Particulate matter trends and quantification of the spring sand-dust contribution in Hohhot, Inner Mongolia, from 2013 to 2017

Wnkng Go , Lingyun Zhu , Zhnyun M , Qingxin Go , Xupu Yu , Sun Wu , Yu Gu

a State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Science, Beijing, China

b Environmental Monitoring Center Station of Inner Mongolia Autonomous, Hohhot, China

c Shanxi Province Institute of Meteorological Sciences, Shanxi Provincial Meteorological Bureau, Taiyuan, China

d Chinese Research Academy of Environmental Sciences, Beijing, China

e Chifeng Ecological Environment Monitoring Centre, Chifeng, China

f Shanxi Province Changzhi Meteorological Bureau, Changzhi, China

ABSTRACT On 10 September 2013, the State Council promulgated the Action Plan for the Prevention and Control of Air Pollution (hereinafter referred to as the Action Plan).To study the variations in the air pollutant concentrations in Hohhot during the implementation of the Action Plan and the effects of various measures, the daily concentrations of air pollutants (fine particulate matter (PM 2.5 ), inhalable PM (PM 10 ), SO 2,and NO 2 ) in Hohhot published by the China Environmental Monitoring Station were obtained to analyze the general meteorological conditions in Inner Mongolia from 2013 to 2017.The variations and causes of the ambient PM concentration in Hohhot were examined, and the quantitative influence of sandstorms on the ambient PM concentration in Hohhot was analyzed by selecting the spring season with frequent sandstorms as an example.The results showed the following.(1) The air quality in Hohhot continuously improved, and compared with those in 2013, the PM 2.5 and PM 10 concentrations decreased by 24.6% and 48.2%, respectively, in 2017.However, the air pollutant concentrations remained high, with the average PM 2.5 and PM 10 concentrations exceeding the national secondary standards by 22.9% and 35.7%, respectively.(2) The reductions in the spring PM 2.5 and PM 10 concentrations were 5.6% and 8.9%, respectively, and the annual decreases in the PM 2.5 and PM 10 concentrations were 3.6 and 15.1 μg m ? 3 yr ? 1 ,respectively, from 2013 to 2017.(3) The absolute contribution ranges of dust weather to the concentrations of PM 2.5,PM 10,and TSP during 2013–17 were 0.6–5.2 μg m ? 3,9.0–16.9 μg m ? 3,and 14.7–30.0 μg m ? 3,respectively,in Hohhot during the spring.

1.Introduction

With the increasing industrialization and urbanization and consumption of energy and resources in China, regional air quality problems have become severe ( Tang et al., 2012,2015a,2015b ; Wu et al., 2016 )and seriously endanger human health and living standards ( Chai et al.,2014 ; Wang et al., 2015a ; Xue et al., 2005 ).To improve the air quality in China, the State Council promulgated the Action Plan for Air Pollution Prevention and Control (hereinafter referred to as the Action Plan)in September 2013, which clearly stated that by 2017, the fine particulate matter (PM) concentration at the city level and above should decrease by 10% over that in 2012, and the number of good air quality days should increase year by year.Subsequently, in May 2018, the Ministry of Ecology and Environment and the local Bureau of Ecology and Environment reported the final assessment results of the implementation of the Action Plan and all 45 key tasks identified in the Action Plan had been completed on schedule; moreover, compared with that in 2013, the average concentration of PM smaller than 10 μm in diameter (PM) in cities at the prefecture level and above decreased by 22.7% in 2017, and the goal of improving the ambient air quality had been fully achieved.In addition, the assessment level of Inner Mongolia was excellent.During this period, to implement regional air pollution control and promote the joint prevention and control of air pollution in Beijing-Tianjin-Hebei (BTH) and its surrounding areas, the Inner Mongolia Autonomous Region government and various departments successively promulgated implementation rules, target measures and solutions for the implementation of the Action Plan and worked with the Ministry of Ecology and Environment to sign the target responsibility letter.

Many studies have evaluated the implementation effect and pollution control of the Action Plan and have provided countermeasures and suggestions.The Chinese Academy of Sciences Special Group on the Formation Mechanism and Control Strategy of Haze evaluated the atmospheric PMpollution control measures in the BTH region after implementation of the Action Plan in 2013 and found that the SO,NO,and PM 2.5 concentrations in the BTH region decreased more significantly in winter than in summer, and it was recommended that the control focus should be shifted toward Hebei Province during implementation.The implementation effects of the Action Plan have been evaluated in different regions of China from 2013 to 2014 and countermeasures have been proposed ( Gao et al., 2016 ).Additionally, the emission reduction level of major air pollutants was calculated before and after the implementation of the Clean Air Action Plan in the Yangtze River Delta (YRD)and Pearl River Delta (PRD), and the improvement in the PMconcentration was simulated under different emission reduction measures and various proposed countermeasures ( Li et al., 2015 ; Huang et al., 2018 ;Zhang et al., 2019 ).Then, the emission inventory was used to simulate and analyze the improvement of the annual average PM 2.5 concentration in different cities and key areas after the Action Plan was completed( Xue et al., 2012 ).In addition, the influences of sand and dust on the ambient air quality in Inner Mongolia should not be underestimated.Model simulation methods have been applied to study and quantify the impact of sand and dust weather on the atmospheric environmental quality.For example, the CALPUFF simulation system was employed to quantitatively analyze the contribution rate of PM 10 to the air quality in Shenyang ( Xia et al., 2007 ).The dust index and sand-dust concentration increase coefficient were adopted to quantitatively study the influence degree of the dust events in the Hexi Corridor on the PMconcentration in Lanzhou ( Tao et al., 2007 ).However, the above research or evaluation work has certain limitations, which are reflected in the following:○There are few reports on the implementation status of the Action Plan in Inner Mongolia and the long-term changes in the atmospheric pollution concentration such as that of PMin Inner Mongolia, where sand and dust events frequently occur and impose a major impact on the ambient air quality in the BTH region.○The government and numerous previous studies have focused only on the long-term variation in the annual PM 2.5 concentration in Inner Mongolia, and thus, the PM 2.5 precursors(SOand NO) and their daily concentration variations have scarcely been jointly considered to explain the mechanism for the changes in the PM 2.5 concentration.○Quantitative sand-dust weather research focused on the air quality has mostly been based on modeling, with the air pollution index (API) or PM 10 observed concentration as a quantitative indicator, but there is little research on the sand-dust weather contribution to the PM 2.5 concentration and long-term changes and impacts.

Based on the PM,PM,SO,and NOconcentrations in Hohhot city released by the China National Environmental Monitoring Centre,Ministry of Ecology and Environment, and meteorological data in Inner Mongolia from 2013 to 2017, the PM and its main gaseous precursor concentrations were analyzed in addition to their variation characteristics before and after implementation of the Action Plan, and the interannual and seasonal variation characteristics and causes of the PM pollution in Hohhot were examined.Additionally, we quantitatively investigated the impact of sand-dust weather on the PM concentration in Hohhot, aimed at proposing a more scientific and targeted basis for environmental air quality assessment in the prevention and control of air pollution to ensure future blue sky conditions in key regions.

2.Methods

2.1.Air quality data

Hourly data on the ambient air quality and concentrations of air pollutants (PM,PM,SO,NO,CO, and O) in Hohhot from 2013 to 2017 were obtained from the China National Environmental Monitoring Center’s National Real-time Urban Air Quality Publishing Platform ( http://113.108.142.147:20035/emcpublish ).The hourly concentration of total suspended particles (TSP) and hourly meteorological data (temperature, relative humidity, sea level pressure, and wind speed) originated from the Sand and Dust Monitoring Network of the Inner Mongolia Environmental Monitoring Center, according to national ambient air quality standard GB3095–2012.The monitoring equipment and operations were in accordance with the automated methods for Ambient Air Quality Monitoring (HJ/T 193–2005).Quality assurance and quality control were implemented following ambient air quality standard GB3095–2012 and the Technical Regulation for Ambient Air Quality Assessment (on trial) (HJ663–2013).

2.2.Methods

This study mainly applied the time series analysis method common in statistics to analyze the annual, monthly, and daily average concentrations of air pollutants (PM 2.5,PM 10,SO 2,and NO 2 ) in Hohhot from 2013 to 2017.The hourly data of the air pollutants (PM 2.5,PM 10,SO 2 ,and NO) for the Ruyi Water Plant in Hohhot in the spring of 2013–2017 were obtained to quantitatively study the impact of sand and dust weather on the atmospheric PM level in Hohhot.Subsequently, the absolute contribution of the air pollutants was calculated based on a previously reported method ( Chen et al., 2013 ), which relies on the difference in the PM concentration between dust and non-dust weather conditions to express the absolute contribution of dust weather to the air quality.The calculation formula is shown in Eq.(1) below.To more accurately reflect the impact of sand and dust weather on the ambient air quality, the proportion of the absolute contribution of dust weather to the secondary standard limit of PM is calculated.The higher the proportion is, the greater the impact of sand and dust weather on the ambient air quality.

3.Results and discussion

3.1.Levels of particulate matter and its gaseous precursors during 2013–2017

In June 2018, the 2017 Inner Mongolia Autonomous Region Ecology and Environment Statement was released by the Environmental Protection Department of the Inner Mongolia Autonomous Region, which revealed that the air quality in Hohhot had improved ( Fig.1 ).The TSP and PMconcentrations were 106 μg m3 and 95 μg m3 in 2017, respectively, which decreased by 37.2% and 48.2% over the 2013 levels,and the annual PM 2.5 and SO 2 concentrations in 2017 were 43 and 29 μg m,respectively.After the implementation of the Action Plan, Hohhot has established and perfected a mechanism to prevent and control atmospheric pollution, mainly focusing on the prevention and control of PMpollution and the comprehensive management of the air quality with industrial pollution as the focus.Additionally, the 2017 China Climate Bulletin (referred to as the Climate Bulletin 2017 ) shows that the atmosphere in Inner Mongolia exhibited a high capacity to remove pollutants in 2017.Due to meteorological factors and the implementation of the Action Plan and related measures, the air quality in Hohhot greatly improved since 2013.However, the PM concentration in 2017 remained relatively high.The annual PMand PMconcentrations exceeded the national ambient air quality standard (NAAQS) by 22.9%and 35.7%, respectively.Moreover, the annual TSP concentrations exceeded the primary national ambient air quality standard (NAAQS,80 μg m3 ) by 32.4%.

The PMand PMconcentrations revealed a decreasing trend in the different seasons from 2013 to 2017.The PM 2.5 concentrations in spring, summer, autumn, and winter decreased by 5.6%, 6.3%, 3.6%,and 1.4%, respectively.The PM 2.5 concentration decreased the most, at 3.8 and 3.6 μg myrin winter and spring, respectively, from 2013–2017.The SOconcentration, the main gaseous precursor of PM,also greatly decreased in winter and spring, at 13.2% yr1 and 12.1% yr1 ,respectively.Moreover, the Climate Bulletin showed that the average atmospheric environmental capacity (reflecting the ability of the atmosphere to diffuse and diffuse pollutants and promote cleaning) in the winter season of 2017 (January–March and October–December) was basically the same as that in 2013 but higher than that from 2014–16.Hence, with the implementation of the Action Plan and related measures, Hohhot city’s SO 2 emission reduction measures have reduced the overall PMconcentration.However, the NOconcentration increased 10.8%, 1.0%, 3.8%, and 8.4% in the spring, summer, autumn, and winter, respectively, of each year, and the highest monthly PMand NO 2 concentrations occurred in the same month from 2015–17, namely, December 2015, November 2016, and January 2017, which might be the reason for the PMconcentration not significantly decreasing in winter.The spring PM 2.5 and PM 10 concentrations in Hohhot over the past five years were the most obvious.On the one hand, this was related to the implementation of the Action Plan and its control measures.On the other hand, the average spring wind speed in Inner Mongolia was relatively high, which promoted PM diffusion.However, spring is also the season with frequent sand-dust events, and their frequency and intensity have major impacts on the increase in PM concentrations.

Fig.1.Annual TSP, PM 10,PM 2.5,SO 2,and NO 2 concentrations in Hohhot from 2013 to 2017 (units: μg m?3).

However, PM 10 and PM 2.5 pollution mainly occurred in spring and winter.The average PMconcentrations in spring, summer, autumn,and winter from 2013–17 were 40.0 ± 5.8 μg m,27.7 ± 1.6 μg m,47.4 ± 17.4 μg m3,and 68.8 ± 10.4 μg m3,respectively.Additionally,the seasonal changes in the SOand NOconcentrations, the main gaseous precursors of PM 2.5,were basically consistent with those in the PMconcentration and exhibited a significant positive correlation with the monthly mean PMconcentration (

p

≤ 0.05), with correlation coefficients of 0.79 and 0.67, respectively.Furthermore, the highest PMconcentration was 129.5 ± 9.1 μg min spring from 2013–17, followed by a concentration of 123.1 ± 14.6 μg m3 in winter, a concentration of 115.0 ± 25.1 μg min autumn, and a concentration of 82.4 ± 2.9 μg m3 in summer, which was 1.2–1.9 times higher than the PM 10 NAAQS.The PMconcentration in May reached 139 ± 30.9 μg m3,which was twice as high as the NAAQS of PM.It was indicated that the PM 10 pollution was the most serious, while the pollution degree was more consistent in spring, and the concentration difference was small.The Chinese Academy of Sciences has found that with the coexistence of sulfur oxides (SO

x

) and nitrogen oxides (NO), the conversion rates of SOand sulfite to sulfate will be significantly accelerated; that is, NOis the key factor promoting the conversion of SO 2 to sulfate ( Wang et al.,2014 ).This conclusion was further verified in a smoke box simulation experiment and confirmed by observation data during the strong haze episode in Beijing during January 2013 ( He et al., 2014 ).The treatment of NOpollution is certainly far less effective than the treatment of PM and SO 2,and the NO 2 concentration even rises instead of decreasing in some cities ( Fig.2 ).

3.2. Spring particulate matter

The PM concentration in northern China is affected by the sanddust source region, Mongolian cyclones and frontal systems in the spring ( Sun et al., 2001 ; Qian et al., 2002 ; Fan et al., 2016 ), and the area is prone to sand-dust weather, which causes severe PM pollution.The average maximum concentrations of TSP, PM,and PMwere 111.8 ± 95.7 μg m3,83.5 ± 135.5 μg m3,and 41.0 ± 30.8 μg m3,respectively, in Hohhot in the spring of 2013–17, and the PMand PMconcentrations were 1.2 and 1.6 times higher, respectively, than the NAAQS (at 70 and 35 μg m3,respectively).However, the concentrations of TSP, PM,and PMin Hohhot in spring also exhibited a decreasing trend.Hence, the concentrations of TSP, PM 10,and PM 2.5 decreased 31.3%, 43.8%, and 46.7%, respectively, from 2013–15, but the maximum PM 2.5 and PMconcentrations in 2017 increased 24.9% and 10.2%, respectively.PM/PM(hereinafter referred to as the

γ

) was 39.2% in Hohhot in the spring of 2013–17, which was 9.1% higher in 2017 than that in 2013.Additionally, the excess rates of PM 2.5,PM 10 ,and TSP in the spring of 2013–17 were 12.0%, 20.6%, and 7.7%, respectively.The number of exceeding days of the PM (PM 2.5,PM 10,and TSP)in Hohhot city in the spring of 2017 was smaller than that in 2013.It was found that in spring, Hohhot was mainly dominated by PM pollution,and the PM concentration generally showed a decreasing trend, which has rebounded in recent years.Furthermore, PM concentrations can be used to describe the intensity of PM pollution, and the

γ

can be used to gauge the contribution of fine particles ( Li et al., 2019 ).In addition,the

γ

can explain the types of pollution and possible sources of pollutants; generally, wind-blown dust and soil particles are dominated by the coarse mode, while industrial and urban aerosols are dominated by the fine mode.In Hohhot, the

γ

increased year by year ( Fig.2 (a)), which indicated that the control effect on primary pollution was obvious, and the air pollution tended to change from primary to secondary pollution.This result is consistent with the findings of other studies conducted in China ( Li et al., 2019 ; Wang et al., 2015b ).

Statistically speaking, the daily variation characteristics of the air pollutant concentration ( Fig.3 (a)) and meteorological elements( Fig.3 (b)) in Hohhot from 2013 to 2017 were analyzed to obtain a comprehensive understanding of the daily variation in the PM concentration and its changes throughout the region: (1) The daily PM variation characteristics basically remain the same, reaching high concentrations from 9:00 to 11:00 LST.As the wind speed increases after 12:00 LST, the conditions are conducive to PM diffusion.As the wind speed decreases at approximately 19:00 LST, the boundary layer structure remains relatively stable.Moreover, the increase in emission sources does not promote the diffusion of particulate pollution.The humidity greatly increases, thereby increasing the PM concentration.

The decreasing trends of the PM 10 and PM 2.5 concentrations at the different times from 2013–17 were basically consistent with the changes in the spring average concentration.In particular, the PMand PMconcentrations declined to varying degrees at the different times.The Action Plan and its control measures in regard to PMand PMin spring in Hohhot were reasonable and effective.The difference was compared to that in 2016, and the concentrations of both PMand TSP in 2017 greatly increased from the early morning to noon and notably decreased in the afternoon, while the proportion of the concentration change in TSP at the different times from 2013–17 was inconsistent with the average decline rate in spring.Although the TSP decreased year by year in the spring of 2013–17, the rate of change at the different times was different.The specific performance entailed that the concentration of TSP from 2014–16 increased in the afternoon over the previous year, and compared to 2016, the concentration of TSP in 2017 increased from the early morning to noon, which might be related to the sand-dust weather conditions.

Fig.2.Monthly mean PM 10,PM 2.5,SO 2,and NO 2 concentrations in Hohhot from 2013 to 2017 (units: μg m ? 3).

Fig.3.(a) Daily changes in the atmospheric pollutant concentration at the Ruyi Water Treatment Plant, Hohhot, in the spring of 2013–17 and (b) daily changes in the meteorological elements at the Ruyi Water Treatment Plant, Hohhot, from 2013 to 2017.

3.3. The absolute contribution of sand-dust weather to the particulate matter concentration

In the spring of 2013–17, 6, 7, 11, 8, and 6 sand-dust weather processes occurred in the Inner Mongolia region in China ( China Meteorological Administration, 2014,2015,2016,2017,2018 ).Against the background of sand-dust weather in Inner Mongolia from 2013 to 2017,with the use of absolute contribution indicators and combined with the daily PM concentrations (TSP, PM 10,and PM 2.5 ) at the Ruyi Water Plant site in Hohhot, Inner Mongolia, the absolute contribution of sand-dust weather to the ambient air quality in Hohhot was calculated ( Fig.4 ).From 2013 to 2017, the absolute contribution of sand-dust weather to the PM 2.5 concentration in Hohhot remained basically stable at 0.6–5.2 μg m.However, the contributions to the concentrations of PMand TSP increased, and the change ranges were from 9.0–16.9 μg m,and 14.7–30.0 μg m3,respectively.Additionally, the sand-dust weather in 2017 contributed the most to the absolute concentrations of PMand TSP (16.9 and 30.0 μg m3,respectively).The sand-dust weather conditions in 2013 contributed the most to the absolute PMconcentration(5.2 μg m3 ), while those in 2015 contributed the least to the absolute concentrations of PM,PM,and TSP (at 0.6, 9.0, and 14.7 μg m,respectively).Hence, sand-dust weather occurred the most frequently in the spring of 2015, and the impact on the air quality was the minimum among the five years.

The daily maximum concentrations (peak concentrations) of PM 2.5 ,PM,and TSP were adopted as a representation of the intensity of the ambient air pollution, and the PM pollution intensity on sand-dust and non-sand-dust days in the spring of 2013–17 was analyzed ( Tables 1 ,2,and 3 ).From 2013 to 2017, the peak PM concentration on sand-dust and non-sand-dust days exhibited a trend of decreasing first and then increasing.Additionally, the PMand PMpeak concentrations on the sand-dust days were both the lowest in 2015 and the highest in 2017, and the PMand PMpeak concentrations on the non-sanddust days were the lowest in 2015 and the highest in 2013.According to the annual change trend of the number of sand-dust days from 2015 to 2017, as shown in Fig.3,the number of sand-dust days decreased year by year.However, the intensity of sand-dust pollution increased year by year.Moreover, the number of sand-dust days was similar in 2013 and in 2017, but the number of days (rate) in regard to PM 2.5,PM 10,and TSP on the sand-dust days was much smaller (lower) in 2017 than that in 2013.On the sand-dust days from 2013–17, TSP and PMwere the primary pollutants, with exceedance rates of 25% and 18.8%, respectively, and on the non-sand-dust days, PM 10 and PM 2.5 were the primary pollutants,with exceedance rates of 9.2% and 5.3%, respectively.The exceedance rates of the concentrations of PM 2.5,PM 10,and TSP in 2017 were 87.5%,50.0%, and 20% lower, respectively, than those in 2013 on the sanddust days.The exceedance rates of the PMand PMconcentrations in 2017 were 73.3% and 73.1% lower, respectively, than those in 2013 on the non-sand-dust days.

The above may be related to the PM prevention and control measures in the Action Plan, which effectively reduced the concentrations of TSP,PM,and PM.In addition, this was related to local scientific and accurate sand-dust storm early warning forecasts, which greatly reduced the impact of sand-dust weather on the air quality.

Table 1 Peak PM 2.5 concentration and days exceeding the standard (rate) on the sand-dust and non-sand-dust days from 2013 to 2017.

Table 2 Peak PM 10 concentration and days exceeding the standard (rate) on the sand-dust and non-sand-dust days from 2013 to 2017.

Table 3 Peak concentration of TSP and days exceeding the standard (rate) on the sand-dust and the non-sand-dust days from 2013 to 2017.

Fig.4.Daily maximum concentrations of TSP, PM 10,and PM 2.5,the number of dust days and the absolute concentration contributions of PM on sand and dust days from 2013 to 2017 in Hohhot.

4.Conclusions

1) The air quality in Hohhot has improved since 2013.Notably, the annual PMand PMconcentrations decreased by 24.6% and 48.2%, respectively.However, the PM 2.5 and PM 10 concentrations were 22.9% and 35.7% higher, respectively, than the NAAQS.

2) PM pollution (PM 2.5 and PM 10 ) mainly occurred in winter and spring, and the gaseous precursor concentrations of PM(SOand NO) were the highest in winter from 2013–17.The reductions in the spring PM 2.5 and PM 10 concentrations were 5.6% and 8.9%, respectively, and the annual decreases in the PMand PMconcentrations were 3.6 and 15.1 μg m3 yr1,respectively, in 2013 and 2017.

3) The peak PM concentration revealed a trend of decreasing first and then increasing in Hohhot on the sand-dust and non-sand-dust days in the spring of 2013–17.The PM concentration was mainly dominated by TSP and PM 10 on the sand-dust days.The non-sand-dust days were mainly dominated by PMand PM.During the same period, the absolute contributions of sand-dust weather to the concentrations of PM,PM,and TSP in Hohhot ranged from 0.6–5.2 μg m,9.0–16.9 μg m,and 14.7–30.0 μg m,respectively.Among them, sand-dust weather contributed the most to the concentrations of PMand TSP in 2017 and to the PMconcentration in 2013.

Declaration of Competing Interest

The authors declare no competing interests.

Funding

This work was supported by the National Key R&D Program of China[grant number 2018YFC1507701], the Key Research and Development(R&D) Projects of Shanxi Province [grant number 201803D31220], and the EDF program: the Belt and Road national greenhouse gas and pollutant co-control study [grant number 2019–434].