Aerosol optical properties under different pollution levels in the Pearl River Delta (PRD) region of China.

To clarify the aerosol hygroscopic growth and optical properties of the Pearl River Delta (PRD) region, integrated observations were conducted in Heshan City of Guangdong Province from October 19 to November 17, 2014. The concentrations and chemical compositions of PM2.5, aerosol optical properties and meteorological parameters were measured. The mean value of PM2.5 increased from less than 35 (excellent) to 35-75 µg/m3 (good) and then to greater than 75 µg/m3 (pollution), corresponding to mean PM2.5 values of 24.9, 51.2, and 93.3 µg/m3, respectively. The aerosol scattering hygroscopic growth factor (f(RH = 80%)) values were 2.0, 2.12, and 2.18 for the excellent, good, and pollution levels, respectively. The atmospheric extinction coefficient (σext) and the absorption coefficient of aerosols (σap) increased, and the single scattering albedo (SSA) decreased from the excellent to the pollution levels. For different air mass sources, under excellent and good levels, the land air mass from northern Heshan had lower f(RH) and σsp values. In addition, the mixed aerosol from the sea and coastal cities had lower f(RH) and showed that the local sources of coastal cities have higher scattering characteristics in pollution periods.

Investigating the characteristics and source analyses of PM2.5 seasonal variations in Chengdu, Southwest China.

In 2015, comprehensive observations were carried out in Chengdu, Sichuan Province, China, to elucidate the seasonal variation characteristics of the concentrations, chemical compositions, and the sources of PM2.5 pollution. The meteorological parameters, gaseous pollutants and chemical compositions of PM2.5 were measured. The annual average concentration of PM2.5 in Chengdu was 67.44 ±â€¯48.78 µg/m3. The highest seasonal PM2.5 mass concentration occurred in winter with an average of 103.04 ±â€¯66.76 µg/m3, followed by spring, autumn, and summer, and the wind speed had an important impact on the diffusion of PM2.5. The seasonal variation characteristics of chemical components in PM2.5 were analysed. The contribution and chemical conversion ability of secondary aerosols increased with increasing of PM2.5 concentration. Source appointment of positive matrix factorization (PMF) shows that the main sources of PM2.5 were secondary aerosols, coal combustion, biomass burning, vehicle emissions, dust and industrial sources, which have more obvious seasonal differences than other sources, and secondary aerosols and coal combustion were the major sources. Conditional probability function (CPF) analysis showed that the local sources of high PM2.5 concentrations were mainly from the eastern and southeastern areas of Chengdu. Potential source contribution function (PSCF), concentration weighted trajectory (CWT) and backward trajectory cluster analyses indicated that the southern, southeast and eastern parts of the Sichuan Basin were the most likely potential sources of PM2.5, and the unique geographical and topographical factors in Chengdu play important roles in the transport and diffusion of pollutants in this region.

A framework for investigating the air quality variation characteristics based on the monitoring data: Case study for Beijing during 2013-2016.

In this study, an analysis framework based on the regular monitoring data was proposed for investigating the annual/inter-annual air quality variation and the contributions from different factors (i.e., seasons, pollution periods and airflow directions), through a case study in Beijing from 2013 to 2016. The results showed that the annual mean concentrations (MC) of PM2.5, SO2, NO2 and CO had decreased with annual mean ratios of 7.5%, 28.6%, 4.6% and 15.5% from 2013 to 2016, respectively. Among seasons, the MC in winter contributed the largest fractions (25.8%~46.4%) to the annual MC, and the change of MC in summer contributed most to the inter-annual MC variation (IMCV) of PM2.5 and NO2. For different pollution periods, gradually increase of frequency of S-1 (PM2.5, 0~75 µg/m3) made S-1 become the largest contributor (28.8%) to the MC of PM2.5 in 2016, it had a negative contribution (-13.1%) to the IMCV of PM2.5; obvious decreases of frequencies of heavily polluted and severely polluted dominated (44.7% and 39.5%) the IMCV of PM2.5. For different airflow directions, the MC of pollutants under the south airflow had the most significant decrease (22.5%~62.5%), and those decrease contributed most to the IMCV of PM2.5 (143.3%), SO2 (72.0%), NO2 (55.5%) and CO (190.3%); the west airflow had negative influences to the IMCV of PM2.5, NO2 and CO. The framework is helpful for further analysis and utilization of the large amounts of monitoring data; and the analysis results can provide scientific supports for the formulation or adjustment of further air pollution mitigation policy.

Characteristics of one-year observation of VOCs, NOx, and O3 at an urban site in Wuhan, China.

A continuous online observation of ozone and its precursors (NOx, VOCs) was carried out in central urban Wuhan from September 2016 to August 2017. The concentration levels of ozone, NOx, VOCs and their variations in urban Wuhan were analyzed, as well as effects of VOCs on ozone photochemical generation and the main controlling factors for ozone production. During the observation period, the average concentrations of ozone and NOx in Wuhan was 22.63 and 30.14 ppbv, respectively, and the average concentration of VOCs was 32.61 ppbv (42.3% alkanes, 13.0% alkenes, 10.0% aromatics, 7.3% acetylene, 9.9% OVOCs, and 10.5% halohydrocarbons). Ozone concentration was higher in spring and summer as compared with autumn and winter, wheras VOCs and NOx concentratios were lower in spring and summer but higher in autumn and winter. Aromatics and alkenes, two of VOCs species, showed the highest contributions to ozone formation potential in Wuhan (35.7% alkenes, 35.4 aromatics, 17.5% alkanes, 8.6% OVOCs, 1.6% halogenated hydrocarbons, and 1.4% acetylene). Among all VOCs species, those with the highest contribution were ethylene, m-xylene, toluene, propylene and o-xylene. The contribution of these five compounds to the total ozone formation potential concentration was 43.90%. Ozone-controlling factors in Wuhan changed within one day; during the early morning hours (6:00-9:00), VOCs/NOx was low, and ozone generation followed a VOCs-limited regime. However, during the peak time of ozone concentration (12:00-16:00), the ratio of VOCs/NOx was relatively high, suggesting that ozone generation followed a NOx-limited regime.

Understanding long-term variations of meteorological influences on ground ozone concentrations in Beijing During 2006-2016.

Recently, ground ozone has become one major airborne pollutant and the frequency of ozone-induced pollution episodes has increased rapidly across China. However, due to the lack of long-term observation data, relevant research on the characteristics and influencing factors of urban ozone concentrations remains limited. Based on ground ozone observation data during 2006-2016, we quantified the causality influence of individual meteorological factors on ozone concentrations in Beijing using a convergent cross mapping (CCM) method. The result indicated that the influence of each meteorological factor on ozone concentrations varied significantly across seasons and years. At the inter-annual scale, all-year meteorological influences on ozone concentrations were much more stable than seasonal meteorological influences. At the seasonal scale, meteorological influences on ozone concentrations were stronger in spring and autumn. Amongst multiple individual factors, temperature was the key meteorological influencing factor for ozone concentrations in all seasons except winter, when wind, humidity and SSD exerted major influences on ozone concentrations. In addition to temperature, air pressure was another meteorological factor that exerted strong influences on ozone concentrations. At both the inter-annual and seasonal scale, the influence of temperature and humidity on ozone concentrations was generally stable whilst that of other factors experienced large variations. Different from PM2.5, meteorological influences on ozone concentrations were relatively weak in summer, when ozone concentrations were the highest in Beijing. Given the generally stable meteorological influences on ozone concentrations and human-induced emissions of VOCs and NOx across seasons, warming induced notable increase in summertime biogenic emissions of VOCs and NOx can be a major driver for the increasing ozone pollution episodes. This research provides useful references for understanding long-term meteorological influences on ozone concentrations in mega cities in China.

VOC characteristics, sources and contributions to SOA formation during haze events in Wuhan, Central China.

Based on detailed data on 102 volatile organic compounds (VOCs) measured continuously from 2016.10.9 to 2016.11.17 in Wuhan, the VOC characteristics, secondary organic aerosol (SOA) characteristics, SOA formation potential (SOAP), potential source regions, sources and contributions during different haze episodes were analyzed. The total VOC (TVOC) concentrations on clear days (visibility >10 km), slight haze days (visibility of 5-10 km), and severe haze days (visibility <5 km) were 34.87 ±â€¯14.89 ppbv, 45.06 ±â€¯26.69 ppbv, and 49.55 ±â€¯24.82 ppbv, respectively. The SOAP on haze days (447.04 ±â€¯253.85 ppbv) was significantly higher than that on clear days (300.62 ±â€¯138.48 ppbv), and aromatics were the dominant contributors to SOA formation under different visibility conditions, accounting for approximately 97% of the total SOAP. The ratio of ethylbenzene to m/p-xylene (E/X) indicated that atmospheric photochemical reactions were slightly stronger on haze days. The ratio of toluene to benzene (T/B) indicated that vehicle exhaust had significant effects on VOCs, but no significant changes occurred during different haze episodes. The ratio of benzene, toluene, ethylbenzene and xylenes (BTEX) to CO indicated that VOCs from solvent usage in painting/coating and industrial emissions increased with increasing haze pollution. Based on backward trajectories and the potential source contribution function (PSCF), short-distance transport was the main source influencing VOC pollution, especially transport from the southwest. Seven sources were identified by positive matrix factorization (PMF): industrial sources, vehicular exhaust, solvent usage in painting/coating, fuel evaporation, liquefied petroleum gas (LPG) usage, biogenic sources and biomass burning. Moreover, solvent usage in painting/coating, vehicle exhaust and LPG usage were the most important sources that significantly aggravated VOC pollution during haze events. The results can provide references for local governments developing control strategies of VOCs during haze pollution events.

Understanding the Influence of Crop Residue Burning on PM2.5 and PM10 Concentrations in China from 2013 to 2017 Using MODIS Data.

In recent years, particulate matter (PM) pollution has increasingly affected public life and health. Therefore, crop residue burning, as a significant source of PM pollution in China, should be effectively controlled. This study attempts to understand variations and characteristics of PM10 and PM2.5 concentrations and discuss correlations between the variation of PM concentrations and crop residue burning using ground observation and Moderate Resolution Imaging Spectroradiometer (MODIS) data. The results revealed that the overall PM concentration in China from 2013 to 2017 was in a downward tendency with regional variations. Correlation analysis demonstrated that the PM10 concentration was more closely related to crop residue burning than the PM2.5 concentration. From a spatial perspective, the strongest correlation between PM concentration and crop residue burning existed in Northeast China (NEC). From a temporal perspective, the strongest correlation usually appeared in autumn for most regions. The total amount of crop residue burning spots in autumn was relatively large, and NEC was the region with the most intense crop residue burning in China. We compared the correlation between PM concentrations and crop residue burning at inter-annual and seasonal scales, and during burning-concentrated periods. We found that correlations between PM concentrations and crop residue burning increased significantly with the narrowing temporal scales and was the strongest during burning-concentrated periods, indicating that intense crop residue burning leads to instant deterioration of PM concentrations. The methodology and findings from this study provide meaningful reference for better understanding the influence of crop residue burning on PM pollution across China.

Comparisons of two serious air pollution episodes in winter and summer in Beijing.

Characteristics of two serious air pollution episodes (9-15 January, as the winter case; and 30 June to 1 July, as the summer case), which occurred in Beijing in 2013 were investigated and compared using multi-method observations and numerical simulations. During these two air pollution episodes, PM2.5 concentrations varied significantly within Beijing, with PM2.5 concentrations in southern parts of Beijing being significantly higher than in northern areas. Typically, heavy air pollution episodes begin in the southern parts and disperse towards the northern parts of Beijing. Clearly, synoptic patterns and the stability of atmospheric circulation patterns were the main factors controlling air pollution in Beijing. During the winter case, a warm center above 900hPa occurred over Beijing. Meanwhile, in the summer case, although there was only a weak inversion, the convective inhibition energy was strong (over 200J/kG). This clearly influenced the duration of the air pollution event. Except for the local accumulation and secondary atmospheric reactions in both cases, regional straw burnings contributed a lot to the PM2.5 concentrations in summer case. Using the CAMx model, we established that regional transport contributed almost 59% to the PM2.5 averaged concentration in Beijing in the winter case, but only 31% in the summer case. Thus, the winter case was a typical regional air pollution episode, while the summer case resulted from local accumulation straw burnings transportation and strong secondary atmospheric reactions. Given that air pollution is a regional problem in China, consistent and simultaneous implementation of regional prevention and control strategies is necessary to improve regional air quality.

Ground ozone concentrations over Beijing from 2004 to 2015: Variation patterns, indicative precursors and effects of emission-reduction.

Based on ozone observation data from urban stations and the Dingling (DL) background station, we investigated the trend of ozone concentrations in Beijing during 2004-2015. For urban stations, both O3_1 h and O3_8 h increased stably with a clear and significant linear pattern and the increase rate was notably higher during the period of May to Sep. Meanwhile, the variation of O3_1 h and O3_8 h for the DL station did not demonstrate a regular pattern. During this period, the differences between the diurnal peak of ozone concentrations at the DL background station and urban stations decreased significantly due to the rapid urbanization of Beijing. Furthermore, we examined simultaneous variations of ozone and its precursors during 2015 Grand Military Parade and 2014 APEC meeting and evaluated the performances of different emission-reduction measures during the two specific events. For 2015 Grand Military Parade, emission-reduction measures were implemented 14 days in advance, which led to a notable decrease of ozone concentrations during the Parade period. For 2014 APEC meeting, emission-reduction measures were not implemented in advance, which led to incomplete VOCs reduction and high VOCs/NOx values, and thus a significant increase of ozone concentrations during the APEC period. The emission-reduction measures during APEC and PARADE periods both slowed down the accumulation and cut down the concentration peaks of ozone. We also analyzed simultaneous concentration variations of ozone and its precursors in long time-series. The results proved that compared with other precursors, NO2/NO was an effective indicator for ozone concentration in Beijing, especially in urban areas. The findings from this research provide useful reference for better monitoring and managing ozone concentrations in Beijing and other cities through properly designed and implemented emission-reduction measures.

Spatio-temporal variations of PM2.5 concentrations and the evaluation of emission reduction measures during two red air pollution alerts in Beijing.

To effectively improve air quality during pollution episodes, Beijing released two red alerts in 2015. Here we examined spatio-temporal variations of PM2.5 concentrations during two alerts based on multiple data sources. Results suggested that PM2.5 concentrations varied significantly across Beijing. PM2.5 concentrations in southern parts of Beijing were higher than those in northern areas during both alerts. In addition to unfavorable meteorological conditions, coal combustion, especially incomplete coal combustion contributed significantly to the high PM2.5 concentrations. Through the CAMx model, we evaluated the effects of emission-reduction measures on PM2.5 concentrations. Through simulation, emergency measures cut down 10% - 30% of the total emissions and decreased the peaks of PM2.5 concentrations by about 10-20% during two alerts. We further examined the scenario if emergency measures were implemented several days earlier than the start of red alerts. The results proved that the implementation of emission reduction measures 1-2 days before red alerts could lower the peak of PM2.5 concentrations significantly. Given the difficulty of precisely predicting the duration of heavy pollution episodes and the fact that successive heavy pollution episodes may return after red alerts, emergency measures should also be implemented one or two days after the red alerts.

Below-cloud wet scavenging of soluble inorganic ions by rain in Beijing during the summer of 2014.

Wet deposition is one of the most important and efficient removal mechanisms in the reduction of air pollution. As a key parameter determining wet deposition, the wet scavenging coefficient (WSC) is widely used in chemical transport models (CTMs) and reported values have large uncertainties. In this study, a high-resolution observational dataset of the soluble inorganic aerosols (SO42-, NO3- and NH4+, hereafter SNA) in the air and in rainwater during multiple precipitation events was collected using sequential sampling and used to estimate the below-cloud WSC in Beijing during the summer of 2014. The average concentrations of SNA in precipitation during the observational period were 7.9 mg/L, 6.2 mg/L and 4.6 mg/L, with the contributions from below-cloud scavenging constituting 56%, 61% and 47% of this, respectively. The scavenging ratios of SNA (i.e., the ratio of the concentrations in rain to concentrations in the air) were used with the height of the cloud base and the precipitation intensity to estimate the WSC. The estimated WSC of SO42- is comparable to that reported elsewhere. The relationship between the below-cloud WSC and the precipitation intensity followed an exponential power distribution (K=aPb) for SNA. In contrast to previous studies, this study considers the differences between the chemical compositions of the SNA, with the highest WSC for NO3-, followed by those of SO42- and NH4+. Therefore, we recommend that CTMs include ion specific WSCs in the future.

[Improvement of Air Quality During APEC in Beijing in 2014].

Variations of air quality, meteorological conditions and the effect of pollution control measures on particle matter concentrations in Beijing were all analyzed during APEC (from 1st to 12th in November) in 2014 based on the atmospheric pollutant monitoring data, monitoring components of PM2.5, meteorological and remote sensing data and CMB model. The results showed that the average concentrations of PM2.5, PM10, SO2, NO2 were 43,62,8,46 [g.m respectively during APEC and the average concentrations of PM2.5, PM10, SO2, NO2 were decreased by 45%, 43%, 64% and 31% compared to those in the same period of the last 5 years (PM2. was the average of the last 2 years); the concentrations of PM25 at different sites were decreased by 27.4%-35.5%; the concentrations of PM2.5 in the center of city and northern mountainous areas were the lowest, which dropped by 30%-45% compared to those in the same period of the last 5 years while in the southern area the decrement was below 25%; the main component SO4(2-), the substance of the crust, and NO3- were decreased by 50%, 76%, 35% respectively compared to those in the same period in 2013 and the chemical mass balance (CMB) model analysis results indicated that contributions of coal boiler, dust, motor vehicle were 2%, 7%, 30% respectively during APEC; air pollution control measures (coal, dust and traffic management) had a significant effect on reducing pollutant emissions and the pollutant emissions control reduced the concentration peak and delayed the accumulation speed.

[Concentration Characteristics of PM2.5 in Beijing During Two Red Alert Periods].

Variations of PM2.5 concentrations and effects of pollution control measures during two red alert periods in 2015 in Beijing were analyzed based on atmospheric pollutant monitoring data. The results showed that during the first red alert, the highest hourly-averaged PM2.5 concentration occurred at 19:00 on 9th December with a value of 282 µg·m-3 and the highest hourly PM2.5 concentration appeared at Yongledian station which is near the southeast border of Beijing, with the peak concentration of 496 µg·m-3. During the second red alert, the highest hourly-averaged concentration of PM2.5 occurred at 20:00 on 22th with a value of 421 µg·m-3. The highest hourly PM2.5 concentration was monitored at Liulihe station which is near the southwest border of Beijing, with the peak concentration of 831 µg·m-3. During the duration period of both red alerts, the concentrations at the southern stations were higher than those at downtown stations and the PM2.5 concentrations at northern stations were found to be the smallest. The difference between these two red alerts was that during the second red alert, the PM2.5 concentrations in southern Beijing were significantly higher than those in the northern area, while the magnitude of this south-to-north gradient was much smaller during the first one. During the second red alert, up to 93% of Beijing area showed an average PM2.5 concentration of above 150 µg·m-3, which was much larger than that in the first one. The meteorological conditions during the two red alerts were both not conducive to the spread of pollutants. Formation of secondary pollutants and regional pollutant transport existed as well. Though the stagnant weather conditions were in favor of the development of severe pollution, large regional-wide pollutant emission was the main reason for these two heavy air pollutions in Beijing. PM2.5 concentrations were decreased by 20%-25% after the implementation of emergency response measures, which showed the significance of emission reduction in air pollution control.

[Characteristics of Ozone Background Concentration in Beijing from 2004 to 2015].

Based on the hourly O3 monitoring data from 2004 to 2015 of Beijing, a comprehensive discussion on the characteristics of O3 concentration at a background station Dingling in Beijing was conducted. The results showed that the annual concentration of O31h was increasing with a growth rate of 4.40 µg·m-3 while the annual concentration of O38h was decreasing with annual average rates of -1.0 µg·m-3 and -1.5 µg·m-3 from May to October in 2004 and 2015. Over the past 3 years, number of O38h severe pollution days increased significantly and the situation of O3 pollution in Beijing became more serious. O3 concentration reached its peak in June in a year and its diurnal peak concentration occurred at about 15:00-18:00 at Dingling station which was 101-1.56 times larger than that in the urban center of Beijing. In different years, the ozone peak concentration at Dingling Station was 1h later than that in the urban center from May to October in diurnal variation and the difference of peak concentration was significantly reduced in recent years, which on the one hand may be related to regional ozone pollution, on the other hand may be related to the expansion of Beijing's urbanization.

[Characteristics of Ozone over Standard and Its Relationships with Meteorological Conditions in Beijing City in 2014].

The spatial-temporal distribution characteristics of O3 and the correlations between O3 and meteorological elements in Beijing urban area were investigated based on the hourly O3 monitoring data from January to December in 2014 released by Beijing Municipal Environmental Monitoring Center. The annual concentration of O3 in Beijing was about 56.18 µg·m-3 in 2014. In the over polluted days during May and September, the O3 concentration could reach as high as 148.05 µg·m-3. The diurnal distribution of ozone presented a clear unimodal pattern with its peak appearing at 15:00 or 16:00 and trough at 06:00 or 07:00 and the concentrations of O3 during 09:00 and 23:00 was significantly higher than those in the Summer time. For the spatial distribution of O3, the concentration was lower in central urban area with the highest concentration appearing at plant garden site in the west of the urban area. Ground weather type of O3 over polluted days was divided into three categories including high-pressure, low-pressure, equalizing field, which accounted for 16%, 36%, 48%, respectively. The concentration of O3 was negatively correlated with the air pressure, humidity and visibility while it was positively correlated with the wind speed and temperature. In one heavy pollution episode of O3 caused by local photochemical pollution and regional transport from May 29th to June 1st in 2014 in Beijing, regional transport showed a very important influence on the concentration of O3 in Beijing.

[Numerical Study on the Characteristics of Regional Transport of PM2.5 in Shandong Province During Spring in 2014].

In this paper, spatial and temporal distribution, transportation and deposition of PM2.5 in Shandong Province in Spring, 2014 were all analyzed by applying PSAT of CAMx model and we also developed a transport matrix of PM2.5 between different cities in Shandong. The results showed that ρ(PM2.5) presented obvious spatial distribution characteristics; ρ(PM2.5) was higher in the western part compared to that in peninsula and ρ(PM2.5) was mainly concentrated below 2 000 m in vertical direction. Simulated horizontal transport flux of PM2.5 was up to 110 µg.(m2.s)-1 and the total deposition amount of PM2.5 was 23. 05 x 10(4) t in Shandong during Spring, 2014. Analysis of regional contribution found that the pollutants mainly came from local districts and the average external transport contribution to the whole Shandong province was about 21. 08% ± 3. 83% while it was 40. 45% ± 5. 96% between different cities; the contribution rates of Jinjinji distrcit, background and boundary conditions gradually increased by 7. 56% and 6. 18% respectively as the altitude increased.

[Formation Mechanism of a Serious Pollution Event in January 2013 in Beijing].

The weather conditions, atmospheric environmental background and formation mechanism of a heavy air pollution episode in Beijing City from January 9th to 15th, 2013 was preliminarily investigated by combining observed data and the WRF meteorology model. The results showed that the average concentration of PM2.5 was 323 µg x m(-3) from January 10th to 14th; the heavy pollution episode was closely related to the local meteorological conditions; the stable atmospheric circulation pattern provided favorable environmental field for the lasting of this heavy air pollution; small wind speed, high humidity, low PBL, and lasting temperature inversion were the main reasons for this heavy air pollution incident; further analysis showed that contributions of regional transmission to the receptor sites in Beijing were between 53% - 69% and there were obvious secondary conversions and transformations; overall regional transportation played a more important role during this serious air pollution incident; the meteorological conditions played a key role in the formation and destruction of the heavy air pollution, therefore we need to strengthen the study on early warning of heavy air pollution, in order to prevent and control the air heavy pollution effectively.