Elucidating severe urban haze formation in China

Compositions and pollutant sources of haze in Beijing urban sites

Haze from urban sites in Beijing was collected with a self-assembled electrostatic dust collector. The sizes and morphologies, thermal properties, and compositions of the particles in the haze were characterized by scanning electronic microscopy (SEM), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and X-ray photoelectric spectroscopy (XPS), respectively. Based on these results, the causes and pollutant sources of the chemicals in the haze were analyzed, and some countermeasures were further advanced to reduce the related pollutant sources. Graphical abstract ᅟ.

Critical role of meteorological conditions in a persistent haze episode in the Guanzhong basin, China

In the present study, the critical role of the meteorological condition in a persistent extreme haze episode that occurred in Guanzhong basin of China on December 16 to 25, 2013 has been investigated. Analyses of the large-scale meteorological conditions on 850hPa during the episode have been performed using the NCEP FNL data set, indicating that synoptic situations generally facilitate the accumulation of pollutants either in horizontal or vertical directions in the basin. The FLEXPART model has been utilized to illustrate the pollutant transport patterns during the episode, further showing the dominant role of synoptic conditions in accumulation of pollutants in the basin. Detailed meteorological conditions, such as temperature inversion, and low-level horizontal wind speed also contribute to the extreme haze episode. In addition, the WRF-CHEM model has been used to evaluate the responses of the surface PM2.5 level to the emission mitigation. Generally, the predicted PM2.5 spatial patterns and temporal variations agree well with the observations at the ambient monitoring sites. Sensitivity studies show that the emissions in the basin need to be mitigated by more than 91% to meet the excellent level of the China National Air Quality Standard under the extremely unfavorable meteorological conditions, demonstrating that it is imperative to implement stringent controls on emissions to improve the air quality.

Characterization of haze episodes and factors contributing to their formation using a panel model

A haze episode is a complex pollution process with high levels of fine particulate matter smaller than 2.5 μm (PM2.5). Understanding factors contributing to their formation is crucial to mitigate PM2.5 pollution, which varies substantially on the daily and city scales. In this study, we attempted to introduce the dynamic panel model that uses the group deviation method to generate unbiased estimates of contributions from different factors by eliminating time-invariant confounding variables. Taking 25 cities in the Yangtze Delta Region (YDR), China, as a case study and we analyzed how natural factors (e.g., wind) and anthropogenic emissions (e.g., sulfur dioxide (SO2)) together contribute to PM2.5 pollution. Results showed that there was significant lag effect on PM2.5 concentration, and approximately 45% of the PM2.5 remained from one day to the next. On the contrary, present day's emission had little effect on its PM2.5 concentration. It suggested that daily variation of PM2.5 concentration was largely affected by natural factors, while the long term PM2.5 pollution such as annual concentration was more determined by anthropogenic emissions. The unbiased estimates of this simple dynamic panel model could well predict the annual changes of PM2.5 concentration with an uncertainty of less than 2% on city scale. Reducing SO2 and nitrogen oxide (NOx) emissions could mitigate PM2.5 pollution to some extent in the YDR; however, to achieve the clean air standard, more pollutants such as ammonia should be added to the emission reduction list. The analyses provide an alternative method to easily quantify contributing factors and their variability to air pollution. It could be helpful to better understand the confounding factors on the assessment of air pollution governance despite the panel model still need to be improved on aspects such as long-range transportation.

Markedly enhanced absorption and direct radiative forcing of black carbon under polluted urban environments

Black carbon (BC) exerts profound impacts on air quality and climate because of its high absorption cross-section over a broad range of electromagnetic spectra, but the current results on absorption enhancement of BC particles during atmospheric aging remain conflicting. Here, we quantified the aging and variation in the optical properties of BC particles under ambient conditions in Beijing, China, and Houston, United States, using a novel environmental chamber approach. BC aging exhibits two distinct stages, i.e., initial transformation from a fractal to spherical morphology with little absorption variation and subsequent growth of fully compact particles with a large absorption enhancement. The timescales to achieve complete morphology modification and an absorption amplification factor of 2.4 for BC particles are estimated to be 2.3 h and 4.6 h, respectively, in Beijing, compared with 9 h and 18 h, respectively, in Houston. Our findings indicate that BC under polluted urban environments could play an essential role in pollution development and contribute importantly to large positive radiative forcing. The variation in direct radiative forcing is dependent on the rate and timescale of BC aging, with a clear distinction between urban cities in developed and developing countries, i.e., a higher climatic impact in more polluted environments. We suggest that mediation in BC emissions achieves a cobenefit in simultaneously controlling air pollution and protecting climate, especially for developing countries.

Hygroscopic Characteristics of Alkylaminium Carboxylate Aerosols

The hygroscopic growth factor (HGF) and cloud condensation nuclei (CCN) activity for a series of alkylaminium carboxylate aerosols have been measured using a hygroscopicity tandem differential mobility analyzer coupled to a condensation particle counter and a CCN counter. The particles, consisting of the mixtures of mono- (acetic, propanoic, p-toluic, and cis-pinonic acid) and dicarboxylic (oxalic, succinic, malic, adipic, and azelaic acid) acid with alkylamine (mono-, di-, and trimethylamines), represent those commonly found under diverse environmental conditions. The hygroscopicity parameter (κ) of the alkylaminium carboxylate aerosols was derived from the HGF and CCN results and theoretically calculated. The HGF at 90% RH is in the range of 1.3 to 1.8 for alkylaminium monocarboxylates and 1.1 to 2.2 for alkylaminium dicarboxylates, dependent on the molecular functionality (i.e., the carboxylic or OH functional group in organic acids and methyl substitution in alkylamines). The κ value for all alkylaminium carboxylates is in the range of 0.06-1.37 derived from the HGF measurements at 90% RH, 0.05-0.49 derived from the CCN measurements, and 0.22-0.66 theoretically calculated. The measured hygroscopicity of the alkylaminium carboxylates increases with decreasing acid to base ratio. The deliquescence point is apparent for several of the alkylaminium dicarboxylates but not for the alkylaminium monocarboxylates. Our results reveal that alkylaminium carboxylate aerosols exhibit distinct hygroscopic and deliquescent characteristics that are dependent on their molecular functionality, hence regulating their impacts on human health, air quality, and direct and indirect radiative forcing on climate.

"APEC Blue": Secondary Aerosol Reductions from Emission Controls in Beijing

China implemented strict emission control measures in Beijing and surrounding regions to ensure good air quality during the 2014 Asia-Pacific Economic Cooperation (APEC) summit. We conducted synchronous aerosol particle measurements with two aerosol mass spectrometers at different heights on a meteorological tower in urban Beijing to investigate the variations in particulate composition, sources and size distributions in response to emission controls. Our results show consistently large reductions in secondary inorganic aerosol (SIA) of 61–67% and 51–57%, and in secondary organic aerosol (SOA) of 55% and 37%, at 260 m and ground level, respectively, during the APEC summit. These changes were mainly caused by large reductions in accumulation mode particles and by suppression of the growth of SIA and SOA by a factor of 2–3, which led to blue sky days during APEC commonly referred to as “APEC Blue”. We propose a conceptual framework for the evolution of primary and secondary species and highlight the importance of regional atmospheric transport in the formation of severe pollution episodes in Beijing. Our results indicate that reducing the precursors of secondary aerosol over regional scales is crucial and effective in suppressing the formation of secondary particulates and mitigating PM pollution.

Characteristics of atmospheric single particles during haze periods in a typical urban area of Beijing: A case study in October, 2014

To investigate the composition and possible sources of particles, especially during heavy haze pollution, a single particle aerosol mass spectrometer (SPAMS) was deployed to measure the changes of single particle species and sizes during October of 2014, in Beijing. A total of 2,871,431 particles with both positive and negative spectra were collected and characterized in combination with the adaptive resonance theory neural network algorithm (ART-2a). Eight types of particles were classified: dust particles (dust, 8.1%), elemental carbon (EC, 29.0%), organic carbon (OC, 18.0%), EC and OC combined particles (ECOC, 9.5%), Na-K containing particles (NaK, 7.9%), K-containing particles (K, 21.8%), organic nitrogen and potassium containing particles (KCN, 2.3%), and metal-containing particles (metal, 3.6%). Three haze pollution events (P1, P2, P3) and one clean period (clean) were analyzed, based on the mass and number concentration of PM2.5 and the back trajectory results from the hybrid single particle Lagrangian integrated trajectory model (Hysplit-4 model). Results showed that EC, OC and K were the major components of single particles during the three haze pollution periods, which showed clearly increased ratios compared with those in the clean period. Results from the mixing state of secondary species of different types of particles showed that sulfate and nitrate were more readily mixed with carbon-containing particles during haze pollution episodes than in clean periods.

Semi-coke briquettes: towards reducing emissions of primary PM2.5, particulate carbon, and carbon monoxide from household coal combustion in China

Direct household use of unprocessed raw coals for cooking and heating without any air pollution control device has caused serious indoor and outdoor environment problems by emitting particulate matter (PM) and gaseous pollutants. This study examined household emission reduction by switching from unprocessed bituminous and anthracite coals to processed semi-coke briquettes. Two typical stoves were used to test emission characteristics when burning 20 raw coal samples commonly used in residential heating activities and 15 semi-coke briquette samples which were made from bituminous coals by industrial carbonization treatment. The carbonization treatment removes volatile compounds from raw coals which are the major precursors for PM formation and carbon emission. The average emission factors of primary PM2.5, elemental carbon, organic carbon, and carbon monoxide for the tested semi-coke briquettes are much lower than those of the tested raw coals. Based on the current coal consumption data in China, switching to semi-coke briquettes can reduce average emission factors of these species by about 92%, 98%, 91%, and 34%, respectively. Additionally, semi-coke briquette has relatively lower price and higher burnout ratio. The replacement of raw coals with semi-coke briquettes is a feasible path to reduce pollution emissions from household activities.

Regional contribution to PM1 pollution during winter haze in Yangtze River Delta, China

To quantify regional sources contributing to submicron particulate matter (PM1) pollution in haze episodes, on-line measurements combining two modeling methods, namely, positive matrix factorization (PMF) and backward Lagrangian particle dispersion modeling (LPDM), were conducted for the period of one month in urban Nanjing, a city located in the western part of Yangtze River Delta (YRD) region of China. Several multi-day haze episodes were observed in December 2013. Long-range transport of biomass burning from the southwestern YRD region largely contributed to PM1 pollution with more than 25% of total organics mass in a lasting heavy haze. The LPDM analysis indicates that regional transport is a main source contributing to secondary low-volatility production. The high-potential source regions of secondary low-volatility production are mainly located in areas to the northeast of the city. High aerosol pollution was mainly contributed by regional transport associated with northeastern air masses. Such regional transport on average accounts for 46% of total NR-PM1 with sulfate and aged low-volatility organics being the largest fractions (>65%).

Sources apportionment of PM2.5 in a background site in the North China Plain

To better understand the sources and potential source regions of PM2.5, a field study was conducted from January 2011 to November 2011 at a background site, the Yellow River Delta National Nature Reserve (YRDNNR) in the North China Plain. Positive matrix factorisation (PMF) analysis and a potential source contribution function (PSCF) model were used to assess the data, which showed that YRDNNR experienced serious air pollution. Concentrations of PM2.5 at YRDNNR were 71.2, 92.7, 97.1 and 62.5 μg m(-3) in spring, summer, autumn and winter, respectively, with 66.0% of the daily samples exhibiting higher concentrations of PM2.5 than the national air quality standard. PM2.5 mass closure showed remarkable seasonal variations. Sulphate, nitrate and ammonium were the dominant fractions of PM2.5 in summer (58.0%), whereas PM2.5 was characterized by a high load of organic aerosols (40.2%) in winter. PMF analysis indicated that secondary sulphate and nitrate (54.3%), biomass burning (15.8%), industry (10.7%), crustal matter (8.3%), vehicles (5.2%) and copper smelting (4.9%) were important sources of PM2.5 at YRDNNR on an annual average. The source of secondary sulphate and nitrate was probably industrial coal combustion. PSCF analysis indicated a significant regional impact on PM2.5 at YRDNNR all year round. Local emission may be non-negligible at YRDNNR in summer. The results of the present study provide a scientific basis for the development of PM2.5 control strategies on a regional scale.

Synergistic formation of sulfate and ammonium resulting from reaction between SO2 and NH3 on typical mineral dust

A synergistic effect between SO₂ and NH₃ on typical mineral dust.

Distinct potential aerosol masses under different scenarios of transport at a suburban site of Beijing

In order to evaluate the secondary aerosol formation potential at a suburban site of Beijing, in situ perturbation experiments in a potential aerosol mass (PAM) reactor were carried out in the winter of 2014. The variations of secondary aerosol formation as a function of time, OH exposure, and the concentrations of gas phase pollutants and particles were reported in this study. Two periods with distinct secondary aerosol formation potentials, marked as Period I and Period II, were identified during the observation. In Period I, the secondary aerosol formation potential was high, and correlated well to the air pollutants, i.e., SO2, NO2, and CO. The maximal secondary aerosol formation was observed with an aging time equivalent to about 3 days of atmospheric oxidation. In period II, the secondary aerosol formation potential was low, with no obvious correlation with the air pollutants. Meanwhile, the aerosol mass decreased, instead of showing a peak, with increasing aging time. Backward trajectory analysis during the two periods confirmed that the air mass in Period I was mainly from local sources, while it was attributed mostly to long distance transport in Period II. The air lost its reactivity during the long transport and the particles became highly aged, resulting in a low secondary aerosol formation potential. Our experimental results indicated that the in situ measurement of the secondary aerosol formation potential could provide important information for evaluating the contributions of local emission and long distance transport to the aerosol pollution.

Annual and diurnal variations of gaseous and particulate pollutants in 31 provincial capital cities based on in situ air quality monitoring data from China National Environmental Monitoring Center

Long-term air quality data with high temporal and spatial resolutions are needed to understand some important processes affecting the air quality and corresponding environmental and health effects. The annual and diurnal variations of each criteria pollutant including PM2.5 and PM10 (particulate matter with aerodynamic diameter less than 2.5 μm and 10 μm, respectively), CO (carbon monoxide), NO2 (nitrogen dioxide), SO2 (sulfur dioxide) and O3 (ozone) in 31 provincial capital cities between April 2014 and March 2015 were investigated by cluster analysis to evaluate current air pollution situations in China, and the cities were classified as severely, moderately, and slightly polluted cities according to the variations. The concentrations of air pollutants in winter months were significantly higher than those in other months with the exception of O3, and the cities with the highest CO and SO2 concentrations were located in northern China. The annual variation of PM2.5 concentrations in northern cities was bimodal with comparable peaks in October 2014 and January 2015, while that in southern China was unobvious with slightly high PM2.5 concentrations in winter months. The concentrations of particulate matter and trace gases from primary emissions (SO2 and CO) and NO2 were low in the afternoon (~16:00), while diurnal variation of O3 concentrations was opposite to that of other pollutants with the highest values in the afternoon. The most polluted cities were mainly located in North China Plain, while slightly polluted cities mostly focus on southern China and the cities with high altitude such as Lasa. This study provides a basis for the formulation of future urban air pollution control measures in China.

Characteristics of ammonia, acid gases, and PM2.5 for three typical land-use types in the North China Plain

Air pollution is one of the most serious environmental problems in China due to its rapid economic development alongside a very large consumption of fossil fuel, particularly in the North China Plain (NCP). During the period 2011-2014, we integrated active and passive sampling methods to perform continuous measurements of NH3, HNO3, NO2, and PM2.5 at two urban, one suburban, and two rural sites in the NCP. The annual average concentrations of NH3, NO2, and HNO3 across the five sites were in the ranges 8.5-23.0, 22.2-50.5, and 5.5-9.7 μg m(-3), respectively, showing no significant spatial differences for NH3 and HNO3 but significantly higher NO2 concentration at the urban sites. At each site, annual average concentrations of NH3 and NO2 showed increasing and decreasing trends, respectively, while there was no obvious trend in annual HNO3 concentrations. Daily PM2.5 concentrations ranged from 11.8 to 621.0 μg m(-3) at the urban site, from 19.8 to 692.9 μg m(-3) at the suburban site, and from 23.9 to 754.5 μg m(-3) at the two rural sites, with more than 70% of sampling days exceeding 75 μg m(-3). Concentrations of water-soluble ions in PM2.5 ranked differently between the non-rural and rural sites. The three dominant ions were NH4(+), NO3(-), and SO4(2-) and mainly existed as (NH4)2SO4, NH4HSO4, and NH4NO3, and their concentrations averaged 48.6 ± 44.9, 41.2 ± 40.8, and 49.6 ± 35.9 μg m(-3) at the urban, suburban, and rural sites, respectively. Ion balance calculations indicated that PM2.5 was neutral at the non-rural sites but acidic at the rural sites. Seasonal variations of the gases and aerosols exhibited different patterns, depending on source emission strength and meteorological conditions. Our results suggest that a feasible pathway to control PM2.5 pollution in the NCP should target ammonia and acid gases together.

Significant concentration changes of chemical components of PM1 in the Yangtze River Delta area of China and the implications for the formation mechanism of heavy haze-fog pollution

Since the winter season of 2013, a number of persistent haze-fog events have occurred in central-eastern China. Continuous measurements of the chemical and physical properties of PM1 at a regional background station in the Yangtze River Delta area of China from 16 Nov. to 18 Dec., 2013 revealed several haze-fog events, among which a heavy haze-fog event occurred between 6 Dec. and 8 Dec. The mean concentration of PM1 was 212μgm(-3) in the heavy haze-fog period, which was about 10 times higher than on clean days and featured a peak mass concentration that reached 298μgm(-3). Organics were the largest contributor to the dramatic rise of PM1 on heavy haze-fog days (average mass concentration of 86μgm(-3)), followed by nitrate (58μgm(-3)), sulfate (35μgm(-3)), ammonium (29μgm(-3)), and chloride (4.0μgm(-3)). Nitrate exhibited the largest increase (~20 factors), associated with a significant increase in NOx. This was mainly attributable to increased coal combustion emissions, relative to motor vehicle emissions, and was caused by short-distance pollutant transport within surrounding areas. Low-volatility oxidized organic aerosols (OA) (LV-OOA) and biomass-burning OA (BBOA) also increased sharply on heavy haze-fog days, exhibiting an enhanced oxidation capacity of the atmosphere and increased emissions from biomass burning. The strengthening of the oxidation capacity during the heavy pollution episode, along with lower solar radiation, was probably due to increased biomass burning, which were important precursors of O3. The prevailing meteorological conditions, including low wind and high relative humidity, and short distance transported gaseous and particulate matter surrounding of the sampling site, coincided with the increased pollutant concentrations mainly from biomass-burning mentioned above to cause the persistent haze-fog event in the YRD area.

Characteristics of On-road Diesel Vehicles: Black Carbon Emissions in Chinese Cities Based on Portable Emissions Measurement

Black carbon (BC) emissions from heavy-duty diesel vehicles (HDDVs) are rarely continuously measured using portable emission measurement systems (PEMSs). In this study, we utilize a PEMS to obtain real-world BC emission profiles for 25 HDDVs in China. The average fuel-based BC emissions of HDDVs certified according to Euro II, III, IV, and V standards are 2224 ± 251, 612 ± 740, 453 ± 584, and 152 ± 3 mg kg(-1), respectively. Notably, HDDVs adopting mechanical pump engines had significantly higher BC emissions than those equipped with electronic injection engines. Applying the useful features of PEMSs, we can relate instantaneous BC emissions to driving conditions using an operating mode binning methodology, and the average emission rates for Euro II to Euro IV diesel trucks can be constructed. From a macroscopic perspective, we observe that average speed is a significant factor affecting BC emissions and is well correlated with distance-based emissions (R(2) = 0.71). Therefore, the average fuel-based and distance-based BC emissions on congested roads are 40 and 125% higher than those on freeways. These results should be taken into consideration in future emission inventory studies.

Daily Estimation of Ground-Level PM2.5 Concentrations over Beijing Using 3 km Resolution MODIS AOD

Estimating exposures to PM2.5 within urban areas requires surface PM2.5 concentrations at high temporal and spatial resolutions. We developed a mixed effects model to derive daily estimations of surface PM2.5 levels in Beijing, using the 3 km resolution satellite aerosol optical depth (AOD) calibrated daily by the newly available high-density surface measurements. The mixed effects model accounts for daily variations of AOD-PM2.5 relationships and shows good performance in model predictions (R(2) of 0.81-0.83) and cross-validations (R(2) of 0.75-0.79). Satellite derived population-weighted mean PM2.5 for Beijing was 51.2 μg/m(3) over the study period (Mar 2013 to Apr 2014), 46% higher than China's annual-mean PM2.5 standard of 35 μg/m(3). We estimated that more than 19.2 million people (98% of Beijing's population) are exposed to harmful level of long-term PM2.5 pollution. During 25% of the days with model data, the population-weighted mean PM2.5 exceeded China's daily PM2.5 standard of 75 μg/m(3). Predicted high-resolution daily PM2.5 maps are useful to identify pollution "hot spots" and estimate short- and long-term exposure. We further demonstrated that a good calibration of the satellite data requires a relatively large number of ground-level PM2.5 monitoring sites and more are still needed in Beijing.

Fine particulate matter (PM 2.5) in China at a city level

This study presents one of the first long term datasets including a statistical summary of PM_2.5 concentrations obtained from one-year monitoring in 190 cities in China. We found only 25 out of 190 cities could meet the National Ambient Air Quality Standards of China, and the population-weighted mean of PM_2.5 in Chinese cities are 61 μg/m³, ~3 times as high as global population-weighted mean, highlighting a high health risk. PM_2.5 concentrations are generally higher in north than in south regions due to relative large PM emissions and unfavorable meteorological conditions for pollution dispersion. A remarkable seasonal variability of PM_2.5 is observed with the highest during the winter and the lowest during the summer. Due to the enhanced contributions from dust particles and open biomass burning, high PM_2.5 abundances are also found in the spring (in Northwest and West Central China) and autumn (in East China), respectively. In addition, we found the lowest and highest PM_2.5 often occurs in the afternoon and evening hours, respectively, associated with daily variation of the boundary layer depth and anthropogenic emissions. The diurnal distribution of the PM_2.5-to-CO ratio consistently displays a pronounced peak during the afternoon periods, reflecting a significant contribution of secondary PM formation.

Assessing Beijing's PM 2.5 pollution: severity, weather impact, APEC and winter heating

By learning the PM 2.5 readings and meteorological records from 2010–2015, the severity of PM 2.5 pollution in Beijing is quantified with a set of statistical measures. As PM 2.5 concentration is highly influenced by meteorological conditions, we propose a statistical approach to adjust PM 2.5 concentration with respect to meteorological conditions, which can be used to monitor PM 2.5 pollution in a location. The adjusted monthly averages and percentiles are employed to test if the PM 2.5 levels in Beijing have been lowered since China's State Council set up a pollution reduction target. The results of the testing reveal significant increases, rather than decreases, in the PM 2.5 concentrations in the years 2013 and 2014 as compared with those in year 2012. We conduct analyses on two quasi-experiments—the Asia-Pacific Economic Cooperation meeting in November 2014 and the annual winter heating—to gain insight into the impacts of emissions on PM 2.5 . The analyses lead to a conclusion that a fundamental shift from mainly coal-based energy consumption to much greener alternatives in Beijing and the surrounding North China Plain is the key to solving the PM 2.5 problem in Beijing.

Real-Time Characterization of Aerosol Particle Composition above the Urban Canopy in Beijing: Insights into the Interactions between the Atmospheric Boundary Layer and Aerosol Chemistry

Despite extensive efforts into the characterization of air pollution during the past decade, real-time characterization of aerosol particle composition above the urban canopy in the megacity Beijing has never been performed to date. Here we conducted the first simultaneous real-time measurements of aerosol composition at two different heights at the same location in urban Beijing from December 19, 2013 to January 2, 2014. The nonrefractory submicron aerosol (NR-PM1) species were measured in situ by a high-resolution aerosol mass spectrometer at near-ground level and an aerosol chemical speciation monitor at 260 m on a 325 m meteorological tower in Beijing. Secondary aerosol showed similar temporal variations between ground level and 260 m, whereas much weaker correlations were found for the primary aerosol. The diurnal evolution of the ratios and correlations of aerosol species between 260 m and the ground level further illustrated a complex interaction between vertical mixing processes and local source emissions on aerosol chemistry in the atmospheric boundary layer. As a result, the aerosol compositions at the two heights were substantially different. Organic aerosol (OA), mainly composed of primary OA (62%), at the ground level showed a higher contribution to NR-PM1 (65%) than at 260 m (54%), whereas a higher concentration and contribution (15%) of nitrate was observed at 260 m, probably due to the favorable gas-particle partitioning under lower temperature conditions. In addition, two different boundary layer structures were observed, each interacting differently with the evolution processes of aerosol chemistry.

Relationships between meteorological parameters and criteria air pollutants in three megacities in China

Meteorological conditions play a crucial role in ambient air pollution by affecting both directly and indirectly the emissions, transport, formation, and deposition of air pollutants. In this study, the relationships between meteorological parameters and ambient air pollutants concentrations in three megacities in China, Beijing, Shanghai, and Guangzhou were investigated. A systematic analysis of air pollutants including PM2.5, PM10, CO, SO2, NO2, and O3 and meteorological parameters including temperature, wind speed (WS), wind direction (WD) and relative humanity (RH) was conducted for a continuous period of 12 months from March 2013 to February 2014. The results show that all three cities experienced severe air quality problems. Clear seasonal trends were observed for PM2.5, PM10, CO, SO2 and NO2 with the maximum concentrations in the winter and the minimum in the summer, while O3 exhibited an opposite trend. Substantially different correlations between air pollutants and meteorological parameters were observed among these three cities. WS reversely correlated with air pollutants, and temperature positively correlated with O3. Easterly wind led to the highest PM2.5 concentrations in Beijing, westerly wind led to high PM2.5 concentrations in Shanghai, while northern wind blew air parcels with the highest PM2.5 concentrations to Guangzhou. In Beijing, days of top 10% PM2.5, PM10, CO, and NO2 concentrations were with higher RH compared to days of bottom 10% concentrations, and SO2 and O3 showed no distinct RH dependencies. In Guangzhou, days of top 10% PM2.5, PM10, CO, SO2, NO2 and O3 concentrations were with lower RH compared to days of bottom 10% concentrations. Shanghai showed less fluctuation in RH between top and bottom 10%. These results confirm the important role of meteorological parameters in air pollution formation with large variations in different seasons and geological areas. These findings can be utilized to improve the understanding of the mechanisms that produce air pollution, enhance the forecast accuracy of the air pollution under different meteorological conditions, and provide effective measures for mitigating the pollution.

Regionally-varying combustion sources of the January 2013 severe haze events over eastern China

Thick haze plagued northeastern China in January 2013, strongly affecting both regional climate and human respiratory health. Here, we present dual carbon isotope constrained (Δ(14)C and δ(13)C) source apportionment for combustion-derived black carbon aerosol (BC) for three key hotspot regions (megacities): North China Plain (NCP, Beijing), the Yangtze River Delta (YRD, Shanghai), and the Pearl River Delta (PRD, Guangzhou) for January 2013. BC, here quantified as elemental carbon (EC), is one of the most health-detrimental components of PM2.5 and a strong climate warming agent. The results show that these severe haze events were equally affected (∼ 30%) by biomass combustion in all three regions, whereas the sources of the dominant fossil fuel component was dramatically different between north and south. In the NCP region, coal combustion accounted for 66% (46-74%, 95% C.I.) of the EC, whereas, in the YRD and PRD regions, liquid fossil fuel combustion (e.g., traffic) stood for 46% (18-66%) and 58% (38-68%), respectively. Taken together, these findings suggest the need for a regionally-specific description of BC sources in climate models and regionally-tailored mitigation to combat severe air pollution events in East Asia.