Search

The Gradient Richardson Number (Ri) is an important parameter for appraising the stability and turbulence exchange at the atmospheric boundary layer (ABL). However, high-resolution measurements of Ri profiles are rarely reported, especially in megacities. In this study, a Doppler wind lidar and a microwave radiometer were simultaneously utilized to measure the 2 km Ri vertical profile in downtown Beijing. These measurements were verified to have high accuracy compared with observations from a 325 m meteorological tower, with root-mean-square errors (RMSEs) of less than 1.66 K, 7.9%, and 1.45 m/s for the temperature, relative humidity, and wind speed (WS) for all altitudes and corresponding Pearson correlation coefficients (R) of 0.97, 0.93, and 0.81. The inter-comparisons of different spatial (25 m, 50 m, 100 m) and temporal resolutions (1 min, 30 min, 1 h) form a 3 × 3 resolution matrix of Ri, in which the 1 h temporal resolution of Ri overestimates the intensity and active area of turbulence. The Ri value retrieved from the 100 m spatial resolution data overestimates these by half as it misidentifies the height of the stable area at the near surface. There are significant differences between the data with a 1 min temporal resolution and a 25 m spatial resolution (defined as the standard resolution of Ri), and the rest of the data in the resolution matrix (defined as data at other resolutions), with an RMSE > 1 and an R < 0.8. The difference between data at the standard resolution and data at other resolutions increases with elevations, which results from frequent weather processes or from water-vapor blocking at higher altitudes. The Ri profiles reveal that the atmospheric layer at altitudes from 100 m to 500 m in daytime is unstable, with Ri < 0, while it is neutral, with 0 < Ri < 0.25, at night-time from 200 m to 400 m. The atmosphere above the ABL in a megacity is rather stable, with Ri > 0.25, whereas below the ABL, it is neutral or unstable, which is due to drastic changes in the WS and temperature that are affected by the topography and surface friction. [ABSTRACT FROM AUTHOR]

The vertical distribution of black carbon (BC), as well as its mixing state, is of great concern due to BC's strong regional climatic and environmental effects. In this study, vertical measurements were conducted through a moveable container based on a meteorological tower in the Beijing urban area during June and July. A total of 112 vertical profiles (0–240 m), including the concentrations of BC, O 3 , NO x and the optical properties of aerosols, were obtained. Based on BC concentration, the vertical profiles could be classified into four categories: uniform, gradual decrease, sharp decrease and sudden increase. The uniform type indicates strong vertical mixing with similar pollutant concentrations along the vertical direction. The gradual and sharp decrease types indicate stable vertical conditions with higher pollutant concentrations on the ground and lower concentrations at higher altitudes. Due to the strong radiation in summer, the vertical profiles exhibited a clear diurnal variation in which ∼ 80 % of profiles were uniform during the daytime and ∼ 40 %–90 % of profiles were of the gradual and sharp decrease types at night. O 3 is an exception, and its concentration generally increases with height, even under strong vertical mixing conditions. The size distribution of the BC core varied slightly along the vertical direction, and the coating thickness, denoted by the diameter ratio between the BC-containing particle and BC core (Dp/Dc), of BC increased with height under stable conditions. Although the coating thickness could increase the absorption ability with an average absorption enhancement of 1.25 at 23:00 LT (local time: UTC + 8), the vertical difference of Dp/Dc (2 %) was much lower than that of BC concentration (∼ 35 %). The vertical variation in absorption ability was mainly caused by the variation in BC concentration. In addition, O 3 and Dp/Dc occasionally increased during 06:00–08:00 but remained stable during 08:00–10:00. Vertical mixing and transportation from upper heights, such as the residual layer, could significantly influence the pollutant properties on the surface during early mornings. This study exhibits a continuous vertical picture of BC and its mixing state in urban areas, which would be helpful for understanding BC's regional environmental effect. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) is a global phenomenon that significantly influences climate. NPF also contributes to haze, with pronounced negative impacts on human health. Theory and observations both show that nucleation is favored during clean days and inhibited during haze episodes due to a high pre‐existing condensation sink (CS). Here we show that the surprising occurrence of NPF during haze days in Beijing is associated with a high concentration of sulfuric acid dimers. With both field observations and model simulations, we demonstrate that downward mixing of sulfur dioxide (SO2) from the residual layer aloft enhances ground level SO2, which in turn elevates sulfuric acid dimer after rapid SO2 oxidation in the polluted air. Our results address a key gap between the source of SO2 and its atmospheric oxidation products during haze conditions in a megacity, Beijing, providing a missing link in a complete chain describing NPF in the polluted atmosphere. Plain Language Summary: New particle formation is a global phenomenon with notable effects on climate and health. Recent studies have argued that atmospheric nucleation during haze episodes with high aerosol loading should not occur. In spite of this, we have observed intensive nucleation events during haze episodes in Beijing and found an unexpected high concentration of sulfur dioxide (SO2), along with a decreased condensation sink (CS) owing to the development of boundary layer. The SO2 originates from emissions by tall stacks outside of Beijing and is subsequently transported downward from the morning residual layer to the surface, this contributes to peaks of SO2 during the day. A decreased loss of sulfuric acid dimer owing to a decreased CS drives nucleation during haze episodes. As the phenomenon of a later morning SO2 peak exactly coincide with nucleation has been widely reported in eastern China, we highlight the importance of emissions of SO2 from tall smokestacks in atmospheric nucleation events. Key Points: Sulfuric acid dimer is involved in new particle formation (NPF) events during haze episodes in BeijingField observation and model simulation confirm downward mixing of sulfur dioxide from residual layerHigh formation rate of sulfuric acid dimer owing to a decreased condensation sink promotes NPF during haze [ABSTRACT FROM AUTHOR]

HONO concentrations in Beijing (BJ) and Sanmenxia (SMX) were simultaneously measured in winter with a duration of 1 month, and the sources and sinks of HONO in the two cities were comparably analyzed. BJ and SMX had different pollution characteristics. Direct vehicle emission made a contribution to observed HONO of about 28% in BJ, whereas it contributed to HONO only about 12% in SMX. Additionally, direct emission from coal combustion in SMX also made a significant contribution to atmospheric HONO, which could achieve to be 13%. In BJ, nighttime NO2 conversion on the aerosol and ground surfaces contributed to HONO sources of 15% and 27%, respectively. In SMX, nighttime NO2 conversion on the aerosol and ground surfaces contributed to HONO sources of 40% and 30%, respectively. Daytime NO2 heterogeneous conversion (including photo‐enhanced conversion) on the aerosol and ground surfaces was important in SMX. Daytime NO3− photolysis contributed 9% HONO in SMX and 4% in BJ. The primary OH production rates from photolysis of daytime HONO in both BJ and SMX were three orders of magnitude faster than those from photolysis of O3, revealing the dominant role of HONO in local atmospheric chemistry of the two cities in winter. Plain Language Summary: Gaseous nitrous acid (HONO), an important source of OH in polluted regions within the boundary layer, remains ambiguous on its formation mechanisms. Here, we chose two sites, Beijing (BJ, a city in the North China Plain) and Sanmenxia (SMX, a city in the Fenwei Plain) in China, for simultaneous measurement. In BJ and SMX, HONO sources mainly came from vehicle emission, homogeneous reaction, and NO2 heterogeneous conversion. Homogeneous reaction was an important HONO source during daytime at two sites. Direct vehicle emission contributed more to observed HONO in BJ than that in SMX. Besides, direct emission from coal combustion in SMX also significantly contributed to atmospheric HONO while BJ was not. In SMX, higher RH and PM2.5 can increase the NO2 uptake coefficient on the aerosol surface. Heterogeneous formation of HONO was more important on the aerosol surface than on the ground surface in SMX while BJ was on the contrary. The primary OH production rates from photolysis of daytime HONO in both BJ and SMX were higher than those from photolysis of O3. Key Points: Simultaneous measurement was carried out in Beijing and Sanmenxia with detailed observation dataThe most important HONO source was homogeneous reaction in the daytime and NO2 heterogeneous reactions in the nighttime at two sitesHONO from coal combustion and NO2 heterogeneous reaction on the aerosol surface was important in Sanmenxia [ABSTRACT FROM AUTHOR]

To investigate the impacts of multiscale circulations on the planetary boundary layer (PBL), we have carried out the PBL thermodynamic structure field experiment with a Doppler wind profile lidar, a microwave radiometer, and a ceilometer from January 2018 to December 2019 in Beijing. We found that the direct regulatory effect of synoptic circulation worked through transporting and accumulating pollutants in front of mountains in the daytime, while the indirect effect of multiscale circulations worked through coupling mechanisms in the nighttime. The horizontal coupling of different-direction winds produced a severe pollution convergent zone. The vertical coupling of upper environmental winds and lower regional breezes regulated the mixing and diffusion of pollutants by generating dynamic wind shear and advective temperature inversion. We also found that the dominating synoptic circulations led to great differences in PBL thermodynamic structure and pollution. The cyclonic circulation resulted in a typical multilayer PBL characterized by high vertical shear (600 m), temperature inversion (900 m), and an inhomogeneous stratification. Meanwhile, strong regional breezes pushed the pollution convergent zone to the south of Beijing. The southwesterly circulation resulted in a mono-layer PBL characterized by low vertical shear (400 m) and inversion (200 m). The westerly circulation led to a hybrid-structure PBL, and the advective inversion was generated by the vertical shear of zonal winds. Strong environmental winds of southwesterly and westerly circulations pushed the severe-pollution zone to the front of mountains. There was no distinct PBL structure under the anticyclone circulation. The study systematically revealed the appreciable effects of synoptic and regional circulations on PBL structure and air quality, which enriched the prediction theory of atmospheric pollution in the complex terrain. [ABSTRACT FROM AUTHOR]

Under a high atmospheric oxidization capacity, the synergistic effect of the physicochemical processes in the atmospheric boundary layer (ABL) caused summer haze pollution in Beijing. The southern and southwestern areas, generally 60–300 km away from Beijing, were seriously polluted in contrast to Beijing, which remained clean. Southerly winds moving faster than 20–30 km h -1 since the early morning primarily caused haze pollution initiation. The PM 2.5 (particulate matter with a dynamic equivalent diameter smaller than 2.5 µm) level increased to 75 µgm-3 over several hours during the daytime, which was simultaneously affected by the ABL structure. Additionally, the O3 concentration was quite high during the daytime (250 µgm-3), corresponding to a high atmospheric oxidation capacity. Much sulfate and nitrate were produced through active atmospheric chemical processes, with sulfur oxidation ratios (SORs) up to ∼0.76 and nitrogen oxidation ratios (NORs) increasing from 0.09 to 0.26, which further facilitated particulate matter (PM) level enhancement. However, the increase in sulfate was mainly linked to southerly transport. At midnight, the PM 2.5 concentration sharply increased from 75 to 150 µgm-3 over 4 h and remained at its highest level until the next morning. Under an extremely stable ABL structure, secondary aerosol formation dominated by nitrate was quite intense, driving the haze pollution outbreak. The PM levels in the southern and southeastern areas of Beijing were significantly lower than those in Beijing at this time, even below air quality standards; thus, the contribution of pollution transport had almost disappeared. With the formation of a nocturnal stable boundary layer (NSBL) at an altitude ranging from 0–0.3 km, the extremely low turbulence kinetic energy (TKE) ranging from 0 to 0.05 m 2 s -2 inhibited the spread of particles and moisture, ultimately resulting in elevated near-surface PM 2.5 and relative humidity (∼90 %) levels. Due to the very high humidity and ambient oxidization capacity, NOR rapidly increased from 0.26 to 0.60, and heterogeneous hydrolysis reactions at the moist particle surface were very notable. The nitrate concentration steeply increased from 11.6 to 57.8 µgm-3 , while the sulfate and organics concentrations slightly increased by 6.1 and 3.1 µgm-3 , respectively. With clean and strong winds passing through Beijing, the stable ABL dissipated with the potential temperature gradient becoming negative and the ABL height (ABLH) increasing to ∼2.5 km. The high turbulence activity with a TKE ranging from 3 to 5 m 2 s -2 notably promoted pollution diffusion. The self-cleaning capacity of the atmosphere is commonly responsible for air pollution dispersion. However, reducing the atmospheric oxidization capacity, through strengthening collaborative control of nitrogen oxide (NOx) and volatile organic compounds (VOCs), as well as continuously deepening regional joint air pollution control, is urgent. [ABSTRACT FROM AUTHOR]

Severe wintertime PM 2.5 pollution in Beijing has been receiving increasing worldwide attention, yet the decadal variations remain relatively unexplored. Combining field measurements and model simulations, we quantified the relative influences of anthropogenic emissions and meteorological conditions on PM 2.5 concentrations in Beijing over the winters of 2002–2016. Between the winters of 2011 and 2016, stringent emission control measures resulted in a 21 % decrease in mean mass concentrations of PM 2.5 in Beijing, with 7 fewer haze days per winter on average. Given the overestimation of PM 2.5 by the model, the effectiveness of stringent emission control measures might have been slightly overstated. With fixed emissions, meteorological conditions over the study period would have led to an increase in haze in Beijing, but the strict emission control measures have suppressed the unfavorable influences of the recent climate. The unfavorable meteorological conditions are attributed to the weakening of the East Asia winter monsoon associated particularly with an increase in pressure associated with the Aleutian Low. [ABSTRACT FROM AUTHOR]

Although much effort has been put into studying air pollution, our knowledge of the mechanisms of frequently occurring intense haze episodes in China is still limited. In this study, using 3 years of measurements of air pollutants at three different height levels on a 325 m Beijing meteorology tower, we found that a positive aerosol–boundary layer feedback mechanism existed at three vertical observation heights during intense haze polluted periods within the mixing layer. This feedback was characterized by a higher loading of PM 2.5 with a shallower mixing layer. Modelling results indicated that the presence of PM 2.5 within the boundary layer led to reduced surface temperature, relative humidity and mixing layer height during an intensive haze episode. Measurements showed that the aerosol–boundary layer feedback was related to the decrease in solar radiation, turbulent kinetic energy and thereby suppression of the mixing layer. The feedback mechanism can explain the rapid formation of intense haze episodes to some extent, and we suggest that the detailed feedback mechanism warrants further investigation from both model simulations and field observations. [ABSTRACT FROM AUTHOR]

As major chemical components of airborne fine particulate matter (PM 2.5), organic carbon (OC) and elemental carbon (EC) have vital impacts on air quality, climate change, and human health. Because OC and EC are closely associated with fuel combustion, it is helpful for the scientific community and policymakers assessing the efficacy of air pollution control measures to study the impact of control measures and regional transport on OC and EC levels. In this study, hourly mass concentrations of OC and EC associated with PM 2.5 were semi-continuously measured from March 2013 to February 2018. The results showed that annual mean OC and EC concentrations declined from 14.0 to 7.7 µgm-3 and from 4.0 to 2.6 µgm-3 , respectively, from March 2013 to February 2018. In combination with the data of OC and EC in previous studies, an obvious decreasing trend in OC and EC concentrations was found, which was caused by clean energy policies and effective air pollution control measures. However, no obvious change in the ratios of OC and EC to the PM 2.5 mass (on average, 0.164 and 0.049, respectively) was recorded, suggesting that inorganic ions still contributed a lot to PM 2.5. Based on the seasonal variations in OC and EC, it appeared that higher OC and EC concentrations were still observed in the winter months, with the exception of winter of 2017–2018. Traffic policies executed in Beijing resulted in nighttime peaks of OC and EC, caused by heavy-duty vehicles and heavy-duty diesel vehicles being permitted to operate from 00:00 to 06:00 (China standard time, UTC + 8, for all times throughout the paper). In addition, the fact that there was no traffic restriction in weekends led to higher concentrations on weekends compared to weekdays. Significant correlations between OC and EC were observed throughout the study period, suggesting that OC and EC originated from common emission sources, such as exhaust of vehicles and fuel combustion. OC and EC levels increased with enhanced SO2 , CO, and NOx concentrations while the O3 and OC levels were enhanced simultaneously when O3 concentrations were higher than 50 µgm-3. Non-parametric wind regression analysis was performed to examine the sources of OC and EC in the Beijing area. It was found that there were distinct hot spots in the northeast wind sector at wind speeds of approximately 0–6 km h -1 , as well as diffuse signals in the southwestern wind sectors. Source areas further away from Beijing were assessed by potential source contribution function (PSCF) analysis. A high-potential source area was precisely pinpointed, which was located in the northwestern and southern areas of Beijing in 2017 instead of solely in the southern areas of Beijing in 2013. This work shows that improvement of the air quality in Beijing benefits from strict control measures; however, joint prevention and control of regional air pollution in the regions is needed for further improving the air quality. The results provide a reference for controlling air pollution caused by rapid economic development in developing countries. [ABSTRACT FROM AUTHOR]

The detailed formation mechanism of an increased number of haze events in China is still not very clear. Here, we found that reduced surface visibility from 1980 to 2010 and an increase in satellite-derived columnar concentrations of inorganic precursors from 2002 to 2012 are connected with each other. Typically, higher inorganic mass fractions lead to increased aerosol water uptake and light-scattering ability in elevated relative humidity. Satellite observation of aerosol precursors of NO2 and SO2 showed increased concentrations during the study period. Our in situ measurement of aerosol chemical composition in Beijing also confirmed increased contribution of inorganic aerosol fraction as a function of the increased particle pollution level. Our investigations demonstrate that the increased inorganic fraction in the aerosol particles is a key component in the frequently occurring haze days during the study period, and particularly the reduction of nitrate, sulfate and their precursor gases would contribute towards better visibility in China. [ABSTRACT FROM AUTHOR]

The planetary boundary layer height (PBLH) is one of the most important parameters in the environmental, weather, and climatic research. Therefore, it is of great significance and application value to accurately estimate the PBLH by using the available conventional reanalysis meteorological datasets. This study established a hybrid machine learning (HML) method by combining three different ensemble algorithms with parallel training to estimate the urban PBLH of Beijing with ERA5 reanalysis data. The Mean Absolute Percentage Error (MAPE) between the ERA5-BLH and the observed BLH is as high as 68%, whereas it decreased to 10% for the PBLH estimated by the HML model. The radiation-related thermal factors of surface temperature and dew point temperature in ERA5, play a critical role in summer in regulating the PBLH, while the dynamic factors of wind speed and pressure dominate in the other seasons. The MAPE for all the seasons decreases by 0.3–1.6% after introducing the measured temperature and humidity profiles. Shapley Additive Explanations (SHAP) method shows that higher RH contributes less for PBLH estimated in spring, while 2 m temperature is positively correlated with HML performance in summer. Finally, an optimal HML model of PBLH is synthesized as the contribution of meteorological elements. The MAPE drops to 5.2%–15.1% throughout the year. The Mean Absolute Error (MAE) is <50 m in autumn and winter, and the maximum MAE is up to 80 m in the summer afternoon, when the convection is intensely developed. Therefore, the HML is capable of accurately estimating the urban PBLH in high resolution, which provides great significances and references for the investigations regarding the atmospheric environment capacity, as well as for advancing weather forecasting. [Display omitted] • An optimal Hybrid ML model is proposed for Beijing urban PBLH estimation. • The HML-based estimates are nearly 60% higher compared to ERA5 PBLH. • Thermal factors dominate PBLH estimation in summer and dynamic factors prevail in other seasons. • Machine learning model is promising to improve the accuracy of EAR5 PBLH even further. [ABSTRACT FROM AUTHOR]

Particulate pollution has remained at a high level in the megacity of Beijing in the past decade. The PM2.5, PM10, aerosol optical depth (AOD), Angstrom exponent(α), and PM2.5/PM10 ratio (the proportion of PM2.5 in PM10) in Beijing were 70±6 μg m −3 , 128±6 μg m −3 , 0.57 ± 0.05, 1.10 ± 0.08, 45 ± 4%, respectively, from 2005 to 2014. The annual means of PM concentration, AOD, α, and PM2.5/PM10 ratio decreased slightly during this decade, meanwhile PM concentration increased in the winter. Furthermore, we found there were thresholds of PM2.5 concentration for aerosol extinction. When the PM concentration was lower than a certain threshold, AOD decreased quickly with the decline of PM concentration. To make the improvement of the particle pollution more noticeable, the PM concentration should be controlled under the threshold. The annual averaged threshold is 63 μg m −3 , and the threshold values reached the maximum of 74 μg m −3 in spring, ranged from 54 to 56 μg m −3 in the three other seasons. The threshold values ranged from 55 to 77 μg m −3 under other relevant factors, including air masses directions and relative humidity. [ABSTRACT FROM AUTHOR]

The interaction of aerosols and the planetary boundary layer (PBL) plays an important role in deteriorating urban air quality. Aerosols from different sources may have different effects on regulating PBL structures owing to their distinctive dominant compositions and vertical distributions. To characterize the complex feedback of aerosols on PBL over the Beijing megacity, multiple approaches, including in situ observations in the autumn and winter of 2016–2019, backward trajectory clusters, and large-eddy simulations, were adopted. The results revealed notable distinctions in aerosol properties, vertical distributions and thermal stratifications among three types of air masses from the West Siberian Plain (Type-1), Central Siberian Plateau (Type-2) and Mongolian Plateau (Type-3). Low loadings of 0.28 ± 0.26 and 0.15 ± 0.08 of aerosol optical depth (AOD) appeared in the Type-1 and Type-2, accompanied by cool and less stable stratification, with a large part (80%) of aerosols concentrated below 1500 m. For Type-3, the AOD and single scattering albedo (SSA) were as high as 0.75 ± 0.54 and 0.91 ± 0.05, demonstrating severe pollution levels of abundant scattering aerosols. Eighty percent of the aerosols were constrained within a lower height of 1150 m owing to the warmer and more stable environment. Large-eddy simulations revealed that aerosols consistently suppressed the daytime convective boundary layer regardless of their origins, with the PBL height (PBLH) decreasing from 1120 m (Type-1), 1160 m (Type-2) and 820 m (Type-3) in the ideal clean scenarios to 980 m, 1100 m and 600 m, respectively, under polluted conditions. Therefore, the promotion of absorbing aerosols below the residual layer on PBL could be greatly hindered by the suppression effects generated by both absorbing aerosols in the upper temperature inversion layer and scattering aerosols. Moreover, the results indicated the possible complexities of aerosol-PBL interactions under future emission-reduction scenarios and in other urban regions. [Display omitted] • Aerosol profiles in Siberian air masses are similar but distributed at higher height. • PBL in Mongolian-plateau air mass is warmer and stabler than these from Siberia. • Aerosols consistently suppress the daytime PBL regardless of their origins. • Promotion of inner-PBL absorption aerosols on PBL is hindered by suppression effect. [ABSTRACT FROM AUTHOR]

With a decrease in fine particulate matter (PM 2.5) due to the implementation of air pollution prevention-control measures, ozone (O 3) has risen in China especially in urban areas recently. It becomes more and more important to control air combined pollution with high concentration of PM 2.5 and O 3. We analyzed the characteristics of air combined pollution in Beijing from 2014 to 2019 in this paper. Results indicated that the trough O 3 showed an upward trend of 4.5 μg m−3 yr−1, while the peak decreased by −1.1 μg m−3 yr−1. There were three typical atmospheric chemical stages in the interaction between O 3 and PM 2.5 in summer through observations. The first was an intense photochemical stage with the synchronous and rapid growth of O 3 and PM 2.5. The O 3 concentration varied from 68 μg m−3 to 206 μg m−3 when PM 2.5 ≤ 35 μg m−3. While secondary inorganic ions (SNA) ranged from 2 to 21 μg m−3 especially SO 4 2− (0.8–12 μg m−3). The second was an aerosol chemical generation stage with the rapid increase of PM 2.5 and SNA under high O 3 concentration. O 3 was maintained at a high concentration between 198 μg m−3 and 241 μg m−3 when PM 2.5 was within the range of 35–125 μg m−3. SNA increased from 12 μg m−3 to 96 μg m−3. The third was a high PM 2.5 pollution stage suppressing the generation of O 3 when PM 2.5 ≥ 125 μg m−3. O 3 began to decline with the growth of PM 2.5 and decreased from 224 μg m−3 to 130 μg m−3. Ultraviolet radiation (UV) decreased from 13 MJ m−3 in the first to 8 MJ m−3 in this stage. [Display omitted] • The O 3 trough increased by 4.5 μg/m3 and the peak decreased by −1.1 μg/m3 per year in Beijing. • There were three typical processes in the interaction between O 3 and PM 2.5 in summer day. • The photochemical process was intense in dry-sunny days with the synchronous growth of O 3 and PM 2.5. • Aerosol chemical process intensified with a rapid growth of PM 2.5 and SNA in high O 3. • O 3 began to decline with the growth of PM 2.5 when PM 2.5 attenuated UV in a high level. [ABSTRACT FROM AUTHOR]

Ground-based and satellite observations indicated heavy anthropogenic aerosol loading over the Beijing metropolitan area from 2004 to 2011. The aerosol optical properties and radiative forcing had strong seasonal variations and large differences in the city, the suburban and the regional background. The annual means of the aerosol optical depth (AOD) at 550nm wavelength were 0.40±0.04 in the Beijing City, 0.51±0.05 in the Xianghe suburban and 0.20±0.02 in the background, while the annual means of Ångström exponent were 1.28±0.07, 1.16±0.06 and 0.99±0.05, respectively. The annual means of single scattering albedo (SSA) and aerosol direct radiative forcing at the top of atmosphere (TOA) at 550nm wavelength were 0.89±0.09 and −0.013±0.002W/m2 in the Beijing City, 0.87±0.09 and −0.004±0.003W/m2 in the Xianghe suburban, and 0.88±0.08 and 0.002±0.001W/m2 in the background. The seasonal maximums of TOA were −0.007±0.001W/m2, 0.012±0.003W/m2 and 0.013±0.003W/m2 in Beijing City, the suburb and the background in spring, respectively. The seasonal minimums of TOA were −0.022±0.003W/m2, −0.014±0.001W/m2 and −0.010±0.002W/m2 in winter. There were no significant weakening trend of the aerosol direct radiative forcing in the large urban area in the period. [ABSTRACT FROM AUTHOR]

Recent epidemiological and toxicological studies have shown associations between particulate matter and human health. However, the estimates of adverse health effects are inconsistent across many countries and areas. The stratification and interaction models were employed within the context of the generalized additive Poisson regression equation to examine the acute effects of fine particles on respiratory health and to explore the possible joint modification of temperature, humidity, and season in Beijing, China, for the period 2004-2009. The results revealed that the respiratory health damage threshold of the PM concentration was mainly within the range of 20-60 μg/m, and the adverse effect of excessively high PM concentration maintained a stable level. In the most serious case, an increase of 10 μg/m PM results in an elevation of 4.60 % (95 % CI 3.84-4.60 %) and 4.48 % (95 % CI 3.53-5.41 %) with a lag of 3 days, values far higher than the average level of 0.69 % (95 % CI 0.54-0.85 %) and 1.32 % (95 % CI 1.02-1.61 %) for respiratory mortality and morbidity, respectively. There were strong seasonal patterns of adverse effects with the seasonal variation of temperature and humidity. The growth rates of respiratory mortality and morbidity were highest in winter. And, they increased 1.4 and 1.8 times in winter, greater than in the full year as PM increased 10 μg/m. [ABSTRACT FROM AUTHOR]

The Atmospheric Environmental Monitoring Network successfully undertook the task of monitoring the atmospheric quality of Beijing and its surrounding area during the 2008 Olympics. The results of this monitoring show that high concentrations of PM pollution exhibited a regional pattern during the monitoring period (1 June-30 October 2008). The PM mass concentrations were 53 μg m, 66 μg m, and 82 μg m at the background site, in Beijing, and in the Beijing-Tianjin-Hebei urban agglomerations, respectively. The PM levels were lowest during the 2008 Olympic Games (8-24 August): 35 μg m at the background site, 42 μg m in Beijing and 57 μg m in the region. These levels represent decreases of 49%, 48%, and 56%, respectively, compared to the prophase mean concentration before the Olympic Games. Emission control measures contributed 62%-82% of the declines observed in Beijing, and meteorological conditions represented 18%-38%. The concentration of fine particles met the goals set for a 'Green Olympics.' [ABSTRACT FROM AUTHOR]

The Beijing-Tianjin-Hebei Atmospheric Environment Monitoring Network was established by the Institute of Atmospheric Physics, Chinese Academy of Sciences. The goals of the network were to monitor and provide warnings of the atmospheric quality in Beijing and its surrounding area during the Beijing 2008 Olympic Games. The results showed that the atmospheric complex pollution exhibited high concentrations of ozone and fine particles and oxidation in summer, with a ubiquitous regional source. The regional mean concentrations of SO2, PM2.5, NO2, and O3&lowbar;8h max (the maximum daily 8 h mean) and Ox were 22±11, 90±40, 25±5, 136±35 and 112±21 µg/m³ in summer, respectively. During the Olympic Games, the mean concentration of SO2, PM2.5, NO2, O3&lowbar;8h max, and Ox were 12.5±4, 56±28, 23±4, 114±29, 95±17 µg/m in the region, respectively, and fell by 51.0%, 43.7%, 13%, 20.2%, and 18.9%, respectively, compared to the prophase mean before the Olympic Games. The concentration of atmospheric pollutants declined significantly and achieved the "Green Olympics" control goal of air quality. After the Olympic Games, SO2, PM2.5 and NOx increased significantly as the temporary atmospheric pollution control measures were terminated. [ABSTRACT FROM AUTHOR]

The spatial and temporal distribution of atmospheric chloromethane (CH 3 Cl) in China was investigated between 2012 and 2014, and the sources of CH 3 Cl were evaluated by positive matrix factorization (PMF) according to PMF 5.0 (USEPA). Various concentrations of CH 3 Cl were found throughout China, ranging from 506.4 ± 110.5 to 1132.1 ± 523.3 pptv, showing the characteristics of high in the southeast and low in the northwest. The correlation between CH 3 Cl and chlorofluorocarbon-12 (CFC-12) at most sites was greater than 0.5 and presented regional characteristics. The distribution of CH 3 Cl in China is related to the degree of economic development, the high-concentration sites were located in the economically developed eastern region. The CH 3 Cl concentrations at western sites were similar to the global average and increased with latitude and altitude, indicating that this region was least affected by anthropogenic activities. The annual average concentrations of atmospheric CH 3 Cl in the northeastern region were below 730 pptv but had strong seasonal fluctuations. Chemical production was the main CH 3 Cl source in the northeast background areas (46%), while industrial combustion contributed 42% of the CH 3 Cl in the northeast urban areas. Large seasonal variations combined with a high relative standard deviation were observed in the southern region. Relative to the southwest, in the southeast, the onset of lower concentrations was delayed until September–October, and higher concentrations persisted for longer. Chemical production and biomass burning were the main sources in the southwest, while Chemical production and solvent use were the main contributors in the southeast. The Beijing area is discussed separately as a super megacity. No obvious seasonal fluctuations were observed in the Beijing area. According to the source analysis, chemical production contributed approximately 60% of the atmospheric CH 3 Cl in the Beijing area, while 15–24% was from solvent release. • High concentrations of CH 3 Cl are concentrated in the central and eastern regions in China. • Chemical production is the main source of atmospheric CH 3 Cl in China. • Industrial combustion is the second source of the CH 3 Cl in the northeast. • Biomass burning and solvent use are important CH 3 Cl contributors in the southern region. [ABSTRACT FROM AUTHOR]

Long-term CO 2 and PM 2.5 measurements in urban areas have important impacts on understanding the roles of urbanization in climate change and air pollution. From 2009 to 2017, CO 2 fluxes were measured by the eddy covariance (EC) system at a height of 140 m on the Beijing Meteorological Tower. The CO 2 fluxes followed a typical two-peak diurnal pattern all year round. The PM 2. 5 concentrations followed a similar diurnal pattern as the CO 2 fluxes in summer but a different diurnal pattern in winter (low in the day and high at night). On a seasonal time scale, both the CO 2 fluxes and the PM 2. 5 concentrations showed a pronounced seasonal variation (high in winter and low in summer). The spatial variations in CO 2 fluxes were dominated by the prevailing land use types within the flux footprint, particularly dense residential areas and heavy traffic roads. On both diurnal and annual time scales, the urban underlying surface was a net source of CO 2. The 9-year average annual total CO 2 flux was 36.4 kg CO 2 ·m−2 yr−1. Depending on the yearly prevailing wind direction, the effect of the heterogeneity correction on the annual total CO 2 fluxes based on the gap-filled dataset could reach up to 3.5%. Over the 9-year period, both the CO 2 fluxes and the PM 2.5 concentrations exhibited a declining interannual trend, and CO 2 fluxes could account for 64% of the interannual variability in PM 2.5 concentrations. In summer, emissions were more likely to control the interannual variability in PM 2.5 concentrations, whereas in winter, meteorological conditions had a greater impact on the interannual variability in PM 2.5 concentrations. Image 1 • The urban underlying surface in Beijing is a net source of CO 2 on multiple timescales. • The effect of the heterogeneity correction on the annual total CO 2 fluxes is within 5%. • Both the CO 2 fluxes and the PM 2.5 concentrations show a declining interannual trend. • There are huge potential co-benefits in carbon mitigation and air pollution abatement. [ABSTRACT FROM AUTHOR]

The high-frequency monitoring of three-dimensional wind fields is crucial in planetary boundary layer meteorology. Doppler wind lidar and meteorological towers are the most important instruments for site observations of three-dimensional wind fields. This study systematically investigated and compared the performances of three wind measurement instruments: A Doppler wind lidar (Windcube 100s), cup anemometer/wind vane and sonic wind anemometer mounted on the 325 m meteorological tower in the polluted urban city of Beijing. The horizontal wind speed measurements of the Doppler wind lidar closely matched those of the cup anemometer and the sonic wind anemometer with high coefficients of determination (R2: 0.79–0.96 and 0.90–0.97, respectively). Moreover, the results also showed good agreement between the three measurements of the prevailing horizontal wind direction. Conversely, there were weak correlations between the vertical wind speeds of the Doppler wind lidar and sonic wind anemometer with low coefficients of determination (R2: 0.30–0.46). With increasing temporal scale, the consistency in the vertical wind increased. In addition, the Doppler wind lidar seemed to correlate better with the sonic wind anemometer at heights exceeding 300 m (R2: 0.48–0.77). Note that there was a remarkable difference between the Doppler wind lidar and 325 m meteorological tower observations as the aerosol concentrations changed rapidly. Different wind measurement instruments have unique advantages and are thus irreplaceable. The Doppler wind lidar is better at measuring larger turbulent eddies. [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosols in the atmosphere play a significant role in climate systems due to their strong ability to absorb solar radiation. The lifetime of BC depends on atmospheric transport, aging and consequently on wet scavenging processes (in-cloud and below-cloud scavenging). In this study, sequential rainwater samples in eight rainfall events collected in 2 mm interval were measured by a tandem system including a single particle soot photometer (SP2) and a nebulizer. The results showed that the volume-weighted average (VWA) mass concentrations of refractory black carbon (rBC) in each rainfall event varied, ranging from 10.8 to 78.9 µg/L. The highest rBC concentrations in the rainwater samples typically occurred in the first fraction from individual rainfall events. The geometric mean median mass-equivalent diameter (MMD) decreased under precipitation, indicating that r BC with larger sizes was relatively aged and preferentially removed by wet scavenging. A positive correlation (R 2 = 0.73) between the VWA mass concentrations of r BC in rainwater and that in ambient air suggested the important contribution of scavenging process. Additionally, the contributions of in-cloud and below-cloud scavenging were distinguished and accounted for 74% and 26% to wet scavenging, respectively. The scavenging ratio of rBC particles was estimated to be 0.06 on average. This study provides helpful information for better understanding the mechanism of rBC wet scavenging and reducing the uncertainty of numerical simulations of the climate effects of rBC. [Display omitted] [ABSTRACT FROM AUTHOR]

The contribution of relative humidity (RH) on Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) was investigated based on in-situ aircraft and ground observations of aerosol and RH from 2007 to 2010 in Beijing, China. The profiles of dry aerosol particle number and size distributions with diameter ranging from 0.1 to 3.0 μm as well as RH were observed during aircraft measurement. The calculated AOD based on aircraft observed dry aerosol particles is lower than MODIS AOD under high RH, but consistent with MODIS AOD after accounting for the aerosol hygroscopic growth effect. A comparison of ground PM 2.5 and MODIS AOD further shows that the ratio of PM 2.5 concentration to AOD decreases with RH. These results collectively suggest that a strong aerosol hygroscopic growth effect on AOD. The averaged contribution of RH (hygroscopic growth) on AOD are estimated as 37% and 22% respectively based on in-situ aircraft and ground measurements, and hence aerosol concentration would be overestimated if the aerosol hygroscopic growth effect is neglected. As a result, it should be cautious with the applications of AOD in studying the complex relationships between aerosol and its climatic effects. [ABSTRACT FROM AUTHOR]

The implementation of emission reduction measures during the Olympics provided a valuable opportunity to study regional photochemical pollution over northern China. In this study, the fifth-generation Pennsylvania State University/National Centre for Atmospheric Research Mesoscale Model and Community Multiscale Air Quality model system was applied to conduct two sets of modelling analyses of the period from July 20 to September 20, 2008, to illustrate the influences of emission reduction measures on regional photochemical pollution over northern China during the Beijing Olympics. The results indicated that the implementation of emission control measures decreased the concentrations of ozone (O 3 ) precursors, namely nitrogen oxide (NOx) and volatile organic compounds (VOCs), throughout the boundary layer. The concentrations of these compounds were reduced by 45% in the central urban area of Beijing at the ground level. Although the average O 3 concentration in the central urban area increased by more than 8 ppbv, the total oxidant concentration decreased significantly by more than 5 ppbv. Greater O 3 concentrations mainly occurred during periods with weak photochemical reactions. During periods of strong photochemical production, the O 3 concentration decreased significantly due to a weakening vertical circulation between the lower and upper boundary layer. Consequently, the number of days when the O 3 concentration exceeded 100 ppbv decreased by 25% in Beijing. The emission control measures altered the sensitivity of the regional O 3 production. The coordinated control region of NOx and VOCs expanded, and the control region of VOCs decreased in size. The reduction of non-point-source emissions, such as fugitive VOCs and vehicles, was more useful for controlling regional photochemical pollution over northern China. [ABSTRACT FROM AUTHOR]

Several air pollution episodes occurred in Beijing before and after the 2014 Asia-Pacific Economic Cooperation (APEC) summit, during which air-pollution control measures were implemented. Within this autumn-winter transit season, domestic heating started. Such interesting period merits comprehensive chemical characterization, particularly the organic species, to look into the influence of additional heating sources and the control measures on air pollution. Therefore, this study performed daily and 6h time resolved PM 2.5 sampling from the 24th October to 7th December, 2014, followed by comprehensive chemical analyses including water-soluble ions, elements and organic source-markers. Apparent alterations of chemical profiles were observed with the initiation of domestic heating. Through positive matrix factorization (PMF) source apportionment modeling, six PM 2.5 sources including secondary inorganic aerosol (SIA), traffic emission, coal combustion, industry emission, biomass burning and dust were separated and identified. Coal combustion was successfully distinguished from traffic emission by hopane diagnostic ratio. The result of this study reveals a gradual shift of dominating sources for PM pollution episodes from SIA to primary sources after starting heating. BaP eq toxicity from coal combustion increased on average by several to dozens of times in the heating period, causing both long-term and short-term health risk. Air mass trajectory analysis highlights the regional influence of the industry emissions from the area south to Beijing. Control measures taken during APEC were found to be effective for reducing industry source, but less effective in reducing the overall PM 2.5 level. These results provide implications for policy making regarding appropriate air pollution control measures. [Display omitted] • Source apportionment of PM 2.5 including organic source tracers in Beijing. • PM 2.5 composition and source changes with regard to domestic heating identified. • Effects of source strength and meteorological changes on PM pollution revealed. • Large increase of BaP eq toxicity in heating period and its main source identified. • Control measure taken during APEC evaluated with new perspective. [ABSTRACT FROM AUTHOR]

Scientifically and efficiently ensuring good air quality for important events is an issue of concern to the government. In addition to analysis based on historical data, advanced prediction before an event is essential for the government having ample time to take effective actions to improve air quality during the event period. Taking "the 2022 Olympic Winter Games (OWG)" as a typical case, a chemical transport model coupled with a tracer-tagged module was used to evaluate the air quality and source apportionment of ambient pollutants in the OWG host cities under historical and predicted meteorological conditions. Driven by the downscaling of meteorological fields from an operational real-time climate forecast system, the potential ability of air quality forecasting three months ahead was investigated, which was meaningful for designing control strategies. Sensitive simulations indicated that under unfavorable meteorological conditions, such as those during February 2014, both Beijing and Zhangjiakou faced a high risk of experiencing haze episodes, even based on current anthropogenic emission intensity. The contribution of the joint prevention and control region to Beijing and Zhangjiakou would become larger under worse meteorological conditions, which favor heavy air pollution. The source apportionment results indicated that strengthened emission control in cities including Beijing, Zhangjiakou and south of Beijing (Baoding, Langfang, Tianjin, and Tangshan) is effective for reducing haze episodes in the host cities. There is still a long way to make accurate daily fine particulate matter predictions on a seasonal-scale in advance; however, it could capture the trends in air quality in host cities around the OWG period three months ahead. The comparison of observations and predictions confirmed and highlighted the role of regional emission controls on the realization of the "OWG blue". • Regional transport became larger under conditions favoring heavy air pollution. • The trend of air quality was captured by seasonal-scale model forecasting. • Joint emission control measures ensured the realization of the "OWG blue". [ABSTRACT FROM AUTHOR]

The aerosol scattering coefficient is mainly affected by various factors, such as aerosol chemical composition, hygroscopicity and relative humidity. A comprehensive observation of the chemical and physical properties of aerosols was conducted in Xianghe, southeast of Beijing, from June 15 to 26, 2018. Five specific cases were analysed during this observation: (1) rainy (RA); (2) low visibility during the day (LD); (3) high visibility during the day (HD); (4) low visibility at night (LN); and (5) high visibility at night (HN). The mass percentages of a secondary inorganic aerosol (nitrate, sulfate and ammonium) measured with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) were 64.5% and 68.3% during LD and LN, which were higher than those (63.6% and 46.1%) observed during HD and HN, respectively. The mass percentages of secondary organic aerosol (SOA) and primary organic aerosol (POA) to organic aerosol were 76.6% and 23.4%, respectively, during LD. The hygroscopic parameters κ were 0.59 and 0.60 during LD and LN, which were slightly higher than those (0.57 and 0.58) observed during HD and HN, respectively, indicating that the variation of aerosol chemical composition had a limited impact on hygroscopicity. The average proportion, measured with a single particle aerosol mass spectrometer (SPAMS), of biomass burning, dust, heavy metal, soot-like, sea salt, secondary aerosols and other particles were 12.3%, 2.9%, 16.2%, 36.1%, 8.8%, 21.0% and 2.7%. The average proportions of soot-like and secondary aerosols were 42.4% and 19.2% from 0.2 to 1.0 μm. Although the variation and height of the mixing layer were similar between LD and HD, clouds reduced the solar radiation during LD, which caused a higher relative humidity (RH) (67.5%) than that (45.9%) observed during HD. A larger wind speed below 850 hpa during HN caused the mixing layer height (MLH) to reach 1910 m, which resulted in a lower RH (64.8%) than that (68.0%) observed during LN. The light scattering enhancement factors f(RH) during low visibility events were almost greater than those observed during high visibility events, regardless of they were measured during the day (0–9.7%) or night (0–13.1%). The aerosol hygroscopic growth factor (GF) maintained a similar trend with RH, indicating that RH was the main factor affecting the hygroscopic growth of the aerosol, resulting in the decrease in visibility. • The hygroscopic parameter κ is calculated by HR-ToF-AMS. • Soot-like and secondary aerosols are the main sources identified by SPAMS. • The hygroscopicity of aerosol is compared under different visibility conditions. [ABSTRACT FROM AUTHOR]

Abstract Carbonaceous aerosols in high emission areas attract worldwide attention of the scientific community and the public due to their adverse impacts on the environment, human health and climate. However, long-term continuous hourly measurements are scarce on the regional scale. In this study, a one-year hourly measurement (from December 1, 2016 to November 30, 2017) of organic carbon (OC) and elemental carbon (EC) in airborne fine particles was performed using semi-continuous OC/EC analyzers in Beijing, Tianjin, Shijiazhuang and Tangshan in the Beijing-Tianjin-Hebei (BTH) region in China, which is one of high emission areas in China, even in the world. Marked spatiotemporal variations were observed. The highest concentrations of OC (22.8 ± 30.6 μg/m3) and EC (5.4 ± 6.5 μg/m3) occurred in Shijiangzhuang while the lowest concentrations of OC (11.0 ± 10.7 μg/m3) and EC (3.1 ± 3.6 μg/m3) were obtained in Beijing and Tianjin, respectively. Pronounced monthly, seasonal and diurnal variations of OC and EC were recorded. Compared to published data from the past two decades for the BTH region, our OC and EC levels were lower, implying some effect of recent measures for improving the air quality. Significant correlations of OC versus EC (p < 0.001) were found throughout the study period with high slopes and correlation coefficients in winter, but low slopes and correlation coefficients in summer. The estimated secondary OC (SOC), based on the minimum R squared (MRS) method, represented 29%, 47%, 38% and 48% of the OC for Beijing, Tianjin, Shijiazhuang and Tangshan, respectively. These percentages are larger than previous ones obtained for the BTH region in the past decade. There were obvious differences in the potential source regions of OC and EC among the four cities. Obvious prominent potential source areas of OC and EC were observed for Beijing, which were mainly located in the central and western areas of Inner Mongolia and even extended to the Mongolian regions, which is different from the findings in previous studies. For all sites, adjacent areas of the main provinces in northern China were found to be important potential source areas. Highlights • A one-year hourly measurement of OC and EC in PM2.5 aerosol was performed in Beijing, Tianjin, Shijiazhuang and Tangshan. • There were marked spatiotemporal variations, with highest levels in Shijiangzhuang and lowest ones in Beijing and Tianjin. • Pronounced monthly, seasonal and diurnal variations of OC and EC were observed. • Secondary OC (SOC) was estimated based on the minimum R squared (MRS) method. • There were obvious differences in the potential source regions of OC and EC among the four cities. [ABSTRACT FROM AUTHOR]

Ozone (O 3) pollution deteriorated in recent years in Beijing; however, the evolution characteristics and formation mechanism of O 3 pollution events (OPEs) lack comprehensive exploration from a long-term perspective. Based on 35-site (Beijing) and multi-city (North China) O 3 and meteorological monitoring and reanalysis data, totally 61 OPEs during 2013–2020 were counted in Beijing to reveal their characteristics, the relationship with regional pollution, and meteorological causes. Mild, moderate, and heavy pollution in all OPEs (203 ± 28 μg/m3 for averaged MDA8 O 3) accounted for 68.4%, 27.8%, and 3.8% of 291 days, respectively. 78.7% of OPEs were concentrated in May–July. The averaged MDA8 O 3 concentration, total days and durations of OPEs increased notably from 2013 to 2020, with a rate of 1.6 μg/m3/yr, 2.7 d/yr, and 0.2 d/yr, respectively. Overall, 90% (53%) of OPEs in Beijing co-occurred with regional pollution covering Beijing-Tianjin-Hebei (North China) under the predominant southerly airflow. Meteorological causes of different levels and spatial distribution of OPEs were explored through multi-scale weather processes. The increasing proportion of southwesterly-westerly synoptic circulation (49%–80%) and mountain-valley breeze (51%–70%), along with enhanced temperature, UV radiation, southerly wind, and temperature inversion and decreased relative humidity, collectively contributed to elevated levels of OPEs. The aforementioned meteorological condition facilitated O 3 photochemical reactions, the transport of pollutants, and hindered diffusion. Three spatial types (urban, northern and southern suburban pollution) of OPEs were determined by the intensity and direction of southerly wind. Additionally, daytime O 3 pollution elevated the mean peak O 3 of nocturnal O 3 enhancement events (NOEs) by 31 μg/m3 (41%) mainly induced by convective storms and low-level jets. Our comprehensive investigations of OPEs provide important scientific basis for the precise prediction and control of O 3 pollution in Beijing. [Display omitted] • OPEs in Beijing co-occur with regional pollution and elevated the peak of NOEs. • Southwesterly-westerly synoptic circulations and mountain-valley breeze enhanced OPEs. • Levels of OPEs increased with enhanced temperature and inversion, radiation, and southerly wind. [ABSTRACT FROM AUTHOR]

Organic carbon (OC) and elemental carbon (EC) in the PM 2.5 of urban Beijing were measured hourly with a semi-continuous thermal-optical analyzer from Jan 1, 2013 to Dec 31, 2014. The annual average OC and EC concentrations in Beijing were 17.0 ± 12.4 and 3.4 ± 2.0 μg/m 3 for 2013, and 16.8 ± 14.5 and 3.5 ± 2.9 μg/m 3 for 2014. It is obvious that the annual average concentrations of OC and EC in 2014 were not less than those in 2013 while the annual average PM 2.5 concentration (89.4 μg/m 3 ) in 2014 was slightly reduced as compared to that (96.9 μg/m 3 ) in 2013. Strong seasonality of the OC and EC concentrations were found with high values during the heating seasons and low values during the non-heating seasons. The diurnal cycles of OC and EC characterized by higher values at night and in the morning were caused by primary emissions, secondary transformation and stable meteorological condition. Due to increasing photochemical activity, the OC peaks were observed at approximately noon. No clear weekend effects were observed. Interestingly, in the early mornings on weekends in the autumn and winter, the OC and EC concentrations were close to or higher than those on weekdays. Our data also indicate that high OC and EC concentrations were closely associated with their potential source areas which were determined based on the potential source contribution function analysis. High potential source areas were identified and were mainly located in the south of Beijing and the plain of northern China. A much denser source region was recorded in the winter than in the other seasons, indicating that local and regional transport over regional scales are the most important. These results demonstrate that both regional transport from the southern regions and local accumulation could lead to the enhancements of OC and EC and likely contribute to the severe haze pollution in Beijing. [ABSTRACT FROM AUTHOR]

Size-segregated water-soluble ionic species (WSIs) were measured using an Anderson cascade impactor from Jul. to Aug. 2008 and from Dec. 2009 to Feb. 2010 in urban Beijing. The results showed that fine particles (PM 2.1 , Dp < 2.1 μm) accounted for ∼49% (summer) and ∼34% (winter) of the total particulate mass, and WSIs accounted for 23–82% of the mass concentration of PM 2.1 . Secondary inorganic aerosols (SIAs, the sum of SO 4 2− , NO 3 − and NH 4 + ) accounted for more than 30% of the fine particles, which were greatly elevated during particle pollution events (PM events), thereby leading to an alteration of the size distributions of SO 4 2− and NO 3 − to nearly single fine-mode distributions peaking at 0.65–2.1 μm. This finding suggests that heterogeneous aqueous reactions were enhanced at high RH values. SIAs also increased during dust events, particularly for coarse mode SO 4 2− , which indicated enhanced heterogeneous reactions on the dust surface. The positive matrix factorization (PMF) model was used to resolve the bulk mass size distributions into condensation, droplet, and coarse modes, representing the three major sources of the particles. The formation of SO 4 2− was attributed primarily to in-cloud or aerosol droplet processes during summer (45%), and the heterogeneous reaction of SO 2 on mineral dust surfaces was an important formation pathway during winter (45%). The formation pathways of NO 3 − in fine particles were similar to those of SO 4 2− , where over 30% were formed by in-cloud processes. This work provides important field measurement-based evidence for understanding the formation pathway of secondary inorganic aerosols in the megacity of Beijing. [ABSTRACT FROM AUTHOR]

Atmospheric loading of black carbon aerosols influences air quality and visibility, and their mixing state and aerosol chemistry were sensitive to both emission scenarios and regional transport. In this study, the aerodynamic size-resolved mixing state of refractory black carbon aerosols (rBC) was investigated using a novel tandem system consisting of an aerodynamic aerosol classifier (AAC), a single particle soot photometer (SP2), and single-particle-aerosol mass spectrometry (SPA-MS) during the XXIV Olympic Winter Games (OWG) at an urban site in Beijing. Particles with aerodynamic diameter (D ae) of 200 nm and 300 nm were selected by AAC to focus on the effect of relative differences in morphology on the coating thickness, coating compositions, and optical properties of rBC particles. We found that rBC-containing particles with D ae of 300 nm were characterized by a rBC core with a count median diameter (CMD) of 108 ± 4 nm that typically have regular morphology (χ : m e a n ∼ 1) , higher relative coating thickness (RCT: mean ∼1.9), and strong light absorption enhancement (E abs : mean ∼1.5). The rBC-containing particles with D ae = 200 nm normally had irregular shapes (χ : m e a n ∼ 1.4) and weak E abs (E abs : mean ∼1.37). Classification based on air mass clustering and air pollution level revealed that the coating thickness of rBC particles only exhibited unimodal patterns under clean air conditions and distinct bimodal distributions under heavy pollution conditions, implying multiple origins of rBC particles, even during OWG periods. The coating thickness and E abs of rBC particles were greater when influenced by the higher RH air from east. Furthermore, the chemical compositions of the rBC coating quantitatively determined by SPA-MS were used to estimate the possible origins of the D ae = 300 nm, featured by a thicker coating thickness and stronger light absorption enhancement. Our results showed that rBC particles were primarily mixed with organics, nitrate, and sulfate during the XXIV OWG period. The organic components have a limited role in increasing rBC coatings under polluted conditions, while the alkali metal ions (K and Na) associated with traffic emissions and the secondary inorganic species favor the formation of thick coatings. Higher RH makes limited contributions to rBC mixed with sulfate and organics only (BCOC*_S) and pure BC. The relatively fresh BC could directly mix with organics and sulfate at low RH levels while evolving to mix with nitrate at high RH conditions. In addition to high concentrations of locally emitted precursors and high RH that tend to form thick coatings of nitrate-related rBC aerosols, secondary transport from the west and southwest also contributes to rBC aerosols mixed with nitrate with relatively thick coatings. • rBC with an aerodynamic diameter (D ae) of 300 nm typically have regular morphology. • rBC of regular shape(D ae = 300 nm)has thicker coatings and stronger absorption enhancement. • During the OWG, rBC particles were primarily mixed with organics, nitrate and sulfate. • rBC particles had multiple sources, even in the OWG periods. • The organic components have a limited role in increasing rBC coatings under polluted conditions. [ABSTRACT FROM AUTHOR]

Extensive studies have linked PM 2.5 and PM 10 with respiratory diseases (RD). However, few is known about causal association between PM 1 and morbidity of RD. We aimed to assess the causal effects of PM 1 on cause-specific RD. Hospital admission data were obtained for RD during 2014 and 2019 in Beijing, China. Negative control exposure and extreme gradient boosting with SHapley Additive exPlanation was used to explore the causality and contribution between PM 1 and RD. Stratified analysis by gender, age, and season was conducted. A total of 1,183,591 admissions for RD were recorded. Per interquartile range (28 μg/m3) uptick in concentration of PM 1 corresponded to a 3.08% [95% confidence interval (CI): 1.66%–4.52%] increment in morbidity of total RD. And that was 4.47% (95% CI: 2.46%–6.52%) and 0.15% (95% CI: 1.44%-1.78%), for COPD and asthma, respectively. Significantly positive causal associations were observed for PM 1 with total RD and COPD. Females and the elderly had higher effects on total RD, COPD, and asthma only in the warm months (Z = 3.03, P = 0.002; Z = 4.01, P < 0.001; Z = 3.92, P < 0.001; Z = 2.11, P = 0.035; Z = 2.44, P = 0.015). Contribution of PM 1 ranked first, second and second for total RD, COPD, and asthma among air pollutants. PM 1 was causally associated with increased morbidity of total RD and COPD, but not causally associated with asthma. Females and the elderly were more vulnerable to PM 1 -associated effects on RD. • Exposure to PM 1 was causally associated with increased morbidity of COPD. • Females and the elderly were more vulnerable to PM 1 -associated effects. • PM 1 contributed higher to respiratory diseases, especially compared to other PM. [ABSTRACT FROM AUTHOR]

Air quality in Beijing-Tianjin-Hebei and its surrounding regions has been significantly improved during the Three-Year Action Plan for Winning the Battle Against Air Pollution from 2018 to 2020. The PM 2.5 concentration can be significantly influenced by meteorological conditions, and the effectiveness of anthropogenic emissions reduction on PM 2.5 over the North China Plain remains unclear. In this study, the Nested Air Quality Model System (NAQPMS) and observations were used to quantitatively examine the contribution of meteorological and emission variations to autumn-winter (November to February) PM 2.5 concentrations in Beijing and its surrounding regions from 2018 to 2020. The model reproduced the temporal evolution of atmospheric pollutants and components well. The results showed that the observed mean PM 2.5 concentrations during the winter were reduced by 15% and 29% in 2019 and 2020, respectively, compared with that in 2018. In 2019, the meteorological conditions over the Beijing-Tianjin-Hebei (BTH) region were poor, causing an increase of PM 2.5 by 7.6% in BTH and 11% in Beijing; therefore, the control of air pollutant emission compensated for the unfavorable influence of meteorology and emission reduction was the decisive factor for PM 2.5 reduction; In 2020, the meteorological conditions were relatively favorable, and the changes in meteorological factors and emission reduction led to a decrease in PM 2.5 concentration by 18.6% and 10.5%, respectively, compared to 2018. Emission reduction also contributed to decrease in nitrate, ammonium, organic matter, element carbon and precursors. This work confirmed the obvious environmental benefit of pollution control measures from 2018 to 2020. There was obvious regional heterogeneity in the effect of meteorology and the impact of adverse meteorological conditions should be considered during the formulation of air pollution control measures. The results can serve as a reference for the formulation of air quality objectives or the evaluation of the environmental effect of pollution control schemes. • The effect of winter meteorology and control measures in 2018 to 2020 was assessed. • Observed winter PM 2.5 reduced by 15% and 29% in 2019 and 2020, respectively, compared to 2018. • Unfavorable meteorology in 2019 partially counteracted the emission control effects. • Emission mitigation played an important role in air quality improvement. [ABSTRACT FROM AUTHOR]

Air quality in China has been continuously improved since clean air action in 2013, yet the visibility was not improved simultaneously. Here we employed a new method by integrating highly time-resolved aerosol compositions with particle light extinction (b ext) into positive matrix factorization to quantify the different contributors to visibility degradation during four seasons in Beijing. Our results show that ammonium nitrate-related factor contributed dominantly to b ext during all seasons (31–48%) and played more significant roles during low-visibility periods. Secondary organic aerosol (SOA) was an important contributor of b ext (27–35%) in autumn and spring while primary OA related sources were more important in winter (37%). An increase in aerosol mass extinction efficiency and similarly important roles of ammonium nitrate and SOA in visibility degradation were also observed during the COVID-19 lockdown period. Our results point towards a future challenge in improving visibility in China due to the increased contributions of nitrate and SOA in PM 2.5. Future emission controls with a priority to decrease nitrate would benefit both air quality and visibility. [Display omitted] • A new method to quantify the source contributions to particle light extinction. • Dominant contributions of ammonium nitrate and SOA to light extinction in Beijing. • Increased aerosol mass extinction efficiency during the COVID-19 period. [ABSTRACT FROM AUTHOR]

To study the aerosol chemistry and regional transport changes in Beijing under different stagnant meteorological conditions, a simultaneous combined observation of the nonrefractory submicron aerosol (NR-PM 1) chemical compositions and vertical wind profiles was conducted at an urban site (Beijing) and a regional background site (Xinglong) in the North China Plain (NCP) from November 2018 to January 2019. The backward trajectories of PM 1 species in Beijing under different pollution stages were calculated. The results showed that the proportion of backward trajectories from southern regions increased from 0% in the clean stage (PM 1 < 35 μg m−3) to 47% in the pollution stage (PM 1 > 75 μg m−3), suggesting the aerosol pollution in winter Beijing was greatly affected by regional transport from southern regions. Although the proportion of trajectories from the northwestern regions of Beijing decreased from the clean stage to the pollution stage, the trajectories still comprised a large proportion (53%), which indicates that the northwestern air mass can also cause heavy pollution in winter. Therefore, two typical episodes with air masses from the southwest (episode 1) and northwest (episode 2) regions were selected for further study. The results showed that episode 1 in Beijing lasted for several days and was initiated by regional transport and dominated by heterogeneous reactions, while episode 2 was dominated by rapid regional transport in a short time. Both water vapor and pollutants were transported from southern regions in episode 1, while only pollutants were transported in episode 2. Because of the stronger heterogeneous reactions in episode 1, inorganic aerosols in Beijing accounted for 71% of PM 1 in episode 1, which was higher than that in episode 2 (56%). In addition, the transport of water vapor from southern China enhanced the secondary aerosol formation in Xinglong and expanded the scope of regional pollution. In comparison, the systematic south wind did not reach the background area in episode 2. Correspondingly, the PM 1 concentration in Beijing was 62% higher than that in Xinglong in episode 2, which was much higher than that in episode 1 (11%). Our results showed that in addition to the enhancement of heterogeneous reactions caused by the increase in humidity, the rapid transport of pollutants because of the sudden change of wind field from north to south can also dominant heavy haze episodes in winter Beijing. • Haze pollution in Beijing was affected by regional transport from southern regions. • Heterogeneous reactions dominate haze episode with air masses from southern regions. • Rapid transportation of pollutants dominates the explosive growth of pollutants. [ABSTRACT FROM AUTHOR]

Comprehensive measurements were conducted in winter 2018 and combined with RMPAS-Chem model simulations to analyze the regional transport mechanisms of atmospheric pollutants over the North China Plain. The instruments used consisted of four Vaisala CL51 ceilometers for planetary boundary layer (PBL) heights and aerosol backscatter profiles, two wind profilers, one radiosonde for the profiles of meteorological variables, and an instrumented King-Air 350 aircraft for the profiles of atmospheric pollutants and meteorological variables. Additionally, observations from Environmental Protection Bureau stations were also analyzed, including hourly concentrations of surface PM 2.5 , SO 2 , NO 2 , CO, and O 3. The results suggest that regional atmospheric pollutant transport is driven by a combination of topography and PBL processes. First, a mountain-induced vertical vortex forms over downwind regions; this elevates ground pollutants to form an elevated pollutant layer (EPL) at an altitude of 1.4–1.7 km. The EPL is then transported to Beijing via an enhanced southerly wind. Finally, the pollutants in the EPL are transported downward to the surface through PBL processes. • The mountain-induced vortex elevated surface pollutants to high layer. • Pollutant in the EPL was transported downward to the surface through PBL process. • Regional atmospheric pollutant transport mechanisms over NCP are suggested. [ABSTRACT FROM AUTHOR]