The variation characteristics and possible sources of atmospheric 1 water-soluble ions in Beijing 2

Abstract: The North China plain (NCP) including Beijing is currently suffering from severe haze 11 events due to high pollution level of PM2.5. To mitigate the serious pollution status, identification of 12 the sources of PM2.5 is urgently needed for the effective control measures. Daily samples of PM2.5 13 were collected in Beijing city as well as a rural area in Baoding, Hebei Province through the year of 14 2014, and the seasonal variation characteristics of water-soluble ions (WSIs) in the PM2.5 were 15 comprehensively analyzed for recognizing their possible sources. The results indicated that the 16 periodic emissions from farmers’ activities made evident contribution to the atmospheric WSIs in 17 Beijing. The relatively high concentration of K in winter and autumn at the two sampling sites 18 confirmed that crop straw burning made evident contribution to atmospheric K in Beijing. The 19 remarkable elevation of Cl at the two sampling sites as well as the evident increase of the Cl/K 20 ratio and the Cl proportion in WSIs during the winter in Beijing were reasonably ascribed to coal 21 combustion for heating by farmers. The unusually high ratio of Cl to Na in summer, the obviously 22 high concentrations of Cl in the rural sampling site and the elevation of Cl proportion in WSIs in 23 Beijing during the maize fertilization could be rationally explained by the use of the prevailing 24 fertilizer of NH4Cl in the vast area of NCP. The abnormally high concentrations of Ca at the two 25 sampling sites and the elevation of Ca proportion during the period of the maize harvest and soil 26 ploughing in Beijing provided convincing evidences that the intensive agricultural activities in 27 autumn made evident contribution to the regional mineral dust. The most serious pollution episodes 28 in autumn were coincident with significant elevation of Ca, indicating that the mineral dust 29 emission from the harvest and soil ploughing not only increased the atmospheric concentrations of 30 the primary pollutants, but also greatly accelerated formation of sulfate and nitrate through 31 heterogeneous reactions of NO2 and SO2 on the mineral dust. The backward trajectories also 32 indicated that the highest concentrations of WSIs usually occurred in the air parcel from 33 southwest/south regions with high density of farmers. In addition, the values of nitrogen oxidation 34 ratio (NOR) and the sulfur oxidation ratio (SOR) were found to be much higher under haze days 35 than under non-haze days, implying that formation of sulfate and nitrate was greatly accelerated 36 through heterogeneous or multiphase reactions of NO2 and SO2 on PM2.5. 37


Introduction
The North China plain (NCP) is frequently suffering from severe haze pollution in recent years (Chan and Yao, 2008;Liang et al., 2016), which has aroused great attention from the general public (Zhang et al., 2014;Guo et al., 2014;Huang et al., 2014;Yang et al., 2015b;Zhang et al., 2015b;Zheng et al., 2015b;Sun et al., 2006).The severe haze pollution is mainly ascribed to elevation of fine mainly focus on June-July and October-November and domestic coal stoves are prevailingly used for heating in winter.The seasonal activities of farmers in the NCP were suspected to make significant contribution to deteriorate the regional air quality, e.g., the most serious pollution events (or haze days) in the NCP were usually coincident with the three seasonal activities of farmers in recent years (Yang et al., 2015b;Huang et al., 2012;Li et al., 2014;Li et al., 2011;Liu et al., 2013;Sun et al., 2013).The serious pollution events during harvest seasons were widely ascribed to crop straw burning (Huang et al., 2012;Li et al., 2014), but the influence of fertilization events and crop straw returning to fields on the regional air quality during the harvest seasons periods was totally neglected.
Strong ammonia (NH3) emission from the vast agricultural fields in the NCP has been found during fertilization events just after harvest of winter wheat in June-July (Zhang et al., 2011), which must accelerate atmospheric ammonium formation.Although crop straws burning by stealth is still prevailing, most residual crops are being returned into the agricultural fields under the advocacy of government for protecting the air quality.Because crop leaves absorbed large quantities of atmospheric particles during crop growing season, the abrupt release of the particles by smashing crop straw for returning in the vast area of the NCP must also make striking contribution to atmospheric particles in the region during the seasonal harvest seasons.In winter, the serious pollutant emissions from the chimney of the farmers' coal stoves can be easily imagined by the strong smog.Although domestic coal consumption only accounts for small fraction of the total, e.g., ~11% in Beijing-Tianjin-Hebei area (http://hbdczx.mep.gov.cn/pub/), the emission strengths of pollutants from farmers' coal stove is usually about 1-2 magnitude greater than those from power plants (Xu et al., 2006), and the farmers coal consumption mainly concentrates on the four months in winter.
The mass concentrations of WSIs and PM2.5 at the sampling site were simultaneously measured by the filter sampling method and the TEOM 1405 Monitor for 24 days (Jan 1-Jan 24, 2015).As shown in Fig. 1a and Fig. 1b, the variation trends of the WSIs and PM2.5 were almost the same with a correlation coefficient (R 2 ) of 0.908, implying that the concentration of WSIs measured could well reveal the pollution status of PM2.5 in Beijing.The average mass concentration of WSIs contributed about 80% to the mass of PM2.5 measured by the TEOM 1405 Monitor, whereas the WSIs accounted for about 50-60% of the total mass concentration measured by the filter sampling method in the NCP (Shen et al., 2009;Li et al., 2013).The mass concentration of PM2.5 measured by the TEOM 1405 Monitor was suspected to be largely underestimated because the volatile even semi-volatile component in PM2.5 can be easily lost at 50°C which is designed in the TEOM 1405 Monitor for avoiding water condensation on the filter (Grover et al., 2005;Liu et al., 2014), e.g., under clean days after serious pollution episodes, the mass concentration of WSIs was even higher than the mass concentration of PM2.5 measured by the TEOM 1405 Monitor (Fig. 1a).To verify above assumption, the concentrations of WSIs on the filters collected by the filter sampling method and the TEOM 1405 Monitor were comparatively measured, and the results are illustrated expected under typical ambient conditions (Finlayson-Pitts et al., 1986).The negative PM2.5 values of the TEOM 1405 Monitor after a serious pollution episode also indicated the serious loss of the volatile component.Although the TEOM 1405 Monitor is widely used for measuring atmospheric PM2.5 in the net stations of China, the pollution levels measured could only represent the lower limits, especially under the clean days after serious pollution episodes in winter.

Daily variations of WSIs in each season
The daily variations of WSIs in each season are illustrated in Fig. 2 and the statistic mass concentrations of the WSIs are summarized in Table 1.It is evident that the daily variations of the WSIs exhibited significantly periodic fluctuation, indicating meteorological conditions played a pivotal role in accumulation and dissipation of atmospheric pollutants.For example, the most frequently high pollution levels of the WSIs in winter were mainly ascribed to the relatively stable meteorological conditions with the low height of boundary layer which favors pollutants accumulation (Wang et al., 2013;Quan et al., 2014;Tian et al., 2014;Wang et al., 2014;Zhang et al., 2015a).Besides meteorological conditions, the extremely high levels of the WSIs during the pollution episodes revealed strong sources of the pollutants around Beijing.
Although the most intensive photochemical reactivity in summer favors sulfate and nitrate formation, the relatively low SO2 concentration, the fast thermal decomposition of ammonium nitrate and the frequent scavenging by rain events must greatly counteract the contribution of the secondary formation, resulting in the lowest pollution levels of the WSIs in summer.In comparison with other seasons, the remarkable elevation of atmospheric SO2 and NOx (see Sect. 3.3) in winter would override the relatively low atmospheric photo-oxidants for their oxidation rates and resulted in the highest mean concentration of WSIs.Although the atmospheric concentrations of SO2 and NOx in autumn were much smaller than in winter and in spring (see Sect. 3.3), the mean concentration of WSIs in autumn was almost the same as that in winter and nearly twice as those in spring and summer, indicating that special mechanisms dominated the secondary formation of the atmospheric principal ions (see Sect. 3.3).

The possible sources for the WSIs
To disclose the contribution of possible sources to the WSIs, the molar composition of the WSIs, the seasonal variation characteristics of typical WSIs, the variation characteristics of the three principal ions during serious pollution episodes, the contribution of secondary formation to atmospheric WSIs, and backward trajectories of air parcels were comprehensively analyzed.

The molar composition of the WSIs
The molar composition of water-soluble ions in each season under three pollution levels is illustrated in Fig. 3.With increasing pollution levels, the noticeable reduction of the proportions of metallic ions (such as Ca 2+ , Na + and Mg 2+ ) and the evident increase of NH4 + , NO3 -and SO4 2-proportions revealed that the three principle ions (NH4 + , NO3 -and SO4 2-) were mainly from atmospheric secondary formation.Compared with SO4 2-, the fast increase of NO3 -proportion with increasing pollution levels indicated that the formation rate of nitrate was faster than that of sulfate under higher pollution levels.It should be mentioned that the increase rate of NO3 -proportion with increasing pollution levels was much slower in summer than in other seasons, validating that nitrate was easily thermal decomposed under high temperature.The conspicuous reduction of Cl -proportion with increasing pollution levels meant Cl -might be mainly from primary sources.

The seasonal variation characteristics of typical WSIs
The seasonal variation characteristics of typical WSIs are illustrated in Fig. 4. For Cl -and K + , their high concentrations mainly occurred in winter and autumn.It should be mentioned that the extremely high concentration of K + in winter on 1 February (Fig. 2) was due to firework for celebrating Chinese lunar year (Jiang et al., 2015;Kong et al., 2015).Sea-salt has long been considered as the source for atmospheric Cl - (Souza et al., 2014), however, the molar ratio of Cl -to Na + measured by this study (Fig. 5) in each season was above 1.30 which was much greater than the value of 1.18 in fresh sea-salt particles (Brewer, 1975), indicating sources other than sea-salt dominated atmospheric Cl -in Beijing.Because K + has been widely used as an indicator for biomass burning (Gao et al., 2011) and crop straw burning by stealth was prevailing in the countryside around Beijing during autumn and winter seasons, crop straw burning was suspected to be a common source for K + and Cl - (Li et al., 2014).The pronounced correlation coefficients (r > 0.6, p < 0.01) between K + and Cl -in the two seasons might be the circumstantial evidence for above suspicion.Several combustion in China (Huang et al., 2014).Because large fraction of coal consumed by farmers for heating in winter was the extra source for atmospheric pollutants in the vast area of North China, the obviously higher Cl -concentrations measured in winter than in other seasons (Fig. 2) indicated that coal combustion by farmers in winter might make great contribution to atmospheric Cl -in Beijing.The source of atmospheric NOx in Beijing is dominated by vehicles and relatively stable in the four seasons, and hence the ratios of Cl -to NOx can largely counteract the influence of accumulation and dispersion due to variation of meteorological factors for identifying the possible extra source of Cl -.The ratio of Cl -to NOx in winter was about a factor of 2 greater than those in other seasons (Fig. 5), confirming that coal combustion by farmers in winter indeed made evident contribution to atmospheric Cl -in Beijing.Previous field investigations in different areas of Chinese mainland also found relatively high Cl -concentration in winter, which was also ascribed to coal combustion (Yu et al., 2013;Wu et al., 2014).In addition, fertilization events in the agricultural fields around Beijing might also make contribution to atmospheric Cl -, because the volatile ammonium chloride is a kind of prevailingly used fertilizer in the NCP, e.g., the extremely high ratios of Cl -to Na + (Fig. 5) were coincident with the cultivation seasons of spring and summer.
For Ca 2+ , remarkably high concentrations occurred in both spring and autumn.The evident elevation of Ca 2+ concentrations in spring has been usually ascribed to the frequent dust storm (Zhao et al., 2013b), but there was still no explanation about the extremely high Ca 2+ concentrations in autumn (Zhao et al., 2013b;Zhang et al., 2013).The three serious pollution events with remarkable elevation of Ca 2+ (Fig. 2) were coincident with the intensive harvest of maize and tillage of the agricultural fields for planting winter wheat in the countryside around Beijing, and hence the extremely high atmospheric mineral particles were absorbed by crop leaves during crop growing season, especially in the North China where atmospheric mineral dust is always at high level (Zhang et al., 2013;Zhao et al., 2013b), a large fraction of the mineral dust absorbed on the leaves of crop would be released into the atmosphere during harvest with crop straw being crushed into pieces for returning to fields which is a prevailing cultivation manner under the advocacy of governments for reducing the influence of crop straw burning on the air quality.
For NH4 + , SO4 2-and NO3 -, remarkably high concentrations also appeared in both winter and autumn.
NH4 + was mainly from the reactions of NH3 with acid gases (such as HNO3) and acid particles, and hence its variation trend was the same as those of SO4 2-and NO3 -.Although atmospheric NH3 has long been considered to be mainly from agricultural activities, their emissions mainly focus on warmer seasons (Krupa, 2003).However, the frequently high concentrations of NH4 + appeared in winter.Beside the slow thermal decomposition of ammonium nitrate, strong NH3 emission sources other than agricultural activities were suspected to be responsible for the frequently high concentrations of NH4 + in the cold winter.Emissions of NH3 from vehicles was regarded as an important source (Liu et al., 2014).In addition, strong emission of NH3 from domestic coal stoves was indeed found by our preliminary measurements (data were not shown).During the serious pollution episodes, the concentrations of SO2 in autumn were almost the same as those in summer and about one magnitude lower than in winter (Fig. 6), but the peak concentrations of SO4 2-in autumn were about two times greater than those in summer and at almost the same level as those in winter.The gaseous phase reaction with OH (Zhao et al., 2013c;Quan et al., 2014), the heterogeneous reaction on mineral dust (He et al., 2014;Nie et al., 2014), and multiphase reactions in the water of particulate matters (Zheng et al., 2015a)  responsible for atmospheric SO4 2-formation.The significant elevation of both Ca 2+ and SO4 2-in autumn implied that the heterogeneous reaction of SO2 on the mineral dust might greatly accelerate the conversion of SO2 to SO4 2-.Although evidently high concentrations of Ca 2+ occurred (Fig. 2 and Fig. 4) in spring and SO2 concentrations were much greater in spring than in autumn (Fig. 6), the SO4 2-concentrations were about a factor of 2 less in spring than in autumn.Atmospheric humidity was suspected to play an important role in the heterogeneous reaction, e.g., the relative humidity was much higher in autumn than in spring during the serious pollution events (Fig. 6).Similar to SO4 2-, the relatively high concentrations of NO3 -during the serious pollution events in autumn were also ascribed to the heterogeneous reaction of NO2 on the mineral dust.

The variation characteristics of the three principal ions during serious pollution episodes
As shown in Fig. 6, the serious pollution episodes with noticeable elevation of various pollutants usually occurred under slow wind speed (less than 2 m s -1 ) and high relative humidity.In comparison with their precursors of SO2 and NOx, however the detailed variation trends of SO4 2-and NO3 -were different, indicating that the elevation of SO4 2-and NO3 -was not simply ascribed to the physical process of accumulation.It is interesting to be noted that the increasing rates of SO4 2-during some serious pollution events especially with elevation of Ca 2+ (such as in spring and autumn) were much slower than those of NO3 -, implying that the atmospheric heterogeneous reaction of NO2 on the mineral dust might be faster than that of SO2.In comparison with summer and winter, the relatively high ratios of NO3 -/SO4 2-in spring and autumn (Fig. 5) also supported above assumption.

Secondary formation for atmospheric sulfate and nitrate
The nitrogen oxidation ratio NOR = nNO3 -/ (nNO3 -+ nNOx) (n refers to molar concentration) and the sulfur oxidation ratio SOR = nSO4 2-/ (nSO4 2-+ nSO2) have been used to estimate the degree of Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-82, 2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 16 March 2016 c Author(s) 2016.CC-BY 3.0 License.secondary formation of NO3 -and SO4 2-, which can counteract the interference of meteorological factors (Chan and Yao, 2008;Yu et al., 2013;Guo et al., 2014;Huang et al., 2014;Yang et al., 2015b;Zheng et al., 2015b).The values of NOR and SOR during haze days and non-haze days in four seasons are listed in Table 2.Both the values of NOR and SOR on non-haze days were found to be the highest in summer and the lowest in winter, well reflecting the seasonal variation of photochemical intensity.Although sunlight intensity greatly reduced at ground level during haze days, the values of NOR and SOR were about a factor of 2 greater during haze days than during non-haze days in the four seasons, implying again that the heterogeneous or multiphase reactions of SO2 and NO2 on atmospheric particles made significant contribution to atmospheric sulfate and nitrate.

The influence of air mass transport on the WSIs in Beijing
To reveal the air mass transport influence on the WSIs in Beijing, three-day backward trajectories for clusters and the corresponding mass concentrations of WSIs in each season were analyzed, and the results are illustrated in Fig. 7.It could be seen that the lowest concentrations of WSIs usually occurred in the northwest/northeast airflow with long distance transport.Because Beijing is surrounded by mountains in the north/northwest/northeast directions where the population is sparse, these clusters brought the relatively clean air mass to accelerate the dissipation of aerosols.The highest concentrations of WSIs (especially for SO4 2-, NO3 -and NH4 + ) were usually observed in the air parcel from southwest/south regions with high density of population.Considering the large fraction (~30%) of air parcel from the southwest/south regions in each season, the human activities in the southwest/south regions made evident contribution to the atmospheric WSIs in Beijing.
of Beijing was also suspected to make evident contribution to the atmospheric WSIs in Beijing, e.g., the remarkable elevations of Cl -in winter and Ca 2+ in autumn were probably from farmers' coal combustion for heating and harvest of maize, respectively.

Comparison with previous studies
The mean concentrations of the three principal ions and some related indicators in Beijing over the past decade are summarized in Table 3.The seasonal variations of the three principal ions reported were quite different, e.g., Huang et al. (2016) found the maximal mean concentrations of SO4 2-and NH4 + in the summer and of NO3 -in the autumn of 2014, whereas in this study all the maximal mean concentrations of the three principal ions appeared in autumn.The mean concentrations of the three ions in autumn in this study were in good agreement with the values reported by Yang et al. (2015).
For the mass concentration ratios of NO3 -/SO4 2-(denoted as N/S), all the investigations exhibited relatively high values in autumn and spring, further confirming that the heterogeneous reaction of NO2 on mineral dust favored nitrate formation (as discussed above).For NOR and SOR, all investigations were in good agreement, with the highest values in summer, the lowest in winter and higher values during haze days than during clean days.Compared with the investigations of 2003, the evident increase of both the concentration of NO3 -and the ratio of N/S in recent years revealed the fast increase of vehicle numbers in the decade made significant contribution to atmospheric nitrate.

Conclusions
The comparison between the mass concentrations of WSIs measured by the filter method and the mass concentrations of PM2.5 measured by the TEOM 1405 Monitor revealed that the mass concentrations of WSIs could well reflect the pollution status of PM2.5 and the mass concentrations Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2016-82,2016   Manuscript under review for journal Atmos.Chem.Phys.Published: 16 March 2016 c Author(s) 2016.CC-BY 3.0 License.

Fig. 1
Fig. 1 Comparison between the filter sampling method and the PM2.5 monitor for the daily average mass concentrations of the WSIs and PM2.5 (Fig. 1a and 1b), and for the 12-day-average molar composition of the WSIs on the filters collected by the two methods during the two 12-day sampling periods (Fig. 1c represents the data collected during the first 12-day; Fig. 1d represents the data collected during the second 12-day.).

Fig. 2
Fig. 2 Daily variations of WSIs in each season (the smooth lines for the WSIs were drawn between the points of the daily data).

Table 3
Summary of three principal ions (μg