Vertical Distribution Of PM2 Research Articles

Identification of particle distribution pattern in vertical profile via unmanned aerial vehicles observation

Below the boundary layer, the air pollutants have been confirmed to present the decreasing trend with the height in most situaitons. However, the disperiosn rate of air pollutants in the vertical profile is rarely investigated in detail, especially through in-situ measurement. With this consideration, we employed an unmanned aerial vehicle equipped with portable monitoring equipments to scrutinize the vertical distribution of PM2.5. Based on the original data, we found that PM2.5 concentration decreases gradually with altitude below the boundary layer and demonstrated an obvious linear correlation. Therefore, the vertical distribution of PM2.5 was quantified by representing the distribution of PM2.5 with the slope of PM2.5 vertical distribution. We used backward trajectories to reveal the causes of outliers (PM2.5 increasing with altitude), and found that PM2.5 in the high altitude came from the southwest. Besides, the relationship between the vertical distribution of PM2.5 and various meteorological factors was investigated using stepwise regression analysis. The results show that the four meteorological factors most strongly correlated with the slope values are: (a) the difference in relative humidity between the ground and the air; (b) the difference in temperature between the ground and the air; (c) the height of the boundary layer; and (d) the wind speed. The slope values increase with increasing the difference in relative humidity between ground and air and the difference in temperature between the ground and the air, and decrease with increasing boundary layer height and wind speed. According to the Random Forest calculations, the ground-to-air relative humidity difference is the most important at 0.718; the wind speed is the least important at 0.053; and the ground-to-air temperature difference and boundary layer height are 0.140 and 0.088, respectively.

Vertical distribution of PM2.5 in Santiago de Chile studied with an unmanned aerial vehicle and dispersion modelling

The vertical structure of temperature in the troposphere is closely related to PM2.5 concentration in polluted regions, with temperature inversions associated to higher PM2.5 at the surface. This fact is more relevant in places surrounded by mountains, like Santiago de Chile, where high pollution events are common in winter. A characterization of the vertical profile of PM2.5 and temperature has been carried out in winter and spring with an unmanned aerial vehicle. Temperature inversions up to 400 m were found only in winter, with full inversions associated to the highest PM2.5 at the surface. Days with full inversion are also characterized by very low wind speeds. In most cases, PM2.5 decreased rapidly with altitude, even when there was mixing layer of considerable height. Consequently, PM2.5 above 300 m was always low, in contrast to other studies, indicating that there are factors that influence the vertical concentration of PM2.5 which are still not well understood. Chemistry transport model (CTM) simulated vertical profiles of PM2.5 agrees qualitatively well with observed concentration profiles in spring, but there is a negative bias in comparison to the measured data and also a stronger simulated vertical gradient than observed. In winter, average simulated PM2.5 concentrations at the surface are similar to observed, but with increasing altitude the simulated data decline faster than measured.

Secondary sources of PM2.5 based on the vertical distribution of organic carbon, elemental carbon, and water-soluble ions in Bangkok

Ambient PM2.5 pollution can affect human health and the environment in global megacities. Secondary PM2.5 is produced by physical and chemical reactions of primary air pollutants. Sources of secondary PM2.5 in Bangkok have been considered in most on-target investigations for the control and management of air quality. This research aims to identify the vertical distribution of secondary PM2.5 sources in Bangkok, Thailand. Vertical distributions of PM2.5 in haze and non-haze periods were collected at different heights (30, 75, and 110 m above ground level) at the Air Pollutants Monitoring Tower, Kasetsart University (KU tower) between November 2020 and July 2021. The carbon components were analyzed by an OCEC analyzer using the IMPROVE thermal/optical reflectance (TOR) protocol, and the ionic species were determined by ion chromatography. The results showed that the organic carbon (OC) to elemental carbon (EC) ratios of PM2.5 tended to increase with altitude at 30, 75, and 110 m in the range of 2.0 - 7.7, 3.4 - 8.0 and 3.0 - 16.6, respectively. In the haze period (Jan-Feb 2021), the SOC within PM2.5 was the highest at 110 m which was 24.5%. In addition, the key components of SOC, NO3−, NH4+, and SO42− within PM2.5 were found in secondary aerosols in Bangkok. The chemical compositions indicated that secondary PM2.5 is mainly contributed by local vehicle exhaust (30 m) and long-range transported biomass burning (75 and 110 m). This research provides useful information that could be applied for urban secondary PM2.5 control and management.

Impact of urban viaducts on the vertical distribution of fine particles in street canyons

In the urbanization process, the viaduct has been regarded as an effective way to relieve metropolis traffic pressures in the urban area. However, due to the double emission source from elevated- and ground-road vehicles, the viaduct-built environment inevitably deteriorates the street canyon's air quality. Besides, the emitted air pollutants are trapped in such street canyons for extended periods. With this consideration, we used measured data to explore the impact of the viaduct on the vertical distribution of PM2.5 in street canyons. First, the experiments were conducted in a residential building near the inner ring of a viaduct in Shanghai, China. Next, the vertical distribution of PM2.5 and its diurnal variation were revealed based on these data. The results indicated the bimodal distribution patterns in the vertical profiles of PM2.5 mass concentrations, with the lower peak at height of 5 m under the viaduct and the upper peak at height of 10 m above the noise barrier on the viaduct. Further, the impact of the viaduct on the vertical distribution of PM2.5 was investigated. Results from both observation and simulation are consistent, revealing that 21–23% of PM2.5 was trapped beneath the viaduct, and the noise barrier causes the PM2.5 to decrease by 4–5%. The traffic source on the viaduct contributes 20–40% of the PM2.5. This study revealed the dispersion patterns of PM2.5, and the impact of the viaduct on it, which could be beneficial to optimizing the viaduct and the noise barrier to release traffic-related air pollutants in street canyons.

Quantitative verification of the turbulence barrier effect during heavy haze pollution events

Under calm and steady weather conditions with low wind speeds, turbulent intermittency frequently occurs in the atmospheric boundary layer (ABL), which can significantly weaken the turbulent diffusion of matter and energy between the surface and atmosphere. The turbulence barrier effect is defined as the phenomenon in which turbulence may disappear at certain heights, and during periods of heavy haze, creating what can seem like a barrier layer that hinders vertical transmissions. Although the turbulence barrier effect can explain the physical mechanisms behind the rapid accumulation of PM2.5 (fine particulate matter with diameters smaller than 2.5 μm) and the influence of turbulent diffusion conditions on the vertical distribution of PM2.5, more direct perspectives such as turbulent flux is still required for quantitative verification. Due of challenges in the acquisition of PM2.5 turbulent flux, carbon dioxide (CO2), which has relatively mature flux acquisition technology, was used as a substitute means of verifying and quantifying this phenomenon. The turbulence data collected during heavy haze events, at from five levels of a 255 m meteorological tower located in Tianjin, were analyzed and used to quantitatively verify the influence of the turbulent barrier effect on PM2.5. The results also revealed that the vertical changes in the turbulent barrier effect were consistent with those of the concentrations and flux of CO2. This means that this knowledge about the turbulent barrier effect can be extended to other mass-transfer processes. The analysis also found that the proportion of counter-gradient transport increases when the occurrences of the turbulent barrier effect are frequent. This work validates the presence of the turbulent barrier effect and is an important foundation for its future parameterization, which will help to accurately identify the matter transport processes in the stable boundary layer and under extreme weather conditions, such as intense pollution events.

Identification of the atmospheric boundary layer structure through vertical distribution of PM2.5 obtained by unmanned aerial vehicle measurements

Variations in the atmospheric boundary layer (ABL) structure present a close relationship with the vertical diffusion of air pollutants. However, the effect of the ABL structure on the vertical distribution of air pollutants is rarely investigated in detail, especially through in-situ measurement. In this study, from November 26th to December 20th, 2017, an unmanned aerial vehicle (UAV) outfitted with sensors was used to monitor meteorological factors and pollutants concentrations (including PM2.5, O3, and BC) in the suburbs of Shanghai. According to the vertical profiles of PM2.5 concentrations, the gradient and the lidar backscatter methods were used to determine the parameters of the ABL structure. Results indicate that the maximum height of the atmospheric boundary layer occurred at noon exceeding 1000 m, and the maximum thickness of the atmospheric entrainment layer occurred at twilight exceeding 360 m during the field campaign. Additionally, the relationships between the structure of ABL and pollutant concentrations were analyzed and quantified. In contrast to the height of the ABL, the concentrations of ground-level aerosol pollutants show a stronger correlation with the thickness of the atmospheric entrainment layer. It also confirms that the entrainment flux ratio was an important factor affecting ground-level ozone concentrations. In summary, the in-situ measurements can provide experimental support for modeling the ABL structure and provide insights for the control of aerosol pollution.

Vertical Distribution of PM2.5 Transport Flux in Summer and Autumn in Beijing

Temporal and spatial distribution characteristics of PM2.5 transport fluxes between Beijing and its adjacent cities in July and October 2016 were estimated quantitatively using the cross-boundary transport flux method based on the WRF-CMAQ model in this study. Vertical distribution of PM2.5 net fluxes was revealed, and hourly evolution characteristics of PM2.5 transport fluxes in typical heavy pollution processes were illustrated. We verified significant seasonal differences in PM2.5 transport fluxes between Beijing and its adjacent cities during the study period. PM2.5 total inflow and outflow fluxes in autumn were 3187 t/d and -2721 t/d, which were significantly higher than that of 2134 t/d and -2172 t/d in summer. In autumn, Beijing received more PM2.5 from adjacent cities, while the results in summer were opposite. Maximum net fluxes appeared at 600 - 800 m and 1000 - 1260 m above the ground in summer and autumn, respectively. The vertical distribution characteristics of pollution days and clean days were consistent, both of which show a net inflow from Baoding and Langfang to Beijing, while Beijing was in a state of net outflow to Chengde. During the whole heavy pollution process, the evolution characteristics of PM2.5 flux at low and high altitudes were consistent, and the intensity of the latter was 2.15~5.30 times of the former. Meanwhile, local discharging was more likely to cause extreme PM2.5 heavy pollution, and a better peak clipping effect can be achieved when emission reduction measures are initiated 1 to 2 days before heavy pollution. The results can provide scientific support to put forward effective joint control measures and obtain insights into the evolutionary mechanism of haze episodes in Beijing.

Evaluation of PM2.5 fluxes in the “2+26” cities: Transport pathways and intercity contributions

To comprehensively evaluate the transport pathways and regional sources of PM2.5 in the “2 + 26” cities, air quality simulations using WRF-Chem were pursued over January, April, July, October 2015. The seasonal characteristics, vertical distributions of PM2.5 fluxes, and transport pathways were investigated. The model reasonably predicted the meteorological parameters (e.g., T2, Q2, WS10, and WD10) and air pollutants in both evaluated domains. The horizontal PM2.5 fluxes were strong in October (1221 kt, −1222 kt) and April (1119 kt, −1142 kt) and weak in January (933 kt, −997 kt) and July (589 kt, −809 kt). The directions of horizontal flux contributors relative to their target cities were entirely different in January and July. The same pattern can also be found at vertical layers. However, there was no uniform direction seen in April and October. A total of 14 pathways at the boundaries of the “2 + 26” cities were identified. The SW-NE pathways, which were dominant in Beijing and Tianjin with flux contributions of 65% and 75%, were found in all four months. The NW-SE pathways, which were the major pathways of Taiyuan (87%), and the W-E pathways, which were the major pathways of Zhengzhou (44%) and Jinan (43%), always showed different directions in January and July, following the directions of prevailing winter and summer monsoon winds. Moreover, Shijiazhuang (42%) and Handan (48%) were dominated by S–N pathways, which showed different patterns throughout the four months. The results could be helpful for implementing policies aimed at ecological compensation and joint air pollution control.

Observed Interactions Among Haze, Fog and Atmospheric Boundary Layer during a Haze-fog Episode in the Yangtze River Delta Region, Eastern China

A severe haze-fog episode occurred in the Yangtze River Delta region of eastern China during 22–30 November, 2018. In this period, the PM2.5 mass concentration and meteorological parameters at the surface were collected at the Station for Observing Regional Processes of the Earth System site in Nanjing. The vertical distributions of PM2.5, humidity and potential temperature below 500 m were observed simultaneously by an unmanned aerial vehicle, and the profile of potential temperature at 1400 local standard time on each day was also observed by radiosonde at the same site. During the first four days, the PM2.5 mass concentration increased, the maximum convective planetary boundary layer height (CBLH) decreased, and the air humidity increased. These are favorable conditions for fog formation. In the latter five days, fog formed on four days, with a lowering of the CBLH and a further increase in PM2.5 mass concentration. We found that the fog top cooling induced a potential temperature jump (i.e., sharp increase of potential temperature) with much warmer temperatures above the cloud top cooling and that this particular thermal structure was maintained until the end of the fog period, which significantly suppressed the daytime development of the planetary boundary layer after fog dissipation. The fog-induced reduction of the CBLH further increased the PM2.5 mass concentration. We also found that the wet deposition of fog on PM2.5 was negligible. The scavenging effect of fog on aerosols only acts during a fog period. When the fog dissipates, the aerosols are liberated from the fog droplets to the atmosphere.

Dynamic prediction of PM2.5 diffusion in urban residential areas in severely cold regions based on an improved urban canopy model

Problems related to air pollution have become more serious over the past few years, and air particulates have attracted attention as one of the most significant air pollutants. The air quality in residential areas is closely related to human health, as urban residents spend most of their time here. Because of the unique climatic characteristics of cities in severely cold regions, their urban energy structure, underlying surface, and architectural form are different from other cities to a certain degree, which causes distinctive particulate pollution characteristics. This study establishes a urban local-scale particle diffusion model for cities in severely cold regions and investigates the particle diffusion process and vertical concentration distribution in a residential area through simulation. The simulation results show that the near-ground PM2.5 mass concentrations are significantly affected by traffic flow. The vertical distribution of PM2.5 mass concentrations in residential areas demonstrates a decreasing trend with an increase in height owing to the effect of vehicle emissions and fugitive road dust. This study systematically studies the vertical distribution of particulate matter in urban settlements in severely cold regions and provides a theoretical basis and technical support for further improving the air quality of such urban settlements.

Vertical distribution of PM2.5 and interactions with the atmospheric boundary layer during the development stage of a heavy haze pollution event

Vertical profiles of PM2.5 (i.e., particulate matter with an aerodynamic diameter of 2.5 µm or less) and meteorological variables (e.g., potential temperature, specific humidity) are crucial to understand formation mechanism including accumulation and dispersion process of PM2.5, as well as interactions between aerosols and the atmospheric boundary layer (ABL). In this study, vertical distributions of PM2.5 are characterized through comprehensive analyses of vertical profiles measured by unmanned aerial vehicle (UAV), Micro Pulse LiDAR, and other surface observational data of a heavy aerosol pollution episode occurring on December 22–25, 2017 in Nanjing, China. Results show that PM2.5 profiles are characterized by a clear three-layer structure with near constant within the mixed layer, a transition layer with a large local gradient in the entrainment zone, and a layer with low concentration and small gradient in the free atmosphere, which shows a large similarity to that of specific humidity. The accumulation of aerosols is found near top of the ABL with the largest increase rate. Vertical distributions of PM2.5 and their evolution are largely constrained by the ABL thermodynamics during daytime, but show much less dependence on the ABL evolution at nighttime. PM2.5 provides an important feedback on the nocturnal boundary layer (NBL) leading to significant modification of vertical distributions of potential temperature and water vapor. Moreover, this study suggests that the current boundary layer parameterization scheme needs refinement with aerosol radiative effect included to further improve the ABL height (ABLH) and air quality predictions.

Sources and vertical distribution of PM2.5 over Shanghai during the winter of 2017

Shanghai, a metropolitan city in China, has suffered from severe air pollution, especially PM2.5, in the last few years. Up to now the contribution of local emission and regional transport to the formation of haze in Shanghai remains unclear. With an aim to characterize the mechanism of haze formation in Shanghai, the present paper attempted to provide an overview of a tethered balloon-based field campaign. According to the backward trajectories, the air mass traveling slowly from Jiangsu province accounted for the highest PM2.5 concentration (66 ± 20 μg/m3). Seventy vertical profiles of PM2.5, NO, NO2, SO2 and O3 within 1000 m were obtained, through which a comparison study on the characteristics of the vertical distributions of air pollutants on clean days and haze days was conducted. When altitude increased, clearly decreasing pattern of PM2.5, NO, and NO2 was observed during the field campaign. Due to the low atmospheric boundary layer, the diffusion of air pollutants was suppressed, which favored the formation of haze. The results of the generalized additive model revealed NO2 could the most significant factor influencing the vertical distribution of PM2.5 in both clean and haze days. This study provides new insight into the sources and vertical distribution of PM2.5, which could offer references for air pollution modeling.

Investigation of variations, causes and component distributions of PM2.5 mass in China using a coupled regional climate-chemistry model

China experiences severe particulate matter (PM) pollution. Although a monitoring network for PM2.5 (diameter < 2.5 μm) has been set up in more than 100 major Chinese cities, insufficient spatial coverage of observations limits the study of the temporal and spatial characteristics, influencing factors, and component of PM2.5. In this study, we conducted a one year air quality simulation using a regional climate–chemistry model and evaluated the simulation's performance based on in situ observations concerning meteorological elements and PM2.5 concentrations. The simulated results showed that, higher PM2.5 concentrations appeared in northern China and the Sichuan Basin, and the maximal value occurred in winter. Furthermore, Vertical PM2.5 concentrations presented a gradual decreasing trend from the surface, whereas in southern coastal cities the profiles were unsteady with a secondary peak in the lower layer. Meteorological conditions were conducive to both pollutant diffusion and removal in summer, whereas stagnant conditions appeared in winter, characterized by high sea level pressure (SLP), the lowest planetary boundary layer height (PBLH), and 2-m temperature (T2). In provincial capital cities, PM2.5 was positively correlated with residential emissions but negatively correlated with precipitation, 10-m wind speed, T2, PBLH, and industrial emissions. Finally, we utilized the simulation results to investigate the component variations of PM2.5. Results indicated that primary PM2.5 components had significantly higher concentrations in northern China where residential heating is the major source of PM2.5 emissions, whereas they had lower concentrations in southern China. Secondary components played a crucial role in PM2.5 mass in eastern China. This study provided a clear perspective of seasonal variations, horizontal and vertical distributions of PM2.5 and its components and influence factors, which could be used in subsequent studies to investigate the formation mechanism and emission sources of PM2.5.

Two Inversion Layers and Their Impacts on PM2.5 Concentration over the Yangtze River Delta, China

AbstractAn integrated winter field campaign was conducted to investigate the atmospheric boundary layer structure and PM2.5 concentration at three sites over the Yangtze River delta (YRD) in China: Shouxian (a rural area), a site in a northern suburb of Nanjing, and Dongshan (a residential area). Two temperature inversion layers and air pollution events occurred simultaneously from 30 to 31 December 2016, local time, over the YRD. It was found that the two inversion layers were related to the presence of a high pressure system, resulting in divergence in the upper boundary layer and radiative cooling near the ground at night. Dominated by agricultural and residential biomass burning, the surface emission sources from the Shouxian rural area were moderately strong. After the formation of the two inversions, the vertical distribution of PM2.5 concentration below the upper inversion layer was uniform as a result of thorough boundary layer mixing in the earlier hours. During nighttime at the Nanjing site, air pollutant plumes from nearby elevated point sources could not easily diffuse downward/upward between the two inversion layers, which led to a distinct peak in the PM2.5 concentration. At the Dongshan site, the emission sources were weak and the nighttime PM2.5 concentration above 100 m was high. The surface PM2.5 concentration gradually increased from early morning to noon, which was attributed to emissions related to the local residents. The results indicated that the vertical distribution of pollutants was affected by a combination of local emissions, vertical boundary layer structure, and horizontal and vertical transports.

Vertical Distribution and Transport of PM2.5 During Heavy Pollution Events in the Jing-Jin-Ji Region

Based on vehicle-borne tethered balloon measurements, the vertical distribution of particulate matter (PM) concentrations were observed in Gaocun in the Wuqing District of Tianjin from December 17 to 19, 2016, during a period of heavy pollution. Using observational data, the transport flux of PM2.5 in the Jing-Jin-Ji region was calculated. The results showed that the mixed layer was low at only 200 m during the heavy pollution period. The vertical distribution of PM2.5 concentrations was closely associated with the heights of mixed layer whereby, below the mixed layer, PM2.5 concentrations were higher. Vertical variation was insignificant, forming a district pollution layer. Above the mixed layer, PM2.5 concentrations rapidly decreased and stabilized at low levels. During the observation period, higher concentrations of PM were found with particle sizes of less than 1.0 μm, and lower concentrations were observed for particle sizes larger than 2.2 μm. The size profiles of PM tallied with relative humidity and the height of the mixed layer. The size distribution was wider during periods of high humidity and with a lower mixed layer height. The greatest PM2.5 transport flux was from the southwest, accounting for 63.3% of the total flux; the highest fluxes occurred at the heights of 46-156 m and 156-296 m. The dominant transport direction was southwest below 300 m, while the dominant transport direction was dispersed over 300 m.

Trends in atmospheric particles and their light extinction performance between 1980 and 2015 in Beijing, China

This study explored the interdecadal variations and their horizontal and vertical light extinction performances of atmospheric particulate matter with aerodynamic diameters ≤2.5 μm (PM2.5), particulate matter with aerodynamic diameters ≤ 10 μm (PM10), and total suspended particulates (TSPs) in Beijing from 1980 to 2015, using data available from historical publications. Prominent declines of PM2.5, PM10, and TSPs were detected with long-term linear trends of −6.7, −4.3, and −1.9 μg m−3 yr−1, respectively. Generally, on the annual scale during the studied period, it was found that PM2.5 displayed negative correlation (R2 = 0.38, p < 0.01) with visibility and positive correlation (R2 = 0.41, p < 0.01) with aerosol optical depth (AOD). Comparably, PM10 exhibited robust negative correlation (R2 = 0.61, p < 0.01) with visibility and positive correlation (R2 = 0.82, p < 0.01) with AOD. The complicated interdecadal variations and light extinction performances of PM2.5 were found, suggesting the changes on particle composition and vertical distribution of PM2.5 in the atmosphere.

Boundary layer structure and scavenging effect during a typical winter haze-fog episode in a core city of BTH region, China

The vertical distribution of PM2.5 and meteorological parameters from ground to upper levels were observed simultaneously using meteorological tower, tethered balloons and aerosol laser radar in Dec of 2016 in the urban area of Tianjin and its southern district, Jinghai. The influence of the vertical structure of boundary layer on a typical haze-fog episode was analyzed. There existed long distance transport of PM in the high layers before the haze formed in Tianjin and the downward airflows brought the PM from the high layer to the ground. In the early stages of this episode, periodic temperature inversions occurred, leading to conspicuous diurnal variations in the vertical profile of the PM2.5. In the middle and late stages of this episode, strong inversion and thick humidity layer were sustained below 400 m, and there were no big daily changes in the vertical profiles of the PM2.5. During the rapid formation period of the fog, the inversion layer was damaged and turbulence was strengthened. During the stationary phase of the fog process, wind and turbulence in the boundary layer became weak again. Rime was the main weather-related, wet cleaning mechanism that lowered pollutants concentration during this fog episode. High concentrations of water soluble ions in the rime samples and the concentrations of those ions in ambient PM2.5 appeared significant decrease during the rime period, which illustrated the scavenging effect of rime.

Vertical Distribution Characteristics of PM2.5 Observed by a Mobile Vehicle Lidar in Tianjin, China in 2016

We present mobile vehicle lidar observations in Tianjin, China during the spring, summer, and winter of 2016. Mobile observations were carried out along the city border road of Tianjin to obtain the vertical distribution characteristics of PM2.5. Hygroscopic growth was not considered since relative humidity was less than 60% during the observation experiments. PM2.5 profile was obtained with the linear regression equation between the particle extinction coefficient and PM2.5 mass concentration. In spring, the vertical distribution of PM2.5 exhibited a hierarchical structure. In addition to a layer of particles that gathered near the ground, a portion of particles floated at 0.6–2.5-km height. In summer and winter, the fine particles basically gathered below 1 km near the ground. In spring and summer, the concentration of fine particles in the south was higher than that in the north because of the influence of south wind. In winter, the distribution of fine particles was opposite to that measured during spring and summer. High concentrations of PM2.5 were observed in the rural areas of North Tianjin with a maximum of 350 μg m–3 on 13 December 2016. It is shown that industrial and ship emissions in spring and summer and coal combustion in winter were the major sources of fine particles that polluted Tianjin. The results provide insights into the mechanisms of haze formation and the effects of meteorological conditions during haze–fog pollution episodes in the Tianjin area.

Investigation of PM2.5 mass concentration over India using a regional climate model

Seasonal variation of PM2.5 (Particulate Matter <2.5 μm) mass concentration simulated from WRF-Chem (Weather Research and Forecasting coupled with Chemistry) over Indian sub-continent are studied. The simulated PM2.5 are also compared with the observations during winter, pre-monsoon, monsoon and post-monsoon seasons of 2008. Higher value of simulated PM2.5 is observed during winter followed by post-monsoon, while lower values are found during monsoon. Indo-Gangetic Basin (IGB) exhibits high amount of PM2.5 (60− 200 μg m−3) throughout the year. The percentage differences between model simulated and observed PM2.5 are found higher (40− 60%) during winter, while lower (< 30%) during pre-monsoon and monsoon over most of the study locations. The weighted correlation coefficient between model simulated and observed PM2.5 is 0.81 at the significance of 98%. Associated RMSE (Root Mean Square Error) is 0.91 μg m−3. Large variability in vertically distributed PM2.5 are also found during pre-monsoon and monsoon. The study reveals that, model is able to capture the variabilities in spatial, seasonal and vertical distributions of PM2.5 over Indian region, however significant bias is observed in the model. PM2.5 mass concentrations are highest over West Bengal (82± 33 μg m−3) and the lowest in Jammu & Kashmir (14± 11 μg m−3). Annual mean of simulated PM2.5 mass over the Indian region is found to be 35± 9 μg m−3. Higher values of PM2.5 are found over the states, where the reported respiratory disorders are high. WRF-Chem simulated PM2.5 mass concentration gives a clear perspective of seasonal and spatial distribution of fine aerosols over the Indian region. The outcomes of the study have significant impacts on environment, human health and climate.

The vertical distribution of PM2.5 and boundary-layer structure during summer haze in Beijing

Between August 1, 2009, and August 16, 2009, physical observations of the atmospheric boundary layer and synchronous vertical observations of atmospheric particles were conducted from the Beijing 325 m meteorological tower, where the particulate matter with a diameter of 2.5 μm or less (PM2.5) analysis meters were stationed at a three-floor platform with altitudes of 8 m, 120 m and 280 m, respectively. Meanwhile, the atmospheric temperatures, relative humidity, wind speeds and wind directions between 8 m and 320 m were observed online at 15 different altitude intervals. The backscattering coefficient of aerosols in the boundary-layer atmosphere within 2.5 km height was also observed using a backscattered laser ceilometer. The observations showed that the PM2.5 pollution in the atmosphere from the ground up to 280 m in Beijing was quite high on August 2009, with a maximum of 200 μg m−3. Within 280 m, the vertical distribution of PM2.5 was inhomogeneous, with a maximum difference of up to 116 μg m−3 between levels in the night residual layer and at the ground. The high concentration of particles in the residual layer reached the ground by the next morning through convection, thus becoming severe pollutants. The PM2.5 in the near-surface layer was directly related to the reduction of ultra-violet radiation (UV), with a correlation coefficient of −0.57. Under steady weather conditions, the topographic mountain-valley breezes in Beijing superposed the land-sea breezes, resulting in a specific breath-like diurnal variation in wind direction. As a result, the PM2.5 mixed and increased in the regional area, leading to serious dust-haze pollution. Within 320 m of the boundary layer, the vertical distributions of temperature, humidity, wind speed and wind direction were inhomogeneous, and these patterns were the major factors influencing the distribution and variation of PM2.5 concentration. Under steady weather conditions, the reverse distribution of relative humidity became more significant, while the low temperatures at higher altitudes facilitated the formation of organic aerosols. Sometimes, the PM2.5 levels increased due to long-range transmission of smoke plume into the residual layer. By the synchronous effect of these factors, the moisture absorption of PM2.5 in the upper layer increased, at times resulting in a “higher-top and lower-bottom” pattern of the PM2.5 distribution.