Source Identification of Acid Rain Arising over Northeast China : Observed Evidence and Model Simulation

Acid rain has already been the environment issue for the past century and is still concerned in Southern and Eastern China. However, Northeastern China, considered in the past as non-acid rain area, is reported high frequency of acid rain occurred recently. Through a measurement and model simulation analysis with linear regression technique, factor speciation, backward trajectory, source-tracing and fraction sampling methods, this study aims to investigate the causes of the acid rain frequently occurred in the two Northeastern cities (i.e., Dandong and Dalian) and to identify the contributing sources of the chemical ions in precipitation. The annual averaged pHvwa of 2007 ranged within 4.15–4.27 and 4.5–5.15 over Dandong and Dalian, which suggested the similar acidity with Southern China. The precipitation acidity in Dandong was found to be sulfur dominant in winter-spring (nssSO4/NO3 = 13.1) than summer-autumn (nssSO4/NO3 = 1.05), whereas in Dalian, estimated ratio nssSO4/NO3 was about 2.35 all over the year, reflecting the strong impact of local NOx emissions from a much more intense mobile traffic in comparison with Dandong. The findings also revealed a typical physico-chemical condition with constant strong influence of regional transport over Dandong and Dalian particularly in summer. Besides, analysis of washout and rainout mechanisms showed that the long range transport in-cloud (rainout) was more pronounced in Dandong (with minor local emissions) than Dalian (with more significant local emissions). This study indicates that acid rain pollution in Northeastern China requires much effort, not only in local emission abatement, but also in regional trans-boundary pollution control.


INTRODUCTION
Acid rain has been one of the most cross-cutting and challenging environmental issues in the past half a century in China (Larssen et al., 2006).Actually, rapid economic development and population growth resulted in increasing emissions and subsequently potential deposition of acidic substances and widespread acid rain in southern China between the 1980s and the mid-1990s (Wang and Wang, 1996;Ding et al., 1997;Ding et al., 2004).To conquer the problem, China had developed the concept of the acid-rain control zone as the main framework for setting priorities in the acid-rain reduction policy.The policy for acid-rain mitigation used a flat emission reduction goal of 20% from 1995 to 2010 within the acid-rain control zone (Larssen et al., 2006).
Nevertheless, in order to improve the knowledge on acid rain and resulting effects in China, a great need still persists for better control of the environmental degradations.Based on the latest 15 years observations (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) of the acid rain network, statistics and analysis results showed that the acid rain still dominated in the vast regions south of the Yangtze River, with an obvious expanding tendency of acid rain zones in central and northern China where alkaline dust from the desert areas, cement production, and construction activities largely neutralized the acids in the past (Annual report of CAMS, 2009;Xie et al., 2009;Tang et al., 2010;Ge et al., 2011).More recently, acid deposition rate in Northeast China strongly increased since 2004, especially in Liaoning Province identified as one of the most affected areas (Hong et al., 2009;Yang et al., 2012a;Zhang et al., 2012;The Liaoning Environmental Bulletin Report in 2007: http://www.lnepb.gov.cn/zfxxgk/hjzl/hjzlzkgb/201011/t20101111_19082.html).According to The Liaoning Environmental Bulletin Report in 2007, increasing trends of acid rain frequency ranging from 1.3% in 2001 to 9.7% in 2007 have been observed over the province.Monitored precipitation chemistry and pH values (rain water pH < 4.5) in 2007 revealed a severe acid rain over the Province where Dandong and Dalian cities were reported as the most affected cities with the lowest acid neutralization capability despite weak local acid precursor emissions (Hong et al., 2009;Zhang et al., 2012).Thereby, as underlined by Zhang et al. (2012), elucidating the role of the regional chemical transport, and more importantly, the contrasting critical lower precipitation pH and higher neutralizing capability between downtown and rural areas, remains crucial for environmental implications in Dalian and Dandong cities.
Based upon the measurement analysis with linear regression technique, factor speciation, backward trajectory and fraction sampling methods, as well as source-tracing simulation method, this study aims to specify the contributing regional sources to the chemical components of the precipitation over Dandong and Dalian cities, where the high frequency acid rain often occurred.The main purpose is to delve into the regional physico-chemical pathways of acidification process over Northeastern China.In collaboration with Liaoning Environmental Monitoring Center, this work is expected to provide valuable insights for policy makers in reducing environmental burden in Liaoning Province.

Measurement Sites
Fig. 1 shows the observation sites located in Dandong and Dalian in Liaoning Province.Dalian is a coastal city with a population of more than 50 million, which could be identified as an area influenced by anthropogenic emissions mainly from shipbuilding industry, traffic, petrochemical industry and the big power plant in surrounding areas (Kong et al., 2010;Zhang et al., 2012).There are three sites in Dalian; one is a rural site located in the southeast of Dalian (Lvshun, abbrev LS, Number 1 th in Fig. 1) and the other two are unban sites, located in the HeiShiJiao Park (Heishijiao, abbrev HSJ, Number 2 th in Fig. 1) and in the industry zone (Ganjinzi, abbrev GJZ, Number 3 th in Fig. 1).In Dandong, where is more influenced by framing rather than industrial activities, there are two sites (one rural, Langtou abbrev LT Number 4 th and one urban Jiancezha, abbrev JCZ, 5 th in Fig. 1).

Sampling Analysis and Quality Control
In 2007, sixty five and seventy samples were collected as a daily frequency in Dandong and Dalian respectively for measurement of nine key chemical species, pH and conductivity.The number of samples in winter, spring, summer and autumn are 6, 18, 29 and 12 for Dandong and 2, 20, 34 and 14 for Dalian, respectively.Besides, the sequential sampling during June and July of 2009 in JCZ, HSJ and GJZ were also measured with four, two and three events, respectively.The pH and the electrical conductivity (EC) of rainfall were measured by Glass electrode pH meter (AR20, Thermo Fisher Scientific Inc.) and EC meter (HI9032, HANNA) immediately.The collected rainfall samples had been refrigerated under 0°C-4°C and analyzed in the laboratory.Four anions (SO 4 2-, NO 3 -, Cl -, F -) and five cations (NH 4 + , Na + , K + , Ca 2+ , Mg 2+ ) were quantitatively measured by ion chromatography (IC, Dionex 600, USA).The relative standard deviation was less than 5% for reproducibility test.The quality assurance was routinely carried out by using Standard Reference Materials produced by National Research Center for Certifed Reference Materials, China.Precision of quality of ionic measurement data was checked by anion-cation balance and by comparison of measured and calculated conductivities.The conductivity was calculated by the ionic concentration based on the following equation: Based on regression and correlation analyses of calculated and measured conductivities (Fig. S1), the slope of the regression line was 1.09 for Dandong and 0.99 for Dalian, while the coefficient of determination was 0.97 and 0.94 respectively.In Dalian, the ratio of calculated vs. measured conductivity was > 1, suggesting that the ions was deficient in observation.Simultaneously, the ratio of total cations to anions was slightly higher than 1 (1.05 as shown in Fig. S2(a)), revealing anion deficiency in the city, in agreement with previous literatures.This might due to some unmeasured anions such as short chain organic acids (Hontoria et al., 2003;Yang et al., 2012).Overall, the ion and conductivity balance in both Dalian and Dandong cities seemed to be reliable according to the acceptable range by US EPA (Table S1), although the organic anions were absent.

Model Descriptions
A three-dimensional regional Eulerian chemical transport model, the Nested Air Quality Prediction Modeling System (NAQPMS) develped by Wang et al.(2000), is utilized in this study.The new version with the cloud and aqueous chemistry precess module, which is more suitable for simulation of wet deposition of acidifying substances in East Asia, is updated by Ge et al. (2014).The NAQPMS model domain used in this study contains 6570 × 5370 km 2 on a Lambert conformal map projection with 30-km gird resolution horizontally and 20 terrain-following layers vertically (Fig. S3).The meteorological conditions are driven by WRF-ARW3.2and the emission as well as more details about the model set up is same as Ge et al. (2014).

Analysis of Precipitation Acidity over Dalian and Dandong
The distribution of precipitation acidity frequency in Dalian and Dandong is displayed in Fig. 2. The precipitation acidity is non-averaged distribution in Dalian.Lower pH values (4-5) of precipitation at HSJ occurred more frequently than those at the other two sites.The VWA pH (4.5) for LS is similar with that at HSJ, with about half of the total samples belonged to acid rain (based on the definition of pH lower than 5.6), which is higher than GJZ (7 percent) but lower than HSJ (69 percent).The rainfall in GJZ was particularly alkaline, probably due to the high concentration of alkalineaerosol such as alkaline calcium and magnesium compounds emitted from anthropogenic activity, although the sulfate and nitrate concentration in precipitation are also high compared to the other two sites in Dalian.In Dandong, the distribution of rainwater acidity frequency seemed to be in the same level both at LT and JCZ with high frequency of low pH (4-6) and more than 50% lowest pH (< 4.5), indicating heavy acid rain over the city.Table 1 shows the statistical results of the raining water acidity analysis in Dalian and Dandong in 2007.Basically, more than 78% rainfall events were acidic in Dandong with no major discrepancy between LT and JCZ and close to the annual volume weight averaged pH (pH vwa ).In Dalian, monitored acid rain frequency was 68% at HSJ station, 46% at LS, and  3.98-7.715.15 * more than 50% samples with pH < 5.6 in an event of rainfall are defined as acid rain.only 7% at GJZ, respectively.In comparison with Dandong, the acid rain was obviously less pronounced in Dalian city with relatively higher yearly pH vwa at HJS, LS (4.5) and GJZ (5.15).However, the acid rain in both cities seemed to be at the similar levels as those observed in Southwestern China (pH are in the range of 4.1-4.6)and Yangzi River (pH vwa 4.5) by (Aas et al., 2007) and (Huang et al., 2008), and more critical than the acidification process observed in Beijing (5 years rainfall records with pH vwa = 6.2) reported by Yang et al. (2012).

Chemistry of Precipitation in Dandong and Dalian Monthly Volume-Weighted Average of Ion Concentration
Calculated monthly averaged ion concentrations are presented in Fig. 3.The ion concentrations in rainfall showed significant seasonal variation with high values in winter and spring and weak in summer in both Dandong and Dalian.These ions were found to be correlated to the volume of precipitation and high concentrations of air pollutants (SO 2 , NO x , etc.), as well as the potentially affecting sources and dominate airflows, such as the anthropogenic heating periods in autumn and wintertime and the mineral dust emissions occurring in springtime reported by (Huang et al., 2008).For instance, high level of SO 2 and NO 2 concentration in winter and low in summer provide the coordinated precursors of the ions in precipitation both in Dandong and Dalian (Fig. S4).Note that, the highest volume of precipitation occurred in August and July, with a discrepancy factor of 2. By assuming that the rainfall in July was sufficient to wash out the air pollutants in a relatively weak local emission area like Dandong, the ion concentrations in water could be determined by the volume of rainfall and this might derive the total ion concentration of August at JCZ (170 µeq L -1 ) and LT (136 µeq L -1 ).However, the theoretical values (or calculated values) of total ion concentrations of August were lower than those measured (186 µeq L -1 and 281 µeq L -1 at JCZ and LT, respectively).Consequently, the air pollutants would not be cleaned out by wet deposition even under the high volume of precipitation (> 100 mm) in Dandong.In Dalian, high volume of rainfall was associated with high ionic concentrations, highlighting the complexity of wet scavenging in Dalian.The findings reveal a typical physicochemical condition over Northeast China with probably constant strong influence of regional transport over Dandong and Dalian, particularly in summer (Huang et al., 2008;Zhang et al., 2008).Actually, in Northeast China, a persistent subtropical high-pressure system situated over the western Pacific, leads to gentle winds favourable for pollutants stagnation and distribution after an important chemical transport controlled by the summer monsoon (Seto et al., 2002;Gao et al., 2005;Wang et al., 2008).On the other hand, the regional warm summertime weather and the growth of the well-mixed boundary layer in that period, act as important triggers of strong photochemical conversion of SO 2 and NO x resulting in high acidic species concentrations (Gao et al., 2005).The variation of planetary boundary layer height (PBLH) is found to play an important role in the stagnation of anthropogenic pollutants concentrations in summer over Northeast China (Wang et al., 2008).The convergence of these physical and chemical reactions undoubtedly towards an accumulation of the chemical components in precipitation over Dandong and Dalian even under high wet scavenge process.

Acidification Process and Neutralization Effect
Among the anions, nssSO 4 2-and NO 3 -were the major species with the yearly volume-weighted averaged concentrations of 71% and 80% of the total anions in Dalian and Dandong, respectively.Usually, the ratio of nssSO 4 2to NO 3 -(nssS/N) is considered as an indicator of the acidification status (Wang and Wang, 1996;Fujita et al., 2000;Tanner, 1999).In 2007, this ratio varied seasonally with 13.1 in winter-spring and 1.05 in summer-autumn over Dandong (Fig. 4).The dominance of sulfur in acidification process was thus more pronounced in winter-spring than that in summer-autumn.The increasing long heating period in winter, potential source of abundant sulfur emissions, might not be significantly balanced by NO x emissions essentially from regional mobile traffic.Over Dalian, the yearly ratio was 2.35 with high concentrations of both nssSO 4 2-and NO 3 -in winter, which is similar with that in Beijing with the value of 2.7 based on the five-year measurements reported by Yang et al. (2012).This reflects the strong impact of local NO x emissions over Dalian where the mobile traffic is actually much more intense than that in Dandong.
Due to the presence of NH 3 and CaCO 3 , the neutralization of the acids such as H 2 SO 4 , HNO 3 and organic acids in precipitation occurs and the pH of precipitation depends on the relative abundance of these acids and bases and of the neutralization reactions amongst them.The neutralization effects of major bases such as CaCO 3 and NH 3 was analyzed by the linear regression model using SO 4 2-and NO 3 -as dependent variables and Ca 2+ and NH 4 + as independent variables (Migliavacca et al., 2005).Table 2 shows the results of the regression coefficient.In Dandong, approximately 69.1% of SO 4 2-(64.5% for (NH 4 ) 2 SO 4 and only 4.6% for CaSO 4 ) could be explained by the two independent variables used in the regression analysis.Similarly, 60.8% NO 3 -could   -and Ca 2+ .In short, the neutralization of precipitation acidity over Dandong was mainly due to existing NH 3 probably emitted from agricultural farms and fertilizer production units.In contrast, the acid neutralization over Dalian was mainly governed by CaCO 3 (large contribution of CaSO 4 (81.2%) and Ca(NO 3 ) 2 (56.4%)) emitted from industrial and construction activities.The difference between Dandong and Dalian could be attributed to the different atmospheric chemistry fields: potentially affecting sources, airflow, precipitation phenomena and topography.Following section will discuss the potential sources of the precipitation chemistry in these two cities.

Source Identification by Factor Analysis
In order to quantify the influence of anthropogenic and natural sources on the chemistry of the precipitation over Dandong and Dalian cities, the principal components analysis (PCA) with mathematical methods, applied worldwide (Lee et al., 2000;Seto et al., 2000;Migliavacca et al., 2005;Huang et al., 2008), has been deployed in this study using SPSS software (version 15.0).The Varimax Rotation was applied to the data set for simplifying the expression of the principal components (PC).Results are presented in Table 3.In Dalian, two factors explained 72% of total variance versus four factors in Dandong for 85% of the total variance.In Dalian, the first factor (F1), with a variation of 40% of the raw data, showed high loading for pH, Cl -, Na + , K + , Ca 2+ and Mg 2+ , reflecting natural sources.Basically, Cl -, Na + and K + characterized the contribution of sea salts (Seto et al., 2000;Huang et al., 2008), while Ca 2+ and Mg 2+ were probably from dust.The second factor (F2), explaining 32% of the total variation, showed high loading for SO 4 2-, NO 3 -, NH 4 + and K + .This factor characterized the anthropogenic sources, of which SO 4 2-, NO 3 -and NH 4 + derived from coal burning, mobile emissions, farming activities and human/ animal excrement.The high loading for K + associated with NH 4 + in F2, could be considered as a signal of biomass burning (Migliavacca et al., 2005).On the other hand, the pH seemed to be mainly associated with Ca 2+ and Mg 2+ .Therefore, the anthropogenic emissions, sea salts and dust were the main sources of rainwater ions in Dalian.In Dandong, F1 explained 29% of the total variation with high loading for F -, Ca 2+ and Mg 2+ mainly emitted from dust.The second factor F2 (23%) represented sea salt sources with high loading for Cl -, Na + and K + .With the total variance (F1 + F2) of 52%, the contribution of natural sources to rainwater ions was found to be more significant in Dandong than that in Dalian.F3 showed higher loading for SO 4 2-, NO 3 -and NH 4 + characterizing anthropogenic emissions, with 20% of the total variance.F4 (13% of the total variance) highlighted good correlation among pH, SO 4 2-(-0.5)and NH 4 + (0.24).Therefore, the pH of the precipitation over Dandong was more influenced by (NH 4 ) 2 SO 4 than CaCO 3 .

Source Identification by Backward Trajectory Analysis
The backward trajectory analysis had been widely used in studying the influence of the rainout and washout processes (Okita et al., 1996;Bertrand et al., 2008;Huang et al., 2008;Yamagata et al., 2009;Zhou et al., 2009;Izquierdo et al., 2012;Shi ChunE, 2014).In this study, the 72-h back trajectories were calculated for four time sequences (00:00, 06:00, 12:00 and 18:00 UTC) per day (rainy day) at 100 m altitude, and clustered using the clustering tool of HYSPLIT4.8(Fig. 5).All rainwater samples were collected and associated with concurrent values of the trajectories and grouped on the basis of their intraday prevailing wind directions.In both Dalian and Dandong cities, four main transport patterns have been dissociated such as local sector with the wind speed lower than 2 m s -1 and air masses from all directions around the sites, W-N-NW sector, E sector, and S-SE sector (Table 4).In Dalian, the local sector (mostly occurring in summer) contributed to about 51% of the total air-mass with dominance of Ca 2+ and Mg 2+ concentrations.The W-N-NW sector (14% of the total air-mass) consisting of air masses mainly from Siberia area in spring and autumn, governed the impacts of dust with the highest concentrations of Ca 2+ (148.6 µeq L -1 ) and Mg 2+ (45.6 µeq L -1 ).The third sector generated from the east including Korea peninsula (21% of the total air-mass, mostly occurring in spring and summer) was dominated by SO 4 2-, NO 3 -and NH 4 + concentrations, slightly higher than local sector loadings.
The pH value of E sector was the lowest in all four sectors, corresponding to the lowest cation concentrations as well (24.5 µeq L -1 and 13.2 µeq L -1 for Ca 2+ and Mg 2+ , respectively).The S-SE sector (14% of the total air mass, from the Pacific and Yelow sea with crossing over Shandong Province) showed the highest concentrations of anthropogenic ions (210.7 µeq L -1 for SO 4 2-, 76.6 µeq L -1 for NO 3 -and 103.5   µeq L -1 for NH 4 + ) as well as relatively high concentrations of sea-salt ions (65.9 µeq L -1 for Cl -and 33.8 µeq L -1 for Na + ).This highlights the contribution from the Pacific and Shandong Province to air pollution over Dalian.In Dandong, the local sector (44% of the total air mass) showed the lowest rainwater pH value with high level concentration of SO 4 2-, NO 3 -and low NH 4 + .The contribution of W-N-NW sector mostly occurring in winter and spring (dust influence) remained same as that in Dalian.It is worthy to note that, the high concentration of F -(30.5 and 20 µeq L -1 ) is loading in local and W-N-NW sector, which is much more higher than that in Dalian (with highest concentration of 17.4 µeq L -1 in W-N-NW sector) although the latter is more affected by the industrial emissions.As we known, the F -in the precipitation is mainly originated from the anthropogenic emission such as coal burning, as well as the natural emissions like the volcanic eruption and the sea salt (Friend, 1989;Saether et al., 1995;Walna et al., 2013).Our findings imply the fluoride in North China as Dandong and Dalian is mainly controlled by the natural emissions instead of the anthropogenic emissions.However, Dandong was particularly influenced by high concentrations of SO 4 2-(304.3 µeq L -1 ) in E sector (in autumn) and low value in S-SE sector (in summer, 61.6 µeq L -1 ).This reveals strong impact of transboundary pollution from Korea peninsula on Dandong.

Source Identification by NAQPMS
The source tracing method coupled in NAQPMS has usually been used for O 3 and PM 10 source-receptor relationship analysis (Li et al., 2008;Wu et al., 2011), and also been investigated the sources of acidity substances deposition in East Asia by Ge et al.(2014).Based on the same method as that in Ge et al.(2014), the source of sulfur and nitrogen in wet deposition in Liaoning has been studied.Utillizing Geographical Information System (ArcGIS), the simulated region are devided into seven areas including: Liaoning, Northeastern China (NEC) except Liaoning, North China Plane (NCP), West China (WC), East China (EC), Southeast China (SEC), Central China (CC) and Outboard (Fig. S3).
The simulated yearly total wet deposition of sulfur (S), nitrogen from NO 3 -(N-NO 3 -) and nitrogen from NH 4 + (N-NH 4 + ) in Liaoning are 1.29 × 10 5 t × S, 4.55 × 10 4 t × N and 5.25 × 10 4 t × N, respectively (Table 5).Among the total wet deposition in Liaoning, almost 47% of S and 38% (83%) of N-NO 3 -(N-NH 4 + ) are from local emission.It should be noted that wet deposition of acidify substances from regional transported are significant, with NCP, EC and Outboard as the three largest imported areas.Fig. 6 shows monthly distributions and the contributions of the S, N-NO 3 -and N-NH 4 + wet deposition from eight areas.The largest amount appears in summer with large contribution from Outboard (e.g., Korea and Japan) and EC (e.g., Shangdongv Province), which is consistent with the resluts of the Backward Trajectory Analysis.The largest local contribution to the N-NH 4 + wet deposition among the three species suggested the significant impacts of the local agriculture acivities.

Influence of Washout and Rainout
Sequential sampling has been carried out in summer in Dandong (at rural site) and Dalian (HSJ & GJZ) to thoroughly evaluate the contribution of long range transport in cloud (rainout) and washout process below-cloud.Fig. 7 shows the averaged concentrations of chemical species of collected fractions.In both Dandong and Dalian, the chemical contents decreased through the precipitation increment.This might be due to scavenging of air pollutants under the cloud into the rainfalls in the first fractions.Based on the method employed by Aikawa andHiraki (2009, 2014) to evaluate the washout and rainout contributions (with the assumption that the chemical ions in later precipitation increment would be attributed to scavenging process through the rainout mechanism), we have estimated the washout and rainout contributions for Dandong and Dalian as prsented in Table 6.Both the rainout and the washout mechanisms accounted for about 50% of the total SO 4 2-deposition in Dandong over rural site.This supports various trends recently published worldwide for rural sites such as 47-53% in France (Bertrand et al., 2008) and 51-49% in Japan (Aikawa et al., 2014).In urban and suburban sites like GJZ and HJS, the conrtibution of the rainout has decreased to 12 % and 35%, similar with that found in Beijing (Liu and Huang, 1993) but lower than that reported in Japan by Aikiwa (2009Aikiwa ( , 2014) ) and Brazil by Migliavacca (2010).This might be due to the fact that coarse sulfate particles below-cloud in the urban cities of northern China are more efficiently scavenged by raindrops than fine particles (Samara and Tsitouridou, 2000).Such explanation is undoubdetely confirmed by resulting high  contributions of Ca 2+ through washout mechanism at all three sites (66%, 78% and 88%), since Ca 2+ exists only in coarse state.The nitrate deposition in Dandong also showed the fifty-to-fifty percent contribution between the the rainout and the washout process in contrast to the strong washout contriburion found in Dalian at HSJ (70%) and GJZ (92%).Many of previous studies reported about one-thrid to two-thrid contributions to rainout and washout over rural and urban areas (Kitada et al., 1993;Liu and Huang, 1993;Aikawa and Hiraki, 2009;2014).In comparison with this study, it is obvious to note that the washout scaveging below-cloud in Dalian (GJZ) under high local emissions, is more significant than that in Dandong with relatively minor local emissions.

CONCLUSION
The measurement analysis performed in this study shows the evidence of acid rain over Dandong and Dalian cities in 2007 (with strong yearly averaged pH vwa ) and provides insights into the mechanisms behind such phenomenon.Based on rainwater volume weighted calculation, factor analysis (with the approach of principal component), backward trajectory description (based on HYSPLIT4), source-tracing simulation and rainout/washout mechanism analysis, this paper had identified various and complex physico-chemical processes such as pollutant accumulation and stagnation over both cities even under high wet scavenge particularly in summer (due to the growth of the well-mixed boundary layer with regional warn weather favourable to strong photochemical conversion of SO 2 and NO x ), long range transport with rainout scavenging in-cloud, and constant seasonal influence of regional air masses on acidification process, as the most plausible triggers of acid rain diffusion over Dandong and Dalian.
In summary, the present findings recommend that an effective acid rain control program over Liaoning should be rigorously conceived at both local and regional scales.In terms of direct environmental implication, regional approach of environmental impact assessment with integration of the trans-boundary pollution should be developed in China.Much effort should be made for regional emission reduction, particularly in the potential polluting surrounding areas of Liaoning Province.Enhanced collaboration with neighboring countries within EANET network might be beneficial.In the further study, associated impacts of the acid rain on local ecosystems, agriculture, soil and aquatic systems in Northeastern China will be investigated.Facing the increasing of the acid precursor's emission, strict strategies with rigorous environmental constraints are necessary to effectively reduce the environmental burden.

Fig. 3 .
Fig. 3. Monthly averaged ion concentrations and monthly precipitation over measurement sites in 2007.

Fig. 4 .
Fig. 4. The ratio SO 4 2-/NO 3 -in the precipitation in Dalian (a) and Dandong (b); red and black circles in (b) are in summerautumn and winter-spring respectively.

Fig. 6 .
Fig. 6.Simulated Source identification of S and N wet deposition volume in (a) and percentage in (b) by NAQPMS.

Fig. 7 .
Fig. 7. Evolution of the chemical content through the rainfall Fraction.

Table 1 .
Overview of the rainfall samples in Dandong and Dalian during 2007.

Table 2 .
Fractions of SO 4 2-and NO 3 -explained by Ca 2+ and NH 4 + in samples of Dandong and Dalian.

Table 3 .
Principle component analysis of the major ions.

Table 4 .
Volume-weighted mean concentrations (µeq L -1 ) of major ions from different air mass clusters in Dalian and Dandong.The number of rain samples in each sector.

Table 5 .
The wet deposition, and the contribution of S and N in Liaoning from different area simulated by NAQPMS (Unit: × 10 3 t S (N) × yr -1 ).