天(?14.62‰～17.54‰, 均值3.56‰±8.49‰)明顯高于晚上(?20.07‰ ～ ?0.25‰, 均值?8.97‰± 5.80‰)。前人把氣溶膠和雨水中δ15N-NH4+值的時(shí)空差異普遍歸因于NH3排放源的時(shí)空差異及氣態(tài)NH3和離子NH4+之間的氮同位素分餾(肖化云等, 2003; 杜鋒, 2012; Altieri et al., 2014; Pan et al., 2018; Ti et al., 2018; Huang et al., 2019; Kawashima, 2019; Wu et al., 2020)。下面將結(jié)合NH氮同位素分餾的影響因素和不同排放源釋放δ15N-NH3特征值來討論南昌市夏季NH氮干沉降樣本中的主要來源。

2.3 δ15N-NH4+值晝夜變化的原因

白天NH干沉降樣本中δ15N-NH4+值(?14.62‰～ 17.54‰, 平均3.56‰±8.49‰)明顯高于晚上(?20.07‰～ ?0.25‰, 平均?8.97‰±5.80‰)(圖5a)。NH干沉降樣本中δ15N-NH4+值與晝夜的溫度變化趨勢(shì)(圖3b) 完全一致, 說明溫度對(duì)NH干沉降樣本中δ15N-NH4+值有顯著的影響。實(shí)驗(yàn)室控制模擬實(shí)驗(yàn)顯示, δ15N-NH4+和δ15N-NH3(aq)(溶液氨)之間的分餾系數(shù)是溫度的函數(shù):=25.94×1000/?42.25(Li et al., 2012), 根據(jù)該公式計(jì)算南昌夏季δ15N-NH4+和δ15N-NH3(aq)之間的分餾系數(shù)介于41.47‰～45.10‰, 高于原位觀測(cè)大氣δ15N-NH3(g)和δ15N-NH4+(p)之間的差異(圖5b)。一些可能的因素導(dǎo)致了理論計(jì)算與實(shí)際觀測(cè)值的不一致。首先, 本研究采樣點(diǎn)位于樓頂, 白天水泥混凝土表溫度高于實(shí)際大氣溫度。假設(shè)白天混凝土表面溫度為60 ℃, 那么根據(jù)Li et al. (2012)的經(jīng)驗(yàn)公式計(jì)算白天δ15N-NH4+和δ15N-NH3(aq)之間的分餾系數(shù)為35.61‰, 與觀測(cè)值29.90‰(圖5b)接近。第二, NH3(g)溶于水后, 會(huì)形成NH3(aq), 然后再電離成NH4+。在NH3(g)?NH3(aq)?NH4+體系中, δ15N-NH3(aq)與δ15N-NH3(g)之間的分餾系數(shù)也是溫度的函數(shù):=13.73×1000/?30.76(Li et al., 2012), 根據(jù)該公式計(jì)算δ15N-NH3(aq)與δ15N-NH3(g)之間的分餾系數(shù)為13.55‰～15.52‰。但是, 由于缺少δ15N-NH3(aq)實(shí)際觀測(cè)數(shù)據(jù), 無法根據(jù)理論計(jì)算來評(píng)估δ15N-NH4+和δ15N-NH3(g)之間的分餾系數(shù)。因此, 在本研究中, 根據(jù)前人的觀測(cè)(圖5b), 我們假設(shè)大氣中δ15N-NH3(g)值為(?20.91±13.42)‰, δ15N-NH4+(p)值為(9.01±10.81)‰, 那么δ15N-NH4+(p)和δ15N-NH3(g)之間的分餾系數(shù)為29.90‰。

圖3 白天和晚上NHx干沉降通量(a), 氣溫(b)和pH(c)

圖4 δ15N-NH4+晝夜日變化

表2 不同地區(qū)夏季干濕沉降樣本中的δ15N-NH4+平均值

NH干沉降樣本是NH3(g)和NH4+(p)沉降的混合體, 因此干沉降樣本中δ15N-NH4+值同時(shí)受到δ15N-NH3(g)值和δ15N-NH4+(p)值的影響。根據(jù)同位素質(zhì)量守恒原理, 可以估算NH3(g)和NH4+(p)對(duì)干沉降樣本中NH4+的貢獻(xiàn)占比。同位素質(zhì)量守恒原理計(jì)算公式為:

(2)

式中:為NH3(g)對(duì)干沉降樣本中NH4+的貢獻(xiàn)占比,為NH4+(p)對(duì)干沉降樣本中NH4+的貢獻(xiàn)占比,1。δ15N-NH3(g)假設(shè)為統(tǒng)計(jì)大氣中δ15N-NH3, δ15N-NH4+(p)假設(shè)為統(tǒng)計(jì)大氣中δ15N-NH4+(圖5b), δ15N-NH4+為實(shí)測(cè)干沉降樣本中δ15N-NH4+值, 由此估算白天和晚上干沉降樣本中NH3(g)和NH4+(p)對(duì)干沉降樣本中NH4+的相對(duì)貢獻(xiàn)如圖5c。結(jié)果顯示白天NH干沉降樣本中NH3(g)貢獻(xiàn)占比(0.21±0.25)遠(yuǎn)低于晚上(0.79±0.25), 因此白天的NH干沉降通量低于晚上(圖3a)。

2.4 NHx的來源

圖6總結(jié)了不同排放源的δ15N-NH3值(白色箱子圖), 假設(shè)δ15N-NH4+(p)值和δ15N-NH3(g)值之間的分餾系數(shù)為29.90‰, 得出不同排放源釋放NH3(g)到大氣中變成NH4+后的氮同位素值如圖6灰色箱子圖。本研究實(shí)測(cè)的NHx干沉降樣本中δ15N-NH4+值正好位于δ15N-NH3(g)值和δ15N-NH4+(p)值之間, 再次驗(yàn)證了NHx干沉降樣本中NH4+同時(shí)來源于NH3(g)和NH4+(p)沉降。根據(jù)白天和晚上氮同位素與不同排放源的δ15N-NH值對(duì)比, 發(fā)現(xiàn)白天NHx干沉降樣本中δ15N-NH4+值更接近煤燃燒和汽車尾氣來源的δ15N-NH3值, 晚上NHx干沉降樣本中δ15N-NH4+值靠近肥料揮發(fā)和養(yǎng)殖廢水來源的δ15N-NH3值。由于NH3(g)?NH3(aq)?NH4+體系中氮同位素分餾過程復(fù)雜、受控因素較多, 利用δ15N量化NH3排放源較為困難。有研究基于不同排放源的δ15N-NH3值和假設(shè)的δ15N-NH分餾系數(shù), 利用貝葉斯混合模型定量了不同來源排放對(duì)生產(chǎn)大氣氣溶膠和雨水中NH的貢獻(xiàn)(Pan et al., 2016)。但是這種量化的結(jié)果存在較大的不確定性。同步觀測(cè)NH3(g)、NH4+(p)的沉降通量以及δ15N-NH3(g)和δ15N-NH4+(p)值可能有助于進(jìn)一步厘清南昌市大氣NH3的排放源。

圖5 白天和晚上NHx干沉降樣本中δ15N-NH4+值(a); 統(tǒng)計(jì)的δ15N-NH3(g)值和δ15N-NH4+(p)值(b; 肖化云等, 2003; 杜鋒, 2012; Altieri et al., 2014; Pan et al., 2018; Ti et al., 2018; Huang et al., 2019; Kawashima, 2019; Wu et al., 2020); 白天和晚上NH3(g)對(duì)干沉降樣本中NH4+的貢獻(xiàn)占比(c)

圖6 一次排放源δ15N-NH3(g)值和考慮分餾后的δ15N-NH4+估算值

3 結(jié) 論

(1) 2019年8月11日至31日采樣期間南昌市晝夜的NH干沉降通量分別為0.2～25.9 μg/(m2·h)(平均14.2±10.0 μg/(m2·h))和0.9～50.2 μg/(m2·h)(平均23.1± 10.4 μg/(m2·h))。白天的NH干沉降通量低于夜晚, 主要與氨氣溶解度隨溫度降低而升高有關(guān)。

(2) 晝夜的δ15N-NH4+值分別為?14.62‰～17.54‰ (平均3.56‰±8.49‰)和?20.07‰ ～ ?0.25‰(平均?8.97‰±5.80‰)。白天的δ15N-NH4+高于夜晚可能與溫度對(duì)氮同位素在NH3(g)?NH3(aq)?NH4+體系中分餾的影響有關(guān)。

(3) 南昌市夏季白天化石燃料燃燒是大氣中NH的主要來源, 夜晚農(nóng)業(yè)肥料揮發(fā)與動(dòng)物排泄則是大氣中NH的主要來源。

致謝:感謝兩位匿名審稿專家對(duì)本文提出的寶貴修改意見。

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Nyctohemeral variations of atmospheric NHdeposition and nitrogen isotopic compositions in Nanchang

YOU Zisheng1, KONG Lujing1, ZHANG Yongyun1, WANG Xiaoman1, ZHANG Haoran1, LIU Min2, LUO Li3*

(1. School of Water Resources and Environmental Engineering, East China University of Technology, Nanchang 330013, Jiangxi, China; 2.Jiangxi Environmental Monitoring Center Station,Nanchang 330013, Jiangxi, China; 3. State Key Laboratory of Marine Resources Utilization in South China Sea, Haikou 570228, Hainan, China)

NH(NH3and NH4+) is the main alkaline substance in the atmosphere, and its dry deposition is one of the important ways to remove NHfrom the atmosphere. To investigate NHdry deposition fluxes and explore the possible sources of atmospheric NH, dry deposition samples were collected between August 11–31, 2019, at East China University of Technology, Nanchang. Concentrations of ammonium (NH4+), values of δ15N-NH4+, and pH values of dry deposition samples were analyzed. NHdry deposition fluxes during daytime (0.2 to 25.9μg/(m2·h)), with an average of 14.2±10.0 μg/(m2·h), were lower than those during the night (0.9 to 50.2 μg/(m2·h)), with an average of 23.1±10.4 μg/(m2·h). Deposition fluxes showed similar pH value patterns, indicating that NH4+levels can regulate the pH values of dry deposition samples. Values of δ15N-NH4+during daytime (?14.62‰ to 17.54‰, with an average of 3.56‰±8.49‰) were higher than those during the night (?20.07‰ to ?0.25‰, with an average of?8.97‰±5.80‰). The diurnal variations of δ15N-NH4+values may be related to the nitrogen isotopic fractionation between the gas phase NH3and ionic NH4+, and between NH3emission sources during the day and night. Based on the δ15N-NH3values from different NH3emission sources and nitrogen isotopic fractionation between the gas phase NH3and ionic NH4+, NHin dry deposition samples may be mainly sourced from fossil fuel combustion during the daytime and predominately from agricultural fertilizer volatilization and animal excretion during the night. An opposite trend with diurnal temperature suggests that high NHdeposition fluxes during the night may be related to the high solubility of ammonia at low temperatures.

NHdry deposition fluxes; nitrogen isotope; Nanchang

X142

A

0379-1726(2022)02-0233-10

10.19700/j.0379-1726.2022.02.007

2020-09-09;

2020-12-28

國家自然科學(xué)基金項(xiàng)目(41763001)、江西省教育廳科技項(xiàng)目(GJJ160580)和東華理工大學(xué)博士科研啟動(dòng)基金項(xiàng)目(DHBK2016105)聯(lián)合資助。

尤子昇(2000–), 男, 本科生, 環(huán)境工程專業(yè)。E-mail: 1164171168@qq.com

羅笠(1982–), 男, 教授, 主要從事氣溶膠化學(xué)和同位素環(huán)境地球化學(xué)研究。E-mail: L.Luo@hainanu.edu.cn