An evaluation of atmospheric Nr pollution and deposition in North China after the Beijing Olympics

Atmospheric Environment 74 (2013) 209e216

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An evaluation of atmospheric Nr pollution and deposition in North China after the Beijing Olympics X.S. Luo a, P. Liu b, A.H. Tang a, *, J.Y. Liu a, X.Y. Zong a, Q. Zhang b, C.L. Kou c, L.J. Zhang d, D. Fowler e, A. Fangmeier f, P. Christie a, g, F.S. Zhang a, X.J. Liu a, * a

Center for Resources, Environment and Food Security, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China Institute of Agricultural Environment and Resource, Shanxi Academy of Agricultural Sciences, Taiyuan 030006, China c Institute of Plant Nutrition, Resources and Environmental Sciences, Henan Academy of Agricultural Sciences, Zhengzhou 450002, China d College of Resources and Environment, Hebei Agricultural University, Baoding 071001, China e Centre for Ecology and Hydrology, Edinburgh EH26 0QB, UK f Institute of Landscape and Plant Ecology, University of Hohenheim, 70593 Stuttgart, Germany g Agri-Environment Branch, Agri-Food and Biosciences Institute, Belfast BT9 5PX, UK b

h i g h l i g h t s
Atmospheric Nr concentration and deposition were reported at six sites in North China.
High Nr concentrations in the air were found in North China several years after the Beijing Olympics.
Annual N dry and total deposition was 35.2e60.0 and 54.4e102.3 kg N ha1 in North China, respectively.
Concentration and deposition of Nr were much higher at urban than at rural sites in North China.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 July 2012 Received in revised form 24 March 2013 Accepted 27 March 2013

North China is known for its large population densities and rapid development of industry and agriculture. Air quality around Beijing improved substantially during the 2008 Summer Olympics. We measured atmospheric concentrations of various Nr compounds at three urban sites and three rural sites in North China from 2010 to 2012 and estimated N dry and wet deposition by inferential models and the rain gauge method to determine current air conditions with respect to reactive nitrogen (Nr) compounds and nitrogen (N) deposition in Beijing  þ  and the surrounding area. NH3, NO2, and HNO3 and particulate NHþ 4 and NO3 , and NH4 eN and NO3 eN in precipitation averaged 8.2, 11.5, 1.6, 8.2 and 4.6 mg N m3, and 2.9 and 1.9 mg N L1, respectively, with large seasonal and spatial variability. Atmospheric Nr (especially oxidized N) concentrations were highest at urban sites. Dry deposition of Nr ranged from 35.2 to 60.0 kg N ha1 yr1, with wet deposition of Nr of 16.3 to 43.2 kg N ha1 yr1 and total deposition of 54.4e103.2 kg N ha1 yr1. The rates of Nr dry and wet deposition were 36.4 and 33.2% higher, respectively, at the urban sites than at the rural sites. These high levels reflect the occurrence of a wide range of Nr pollution in North China and suggest that further strict air pollution control measures are required. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Air pollution Reactive N Dry and wet deposition Inferential method

1. Introduction Reactive nitrogen (Nr) in the environment produced by human activities has increased more than ten-fold over the past 150 years since the industrial revolution (1860) and will continue to increase

* Corresponding authors. E-mail addresses: [email protected] (A.H. Tang), [email protected], [email protected] (X.J. Liu). 1352-2310/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2013.03.054

because of the increasing demand for food and energy worldwide (Galloway et al., 2008). Global application of N fertilizers produced using the HabereBosch process has fed nearly 50% of the newly increased world population (Erisman et al., 2008). Additionally, fossil fuel combustion has facilitated the development of industry and transportation and improved the quality of life of people in developed countries (Compton et al., 2011). Unfortunately, atmospheric emissions of Nr such as NH3 and NOx (sum of NO and NO2) can also promote the formation of small particles which lower air quality and damage human health (Tainio et al., 2009). Increases in

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atmospheric Nr emissions will also lead to elevated Nr dry and wet deposition to the land surface, leading to series of negative effects on ecosystems such as loss of biodiversity in grasslands (Stevens et al., 2004; Song et al., 2012) and forests (Rattray and Sievering, 2001), soil acidification and eutrophication (Erisman and Pul, 1994), and increased N2O emission which impacts the global greenhouse gas budget (Sutton et al., 2011). Therefore atmospheric Nr pollution and deposition induced by excessive anthropogenic Nr emissions have become an environmental concern worldwide (Compton et al., 2011). Rapid socioeconomic development in China has led to large N fertilizer consumption and energy consumption over the past 30 years. For example, synthetic N fertilizer consumption, from 12.1 Tg N in 1980 to >30 Tg N in 2010 (Liu et al., 2013), and total energy consumption was equivalent to 3.25 billion tons of standard coal in 2010, about 5 times more than in 1978 (China Statistical Yearbook, 2011). These intensive human activities stimulate the huge Nr emission over China. However, the impacts of such high anthropogenic Nr emissions on atmospheric N deposition and their subsequent implications have not been evaluated systematically to date (Liu et al., 2011). Cities in North China such as Beijing and Tianjin, and provinces such as Hebei, Henan, Shanxi and Shandong have large population densities, intensive agriculture and highly developed industries and transport systems. Previous studies (Zhang et al., 2008; Shen et al., 2009; He et al., 2010) have demonstrated high levels of Nr pollution and N deposition in rural regions. Substantial air pollution by particulate matter (e.g. PM10 and PM2.5) has also been found in the megacity of Beijing (Chan and Yao, 2008). The policy of the national government has been to reduce air pollution (Fang et al., 2009). The 2008 Beijing Summer Olympics provided a unique opportunity to check the effects of the pollution control measures, including motor vehicle restrictions, reducing the output from the most polluting factories, and limiting pollutant emissions from coal combustion facilities in Beijing and surrounding areas (Zhou et al.,

2010). Air quality in terms of PM10 and PM2.5, NH3 and NO2 improved substantially in Beijing during the Olympics (Wang et al., 2010; Shen et al., 2011a) but air pollution (as indicated by PM10, PM2.5, NH3, NO2, SO2, and the particulate ions) recovered quickly almost to previous levels within two months following the Olympics (Shen et al., 2011a). During the three years since the Olympics (2009e2011) the central government encouraged high-quality economic growth with a target of 40e45% less GHG emission per unit GDP by 2020 and also the concept of ‘green GDP’ (referring to less environmental cost per unit GDP), but there is still a challenge for air quality especially in Beijing, the megacity of North China (Zhang et al., 2012). Moreover, there have been many uncertainties associated with previous studies on N deposition in this area. These include the separate evaluation of dry and wet deposition and uncertainties in the estimation of dry N deposition (Zhang et al., 2008; Shen et al., 2009). Six monitoring sites representing urban and rural regions in North China were selected for this study. Atmospheric Nr concentrations were measured and N dry and wet deposition rates were evaluated using a DELTA system combined with inferential modeling (dry) and the rain gauge method (bulk). The objectives were to verify whether there is still higher atmospheric Nr pollution in north China (including a comparison of urban and rural Nr levels after the Beijing Olympics) and to provide improved estimates of N dry and wet deposition in this region. 2. Materials and methods 2.1. Sampling sites Sampling was conducted at six sites located in Beijing and in Hebei, Shanxi, and Henan provinces (Fig. 1). The three urban sites are CAU, BD, and ZZ. CAU, located at the west campus of China Agricultural University (40 010 N, 116170 E), is near the fifth ring road in Beijing. The BD site is in Baoding city in Hebei province and

Fig. 1. Geographical distribution of the six monitoring sites in North China. Urban sites (CAU, BD, ZZ); Rural sites (SZ, SY, QZ).

X.S. Luo et al. / Atmospheric Environment 74 (2013) 209e216

located at Hebei Agricultural University (38 080 N, 115 040 E). The ZZ site is located at the centre of Zhengzhou, the capital city of Henan province and Henan Academy of Agricultural Sciences (34 470 N, 113 400 E). Three rural sites are SZ, SY, and QZ. SZ (40 080 N, 116110 E) is about 20 km northwest of CAU and is a more rural site with both agricultural land and small towns. SY (39 260 N, 112 550 E) belongs to a remote rural site with a small population density and mixed arable fields and pastures in Shanyin county in Shanxi province. QZ (36 520 N, 115 010 E) is a typical rural agricultural site with a recently constructed industrial district in the South of Hebei province, and the sampling site was in China Agricultural University’s Quzhou experimental station. The cropping system at SZ and QZ was a winter wheat and summer maize rotation system. The climatic conditions are similar at the Beijing, Hebei and Henan sampling sites, all of which belong to the temperate monsoon system. The climate at SY is continental monsoon. The sampling and sample handling processes were strictly controlled to avoid any pollution of or changes in the chemical composition of the samples. There were also strict quality control and quality assurance (QA and QC) measures during sample analysis in our laboratory, the Key Laboratory of PlanteSoil Interactions, Chinese Ministry of Education. 2.2. Samplers used for dry deposition 2.2.1. DELTA system  þ Atmospheric NH3, HNO3, and particulate NHþ 4 and NO3 (pNH4 and pNO ) were collected by the DELTA system designed by the 3 Centre for Ecology and Hydrology in Edinburgh. The DELTA system has been used widely in the UK and EU environmental monitoring networks. A mini pump in the system provides sampling rates of 0.3e0.4 L min1. When a laminar air stream passes through the denuder coated on the inside with acid solution or alkaline solution, nitric acid or ammonia is captured by the acid or alkaline walls while aerosols pass through and can be captured by aerosol filters. Detailed information about the DELTA system was given by Tang et al. (2009). Sampling height was 1.6 m above the ground. We þ used 37-mm-diameter filters to capture pNO 3 and pNH4 , which were larger than the filters used in the NitroEurope network (25 mm). All samples were prepared and measured in the laboratory at China Agricultural University. The sampling period was one sample per month. After the samples were taken back from the monitoring sites they were immediately placed in a freezer at 4  C. The samples were analyzed within one month of collection. The denuders capturing HNO3 and the filters capturing pNO 3 were extracted with 10 ml 0.05% H2O2 solution. The denuders capturing NH3 and the filters capturing pNHþ 4 were extracted with 10 ml high purity water. Ammonium and nitrate in the solutions were measured with an AA3 continuous-flow analyzer (Bran þ Luebbe GmbH, Norderstedt, Germany). Nr data cover two complete years at CAU, ZZ, SZ (April 2010eMarch 2012) and QZ (May 2010eApril 2012), but only one year at BD (JanuaryeDecember 2011) and SY (from April 2010 to March 2011) since the land use changed and led us to stop sampling. 2.2.2. Passive samplers Atmospheric NO2 was collected with passive samplers using Gradko diffusion tubes from ECN (UK Environmental Change Network website). The samplers consisted of tubes, two caps, and stainless steel mesh disks. Two dry disks were placed in the caps and 30 ml of a 20% aqueous solution of triethanolamine was pipetted into the gray cap to prepare them for use. The samplers were hung 2 m above the ground and exposed for two weeks in the air. To estimate the NO2 concentration, the disks were extracted

211

with a solution containing sulphanilamide, H3PO4 and N-1Naphthylethylene-diamine dihydrochloride and then measured with a colorimetric method at a wavelength of 542 nm. 2.3. Precipitation collectors Rainwater samples were collected on a daily basis with precipitation collectors (SDM6, Tianjin Weather Equipment Inc., China). The collectors were located in flat and open areas without surrounding obstacles to prevent contamination. After collection, the rainwater samples were stored in a refrigerator at 4  C until  analysis. All samples were analyzed for NHþ 4 eN and NO3 eN by continuous flow analysis as described above. Wet deposition of inorganic N was calculated according to the N concentration and the amount of precipitation (Liu et al., 2006). We collected bulk N deposition in one complete year at CAU, BD, SZ, QZ (Januarye December 2011) and SY (April 2010eMarch 2011). Bulk deposition at ZZ was not collected in the sampling period and no wet N deposition results were used. 2.4. Method for estimation of N dry deposition According to the theory of inferential models, the trace gas deposition flux (F) is the product of the concentration of Nr species at a specific height (Cz) and the deposition velocity (Vd), and the formula of Hicks et al. (1987) was used:

F ¼ Cz Vd

(1)

Deposition velocity can be expressed as:

Vd ¼ ðRa þ Rb þ Rc Þ1

(2)

Here, Ra is the aerodynamic resistance, Rb is the quasi-laminar boundary layer resistance, and Rc is the surface or canopy resistance. In the neutral atmospheric stability Ra and Rb can be estimated according to Hicks et al. (1987) and Smith et al. (2000):

Ra ¼ uu*2

(3)

Rb ¼ 2ðSc =Pr Þ2=3 =ku*

(4)

where k is von Karman’s constant, 0.41; u is the horizontal wind speed; u* is the friction velocity; Sc/Pr is the ratio of the Schmidt number for the gas to the Prandtl number for air, for which values can be obtained from Erisman and Pul (1994). Friction velocity and wind speed values were obtained from a sonic anemometer produced by the Campbell Company and installed at QZ experimental station. There were data from one year only (2011) for the estimation of dry deposition at all the sites. The height of the sonic anemometer was 2.5 m above the ground. Surface or canopy resistance (Rc) is an important parameter in estimation of dry deposition and its estimation requires different methods of calculation for different gases or particles. For example, NO2 is mainly absorbed by the stomata, so Rc ¼ Rstomata (Smith et al., 2000), and for HNO3 most dry deposition models assume that Rc ¼ 0. The canopy resistance of NH3 is difficult to derive directly, and temperature, radiation, and leaf area index (LAI) must be considered together for estimation (Smith et al., 2000). In the present study we assumed the canopy or surface resistance to be constant. HNO3 canopy resistance was assumed to be 0. The Rc of NO2 and NH3 was modified from the literature (Trebs et al., 2006). Rc of NO2 was assumed to be 75 s m1, and Rc of NH3 was assumed to be 100 s m1. Because of a lack of suitable methods to estimate

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þ the in-situ deposition velocities of pNO 3 and pNH4 we adopted the  and pNO of Shen et al. (2009). deposition velocities of pNHþ 4 3

3. Results

 3.3. NHþ 4 eN and NO3 eN concentrations in precipitation

3.1. Atmospheric concentrations of Nr compounds Annual mean concentrations of atmospheric Nr compounds are shown in Fig. 2. Large spatial variation in Nr concentrations was found at different sites in north China. QZ and CAU had the highest annual mean NH3 concentration (10.0 mg N m3), followed by BD (9.4 mg N m3), SY (8.1 mg N m3) and ZZ (8.0 mg N m3). The lowest concentration was found at SZ (7.4 mg N m3). Overall, there was no notable difference in NH3 concentration across the urban (CAU, BD and ZZ) and rural (SZ, QZ and SY) sites. The concentration of NO2 was evidently higher at the urban sites (average 14.3 mg N m3) than the rural sites (average 9.0 mg N m3). Atmospheric concentrations of HNO3 were relatively low and 20% higher at the urban sites (1.3e2.2 mg N m3) than the rural sites (1.0e1.9 mg N m3). 3 across all six sites. Average concentration of pNHþ 4 was 8.4 mg N m 3 (6.1e10.8 m g N m ) were found at the High concentrations of pNHþ 4 Beijing, Hebei, and Henan sampling sites but a much lower concentration (2.2 mg N m3) was observed at the remote rural site SY. 3 Average concentrations of pNO 3 at the urban sites (6.1 mg N m ) were substantially higher (P < 0.05) than those at the rural sites (3.1 mg N m3). The urban sites showed higher atmospheric concentrations of NO2, HNO3 and pNO 3 than the rural sites but no significant differences in reduced Nr concentrations were found between the urban and rural sites (Fig. 2). 3.2. Seasonal variation in Nr Seasonal variation in Nr concentrations at the six monitoring sites is shown in Fig. 3. The highest NH3 concentrations appeared in spring at BD and autumn at CAU while the peak values of NH3 were found in summer at SZ, SY, QZ and ZZ. NH3 concentrations were highest in summer at all rural regions. Concentration of NH3 at urban sites was similar in all seasons except winter. NH3 concentrations were lowest in winter across all sampling sites (except SY). Atmospheric pNHþ 4 concentrations were the highest in summer. Atmospheric concentrations of HNO3 were also highest in the summer. pNO 3 concentrations were highest in the autumn or winter. NO2 concentrations did not show the same trend across the

18

-3

Atmospheric concentration (ug N m )

different sampling sites. The highest value of atmospheric NO2 occurred in winter at BD and QZ but NO2 peaked in autumn at CAU, SZ and ZZ and in summer at SY.

CAU BD ZZ SZ SY QZ

16 14 12

 The average concentrations of NHþ 4 eN and NO3 eN in precipi1 at the five sampling sites tation were 2.9 and 1.9 mg N L  (excluding ZZ) (Table 1). The concentrations of NHþ 4 eN and NO3 eN were 78% and 81% higher (P < 0.05) at urban than at rural sites. The  seasonal variation in NHþ 4 eN and NO3 eN was consistent with the Nr concentrations in the air. For example, SZ and QZ had the highest þ concentrations of NH3 and pNHþ 4 in summer, so the NH4 eN in rainfall was also high in the summer (data not shown).

3.4. Dry deposition of Nr compounds Meteorological data were recorded every 30 min at QZ, a site representative of North China. We used the monthly average wind speed and u* values to calculate the monthly N dry deposition for QZ and all other sites. Wind speed ranged from 0.89 to 3.23 m s1, and the u* value ranged from 0.10 to 0.28 s m1 during the sampling period (Fig. 4). Meteorological parameters of wind speed and u* value were higher in spring and summer. The deposition velocities of HNO3, NH3 and NO2 ranged from 0.6 to 1.6 cm s1, 0.4 to 0.6 cm s1 and 0.4 to 0.7 cm s1, respectively, and the average deposition velocities of HNO3, NH3, and NO2 were 1.1, 0.5, and 0.6 cm s1 (Fig. 5). Assuming that all the sampling sites have the same land use pattern, that surface or canopy resistance can be regarded as constants, and that the same deposition velocities as at QZ apply to all sampling sites, the total N dry deposition ranged from 35.2 to 60.0 kg N ha1 and averaged 49.4 kg N ha1 in the year 2011 across all six sampling sites (Table 2). NH3 and NO2 were the dominant Nr species in the N dry deposition. Annual N dry deposition at the urban sites (average 57.0 kg N ha1) was 36.4% higher than that at the rural sites (average 41.8 kg N ha1), attributable mainly to the high concentration and dry deposition of NO2 in the urban environment.  3.5. Wet deposition of NHþ 4 eN and NO3 eN

Rainfall ranged from 388 to 730 mm yr1 across the sampling sites in 2011 (Table 1), showing large spatial variation in precipitation over this region. Most rainfall events occurred in the summer (Fig. 6). Wet deposition of inorganic N averaged 28.2 kg N ha1 yr1 and ranged from 16.3 to 43.2 kg N ha1 yr1 across the sites (Table 1). NHþ 4 eN was the dominant N deposition form in the precipitation which occurred mainly in the summer (Fig. 6) and the highest wet N deposition was observed in July, especially for NHþ 4e N deposition at both urban and rural sites (Fig. 6), because July had high rainfall and concentrations of NH3 and pNHþ 4 were also high. Wet N deposition was 33.2% higher at urban than at rural sites.

10 8

4. Discussion

6

4.1. Nr concentrations in the air and precipitation in North China

4

After the 2008 Olympic Games were allocated in 2001 the Chinese Government took action to control air pollution in Beijing and the surrounding regions (Cermak and Knutti, 2009). In the present study the data indicate consistent particulate Nr concentrations outside the period of the games and show that other cities (such as Baoding and Zhengzhou) had higher concentrations of  pNHþ 4 and pNO3 than Beijing. We also found high concentrations of NO2 and pNO 3 even in rural regions, reflecting the widespread

2 0 NH3

+

pNH4

HNO3

-

pNO3

NO2

Fig. 2. Average concentrations of Nr at the six sampling sites during the sampling periods. The sampling sites were grouped as urban (CAU, BD and ZZ) and rural (SZ, QZ and SY) sites. Bars are standard errors of the means.

X.S. Luo et al. / Atmospheric Environment 74 (2013) 209e216

concentration of Nr(ug N m-3)

Concentration of Nr(ug N m-3)

20

20

a

CAU BD ZZ SZ SY QZ

15

10

5

5

0

0

20

20

CAU BD ZZ SZ SY QZ

15

10

5

5

NH3

pNH4+

HNO3

CAU BD ZZ SZ SY QZ

d

CAU BD ZZ SZ SY QZ

15

10

0

b

15

10

c

213

0

pNO3-

pNH4+

NH3

NO2

HNO3

pNO3-

NO2

Fig. 3. Seasonal variation in atmospheric Nr concentration at the six sampling sites in North China: (a) spring; (b) summer; (c) autumn; and (d) winter. The first three columns (CAU, BD and ZZ) denote urban sites and the second three columns (SZ, QZ and SY) denote rural sites. Bars are standard errors of the means.

The high levels of Nr air pollution in North China have also been shown by the high concentrations of inorganic N in precipitation (Table 1). Both annual volume-weighed NH4eN and NO3eN concentrations in precipitation were higher at the urban sites than at the rural sites. Distinct seasonal variation in NH4eN and NO3eN concentrations was found across all sites (Fig. 6). High temperatures and humidity in summer and high concentrations of SO2 in winter will stimulate the formation of pNHþ 4 and heating from coal leads to higher concentrations of pNO 3 and NO2 in autumn and

.30

3.5 Wind speed u*

.25

2.5 .20 2.0 .15 1.5 .10 1.0

Table 1 Bulk N deposition at five sampling sites in North China in the year 2011.

a

Dec-2011

Oct-2011

Nov-2011

Sep-2011

43.2 27.8 35.4 19.2 16.3

0.00

Jul-2011

15.8 12.1 11.1 9.2 6.6

0.0

Aug-2011

27.4 15.7 23.2 10.0 9.7

Sampling period from April 2010 to March 2011 at SY, the same below.

) Jun-2011

Total

Apr-2011

2.17 2.22 1.61 1.64 1.71

yr

NO 3 eN

May-2011

3.75 2.90 3.37 1.79 2.49

N deposition (kg N ha NHþ 4 eN

Mar-2011

730.3 542.6 690.5 558.3 387.5

)

.05

.5 1

Feb-2011

CAU BD SZ SYa QZ

Rainfall N concentration (mg N L (mm) NO NHþ 4 eN 3 eN

1

Jan-2011

Site

1

The value of u*(m s-1)

3.0

Wind speed(m s-1)

transfer of NOx from industry, power plants and transportation sectors, most likely transferring from urban to rural areas in North China. There was a substantial Nr gradient between the urban and rural sites (e.g. CAU and SZ) during the sampling period (Figs. 2 and þ 3). Air concentrations of HNO3, NH3, NO2, pNO 3 , and pNH4 at CAU were 15.5, 25.9, 35.8, 37.6, and 28.8% higher, respectively, than at SZ. Similar higher Nr levels were found in urban than rural areas (Figs. 2 and 3). This indicates that urban areas are important Nr sources and have high Nr (especially oxidized N) with potential for transport from the urban to the surrounding rural areas. Agricultural activities are the prime source of NH3, and high application rates of N fertilizers and intensive livestock farms contribute to the high NH3 concentrations in rural North China (Shen et al., 2011b). Motor vehicles, waste disposal facilities, and manufacturing industries are additional NH3 sources (Anderson et al., 2003; Battye et al., 2003). Our results also show high NH3 pollution in urban areas due to the complex mosaic of NH3 emission sources in urban areas. The high concentrations of NH3 found in North China in this study are consistent with results from the global satellite estimates of NH3 distribution (Clarisse et al., 2009).

Month-Year Fig. 4. Monthly variation in wind speed and friction velocity at QZ site in central North China.

X.S. Luo et al. / Atmospheric Environment 74 (2013) 209e216 14

1.8

12

NO2

Bulk N deposition(kg N ha -1)

NH3

1.4 1.2

NH -N

300

NO -N Rainfall

10

250

8

200

6

150

4

100

.4

2

50

.2

0 14

1.0 .8 .6

0 350

bb

b

Month-Year Fig. 5. Monthly deposition velocities for NH3, HNO3 and NO2 at QZ site in central North China.

winter, and this trend is consistent with other reports about the seasonal variation in Nr concentrations in North China (Shen et al., 2009; Pan et al., 2012; Yang et al., 2012).

12

NH -N

300

NO -N

-1

Bulk N deposition(kg N ha )

Dec-2011

Nov-2011

Oct-2011

Sep-2011

Aug-2011

Jul-2011

Jun-2011

May-2011

Apr-2011

Mar-2011

Feb-2011

Jan-2011

0.0

Rainfall

10

250

8

200

6

150

4

100

2

50

0

4.2. Re-quantification of dry and bulk deposition of Nr in North China There have been some studies on N deposition in North China, mainly in rural areas. Bulk N deposition in Beijing area were found to range from 26.6 to 38.3 kg N ha1 yr1, with an average value of about 30 kg N ha1 yr1 (Liu et al., 2006). Total N deposition in maizee wheat rotation systems in North China was estimated to be 99e 117 kg N ha1 yr1 using a 15N dilution method (He et al., 2010). In the present study we calculated the deposition velocities combining the meteorological conditions in North China, and the amount of dry N deposition (average 49.4 kg N ha1 yr1, Table 2) was slightly lower than our earlier estimated value (55 kg N ha1 yr1) (Shen et al., 2009) but substantially higher than the average dry deposition (36 kg N ha1 yr1) in Beijing and its surrounding areas (Pan et al., 2012). Dry N deposition ranged from 35.2 to 60.0 kg N ha1 yr1 over the six sampling sites and dry N deposition was higher than wet N deposition (16.3e43.2 kg N ha1 yr1) in this area. Total inorganic N deposition amounted to 54.4e103.2 kg N ha1 yr1. More emphasis needs to be placed on the environmental effects of these high N deposition rates and their potential impact on other regions. There are also some major uncertainties regarding our dry N deposition study. For example, there are uncertainties associated with the estimation of bi-directional fluxes of NH3, the differences among different land use patterns in deposition velocity, and the Table 2 Atmospheric N dry deposition at six sampling sites in 2011 (kg N ha1 yr1). Nr species

HNO3 NH3 NO2 pNHþ 4 pNO3 Total

Sampling site CAU

BD

ZZ

SZ

SY

QZ

6.4 17.2 27.8 5.0 3.7 60.0

7.0 15.3 20.2 7.1 5.1 54.7

5.0 11.4 27.0 7.9 5.1 56.4

5.6 13.6 17.2 3.5 2.4 42.3

3.4 13.3 15.6 1.7 1.2 35.2

5.2 16.7 14.9 7.7 3.3 47.9

Rainfall(mm)

Deposition velocities(cm s-1)

a

HNO3

1.6

350

aa

Rainfall(mm)

214

0 Jan

Feb Mar

Apr May Jun

Jul

Aug Sep

Oct

Nov Dec

Month Fig. 6. Distribution of monthly average bulk N deposition and precipitation at the monitoring sites in North China during the monitoring periods: (a) urban sites; (b) rural sites. þ deposition velocities of pNO 3 and pNH4 . Furthermore, our calculations using monthly averaged data to estimate deposition velocities will have lower accuracy than the US and EU networks in which calibration used 30-min weather datasets (Baumgardner et al., 2002; Flechard et al., 2011), thus further work is required to increase the reliability of dry N deposition values in the future.

4.3. Implications of pollution control measures on air quality and Nr deposition Our results show that atmospheric concentrations and deposition of Nr were still high in North China about three years after the Beijing Olympic Games. Although the national government took effective measures during and after the games to control air pollution in Beijing because it is the national economic and political center, provincial cities were ignored. Local governments (outside Beijing) will need to balance the trade-off between economic growth and environmental protection. Emissions from the transportation sector are a major source of air pollution in some Chinese cities (Zhou et al., 2010). Hao et al. (2005) estimated that NOx emissions from motor vehicles accounted for 35% and power plants 27% of total emissions in Beijing, with 74% of the ground level NOx due to vehicular emissions. New power plants are developing very quickly in the provinces of North China and play a major role in NOx emission. However, no new power plants were built in the megacity Beijing over the period 2005e2007 and emissions from industries and transportation still grew rapidly (Wang et al., 2012). Our study confirms the presence of high concentrations of pNO 3 and NO2 even in rural regions, reflecting the widespread transfer of NOx

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from industry, power plants and transportation sectors, most likely from urban to rural areas. The high concentrations of pNO 3 and NO2 in both rural and urban areas may be closely linked to the rapid increase in the number of motor vehicles (especially private cars) after 2008 (Liu et al., 2013). The rapid increase in motor vehicles may also contribute substantially to the NH3 pollution in the urban areas of North China (Shen et al., 2009). Ambient meteorological conditions also show a close relationship with heavy air pollution in North China. High temperatures, high humidity and low wind speeds will foster PM and ozone pollution in the summer in Beijing (Streets et al., 2007). The high levels of Nr air pollution in North China have been indicated by the high concentrations and wet deposition of inorganic N in precipitation. Although the rainfall in North China is about half that in the southeast (e.g. Tai Hu lake region), the total annual N wet deposition in rainwater in the north is still higher than in the southeast due to higher N concentrations (Xie et al., 2008; Yang et al., 2010). High temperatures and humidity in summer and high concentrations of SO2 in winter will stimulate the formation of pNHþ 4 , and heating from coal leads to higher concentrations of pNO 3 and NO2 in autumn and winter, and this trend is consistent with other reports about the seasonal variation of pNO 3 and NO2 in North China (Shen et al., 2009; Yang et al., 2012). Our study demonstrates that pollution by Nr in air and rain is still heavy in North China. Some developed countries have taken measures to control NOx and SO2 emissions and have made some progress. Emissions of SO2 and NOx in the European Union declined by 66 and 32%, respectively, over the period 1990e2005; NOx and SO2 emissions from power plants and motor vehicles in the US were also reduced between 1980 and 2005 (Monks et al., 2009). Population growth and urbanization do not need to result in increasing emissions of air pollutants. A good example is the US where during the last 40 years, despite a doubling of the population and tripling of vehicle use, the quality of air in the greater Los Angeles area has improved and atmospheric concentrations of O3, CO, NO2 and PM2.5 have decreased by 50, 72, 58 and 44%, respectively (Parrish et al., 2011). In spite of some challenges to decrease Nr and other forms of pollution in North China, as mentioned earlier, there is much to learn from the successful experience in greater Los Angeles. Overall the government must devote more resources to environmental protection and cooperation among various government and nongovernment agencies and improve energy efficiency and nitrogen fertilizer use efficiency in the future by technological innovation and policy enforcement (Wang and Hao, 2012; Liu et al., 2013). 5. Conclusions We found severe atmospheric Nr pollution (NH3, NO2, HNO3,  pNHþ 4 and pNO3 ) at six sampling sites in North China after the Beijing Olympics. Atmospheric concentrations of Nr were higher at urban monitoring sites (CAU, BD and ZZ) than at rural sites (SZ, SY and QZ). NH3 concentrations at urban sites were comparable with or even higher than those at rural sites, reflecting the contribution of complicated emission sources in Chinese cities. A substantial atmospheric Nr gradient between the rural and urban (e.g. CAU and SZ) sites suggests that the city itself can be an important source of atmospheric Nr pollution with consequent transport to surrounding areas. Dry and wet inorganic N deposition rates ranged from 35.2 to 60.0 kg N ha1 yr1 and from 16.3 to 43.2 kg N ha1 yr1, respectively, with total annual deposition of 54.4 to 103.2 kg N ha1 in this region. Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant 41071151), the innovative group grant

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