In January 2013, a long-lasting episode of severe haze occurred in central and eastern China, and it attracted attention from all sectors of society. The process and evolution of haze pollution ...episodes were observed by the ＂Forming Mechanism and Con- trol Strategies of Haze in China＂ group using an intensive aerosol and trace gases campaign that simultaneously obtained data at 11 ground-based observing sites in the CARE-China network. The characteristics and formation mechanism of haze pollu- tion episodes were discussed. Five haze pollution episodes were identified in the Beijing-Tianjin-Hebei （Jing-Jin-Ji） area; the two most severe episodes occurred during 9-15 January and 25-31 January. During these two haze pollution episodes, the maximum hourly PMz5 mass concentrations in Beijing were 680 and 530 ~tg m-3, respectively. The process and evolution of haze pollution episodes in other major cities in the Jing-Jin-Ji area, such as Shijiazhuang and Tianjin were almost the same as those observed in Beijing. The external cause of the severe haze episodes was the unusual atmospheric circulation, the depres- sion of strong cold air activities and the very unfavorable dispersion due to geographical and meteorological conditions. How- ever, the internal cause was the quick secondary transformation of primary gaseous pollutants to secondary aerosols, which contributed to the ＂explosive growth＂ and ＂sustained growth＂ of PM2.5. Particularly, the abnormally high amount of nitric ox- ide （NOx） in the haze episodes, produced by fossil fuel combustion and vehicle emissions, played a direct or indirect role in the quick secondary transformation of coal-burning sulphur dioxide （SO2） to sulphate aerosols. Furthermore, gaseous pollutants were transformed into secondary aerosols through heterogeneous reactions on the surface of fine particles, which can change the particle＇s size and chemical composition. Consequently, the proportion of secondary inorganic ions, such as sulphate and nitrate, gradually increased, which enhances particle hygroscopicity and thereby accelerating formation of the haze pollution.
Abstract
Severe events of wintertime particulate air pollution in Beijing (winter haze) are associated with high relative humidity (RH) and fast production of particulate sulfate from the oxidation ...of sulfur dioxide (SO
2
) emitted by coal combustion. There has been considerable debate regarding the mechanism for SO
2
oxidation. Here we show evidence from field observations of a haze event that rapid oxidation of SO
2
by nitrogen dioxide (NO
2
) and nitrous acid (HONO) takes place, the latter producing nitrous oxide (N
2
O). Sulfate shifts to larger particle sizes during the event, indicative of fog/cloud processing. Fog and cloud readily form under winter haze conditions, leading to high liquid water contents with high pH (>5.5) from elevated ammonia. Such conditions enable fast aqueous-phase oxidation of SO
2
by NO
2
, producing HONO which can in turn oxidize SO
2
to yield N
2
O.This mechanism could provide an explanation for sulfate formation under some winter haze conditions.
The rapidly spread coronavirus disease (COVID-19) has limited people's outdoor activities and hence caused substantial reductions in anthropogenic emissions around the world. However, the air quality ...in some megacities has not been improved as expected due to the complex responses of aerosol chemistry to the changes in precursors and meteorology. Here we demonstrate the responses of primary and secondary aerosol species to the changes in anthropogenic emissions during the COVID-19 outbreak in Beijing, China along with the Chinese New Year (CNY) holiday effects on air pollution by using six-year aerosol particle composition measurements. Our results showed large reductions in primary aerosol species associated with traffic, cooking and coal combustion emissions by 30–50% on average during the CNY, while the decreases in secondary aerosol species were much small (5–12%). These results point towards a future challenge in mitigating secondary air pollution because the reduced gaseous precursors may not suppress secondary aerosol formation efficiently under stagnant meteorological conditions. By analyzing the long-term measurements from 2012 to 2020, we found considerable increases in the ratios of nitrate to sulfate, secondary to primary OA, and sulfur and nitrogen oxidation capacity despite the overall decreasing trends in mass concentrations of most aerosol species, suggesting that the decreases in anthropogenic emissions have facilitated secondary formation processes during the last decade. Therefore, a better understanding of the mechanisms driving the chemical responses of secondary aerosol to the changes in anthropogenic emissions under complex meteorological environment is essential for future mitigation of air pollution in China.
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•Different responses of primary and secondary species to the COVID-19 outbreak•30–50% decreases in primary aerosol species during the Chinese New Year holiday•Much smaller changes in secondary species compared with primary aerosol species•Large increases in sulfur and nitrogen oxidation capacity during the last decade
Elements in fine particles (PM2.5) have adverse impacts on ecosystems and human health. Using an online multi-heavy metal analyzer, one-year continuous hourly measurements were performed for thirteen ...elements, namely, K, Ca, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Ba, Hg and Pb, from June 1, 2016, to May 31, 2017, in urban Beijing. The total concentrations of 13 elements were in the range of 114–136,574 ng/m3, with a mean concentration of 2534 ± 5563 ng/m3. The four dominated elements were K (900 ± 3554 ng/m3), Fe (738 ± 1485 ng/m3), Ca (493 ± 1473 ng/m3) and Zn (174 ± 189 ng/m3). The annual mean concentrations of Cr and As exceeded the guideline values of the World Health Organization and the Chinese National Ambient Air Quality Standard. In the absence of firework-burning episodes and heavy dust storms, K, Cr, Mn, Cu, Zn, As, Se, Hg and Pb showed higher concentrations in autumn and winter, whereas lower concentrations were observed in spring and summer. The higher concentrations of Ca, Fe, and Ba observed in spring were associated with the frequent occurrence of dust. All elements showed higher concentrations on weekends than on weekdays. The diel patterns of Fe, Ba, Ca, Cu, Zn, Ni and Mn showed higher concentrations during peak traffic periods, whereas those of As, Cr, Hg, K, Pb and Se showed lower concentrations in the daytime but higher ones in the nighttime. The potential source function (PSCF) identified that neighboring provinces were the major source regions for most elements, whereas Ni mainly came from Shandong and the eastern ocean in spring and summer. Health risks of PM2.5 elements (Cr, Mn, Ni, As, Ba and Pb) via the inhalation pathway were estimated. Mn (As) and As posed the highest noncarcinogenic and carcinogenic risks to human health, respectively. Overall, this work can provide more detailed information on the characteristics of elements for the science community and be used in other receptor modelling and health risk assessment studies.
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•Annual average concentrations of As and Cr exceeded the guideline values.•All studied elements showed higher concentrations on weekends than on weekdays.•Neighboring provinces are the major source regions of elements in PM2.5 in Beijing except Ni.•Controlling coal combustion could be a more effective way to protect public health.
Haze in China has been increasing in frequency of occurrence as well as the area of the affected region. Here, we report on a new mechanism of haze formation, in which coexistence with NOx can reduce ...the environmental capacity for SO2, leading to rapid conversion of SO2 to sulfate because NO2 and SO2 have a synergistic effect when they react on the surface of mineral dust. Monitoring data from five severe haze episodes in January of 2013 in the Beijing-Tianjin-Hebei regions agreed very well with the laboratory simulation. The combined air pollution of motor vehicle exhaust and coal-fired flue gases greatly reduced the atmospheric environmental capacity for SO2, and the formation of sulfate was found to be a main reason for the growth of fine particles, which led to the occurrence of haze. These results indicate that the impact of motor vehicle exhaust on the atmospheric environment might be underestimated.
Particulate matter (PM) pollution is a serious issue that has aroused great public attention in Beijing. To examine the seasonal characteristics of aerosols in typical pollution episodes, ...water-soluble inorganic ions (SO42−, NO3−, NH4+, Cl−, K+, Na+, Ca2+ and Mg2+) in size-segregated PM collected by an Anderson sampler (equipped with 50% effective cut-off diameters of 9.0, 5.8, 4.7, 3.3, 2.1, 1.1, 0.65, 0.43μm and an after filter) were investigated in four intensive campaigns from June 2013 to May 2014 in the Beijing urban area. Pronounced seasonal variation of TWSIs in fine particles (aerodynamic diameter less than 2.1μm) was observed, with the highest concentration in summer (71.5±36.3μg/m3) and the lowest in spring (28.1±15.2μg/m3). Different ion species presented different seasonal characteristics of mass concentration and size distribution, reflecting their different dominant sources. As the dominant component, SO42−, NO3− and NH4+ (SNA) in fine particles appeared to play an important role in the formation of high PM pollution since its contribution to the TWSIs and PM2.1 mass increased significantly during pollution episodes. Due to the hygroscopic growth and enhanced secondary formation in the droplet mode (0.65–2.1μm) from clean days to polluted days, the size distribution peak of SNA in the fine mode tended to shift from 0.43–0.65μm to 0.65–2.1μm. Relative humidity (RH) and temperature contributed to influence the secondary formation and regulate the size distributions of sulfates and nitrates. Partial correlation analysis found that high RH would promote the sulfur and nitrogen oxidation rates in the fine mode, while high temperature favored the sulfur oxidation rate in the condensation mode (0.43–0.65μm) and reduced the nitrogen oxidation rate in the droplet mode (0.65–2.1μm). The NO3−/SO42− mass ratio in PM2.1 (73% of the samples) exceeded 1.0, suggesting that vehicle exhaust currently makes a greater contribution to aerosol pollution than stationary sources in Beijing.
•Seasonal characteristics of size-segregated ions in pollution episodes were analyzed.•Contribution of droplet-mode secondary ions was clearly elevated on polluted days.•Relative humidity and temperature affect secondary formation of sulfate and nitrate.
Accurate determination of the atmospheric particulate matter mass concentration and chemical composition is helpful in exploring the causes and sources of atmospheric enthalpy pollution and in ...evaluating the rationality of environmental air quality control strategies. Based on the sampling and chemical composition data of PM
2.5
in different key regions of China in the CARE-China observation network, this research analyzes the environmental air quality data released by the China National Environmental Monitoring Centre during the studied period to determine the changes in the particulate matter mass concentration in key regions and the evolution of the corresponding chemical compositions during the implementation of the
Action Plan for Prevention and Control of Air Pollution
from 2013–2017. The results show the following. (1) The particulate matter mass concentration in China showed a significant downward trend; however, the PM
2.5
annual mass concentration in 64% of cities exceeds the New Chinese Ambient Air Quality Standard (CAAQS) Grade II (GB3095-2012). The region to the east of the Taihang Mountains, the Fenhe and Weihe River Plain and the Urumqi-Changji regions in Xinjiang, all have PM
2.5
concentration loading that is still high, and heavy haze pollution occurred frequently in the autumn and winter. (2) During the heavy pollution in the autumn and winter, the concentrations of sulfate and organic components decreased significantly. The mean
S
O
4
2
−
concentration in PM
2.5
decreased by 76%, 12%, 81% and 38% in Beijing-Tianjin-Hebei (BTH), the Pearl River Delta (PRD), the Sichuan-Chongqing region (SC) and the Fenhe and Weihe River Plain, respectively. The mean organic matter (OM) concentration decreased by 70%, 44%, 48% and 31%, respectively, and the mean concentration of
N
H
4
+
decreased by 68%, 1.6%, 38% and 25%, respectively. The mean elemental carbon (EC) concentration decreased by 84% and 20% in BTH and SC, respectively, and it increased by 61% and 11% in the PRD and Fenhe and Weihe River Plain, respectively. The mean concentration of mineral and unresolved chemical components (MI) dropped by 70%, 24% and 13% in BTH, the PRD and the Fenhe and Weihe River Plain, respectively. The change in the PM
2.5
chemical composition is consistent with the decrease of the PM
2.5
mass concentration. (3) In 2015, the mean OM concentration contributions to fine particles and coarse particles were 13–46% and 46–57%, respectively, and the mean MI concentration contributions to fine particles and coarse and particles were 31–60% and 39–73%, respectively; these values are lower than the 2013 values from the key regions, which is the most important factor behind the decrease of the particulate matter mass concentration. From 2013 to 2015, among the chemical components of different particle size fractions, the peak value of the coarse particle size fraction decreased significantly, and the fine particle size fractions of
S
O
4
2
−
,
N
O
4
−
,
a
n
d
N
H
4
+
decreased with the decrease of the particulate matter mass concentration in different particle size fractions. The fine-particle size peaks of
S
O
4
2
−
,
N
O
4
−
,
a
n
d
N
H
4
+
shifted from 0.65–1.1 μm to the finer size range of 0.43–0.65 μm during the same time frame.
We investigate the rapid formation and evolutionary mechanisms of an extremely severe and persistent haze episode that occurred in northern China during winter 2015 using comprehensive ground and ...vertical measurements, along with receptor and dispersion model analysis. Our results indicate that the life cycle of a severe winter haze episode typically consists of four stages: (1) rapid formation initiated by sudden changes in meteorological parameters and synchronous increases in most aerosol species, (2) persistent evolution with relatively constant variations in secondary inorganic aerosols and secondary organic aerosols, (3) further evolution associated with fog processing and significantly enhanced sulfate levels, and (4) clearing due to dry, cold north-northwesterly winds. Aerosol composition showed substantial changes during the formation and evolution of the haze episode but was generally dominated by regional secondary aerosols (53-67%). Our results demonstrate the important role of regional transport, largely from the southwest but also from the east, and of coal combustion emissions for winter haze formation in Beijing. Also, we observed an important downward mixing pathway during the severe haze in 2015 that can lead to rapid increases in certain aerosol species.
Organic carbon (OC) and elemental carbon (EC) in PM2.5 were measured hourly with a semicontinuous thermal-optical analyzer in urban Beijing, China, from Mar 1, 2013 to Feb 28, 2014. The annual mean ...concentrations of OC and EC in Beijing were 14.0 ± 11.7 and 4.1 ± 3.2 μg/m3, respectively. The concentrations observed in this study were lower than those of other reports over the past ten years; however, the concentrations were higher than those reported from most of the megacities in North America and Europe. These findings suggest that OC and EC remained at high levels despite the implementation of strict control measures to improve air quality. The OC and EC concentrations exhibited strong seasonality, with high values in the autumn and winter but low values in the spring and summer in Beijing. The diurnal OC and EC cycles were characterized by higher values at night and in the morning because of primary emissions, accumulations and low boundary-layer heights. Due to increasing photochemical activity, a well-defined OC peak was observed at approximately noon. The OC and EC concentrations followed typical lognormal patterns in which more than 75% of the OC samples had concentrations between 0.9 and 18.0 μg/m3 and 75% of the EC samples had concentrations between 0.4 and 5.6 μg/m3. An EC tracer method and combined EC tracer and K+ mass balance methods were used to estimate the contributions from secondary formation and biomass burning, respectively. High secondary organic carbon (SOC) concentrations were found in the autumn and winter due to low temperatures, which are favorable for the absorption and condensation of semi-volatile organic compounds on existing particles. High correlations were found between the estimated SOC in PM2.5 and the observed OOA (oxidized organic aerosol) in PM1; thus, the method proved to be effective and reliable. The annual average OCBiomass burning (OCbb) contribution to the total OC concentration was 18.4%, suggesting that biomass burning is a substantial pollution factor in Beijing.
•Semi-continuous measurements of carbonaceous aerosols were obtained in Beijing.•Seasonal, weekly and diurnal variations of OC and EC are reported.•Clean energy strategies resulted in an effective reduction of OC and EC.•High SOC concentrations were observed in autumn and winter.•Biomass burning emissions accounted for 18.4% of OC.
This study aimed to characterize PM2.5-bound trace elements in Northern Zhejiang Province (NZP), one of the most economically prosperous regions in China, and assess the associated health risks for ...the general populations. A year-long sampling campaign was conducted at four sites representative of urban, suburban, and rural areas of NZP. The average of the sum of twenty trace elements in PM2.5 was 2.8 ± 0.4 μg m−3, dominated by K, Al, Fe, Mg, Zn, and V (>100 ng m−3). The highest total elements’ concentration occurred in winter, followed by autumn, spring, and summer. Enrichment factors and principal component analysis (PCA) revealed that the major sources of trace elements in NZP were fossil fuel combustion, biomass burning, crustal dust, traffic, and industrial emissions. Elevated concentrations of certain elements reflected featured sources in different areas, e.g., V and Ni from heavy oil combustion in the port city, and Cu, Fe and Ba from traffic emissions in urban areas. Arsenic (As) represented the major non-cancer risk driver as its hazard quotient was 8.7. The cumulative cancer risk from all the carcinogenic elements was 1.7 × 10−3 in NZP, exceeding the upper limit (10−4) of the acceptable risk range. As and Cr contributed 33% and 66%, respectively, and thus were regarded as cancer risk drivers. The high health risks from PM2.5-bound elements warrant future actions to control their emissions in this region. Priorities should target industrial operations and coal combustion emissions, as informed by the risk drivers.
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•Trace element levels exhibited a seasonality of winter > autumn > spring > summer.•Elements were mostly anthropogenic except for Mg, Sr, and Fe.•Shipping emissions dominated the port city.•Arsenic was the non-cancer risk driver with a hazard quotient of 8.7.•The cumulative cancer risk was 1.7 × 10−3, with chromium and arsenic as drivers.
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