Interactive comment on “ Heavy air pollution episodes in Beijing during January 2013 : inorganic ion chemistry and source analysis using Highly Time-Resolved Measurements in an urban site ”

Abstract. Heavy air pollution episodes occurred in Beijing in January 2013 attracted intensively attention around the whole world. During this period, the authors conducted highly time-resolved measurements of water soluble ions associated with PM 2.5 at an urban site, and attempted to distinguish the ion chemistry and potential sources. In this study, hourly mean concentrations of Cl − , NO 3 − , SO 4 2− , Na + , NH 4 + , K + , Mg 2+ and Ca 2+ were measured during the air pollution episode in January 2013, and the ions were found to exist mainly in the form of (NH 4 ) 2 SO 4 , NH 4 NO 3 , NaCl and KCl in aerosol particles by correlation and linear analysis. SO 4 2− and NO 3 − were observed peak concentrations in 10–15, 18–20, 21–24, and 26–30 January during this monitoring campaign. The percentage of SO 4 2− and NH 4 + in total ions concentrations exhibited an increasing trend with the enhancement of PM 2.5 concentration, indicating high concentrations of SO 4 2− and NH 4 + had played important roles in the formation of air pollution episodes. Ratio of [NO 3 − ]/[SO 4 2− ] was calculated, finding the sources of SO 4 2− would contribute more to the formation of PM 2.5 than mobile sources. Diurnal variations of SO 4 2− , NO 3 − , NH 4 + were examined, and all of them exhibited similar pattern with high concentration in night and relative low level at daytime. Emission from coal combustion, remote transportation at night or impact of meteorological was likely to be responsible for the high level of SO 4 2− , NH 4 + andNO 3 − . Potential sources were identified by applying PMF. Secondary nitrate, secondary sulfate, coal combustion and biomass burning, as well as fugitive dust were considered as the major contributors to total ions.


Introduction
As the capital of China, Beijing (39.9 • N, 116.ergy consumption, vehicle quantities and urban expansion, plus the fugitive dust from surrounding soil or constructive activities.Since 2000, air pollution control measures have been designed and taken to reduce local emission and improve air quality.However, after 2008 Olympic Games, the regional pollution and visibility in the whole area has become worsen (Zhang et al., 2010) and serious air pollution episodes occurred frequently (Cao et al., 2012;Huang et al., 2010;Sun et al., 2013;Wang et al., 2009).High concentration of PM 2.5 was believed to be largely responsible for the deterioration of air quality and visibility.With the implementation of China's new National Ambient Air Quality Standard for PM 2.5 in 2012, more challenges arose to improve air quality in megacities (Hu et al., 2014).Some previous studies found secondary inorganic aerosol (SIA), such as sulfate, nitrate and ammonium were the dominant ions in atmospheric PM 2.5 of Beijing (Cao et al., 2012;Duan et al., 2003;Pathak et al., 2009;Yao et al., 2002).These components have effects on the hygroscopicity and acidity of aerosol, which are important factors in influencing aerosol-phase chemistry and uptake of gaseous species by particles (Ocskay et al., 2006;Xue et al., 2011;Shon et al., 2012).He et al. (2001) and Ye et al. (2003) reported SO 2− 4 , NO − 3 , and NH + 4 accounted for about one-third of the total PM 2.5 mass in Shanghai and Beijing.Yao et al. (2002) investigated the formation of SO 2− 4 and NO − 3 in PM 2.5 in understanding the origin of these species.They found that a large part of these species might be formed through the direct emissions of SO 2 , NO x , and NH 3 .Wang et al. (2005) collected daily PM 2.5 aerosol samples at five sites in Beijing for a 3 year period from 2001 to 2003, and analyzed concentrations of the watersoluble ions, finding that the inorganic ions existed mainly in the form of (NH 4 ) 2 SO 4 , NH 4 NO 3 , NaCl, KCl, and CaCl 2 in aerosol particles.Zhang et al. (2013) collected daily PM 2.5 samples between April 2009 and January 2010 at Beijing, finding SO 2− 4 ranked the highest among the water-soluble ions analyzed, with an annual mean of 13.6 ± 12.4 µg m −3 , followed by NO − 3 (11.3± 10.8 µg m −3 ), NH + 4 (6.9 ± 7.1 µg m −3 ), Ca 2+ (1.6 ± 1.4 µg m −3 ), Cl − (1.4 ± 2.2 µg m −3 ), K + (0.92 ± 0.75 µg m −3 ), Na + (0.46 ± 0.55 µg m −3 ), and Mg 2+ (0.16 ± 0.13 µg m −3 ).Figures

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Full Most of previous studies used filter-based methods to collect PM samples with each sample covering hours to days.Such low temporal resolution data have limitations when used for investigating secondary aerosol formation and time evolution.Highly time-resolved measurements were considered to be helpful for a wide range of PM 2.5 components, including inorganic compounds.Data from the high resolution instruments offer significant advantages over traditional 24 h integrated filter-based measurements (Vedantham et al., 2014).To investigate the impacts of control measures and regional transport in 2008 Olympic Games, Gao et al. (2013)  and NH + 4 were dominant ions.In the first beginning to 2013, Beijing's January air pollution episodes drew international media attention.Beijing, along with the rest of the mideastern region of China, experienced massive, severe air pollution episodes (Ouyang, 2013).
The high concentration of PM 2.5 was believed to be largely responsible for the deterioration of air quality and visibility.Five haze pollution episodes were identified in the Beijing-Tianjin-Hebei area, with the two most severe episodes occurring during 9-15 and 25-31 January.During these two haze pollution episodes, the maximum hourly PM 2.5 mass concentrations in Beijing were 680 and 530 µg m −3 , respectively (Wang et al., 2014a).As urgent countermeasure, some industries and construction activities were suspended.Heavy air pollution episode also aroused some adverse health effects.The term "Beijing cough" has been in use since as early as the 1990s among foreigners, many of whom experienced chronic respiratory problems when they arrived in Beijing due to the city's dry and polluted air.But it did not become well-known un-Figures

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Full til recently, when more health problems directly attributable to the current heavy air pollution (Chen et al., 2013).By now, there have been several studies related on the occurring of these heavy pollution episodes in Beijing.Wang et al. (2014a) and Ji et al. (2014) analyzed the mechanism for the formation of heavy pollution episode, concluding that the external cause of the severe haze episodes was the unusual atmospheric circulation, the depression of strong cold air activities and the very unfavorable dispersion due to geographical and meteorological conditions, and internal cause was the quick secondary transformation of primary gaseous pollutants to secondary aerosols.Secondary aerosol was considered as one of the most important reasons of heavy air pollution episodes.Wang et al. (2014a) also revealed the two stage of aerosol growth.i.e. the "explosive growth" and "sustained growth".Huang et al. (2014) found anomalous meteorological conditions in 2013, which was different from the normal climatology from 2007-2012, were especially favorable for haze formation, and explained the formation mechanism of this episode in regard of aerosol chemistry based on the field measurement and meteorological analysis during the first half of January 2013.Zhang et al. (2014) achieved the similar conclusion that anomalous meteorology was found for this long-lasting air pollution episode, explaining about 2/3 of the variance of daily visibility evolution.Model simulation indicated that regional transport played an important role in the formation of regional haze over the Beijing-Tianjin-Heibei area (Wang et al., 2014b) To better understand the compositions of PM 2.5 and their impact on air quality in the heavy air pollution episodes during January 2013, this study analyzed the highly time-resolved measurements of inorganic ions associated with PM 2.5 , and investigated the characteristics of aerosol inorganic ions, major chemical forms, as well as potential sources.Different from previous studies which paid close attention to meteorological analysis, this paper mainly focused on specific ion compositions, which was considered as one of the major contributor to air pollution episode, and had an in-deep discussion on their role in forming the episode.The paper will help us have a more comprehensive understanding of air pollution episode in Beijing.

Sampling sites and meteorological conditions
The field campaign was conducted in the January 2013, and the sampling site was on the rooftop of a building in the Chinese Research Academy of Environmental Sciences (CRAES, 40 • 2 29.46 N, 116 • 24 51.00 E), which is located outside the 4 km north of the 5th Ring Road and 15 km from the city center as shown in Fig. 1.
The wind rose of Beijing in January is depicted in Fig. 2, and the corresponding calm wind frequencies are listed in Table 1.Firstly, the prevailing wind directions (frequency higher than 10 %) were SW (12.50 %), ENE (11.33 %), N (10.94 %) and WSW (10.74 %).Compared with the statistical results of Zhao et al. (2013) for 2009 and 2010, the frequency of WSW, SW and SSW were higher in this study.Su et al. (2004) concluded that southwest transport pathway was one of the three major pathways for outside pollutants being transported to Beijing.The CMAQ model simulation of secondary aerosols around Beijing in summer of 2003 by Liu et al. (2005) suggested that when wind from southern and southwestern directions prevails in Beijing, high concentrations of vapor and NH 3 was brought in, which would enhanc the efficiency of the secondary aerosol production.Secondly, the average temperature in January 2013 (−5.1 • C) was lower than that in 2009 and 2010 (−2.0 • C) (Zhao et al., 2013).Lower temperature would lead to more demand for fossil fuel, and more air pollutants could be emitted.Thirdly, the calm wind frequency (6.64 %) was higher than that in 2009 and 2010 (3.21 %), which was perhaps an important reason for the air pollution episode in January 2013.

Instruments
The hourly concentrations of Cl − , NO sharp-cut cyclone at a volumetric-flow controlled rate of 3 L min −1 to remove the larger particles from the air stream.Then the air stream is drawn through a liquid diffusion denuder where water-soluble acid gases (e.g., SO 2 , NH 3 , and HNO 3 ) are removed.In order to achieve high collection efficiencies, the particles-laden air stream next enters an aerosol super-saturation chamber to enhance particle growth.An inertial particle separator collects these enlarged particles, which are dissolved in water solution and then injected into the ion chromatograph.The detection limits of ions were shown in Table 2. Hourly concentrations of PM 2.5 were obtained from Chaoyang Meteorological Bureau, which is 10 km away from the CRAES sampling site.

Data analysis
Data were analyzed using SPSS 17.0 for Windows (SPSS Inc., 2008) for the correlation analysis and linear regression.EPA PMF 4.2 (USEPA, 2011) was applied to identify the potential contributors to the ion species.A detailed introduction to Positive Matrix Factorization (PMF) is shown in the Supplement.

Characterizations of ionic species
Water-soluble ions comprise a large part of aerosol particles and play an important role in the atmosphere.Na represent the secondary pollution sources from the transformation of their precursors of NH 3 , SO 2 and NO x (Wang et al., 2005).Cl − is usually considered to be from coal combustion (He et al., 2001), and K + is from biomass burning (Duan et al., 2004).Introduction

Conclusions References
Tables Figures

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Full The concentrations of ionic species in the whole sampling period were shown in Table 3. Hourly concentrations of individual ions from AIM measurements varied significantly in a broad range.Emission intensities of gaseous precursors, oxidation or conversion rate, local atmospheric mixing, and regional transport all would affect their concentration levels (Hu et al., 2014).Wang et al. (2014a) reported that five air pollution episodes were identified in the Beijing-Tianjin-Hebei (Jing-Jin-Ji) area in this time period, and the two most severe episodes occurred during 9-15 and 25-31 January.This study also analyzed the continuous variations of ions in Fig. 3 To better understand the ion species characterizations in January 2013, the main ion concentrations measured by manual sampling in the winter of Beijing from other study were summarized in Table 4.We can find that the concentrations of SO

Ratio of [NO
The mass ratio of  (Arimoto et al., 1996).4 ] ratio decrease with the increase of PM 2.5 level.When the PM 2.5 concentrations were lower than 75 µg m −3 , the ratio were larger than 1, indicating predominance of mobile source over stationary source of pollutants.When PM 2.5 concentration increased, the stationary sources would contribute more pollutants than mobile sources.The results also confirmed that SO 2− 4 was one of the most important compositions which contributed to the PM 2.5 heavy pollution.

Diurnal variation
The diurnal variations of ion species in PM 2.5 in the sampling period were shown in Fig. 7. On the whole, SO radiation and the O 3 and SO 2 concentrations.In this study, the O 3 concentration was low, especially in the pollution episodes (< 15 µg m −3 ) (Wang et al., 2014a).Such low O 3 levels cannot supply enough oxidizing capacity in the atmosphere; therefore, other oxidation reactions, regional transport at night or the impact of meteorological may also be responsible for the high levels of SO 2− 4 , NH + 4 and NO − 3 .

The speciation of major ions
The chemical forms of those major ions, i.e.SO  5 showed the correlation coefficients among these major ions.Ammonia is an important alkaline gas in the atmosphere.Ammonia in air is neutralized first by H 2 SO 4 to form (NH 4 ) 2 SO 4 or NH 4 HSO 4 , and then the remaining is neutralized by reaction with HNO 3 to form NH 4 -NO 3 .
In this study, among all cations, NH + 4 was highly significantly correlated with NO − 3 and SO 2− 4 as shown in Table 6.

Source analysis
The mass concentrations of all ions species were input into EPA PMF 4.2 model to identify the potential sources.Four factors were isolated, representing potential sources, including secondary nitrate, secondary sulfate, coal combustion and biomass burning, and fugitive dust.The factor profiles of PM 2.5 bound ion species at the monitoring site are shown in Fig. 8. Factor 1 and factor 2 were considered as secondary nitrate and sulfate, with high loading of NO − 3 and NH + 4 in factor 1, as well as SO 2− 4 and NH + 4 in factor 2. NO − 3 is mainly converted from ambient NO x , which is emitted by both vehicle exhaust and fossil fuel combustion.The precursor of aerosol SO 2− 4 is SO 2 , which may originate from biomass burning and fossil fuel combustion.Cl − and K + were found to be the main contributors to factor 3, originating mainly from coal and biomass combustion.Factor 4 is identified as fugitive dust, such as soil dust, constructive dust, and paved or unpaved road dust, including high contributions of Ca 2+ and Mg 2+ .
The factor contributions to the ion concentrations are illustrated in Fig. 9. Factors 1 and 2 were the biggest contributors to NO − 3 and SO 2− 4 , respectively.Both factors also comparably contribute to the concentration of NH + 4 .Factor 3 was the main source for Cl − and K + , while factors 1 and 2 also took some portions for Cl − and K + , respectively.Na + , Mg 2+ and Ca 2+ were mostly contributed by factor 4.

Conclusions
In this study, we reported in-situ measurements of water-soluble inorganic ions associated with PM Potential sources were identified by applying PMF.Secondary nitrate, secondary sulfate, coal combustion and biomass burning, as well as fugitive dust were considered as the major contributors to total ions.Introduction

Conclusions References
Tables Figures

Conclusions References
Tables Figures

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Full  Full  Full  Full 4 • E) has more than 20 million inhabitants distributed over 16 800 km 2 .The city has been facing serious air pollution.During the past two decades, Beijing experienced a rapid increase and development in en-Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2.5 simultaneously at an urban site and a downwind rural site in Beijing during the 2008 Olympics.The mean concentrations of SO 2− 4 , NO − 3 , and NH + 4 were 18.23, 9.47, and 9.70 µg m −3 , respectively, at the rural site and 20.74, 8.83, and 10.85 µg m −3 , respectively, at the urban site.Hu et al. (2014) monitored hourly water-soluble inorganic ions in PM 2.5 and gaseous precursors during June-November 2009 at an urban site in Beijing.The average mass concentration of the total water-soluble ions was 44 µg m −3 , accounting for 38 % of PM 2 Discussion Paper | Discussion Paper | Discussion Paper | ciated with PM 2.5 were simultaneously measured by an ambient ion monitor (Model URG 9000B, URG Corporation, USA).This instrument draws air in through a PM 2.5 Discussion Paper | Discussion Paper | Discussion Paper | + , Mg 2+ , and Ca 2+ are mainly from crustal sources, such as re-suspended road dust, soil dust, and construction dust, and Discussion Paper | Discussion Paper | Discussion Paper | in different studies.Huang et al. (2014) collected PM 2.5 samples during 4-9 January (light air pollution) and 10-15 January (heavy air pollution), and analyzed daily concentrations of ions.We calculated the mean ions concentrations during corresponding time periods as shown in Table5.The concentrations of Cl − Discussion Paper | Discussion Paper | Discussion Paper | sponding concentrations of air pollutants.The regulation divided the daily concentrations of PM 2.5 into six classes: 0-35, 35-75, 75-115, 115-150, 150-250  and larger than 250 µg m −3 , corresponding AQI as follows: 0-50 (excellent), 51-100 (good), 101-150 (light pollution), 151-200 (medium pollution), 201-300 (heavy pollution) and more than 300 (severe pollution).According to the PM 2.5 concentration classification, this study divided the ions concentrations into six categories and compared their distribution trend in Fig.4.Except Mg 2+ and Ca 2+ , all ions concentrations showed an increasing trend with the deterioration of PM 2.5 pollution, indicating the ion species (except Mg 2+ and Ca 2+ ) contributed to the increase of PM 2.5 concentration.To further investigate the relative abundance variation trend, percentage of ion species in different PM 2.5 concentration ranges were calculated as shown in Fig.5increasing proportions of total ions (the increasing of NH + 4 abundance was not significant, from 12.7 % under the PM 2.5 concentration less than 35 µg m −3 to 18.5 % under the PM 2.5 concentration larger than 250 µg m −3 ).The different variations of species percentage in total ions indicate more to the PM 2.5 pollution.This result is similar with the conclusion ofWang et al. (2014a), who considered the increasing proportion of sulfate enhanced particle hygroscopicity and thereby accelerating formation of the haze pollution.
[NO − 3 ]/[SO 2− 4 ] has been used as an indicator of the relative importance of stationary vs. mobile sources of sulfur and nitrogen in the atmosphere.ratios indicate the predominance of mobile sources over stationary sources of pollutants the observation period was 0.68 ± 0.44, which was comparable with the results ofWang et al. (2014a), but higher than the 11119 Discussion Paper | Discussion Paper | Discussion Paper | value measured during the winters of 2001-2003 in Beijing (0.49).Wang et al. (2005) suggested that the contribution of mobile sources (e.g., motor vehicles) increased in Beijing, in accord with the adjustment of the energy structure in recent years.was also classified according to the PM 2.5 concentration ranges.As shown in Fig.6, we can find that the [ diurnal patterns with a broad nighttime maximum and a relatively low concentration at daytime, while the variations of Na + , Mg 2+ and Ca 2+ were small.Compared with another highly resolved measurements for ion species at the same site during summer of 2008(Gao et al., 2013), the diurnal cycle of SO 2− 4 in this study was completely opposite, i.e., the highest concentration of SO 2− 4 occurred at daytime and the lowest at night during summer campaign of 2008.Also, for some ions, such as SO 2− 4 , Cl − and K + , high levels at night and low concentrations at daytime could be the characterizations of coal combustion.The sampling site is located outside the 5th ring of Beijing, and there still exists considerable coal combustion boilers for domestic heating.The emission from these coal combustion facilities would be one of the important reasons that led to the diurnal variations.Gao et al. (2013) found the SO 2− 4 concentrations in summer rapidly increased from the early morning to the late afternoon synchronously with the enhancement of solar Introduction Discussion Paper | Discussion Paper | Discussion Paper | The slope of the regression between NH + 4 and SO 2− 4 (µeq vs. µeq) for the whole data set was 1.33, indicating the complete neutralization of suggested that (NH 4 ) 2 SO 4 , instead of NH 4 HSO 4 , was the major species formed by SO 2− 4 and NH + 4 .Moreover, the slope between NH + 4 only Cl − can supply negative charge to balance the Na + , K + .Considering the correlation coefficients of Cl − between Na + , K + , were comparable, the mixture of NaCl and KCl were likely to be the major form of ions (NH 4 ) 2 SO 4 and NH 4 NO 3 .Discussion Paper | Discussion Paper | Discussion Paper | 2.5 at an urban site of Beijing during the air pollution episode in January 2013.The hourly concentrations of Cl − , NO − 3 , SO 2− 4 , Na + , NH + 4 , K + , Mg 2+ and Ca 2+ were measured.Peak concentrations of the ions were observed in four periods(10)(11)(12)(13)(14)(15)(18)(19)(20)(21)(22)(23)(24)(26)(27)(28)(29)(30).SO 2− 4 and NO − 3 exhibited high concentrations in three periods.ions concentrations kept an increasing trend with the enhancement of PM 2.5 concentration, thus high concentrations of SO 2considered as one of the main reasons of air pollution episodes.NH + 4 also played an important role in the formation of PM 2.5 .Based on the correlation analysis, the observed ions existed mainly in the form of (NH 4 ) 2 SO 4 , NH 4 NO 3 , NaCl and KCl in aerosol particles.The ratio of [NO − 3 ]/[SO 2− 4 ] also proved the role of SO 2− 4 in the air pollution episodes.With the aggravation of air quality, the [NO − 3 ]/[SO 2− 4 ] displayed a decreasing trend, indicating the SO 2 emitters, such as some stationary sources, contributed more to PM 2.5 aerosols formation, rather than mobile sources, which are considered as the sources of NO observed.All of them exhibited similar patterns with high concentration in night and relatively low level at daytime.Considering the lack of strong oxidative atmosphere during monitoring period, pollutant transport at night may be responsible for the high level of SO 2− 4 , NH + 4 and NO − 3 .
Discussion Paper | Discussion Paper | Discussion Paper | Table 1.Meteorological conditions of January in Beijing.Meteorological parameter Temperature ( • C) Relative humidity (%) Atmospheric press (Hpa) Wind speed (m s −1 ) Calm wind frequency (Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
significant variations before 28 January.In the last several days, the concentrations of Ca 2+ and Mg 2+ decreased dramatically.Most constructive activities related with the emission of Ca 2+ and Mg 2+ were suspended by government during this episode.This would perhaps be the reason of lower concentrations of Ca 2+ and Wang et al. (2005))fied by bivariate correlations.Verma et al. (2010)andWang et al. (2005)used bivariate correlations to identify the possible chemical forms of ions in PM 2.5 .Since concentrations of all the ions are not normally distributed (k-s test, p < 0.05), Spearman Correlation analysis was applied.Table

Table 2 .
The detection limit of URG9000.

Table 4 .
Comparisons of the concentrations of ions in several studies in Beijing during winter by manual samplers (µg m −3 ).

Table 5 .
Comparisons of the concentrations of ions between this study and a simultaneous study (µg m −3 ).