Mountain-valley Breeze Research Articles

Atmospheric Circulation in the Furnas Reservoir Region, MG: Sensitivity Experiments with RegCM5 in Convection-Permitting Mode

In the 1950s, the Furnas reservoir was created, an artificial lake surrounded by hills, for the operation of the Furnas Hydroelectric Power Plant in southeastern Brazil. Along the shores of the Furnas reservoir, there are no meteorological stations, and only three are found at a distance greater than 10 km from the reservoir, which makes it challenging to assess the lake's influence on the local climate. To address this issue, the Regional Climate Model version 5 (RegCM5) was run in convection-permitting (CP) mode for two months, with a horizontal resolution of 1 km, to investigate the near-surface atmospheric circulation patterns around the reservoir. Three numerical experiments were conducted using RegCM5-CP nested in ERA5 reanalysis: a control simulation (CTRL), an experiment where the topography was assumed to be flat throughout the domain (expTOPO), and an experiment where the water body was replaced by vegetation (expVEG). The first month of simulation in each experiment was excluded from the analysis as it was considered a spin-up period. RegCM5-CP has good performance in simulating the diurnal cycle of the 2-m air temperature and reasonable performance for the 10-m wind intensity compared to observed data from three meteorological stations in the domain of simulation (but distant from the reservoir shore). Despite the geographic complexity of the Furnas reservoir, the experiments revealed coupling between lake and valley breezes during the daytime and land and mountain breezes during nighttime in different segments of the reservoir, such as in Guapé (near the dam). However, mountain-valley breezes are more dominant.

Legacy and alternative perfluoroalkyl acids in the Yellow River on the Qinghai-Tibet Plateau: Levels, spatiotemporal characteristics, and multimedia transport processes

The source region of the Yellow River (SRYR) located in the northeast of the Qinghai-Tibetan Plateau is not only the largest runoff-producing area in the Yellow River Basin, but also the most important freshwater-supply ecological function area in China. In this study, the short-term spatiotemporal distribution of selected legacy and alternative perfluoroalkyl acids (PFAAs) in the SRYR was first investigated in multiple environmental media. Total PFAA concentrations were in the range of 1.16-14.3 ng/L, 4.25-42.1 pg/L, and 0.21-13.0 pg/g dw in rainwater, surface water, and sediment, respectively. C4-C7 PFAAs were predominant in various environmental matrices. Spatiotemporal characteristics were observed in the concentrations and composition profiles. Particularly, the spatial distribution of rainwater and the temporal distribution of surface water exhibited highly significant differences (p<0.01). Indian monsoon, westerly air masses, and local mountain-valley breeze were the driving factors that contributed to the change of rainwater. Rainwater, meltwater runoff, and precursor degradation were important sources of PFAA pollution in surface water. Organic carbon content was a major factor influencing PFAA distribution in sediment. These results provide a theoretical basis for revealing the regional transport and fate of PFAAs, and are also important prerequisites for effectively protecting the freshwater resource and aquatic environment of the Qinghai-Tibetan Plateau.

Ozone pollution aggravated by mountain-valley breeze over the western Sichuan Basin, Southwest China

The impact of thermally driven mountain-valley breezes (MVB) on the atmospheric environment remains poorly understood, especially in ozone (O3)-polluted regions with complex underlying topography. To address this knowledge gap, we focused on the western Sichuan Basin (SCB), situated immediately east of the Tibetan Plateau (TP), which is considered susceptible to MVB coupled with severe O3 pollution in southwest China. We revealed the MVB driving diurnal O3 variations and meteorological mechanisms using surface observations and ERA5 reanalysis data. Local MVB days accounted for up to 47% of cases in the summers of 2015–2022. Driven by the MVB, the near-surface O3 concentrations increased by 8.8%, with 12.7% and 50.0% deterioration in the O3 light and moderate exceedance rates, respectively, on the western SCB edge. The daytime upslope valley breeze with 20% higher wind speed drove the westward transport of rich O3 and precursors from the upwind-polluted inner SCB towards its western edge, and the O3 photochemical production, followed by intensifying solar radiation and air temperature, gave rise to 14.8% of surface O3 concentrations over the western SCB edge. The nighttime downward mountain breeze with a 20% increase in wind speed could transport the rich O3 in the mountainous area to the basin edge, causing O3 levels to increase by 2.8%. In summary, we quantitatively assessed the impacts of MVB on changes in O3 concentrations and air quality along with its meteorological mechanisms, facilitating a comprehensive understanding of meteorological drivers in the atmospheric environment.

Effects of boundary layer variations on physicochemical characteristics of aerosols in mid-low-altitude regions

Variations in the height of the boundary layer have a critical impact on the vertical transport of near-surface aerosols. Variations can affect the interactions between aerosols and clouds/fog by altering the scattering and absorption of solar radiation, significantly changing radiative forcing, convective precipitation, and regional climate. In this study, we simultaneously monitored air pollution and meteorological factors in a flat urban area (YunTech site, 50 m asl) and its peripheral mountainous region (MeiShan site, 980 m asl), analyzed the characteristics of pollutants under different atmospheric conditions, and explored the differences in the chemical reaction mechanisms of aerosols at various altitudes, aiming to clarify the evolution of the boundary layer in urban and suburban areas and its impact on the transport of pollutants. The results show that even without anthropogenic emissions, urban ground-level pollutants could be transported to peripheral mountainous areas through boundary layer height variations and local circulations, such as mountain-valley breezes. The PM2.5 concentration was higher at the urban site (average 31.14 ± 14.82μgm−3) and could be transported aloft by valley winds, leading to the gradual accumulation of daytime PM2.5 with an afternoon peak at the mountain site. Moreover, the nitrogen oxidation rate (NOR = [NO3−]/[NO3−] + [NO2]) exhibited clear site variations, the mountain site (average 0.41 ± 0.20) was higher than the urban site (average 0.19 ± 0.07), likely due to the atmospheric environment with thick clouds/fog and strong oxidation capacity in the mountain area. Our study has verified that aerosol characteristics, origins, formation pathways and transport mechanisms at the two measurement sites are significantly different under different conditions, which provides a theoretical basis for future air pollution prevention and regional climate research.

Comprehensively exploring the characteristics and meteorological causes of ozone pollution events in Beijing during 2013–2020

Ozone (O3) pollution deteriorated in recent years in Beijing; however, the evolution characteristics and formation mechanism of O3 pollution events (OPEs) lack comprehensive exploration from a long-term perspective. Based on 35-site (Beijing) and multi-city (North China) O3 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 O3) 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 O3 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 O3 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 O3 pollution elevated the mean peak O3 of nocturnal O3 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 O3 pollution in Beijing.

Diurnal Variations in High Time-Resolved Molecular Distributions and Formation Mechanisms of Biogenic Secondary Organic Aerosols at Mt. Huang, East China.

The molecular characteristics and formation mechanism of biogenic secondary organic aerosols (BSOAs) in the forested atmosphere are poorly known. Here, we report the temporal variations in and formation processes of BSOA tracers derived from isoprene, monoterpenes, and β caryophyllene in PM2.5 samples collected at the foot of Mt. Huang (483 m a. s. l) in East China during the summer of 2019 with a 3 h time resolution. The concentrations of nearly all of the detected species, including organic carbon (OC), elemental carbon (EC), levoglucosan, and SIA (sum of SO42-, NO3-, and NH4+), were higher at night (19:00-7:00 of the next day) than in the daytime (7:00-19:00). In addition, air pollutants that accumulated by the dynamic transport of the mountain breeze at night were also a crucial reason for the higher BSOA tracers. Most of the BSOA tracers exhibited higher concentrations at night than in the daytime and peaked at 1:00 to 4:00 or 4:00 to 7:00. Those BSOA tracers presented strong correlations with O3 in the daytime rather than at night, indicating that BSOAs in the daytime were primarily derived from the photo-oxidation of BVOCs with O3. The close correlations of BSOA tracers with SO42- and particle acidity (pHis) suggest that BSOAs were primarily derived from the acid-catalyzed aqueous-phase oxidation. Considering the higher relative humidity and LWC concentration at night, the promoted aqueous oxidation was the essential reason for the higher concentrations of BSOA tracers at night. Moreover, levoglucosan exhibited a robust correlation with BSOA tracers, especially β-caryophyllinic acid, suggesting that biomass burning from long-distance transport exerted a significant impact on BSOA formation. Based on a tracer-based method, the estimated concentrations of secondary organic carbon (SOC) derived from isoprene, monoterpenes, and β caryophyllene at night (0.90 ± 0.57 µgC m-3) were higher than those (0.53 ± 0.34 µgC m-3) in the daytime, accounting for 14.5 ± 8.5% and 12.2 ± 5.0% of OC, respectively. Our results reveal that the BSOA formation at the foot of Mt. Huang was promoted by the mountain-valley breezes and anthropogenic pollutants from long-range transport.

Attenuation of mountain-valley circulations on PM2.5 pollution over the western Sichuan basin, southwest China

The Sichuan Basin (SCB), located immediately to the east of the Tibetan Plateau (TP) in southwest China, is identified as a region with severe PM2.5 pollution, especially in the western SCB region. To understand the terrain effect on the atmospheric environment change in detail, this study investigated the effect degree and meteorological mechanism of thermally driven mountain-valley breeze (MVB) circulations on wintertime PM2.5 in the western SCB region, based on the near-surface observations of PM2.5 and the ERA5 reanalysis data of meteorology. The results showed that the western SCB edge exhibited a significant diurnal change of MVB, shifting between daytime upslope easterly flows and nighttime downslope westerly flows. The frequency of the MVB circulations was accounted for 39% days in December 2017, with the mountain and valley breeze-controlling periods being from 01:00 to 05:00 and 14:00 to 17:00 local time, respectively. Notably, the hourly PM2.5 reductions of 13.9 ± 4.6 μg m−3 was averaged during the MVB days in the western SCB edge, resulting in a decrease of 46.4 ± 14.0% in PM2.5 pollution with the MVB, which indicates that the MVB could alleviate PM2.5 pollution in improving air quality over the western SCB region. The daytime MVB circulations drove the transport of PM2.5-rich air mass in the atmospheric boundary layer from the polluted western SCB edge to the surrounding regions, causing 22% attenuation in near-surface PM2.5 concentrations during the valley breeze-controlling period. The nocturnal MVB circulations carried clean TP air eastward downslope along the eastern slope of the TP into the polluted western SCB region, mitigating PM2.5 levels by 20% during the mountain breeze-controlling period.

An Overview of Extreme Storm Trends in Java Island using Storm Severity Index (SSI)

Extreme winds can lead to extreme storms, which are among the most frequent forms of extreme weather in Indonesia. With climate change predicted to increase the adverse impact of extreme weather, it is crucial to keep track of and gauge the severity of storm events. Storm Severity Index (SSI) can be used to measure the severity of storms, which is defined by their intensity, duration, and extent. SSI has been used in continental areas. However, the application of SSI in the maritime continent is still limited. This study explores the application of SSI to measure extreme storms in Indonesia based on wind gust speed. This study uses wind gust data in 1959-2021 from ERA5 reanalysis data and calculates the SSI based on the 98th percentile and the 50-year return value of the wind data. This study also explores the meteorological events that coincide with high SSI values. This study found the SSI able to capture high wind gust events in Java Island. This study finds high SSI value coincides with tropical cyclone events in the Indian Ocean. Topography affects the area on Java Island that has a high SSI value. Further study should evaluate the impact of the coastal and non-coastal areas to reduce the significant effect of sea-land breeze or mountain-valley breeze. In addition, future research also needs to address the significance level of the duration of events in a severe storm to enhance the information on severity.

Particulate organic nitrates at Mount Tai in winter and spring: Variation characteristics and effects of mountain-valley breezes and elevated emission sources

Particulate organic nitrates, among the major components of secondary organic aerosols and fine particles, play important roles in regional nitrogen cycle, ozone budget, and cloud condensation nuclei formation. However, the pollution characteristics of particulate organic nitrates at mountain areas and the effects of anthropogenic pollutant transport remain poorly understood. In this study, field sampling and measurements were conducted at a high-elevation mountain site over North China Plain in winter and spring. Total five kinds of particulate organic nitrates in fine particles were determined by ultra-high performance liquid chromatography-electrospray mass spectrometry. The average total concentrations of particulate organic nitrates were 330 ± 121 ng m-3 and 247 ± 63 ng m-3 in winter and spring. The monoterpene-derived organic nitrates were the dominant components in both seasons with their contribution higher than 70%, accounting for 1.2 ± 0.8% and 2.0 ± 1.0% in organic aerosols in winter and spring, respectively. The significantly higher levels of particulate organic nitrates in winter than spring was ascribed to the strong effects of mountain-valley breezes and coal combustion plumes. The increasing concentrations of NOx and particulate matters brought by the valley breeze at daytime facilitated the formation of MHN215, OAKN359, and OAHN361, while the rising SO2 abundance and the sulfate aerosols transported by elevated emission sources affected the formation of MDCN247 at nighttime.

Meteorology and topographic influences on nocturnal ozone increase during the summertime over Shaoguan, China

Ozone (O3) is a typical photochemical product, generally with a maximum value in the late afternoon and a minimum value at night. This study aimed to investigate the variation characteristics of nocturnal O3 in a subtropical urban city (Shaoguan, Guangdong Province, China). The measurement campaigns were conducted from 23 July to July 28, 2020. O3 lidar, aerosol lidar, and wind profile lidar were utilised to detect the vertical distributions of the O3 concentrations, aerosol extinction coefficient, and wind velocity. The topographic features, pollutant data (PM10, PM2.5, O3, SO2, NO2, and CO), and meteorological parameters (temperature, humidity, wind speed, and wind direction) were also analysed. The results showed that the concentration of nocturnal O3 over Shaoguan peaked at midnight. The horizontal wind was dominated by southwest and south winds, and the vertical wind was dominated by downdrafts during the fixed-point observations in Shaoguan. Horizontal transport might have a high impact on the increase in the nocturnal O3 because of high pollutant emissions in upwind cities. This was further justified by the 24-h back trajectories of the air mass generated by the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Vertical transport of O3 in the lower or middle troposphere might be induced by the subtropical high and nocturnal mountain-valley breezes. Thus, the vertical mixing of O3 affects the O3 concentration at night.

Characteristics of Lake Breezes and Their Impacts on Energy and Carbon Fluxes in Mountainous Areas

In mountainous lake areas, lake-land and mountain-valley breezes interact with each other, leading to an “extended lake breeze”. These extended lake breezes can regulate and control energy and carbon cycles at different scales. Based on meteorological and turbulent fluxes data from an eddy covariance observation site at Erhai Lake in the Dali Basin, southwest China, characteristics of daytime and nighttime extended lake breezes and their impacts on energy and carbon dioxide exchange in 2015 are investigated. Lake breezes dominate during the daytime while, due to different prevailing circulations at night, there are two types of nighttime breezes. The mountain breeze from the Cangshan Mountain range leads to N1 type nighttime breeze events. When a cyclonic circulation forms and maintains in the southern part of Erhai Lake at night, its northern branch contributes to the formation of N2 type nighttime breeze events. The prevailing wind directions for daytime, N1, and N2 breeze events are southeast, west, and southeast, respectively. Daytime breeze events are more intense than N1 events and weaker than N2 events. During daytime breeze events, the lake breeze decreases the sensible heat flux (Hs) and carbon dioxide flux ( $$\boldsymbol{F}_{\text{CO}_{2}}$$ ) and increases the latent heat flux (LE). During N1 breeze events, the mountain breeze decreases Hs and LE and increases $$\boldsymbol{F}_{\text{CO}_{2}}$$ . For N2 breeze events, the southeast wind from the lake surface increases Hs and LE and decreases $$\boldsymbol{F}_{\text{CO}_{2}}$$ . Results indicate that lakes in mountainous areas promote latent heat mixing but suppress carbon dioxide exchange.

Cable-car measurements of vertical aerosol profiles impacted by mountain-valley breezes in Lushan Mountain, East China

In-situ field observations of vertical aerosol profiles for one month in complex terrain (Lushan Mountain, China) were carried out using a cable car, which resolved detailed vertical distributions of mountain aerosols with low-cost operation. Cable-car observations were conducted during the early morning and late afternoon, when mountain and valley winds dominated, respectively. The diurnal aerosol variations at the top and foot of Lushan Mountain were analyzed based on environmental and meteorological stations. The observations indicated that the mountain-valley breezes notably impacted the mountain-area aerosol distribution under weak weather conditions. More uniform aerosol profiles for the afternoon than the morning, with their decreasing rates of PM2.5 (particles with diameters less than 2.5 μm) were 1.64 and 2.28 μg m−3/100 m, respectively. The PM2.5/PM10 ratio at the mountain top increased from 0.69 to 0.81, and that at the mountain base decreased from 0.75 to 0.70 from morning to afternoon. The PM2.5 concentration decreased in and around Lushan Mountain from daytime to nighttime, with the impacted diameter of the 300-m topography line being smaller than ~5 km, while the concentration increased in Jiujiang City. The relative decreasing rate of PM2.5 was higher at the mountain top site (~20%) than at the base site (~2%) from daytime to nighttime. Moreover, uniform aerosol profiles could have been caused by regional transport through a relatively strong low-level synoptic flow (~5 m s−1) and the mountain's dynamic lifting effect.

Key Points in Air Pollution Meteorology.

Although emissions have a direct impact on air pollution, meteorological processes may influence inmission concentration, with the only way to control air pollution being through the rates emitted. This paper presents the close relationship between air pollution and meteorology following the scales of atmospheric motion. In macroscale, this review focuses on the synoptic pattern, since certain weather types are related to pollution episodes, with the determination of these weather types being the key point of these studies. The contrasting contribution of cold fronts is also presented, whilst mathematical models are seen to increase the analysis possibilities of pollution transport. In mesoscale, land–sea and mountain–valley breezes may reinforce certain pollution episodes, and recirculation processes are sometimes favoured by orographic features. The urban heat island is also considered, since the formation of mesovortices determines the entry of pollutants into the city. At the microscale, the influence of the boundary layer height and its evolution are evaluated; in particular, the contribution of the low-level jet to pollutant transport and dispersion. Local meteorological variables have a major influence on calculations with the Gaussian plume model, whilst some eddies are features exclusive to urban environments. Finally, the impact of air pollution on meteorology is briefly commented on.

Diurnal variation of summer precipitation modulated by air pollution: observational evidences in the beijing metropolitan area

Studying precipitation diurnal variation characteristics is crucial to better understand their formation mechanism. Moreover, it is fundamental in assessing regional climate variability and to validate the effectiveness of cloud and precipitation parameterization schemes in weather and climate models. Based on observational data from 2008 to 2017, this manuscript objective is to assess the aerosol effects on summer precipitation on an hourly-scale diurnal variation basis in the Beijing metropolitan area. The results put in evidence that the precipitation frequency and duration on polluted days were 25.0% and 14.8% lower with respect to clean days. No significant differences were observed in total daily accumulated rainfall between clear and polluted days. While heavy precipitation mean intensity on polluted days increased by 13.5%, which is potentially linked with aerosol microphysical effects. Note that precipitation occurs earlier on polluted days, with peak time of 1 ∼ 2-hour in advance compared with clean days over the urban areas of Beijing, which may be primarily ascribed to the influence of advanced turning local circulation of mountain-valley breezes and urban heat island circulation due to aerosol-radiation effects.

Sub-national variability of wind power generation in complex terrain and its correlation with large-scale meteorology

The future European electricity system will depend heavily on variable renewable generation, including wind power. To plan and operate reliable electricity supply systems, an understanding of wind power variability over a range of spatio-temporal scales is critical. In complex terrain, such as that found in mountainous Switzerland, wind speeds are influenced by a multitude of meteorological phenomena, many of which occur on scales too fine to capture with commonly used meteorological reanalysis datasets. Past work has shown that anticorrelation at a continental scale is an important way to help balance variable generation. Here, we investigate systematically for the first time the possibility of balancing wind variability by exploiting anticorrelation between weather patterns in complex terrain. We assess the capability for the Consortium for Small-scale Modeling (COSMO)-REA2 and COSMO-REA6 reanalyses (with a 2 and 6 km horizontal resolution, respectively) to reproduce historical measured data from weather stations, hub height anemometers, and wind turbine electricity generation across Switzerland. Both reanalyses are insufficient to reproduce site-specific wind speeds in Switzerland’s complex terrain. We find however that mountain-valley breezes, orographic channelling, and variability imposed by European-scale weather regimes are represented by COSMO-REA2. We discover multi-day periods of wind electricity generation in regions of Switzerland which are anticorrelated with neighbouring European countries. Our results suggest that significantly more work is needed to understand the impact of fine scale wind power variability on national and continental electricity systems, and that higher-resolution reanalyses are necessary to accurately understand the local variability of renewable generation in complex terrain.

The transboundary transport of air pollutants and their environmental impacts on Tibetan Plateau

The Tibetan Plateau is the largest high altitude landform on Earth, with an area of over 2.5×106 km2 and an average elevation of ~4000 m above sea level. With a unique multisphere environmental system, the Tibetan Plateau provides an important ecological sheltering function for China and other parts of Asia. The Tibetan Plateau is one of the world’s most pristine regions, benefiting from a sparse population with negligible local influence on its environment. However, it is surrounded by some of the most polluted areas in the world, such as South Asia, East Asia, and Southeast Asia. With the atmospheric circulation, such pollutants may impact the Tibetan Plateau through long-range transport. Clearly, the scientific research on the transboundary transport of pollutants is not only important for the understanding of multisphere interactions on the earth surface, but also could meet the national strategic needs for ecological and environmental protection. Long-term monitoring combined with short-term intensive observation campaigns, were used to comprehensively summarize the latest research progress regarding the spatial-temporal distribution and transport mechanism of air pollutants, as well as their climate and ecological impacts, which were achieved during the Second Tibetan Plateau Scientific Expedition. With respect of historical trends reconstructed from environmental archives, e.g., glacial ice cores and lake sediments, the black carbon and heavy metals like mercury show a dramatic increase since 1950s, which reflect the enhanced emission of air pollutants in Asia. On-line observation data and WRF-Chem modeling indicate that upper air circulation and local mountain-valley breeze system are the main drivers of trans Himalaya air pollution from South Asia. A regional climate-chemistry model coupled with an aerosol-snow/ice feedback module was used to reveal the natural and anthropogenic light-absorbing aerosols’ radiative effects over the Tibetan Plateau. Results indicated that the mineral dust both in the atmosphere and snow induced 0.1−0.5°C warming over the western Tibetan Plateau and Kunlun Mountains in spring. Meanwhile, dust aerosols caused snow water equivalent to decrease by 5−25 mm over the western TP, Himalayas and Pamir Mountains in winter and spring. The radiative effects of BC-in-snow induced surface temperature increased by 0.1− 1.5°C and snow water equivalent decreased by 5−25 mm over the western Tibetan Plateau and Himalayas. According to the observations the black carbon and dust found in the snow and ice on the surfaces of glaciers were responsible for on average 20% of the albedo reduction within the TP region. Those atmospherically transported pollutants also have obvious negative impacts on the ecosystem in Tibetan Plateau. For example, bioaccumulation of DDTs have been found in Tibetan terrestrial and aquatic food chains, and newly emerging compounds such as polyfluoroalkyl substances and hexabromocyclododecanes have been widely detected in wild fish species. Therefore, the corresponding ecological risks are of great concern. In the future, it is necessary to quantify the extent of atmospherically transported pollution and model the pollutant fate under the future environmental scenarios as well as establish environmental and health risk.

Aerosol optical properties and the mixing state of black carbon at a background mountainous site in Eastern China

In-situ measurements of aerosol optical properties were conducted at Mt. Huang from September 23 to October 28, 2012. Low averages of 82.2, 10.9, and 14.1 Mm−1 for scattering coefficient (σsp, neph, 550), hemispheric backscattering coefficient (σhbsp, neph, 550), and absorption coefficient (σap, 550), respectively, were obtained. Atmospheric aging process resulted in the increase of σap, 550 but the decrease of the single scattering albedo (ω550) at constant aerosol concentration. However, the proportion of non-light-absorbing components (non-BCs) was getting higher during the aging process, resulting in the increase of aerosol diameter, which also contributed to relatively higher σsp, neph, 550 and ω550. Diurnal cycles of σsp, neph, 550 and σap, 550 with high values in the morning and low values in the afternoon were observed closely related to the development of the planetary boundary layer and the mountain-valley breeze. BC mixing state, represented by the volume fraction of externally mixed BC to total BC (r), was retrieved by using the modified Mie model. The results showed r reduced from about 70% to 50% when the externally mixed non-BCs were considered. The periodical change and different diurnal patterns of r were due to the atmospheric aging and different air sources under different synoptic systems. Local biomass burning emissions were also one of the influencing factors on r. Aerosol radiative forcing for different mixing state were evaluated by a “two-layer-single-wavelength” model, showing the cooling effect of aerosols weakened with BC mixing state changing from external to core-shell mixture.

Influence of Mountain Valley Breeze and Sea Land Breeze in Winter on Distribution of Air Pollutants in Beijing-Tianjin-Hebei Region

To reveal the effect of Mountain Valley Breeze (MVB) and Sea Land Breeze (SLB) in winter on the spatial-temporal distribution of air pollutants in the Beijing-Tianjin-Hebei region, hourly data from Automatic Weather Stations (AWS) and hourly air pollutant concentration data in December 2016 from the China National Environmental Monitoring Center were used to calculate the average wind vector fields and PM2.5 concentration fields. The change rule of MVB and SLB and its influence on the distribution of PM2.5 concentration were analyzed. The prevailing factor for the MVB days was the southerly wind (valley wind) in the Beijing-Tianjin-Hebei region from noon to afternoon, this valley wind transports air pollutants from the eastern areas of the Taihang Mountains and southwestern areas of Beijing northward. In the evening, "herringbone" convergence lines formed between the emerging mountain breeze in the western and northern parts of Beijing, as well as in the piedmont areas of the Taihang Mountains, and the southerly wind. The PM2.5 concentration increased in Beijing, Langfang, Baoding, Shijiazhuang, and Xingtai according to the concentration of the convergence lines. For the SLB days, the PM2.5 concentration increased in the piedmont areas of the Taihang Mountains due to the influence of valley wind from noon to afternoon. For the MVB days, from noon until midnight, the sea breeze appeared in the eastern coastal areas and reached the southeastern part of Tianjin, the PM2.5 concentration increased towards the front of the sea breeze. The influence of MVB and SLB on the distribution of air pollutants in the heavy pollutions process were surveyed by analyzing the temporal variation relationship between the vertical distribution of wind over 0-325 m tower (at the Institute of Atmospheric Physics) and PM2.5 concentration of urban area, and by using the Cressman method to interpolate the 10 m wind data and PM2.5 concentration data to 2D grid field. From noon to afternoon, the air pollutants were blown to Beijing by valley wind. In the evening, the air pollutants converged near the convergence lines, which were formed by the mountain breeze and southerly wind. The severe pollution zone formed in the plains of Beijing and south of Beijing. From midnight to early morning, the air pollutants in Beijing were gradually blown away by the mountain breeze and stayed south of Beijing and northwest of Tianjin. In the winter, the effect of MVB on the recycling and accumulation of air pollutants plays an important role in severe atmospheric pollution incidents in Beijing, south of Beijing, and the eastern areas of the Taihang Mountains.

Altitudinal patterns of wind transport and deposition of Yungas tree pollen in northwestern Argentina: Implications for interpreting the Quaternary fossil record

The presence of pollen from Yungas tree species in fossil sequences located above the tree-line has led to alternative interpretations of past climate changes in tropical and subtropical Andes. We studied atmospheric and surface pollen deposition rates of Yungas trees along an altitudinal gradient (1700–3800 m a.s.l.) to understand the role of local wind patterns in modern pollen transport to mountain regions. Pollen from the Montane forest (Alnus and Juglans) and the Montane rainforest (Anadenanthera, Celtis and Myrtaceae) becomes airborne and is transported upslope well above the tree-line. The resulting patterns of pollen deposition show a secondary maximum in the Montane grassland, consistent with the development of a mountain-valley breeze system during the flowering period of the pollen producer species. Given that the pattern is also preserved in the surface sediment, we propose that these tree pollen types are good indicators of mesoscale atmospheric circulation and could provide a proxy for this circulation system in Quaternary records. This alternative approach to interpreting fossil pollen records of Yungas species in highlands of the Central Andes has implications for regional reconstructions of past climate.

Impacts of sea-land and mountain-valley circulations on the air pollution in Beijing-Tianjin-Hebei (BTH): A case study.

In the study, observational data analyses and the WRF-CHEM model simulations are used to investigate the role of sea-land and mountain-valley breeze circulations in a severe air pollution event occurred in Beijing-Tianjin-Hebei (BTH) during August 9-10, 2013. Both the wind observations and the model simulations have clearly indicated the evolution of the sea-land and mountain-valley breeze circulations during the event. The WRF-CHEM model generally reproduces the local meteorological circulations and also performs well in simulating temporal variations and spatial distributions of fine particulate matters (PM2.5) and ozone (O3) concentrations compared to observations in BTH. The model results have shown that the offshore land breeze transports the pollutants formed in Shandong province to the Bohai Gulf in the morning, causing the formation of high O3 and PM2.5 concentrations over the gulf. The onshore sea breeze not only causes the formation of a convergence zone to induce upward movement, mitigating the surface pollution to some degree, also recirculates the pollutants over the gulf to deteriorate the air quality in the coastal area. The upward valley breeze brings the pollutants in the urban area of Beijing to the mountain area in the afternoon, and the downward mountain breeze transports the pollutants back during nighttime. The intensity of the mountain-valley breeze circulation is weak compared to the land-sea breeze circulation in BTH. It is worth noting that the local circulations play an important role when the large-scale meteorological conditions are relatively weak.