Impact of aerosol-boundary layer interactions on PM2.5 pollution during cold air pool events in a semi-arid urban basin

Global emission reductions still must address winter fine particulate matter (PM2.5) pollution in urban basins with enclosed terrains and frequent cold air pool (CAP) events when the temperatures within the basin are colder than above it. The effects of urban basin aerosol-boundary layer interactions on PM2.5 pollution during CAP events remain unclear. Intensive boundary layer observations in January 2021 and numerical models were used to investigate this issue in the semi-arid urban Lanzhou Basin of China. The results showed that CAPs formed because of the synoptic weather system that exacerbated the warming over the basin. The CAPs in this experiment were characterized by stronger temperature inversion (TI) layers in the vertical direction and lower relative humidity, lower wind speed, and weaker turbulence at the bottom of the basin compared to other conditions. The strong TI layers below the top of the basin inhibited the vertical dispersion of pollutants in the basin and concentrated the PM2.5 within a height of 0.3 km from the bottom of the basin. During CAP events, the proportion of elemental carbon in PM2.5 increased, whereas that of secondary inorganic species decreased. Aerosol absorption increased faster than scattering during CAP events. Therefore, the mean single scattering albedo decreased from 0.85 during non-CAP periods to 0.81 during CAP events. Radiosonde-sounding observations and numerical simulations indicated that aerosols accumulating in the lower basin heated the atmosphere during the daytime and facilitated boundary layer development via the "stove effect" (absorption aerosol heats lower atmosphere to promote boundary layer development). No significant "dome effect" (absorption aerosol heats the upper boundary layer to suppress boundary layer development) occurred during the two CAP events. These findings provide a theoretical basis for scientifically-guided PM2.5 pollution control in winter in isolated urban basins.

Inverse effects of aerosol radiative forcing on heavy PM2.5 pollution of local accumulation and regional transport over Central China

Regional transport of air pollutants is a crucial factor influencing atmospheric environment, and aerosol radiative forcing (ARF) feedback to atmospheric boundary layer (ABL) structure and ambient air pollution is yet to be comprehensively understood over the receptor region of regional transport. By simulating meteorology and air pollutants during a heavy PM2.5 pollution event with WRF-Chem model, we quantitatively investigated the ARF and ABL interaction for PM2.5 pollution over the Twain-Hu Basin (THB), a key receptor region of regional transport over central China. Driven by northerly winds, PM2.5 was transported from upstream north China to downstream THB accompanied by high PM2.5 levels in the free troposphere. The ARF exacerbated local PM2.5 accumulation by up to 20 μg m-3 and inhibited the impact of regional transport on PM2.5 levels in the ABL with reducing near-surface PM2.5 concentrations of 5 μg m-3 over the THB. The ARF-intensified air temperature inversion at the top of ABL was unfavorable for the transported air pollutants crossing the ABL top to the near-surface layer, thus weakening the impact of regional PM2.5 transport on air quality in the receptor region. Meanwhile, the ARF of transported PM2.5 induced updrafts in the free troposphere, promoting vertical mixing of air pollutants with positive feedback on increasing secondary PM2.5 concentrations in the free troposphere. The ARF induced more and less secondary PM2.5 formations respectively in the free troposphere and the near-surface layer during the regional transport period of air pollution. These results enhance our comprehension of aerosol-meteorology feedback in regional changes of atmospheric environment with inverse effects of ARF on PM2.5 pollution of local accumulation and regional transport.

High black carbon episodes over a polluted metropolis near the land-sea boundary and their impact on associated atmospheric dynamics

The present investigation outlines the crucial factors that influence the black carbon (BC) concentrations over a polluted metropolis, Kolkata (22.57° N, 88.37° E), India. Located in the eastern part of the Indo Gangetic Plain (IGP) outflow region and close to the land-ocean boundary, Kolkata is subject to contrasting seasonal maritime airflow from the Bay of Bengal and continental air mass from the IGP and Tibetan plateau region, which modulates the local concentration of BC. The origin of aerosol transport and associated atmospheric dynamics with high and low BC activities over Kolkata are examined during 2012-2015 using data from multi-technique sources which include measurements of ground-based instruments of aethalometer and multi-frequency microwave radiometer, reanalysis data from ERA-5 and MEERA-2, and model outputs from HYPSLIT back trajectory model simulations. The study highlights the control of IGP wind inflow on the occurrence of anomalous enhancements in BC concentration during weekends and holidays when local emissions are low. High BC events are associated with enhanced atmospheric heating below the boundary layer (2000 m) and significant negative surface radiative forcing. The response of the boundary layer to high and low BC episodes, shown in the diurnal variation in comparison with the seasonal mean, is investigated. Dominant suppression of morning and night-time boundary layer height is observed on high BC days. During the daytime in pre-monsoon, post-monsoon, and winter seasons, boundary layer height peaks are found to be strongly controlled by high BC episode occurrences as obtained from the hourly data of ERA-5.

The interaction between black carbon and planetary boundary layer in the Yangtze River Delta from 2015 to 2020: Why O3 didn't decline so significantly as PM2.5

Since the Air Pollution Prevention and Control Action Plan (air clean plan) issued in 2013, air quality has been in continuous improvement. The second stage of air clean plan since 2018 was focused on O₃ controlling, but it still didn't decline so significantly as PM_2.5. This study conducted a long-term observation on black carbon (BC) and utilized the observational data of other air pollutants (PM_2.5, PM₁₀, NO₂, SO₂, CO and O₃), the meteorological elements and the vertical sounding data of PBL in Nanjing. In the daytime (08:00-20:00), PM_2.5 kept decreasing from 2015 to 2020 at the rate of 4.8 μg⋅m^-3⋅a^-1, however, BC increased at the rate of 0.6 μg⋅m^-3⋅a^-1, which has led to the continuous growth of BC/PM_2.5 (0.9%⋅a^-1). However, during this period, O₃ was relatively stable and, in 2020, it returned below its value in 2015 after slight increases in 2017 and 2018. Meanwhile, the average surface temperature had increased by around 1.0 °C during 2015-2019 at the rate of 0.3 °C⋅a^-1. Also, the average height of the inversion layer had increased significantly by 494.0 and 176.7 m at 20:00 and 08:00, whose growth ratio was up to 57% and 25%, respectively. The above observation results have formed a set of chain reactions as follows. The growth of the surface BC caused the surface temperature to rise due to the increasing heating effect of BC. The continuous growth of the surface temperature made it easier for the PBL height to develop, which led to the lift of the inversion layer in the PBL and the larger atmospheric environment capacity. Ultimately, it is conducive to the diffusion of the near surface pollutants, thus helping reduce their concentrations, which offsets the increasing tendency of O₃ and add to the decreasing trend of PM_2.5. This phenomenon is the most remarkable in summer, with the fastest increasing rate of temperature (0.8 °C⋅a^-1) and O₃ (3.9 μg⋅m^-3⋅a^-1) during 2015-2019 (excluding 2020 to erase the great effect of COVID-19 lockdown on emissions).

Vertical profiles of the transport fluxes of aerosol and its precursors between Beijing and its southwest cities

The influence of regional transport on aerosol pollution has been explored in previous studies based on numerical simulation or surface observation. Nevertheless, owing to inhomogeneous vertical distribution of air pollutants, vertical observations should be conducted for a comprehensive understanding of regional transport. Here we obtained the vertical profiles of aerosol and its precursors using ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) at the Nancheng site in suburban Beijing on the southwest transport pathway of the Beijing-Tianjin-Hebei (BTH) region, China, and then estimated the vertical profiles of transport fluxes in the southwest-northeast direction. The maximum net transport fluxes per unit cross-sectional area, calculated as pollutant concentration multiply by wind speed, of aerosol extinction coefficient (AEC), NO₂, SO₂ and HCHO were 0.98 km^-1 m s^-1, 24, 14 and 8.0 μg m^-2 s^-1 from southwest to northeast, which occurred in the 200-300 m, 100-200 m, 500-600 m and 500-600 m layers, respectively, due to much higher pollutant concentrations during southwest transport than during northeast transport in these layers. The average net column transport fluxes were 1200 km^-1 m² s^-1, 38, 26 and 15 mg m^-1 s^-1 from southwest to northeast for AEC, NO₂, SO₂ and HCHO, respectively, in which the fluxes in the surface layer (0-100 m) accounted for only 2.3%-4.2%. Evaluation only based on surface observation would underestimate the influence of the transport from southwest cities to Beijing. Northeast or weak southwest transports dominated in clean conditions with PM_2.5 <75 μg m^-3 and intense southwest transport dominated in polluted conditions with PM_2.5 >75 μg m^-3. Southwest transport through the middle boundary layer was a trigger factor for aerosol pollution events in urban Beijing, because it not only directly bringing air pollutants, but also induced an inverse structure of aerosols, which resulted in stronger atmospheric stability and aggravated air pollution in urban Beijing.

Comprehensive study of aerosols properties over various terrain types

Aerosols are a crucial part of the climate system. Numerous factors, including aerosols, govern Earth's radiation balance. Different aerosols have distinct radiational effects on the earth system, and thus the slight change in their composition may lead to a drastic change in their radiative effects. Aerosols' chemical and physical properties also depend on generation processes, generation source, and geographical location. Significant spatio-temporal inconsistency is noticed in the distribution of aerosols. It makes it much difficult task to assess their radiative properties. We attempted to explore aerosol's optical properties and wavelength dependence over different locations. We have used AERONET (Aerosol Robotic Network) data over various stations (Kanpur, Jaipur, Gandhi College, Pune) with varying terrain properties in the Indian continent. We have studied the variation of different optical parameters: aerosol optical depth (AOD), single scattering albedo (SSA), and Angstrom exponent (α), and their wavelength dependence. This study indicated that Jaipur is the cleanest site, with dust aerosols as a primary aerosol. Though over Pune also aerosol concentration was relatively low but the anthropogenic aerosols contributed primarily over this site. Over the Indo-Gangetic Plain (IGP) sites, dust aerosols dominated the pre-monsoon season, while anthropogenic aerosols dominated the post-monsoon and winter seasons. The scatter plot of AOD with α gives the details of different aerosols (desert dust, continental aerosols, mixed aerosol, biomass burning aerosols, and sulfate aerosols) in the different seasons and places. This study provides an overview of aerosol properties, dominant aerosols in the aerosol system, and their seasonal and spectral variation.

Climatic–Environmental Effects of Aerosols and Their Sensitivity to Aerosol Mixing States in East Asia in Winter

To establish the direct climatic and environmental effect of anthropogenic aerosols in East Asia in winter under external, internal, and partial internal mixing (EM, IM and PIM) states, a well-developed regional climate–chemical model RegCCMS is used by carrying out sensitive numerical simulations. Different aerosol mixing states yield different aerosol optical and radiative properties. The regional averaged EM aerosol single scattering albedo is approximately 1.4 times that of IM. The average aerosol effective radiative forcing in the atmosphere ranges from −0.35 to +1.40 W/m2 with increasing internal mixed aerosols. Due to the absorption of black carbon aerosol, lower air temperatures are increased, which likely weakens the EAWM circulations and makes the atmospheric boundary more stable. Consequently, substantial accumulations of aerosols further appear in most regions of China. This type of interaction will be intensified when more aerosols are internally mixed. Overall, the aerosol mixing states may be important for regional air pollution and climate change assessments. The different aerosol mixing states in East Asia in winter will result in a variation from 0.04 to 0.11 K for the averaged lower air temperature anomaly and from approximately 0.45 to 2.98 μg/m3 for the aerosol loading anomaly, respectively, due to the different mixing aerosols.

Drought self-propagation in drylands due to land-atmosphere feedbacks

Reduced evaporation due to dry soils can affect the land surface energy balance, with implications for local and downwind precipitation. When evaporation is constrained by soil moisture, the atmospheric supply of water is depleted, and this deficit may propagate in time and space. This mechanism could theoretically result in the self-propagation of droughts, but the extent to which this process occurs is unknown. Here we isolate the influence of soil moisture drought on downwind precipitation using Lagrangian moisture tracking constrained by observations from the 40 largest recent droughts worldwide. We show that dryland droughts are particularly prone to self-propagating, because evaporation tends to respond strongly to enhanced soil water stress. In drylands precipitation can decline by more than 15% due to upwind drought in during a single event, and up to 30% during individual months. In light of projected widespread reductions in water availability, this feedback may further exacerbate future droughts.

Atmospheric Deposition on the Southwest Coast of the Southern Basin of Lake Baikal

A precipitation monitoring station in Listvyanka was set up to determine the potential impact of the coastal area on the state of the adjacent air environment above Lake Baikal on its southwest coast. This article presents the results of studying the chemical composition of atmospheric deposition (aerosols and precipitation) at this station in 2020, and of their comparison with the data from previous years (from 2000 to 2019). In 2020, the ionic composition of atmospheric aerosols and precipitation had changed compared to previous years. In the modern period, the total amount of ions in aerosols, accounting for 0.46 ± 0.40 μg∙m−3, was lower by an order of magnitude than between 2000 and 2004. The average annual total amount of ions in precipitation in Listvyanka was almost unchanged from the average values in 2000–2010 and was 10% lower than that from 2011 to 2019 (7.3 mg/L). The ratio of major ions of sulphates and ammonium changed in the aerosol composition: compared to the period from 2000 to 2004, in 2020, the contribution of ammonium ions had decreased significantly, from 32% to 24%; the contribution of sulphates had increased to 43%, and the contribution of calcium had increased from 8 to 13%. Since 2010, the contribution of K+ ions has increased to 8–10%, indicating the effect of smoke aerosols from wildfires. In precipitation, despite the dominance of sulphates (26%) and calcium (18%) throughout the year, the contribution of nitrates increases to 19% during the cold season (from October to March), while the contribution of ammonium ions and hydrogen ions increases to 13% and 17%, respectively, in the warm season (from April to September). In 2020, as in previous research years, the acidity of precipitation at the Listvyanka station was elevated (pH 5.1 ± 0.5); 50% of precipitation in 2020 had pH ˂ 5. We quantified ions in atmospheric aerosols and precipitation on the underlying surface of the coastal southwestern part of Lake Baikal. Ion fluxes with precipitation were the highest in the warm season, which corresponds to the annual maximum precipitation. Unlike previous years (from 2000 to 2010 and from 2011 to 2019), wet deposition of most ions—especially calcium, ammonium and nitrates—had decreased in 2020. There was a 35-fold decrease in nitrogen fluxes and a 5-fold decrease in sulphur fluxes in aerosols, as well as 1.6-fold and 1.3-fold decreases, respectively, in precipitation.

Impacts of the San Francisco Bay Area shelter-in-place during the COVID-19 pandemic on urban heat fluxes

The purpose of this study was to make quantitative connections between changes in social and economic activities in northern California urban areas and related Earth system environmental responses to the COVID-19 pandemic in 2020. We tested the hypothesis that the absence of worker activities during Shelter-in-Place in the San Francisco Bay Area detectably altered the infrared heat flux from parking lots, highways, and large building rooftops, caused primarily by quantitative changes in the reflective properties in these different classes of urban surfaces. The Landsat satellite's thermal infrared (TIR) sensor imagery for surface temperature (ST) was quantified for all the large urban features in the Bay Area that have flat (impervious) surfaces, such parking lots, wide roadways, and rooftops. These large impervious surface features in the five-county Bay Area were first delineated and classified using sub-meter aerial imagery from the National Agriculture Imagery Program (NAIP). We then compared Landsat ST data acquired on (or near) the same dates from the three previous years (2017-2019) for all these contiguous impervious surfaces. Results showed that all the large parking lots, roadway corridors, and industrial/commercial rooftops across the entire Bay Area urban landscape were detected by Landsat ST time series as significantly cooler (by 5^o C to 8^o C) during the unprecedented Shelter-in-Place period of mid-March to late-May of 2020, compared to same months of the three previous years. The explanation for this region-wide cooling pattern in 2020 that was best supported by both remote sensing and ground-based data sets was that relatively low atmospheric aerosol lower (PM_2.5) concentrations from mid-March to late May of 2020 resulted in weaker temperature inversions over the Bay Area, higher diurnal surface mixing, and lowered urban surface temperatures, compared to the three previous years.

Explosive Secondary Aerosol Formation during Severe Haze in the North China Plain

Severe haze events with exceedingly high-levels of fine aerosols occur frequently over the past decades in the North China Plain (NCP), exerting profound impacts on human health, weather, and climate. The development of effective mitigation policies requires a comprehensive understanding of the haze formation mechanisms, including identification and quantification of the sources, formation, and transformation of the aerosol species. Haze evolution in this region exhibits distinct physical and chemical characteristics from clean to polluted periods, as evident from increasing stagnation and relative humidity, but decreasing solar radiation as well as explosive secondary aerosol formation. The latter is attributed to highly elevated concentrations of aerosol precursor gases and is reflected by rapid increases in the particle number and mass concentrations, both corresponding to nonequilibrium chemical processes. Considerable new knowledge has been acquired to understand the processes regulating haze formation, particularly in light of the progress in elucidating the aerosol formation mechanisms. This review synthesizes recent advances in understanding secondary aerosol formation, by highlighting several critical chemical/physical processes, that is, new particle formation and aerosol growth driven by photochemistry and aqueous chemistry as well as the interaction between aerosols and atmospheric stability. Current challenges and future research priorities are also discussed.

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 PM_2.5 (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 PM_2.5, as well as interactions between aerosols and the atmospheric boundary layer (ABL). In this study, vertical distributions of PM_2.5 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 PM_2.5 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 PM_2.5 and their evolution are largely constrained by the ABL thermodynamics during daytime, but show much less dependence on the ABL evolution at nighttime. PM_2.5 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.

Entanglement of near-surface optical turbulence to atmospheric boundary layer dynamics and particulate concentration: implications for optical wireless communication systems

Localized reduction in optical turbulence due to enhanced atmospheric heating caused by the solar absorption of aerosol black carbon (BC) is reported. Immediate response of atmospheric turbulence to BC-induced atmospheric warming strongly depends on the available solar radiation (time of the day), BC concentration, and atmospheric boundary layer dynamics. Besides the significant climate implications of a reduction in turbulence kinetic energy, a large reduction in the refractive index structure parameter (Cn2) resulting from BC-induced warming would affect the atmospheric propagation of laser beams. Interestingly, aerosols contribute significantly (up to 25%) to the signal deterioration in optical wireless communication systems during convectively stable atmospheric conditions when higher signal-to-noise ratios are expected otherwise due to the reduced thermal convection. Competing effects of the fractional contributions of aerosol extinction and scintillations on beam attenuation are reported; daytime being largely dominated by scintillation effects while the nighttime being dependent on the ambient aerosol concentration as well. We put forward the entanglement of optical turbulence to aerosol concentration, atmospheric boundary layer dynamics, and surface-reaching solar radiation, and discuss the possible implications for optical propagation.

Refractive Indices of Biomass Burning Aerosols Obtained from African Biomass Fuels Using RDG Approximation

Biomass burning (BB) aerosols contribute to climate forcing, but much is still unknown about the extent of this forcing, owing partially to the high level of uncertainty regarding BB aerosol optical properties. A key optical parameter is the refractive index (RI), which influences the absorbing and scattering properties of aerosols. This quantity is not measured directly, but it is obtained by fitting the measured scattering cross section and extinction cross section to a theoretical model using the RI as a fitting parameter. We used the Rayleigh–Debye–Gans (RDG) approximation to retrieve the complex RI of freshly emitted BB aerosol from two fuels (eucalyptus and olive) from Africa in the spectral range of 500–580 nm. Experimental measurements were carried out using cavity ring-down spectroscopy to measure extinction over the range of wavelengths of 500–580 nm and nephelometry to measure scattering at three wavelengths of 450, 550, and 700 nm for size-selected BB aerosol particles. The fuels were combusted in a tube furnace at a temperature of 800 °C, which is representative of the flaming stage of burning. Filter samples were collected and imaged using tunneling electron microscopy to obtain information on the morphology and size of the particles, which was used in the RDG calculations. The mean radii of the monomers were 27.8 and 31.5 nm for the eucalyptus and the olive fuels, respectively. The components of the retrieved complex RI were in the range of 1.31 ≤ n ≤ 1.56 and 0.045 ≤ k ≤ 0.468. The real and complex parts of the RI increase with increasing particle mobility diameter. The real part of the RI is lower, and the imaginary part is higher than what was recommended in literature for black carbon generated by propane or field measurements from fires of mixed wood samples. Fuel dependent results from controlled laboratory experiments can be used in climate modeling efforts and to constrain field measurements from biomass burning.

Observational study of aerosol-induced impact on planetary boundary layer based on lidar and sunphotometer in Beijing

Atmospheric aerosols have been found to influence the development of planetary boundary layer (PBL) and hence to aggravate haze pollution in megacities. PBL height (PBLH) determines the vertical extent to which the most pollutant effectively disperses and is a key argument in pollution study. In this study, we quantitatively evaluate aerosol radiation effect on PBL, as well as assessment of surface cooling effect and atmosphere heating effect. All the data are measured at a site of Beijing from 2014 to 2017, of which PBLH is retrieved from micro pulse lidar and aerosol optical depth (AOD) from sunphotometer. Case study shows qualitatively that relative high aerosol load reduces PBLH, and in turn causes a high surface PM_2.5 concentration. We preliminarily reveal the influential mechanism of aerosol on PBL. The influence of aerosol on the radiation flux of PBL is analyzed, with the correlation coefficient (R) of 0.938 between AOD and radiative forcing of BOA (RF_BOA) and R = 0.43 between RF_BOA and PBLH. Also, AOD is found to negatively correlate with PBLH (R = -0.41). With the increase of AOD, the cooling effect of surface is enhanced, and further impede the development of PBL. Due to aerosol-induced reduction of PBLH, near surface PM_2.5 concentration surges and presents an exponential growth following AOD. Then, it is speculated and testified that the relationship between SSA (single scatting albedo) and PBLH would be determined by the location of absorbing aerosol within PBL. The upper PBL absorbing aerosol may decrease PBLH, while the lower absorbing aerosol appear to enhance PBLH. The study probably can provide effective observational evidence for understanding the effect of aerosol on PBL and be a reference of air pollution mitigation in Beijing and its surrounding areas.

Severe haze in northern China: A synergy of anthropogenic emissions and atmospheric processes

Regional severe haze represents an enormous environmental problem in China, influencing air quality, human health, ecosystem, weather, and climate. These extremes are characterized by exceedingly high concentrations of fine particulate matter (smaller than 2.5 µm, or PM_2.5) and occur with extensive temporal (on a daily, weekly, to monthly timescale) and spatial (over a million square kilometers) coverage. Although significant advances have been made in field measurements, model simulations, and laboratory experiments for fine PM over recent years, the causes for severe haze formation have not yet to be systematically/comprehensively evaluated. This review provides a synthetic synopsis of recent advances in understanding the fundamental mechanisms of severe haze formation in northern China, focusing on emission sources, chemical formation and transformation, and meteorological and climatic conditions. In particular, we highlight the synergetic effects from the interactions between anthropogenic emissions and atmospheric processes. Current challenges and future research directions to improve the understanding of severe haze pollution as well as plausible regulatory implications on a scientific basis are also discussed.

The Effect of Aerosol Radiative Heating on Turbulence Statistics and Spectra in the Atmospheric Convective Boundary Layer: A Large-Eddy Simulation Study

Turbulence statistics and spectra in a radiatively heated convective boundary layer (CBL) under aerosol pollution conditions are less investigated than their counterparts in the clear CBL. In this study, a large-eddy simulation (LES) coupled with an aerosol radiative transfer model is employed to determine the impact of aerosol radiative heating on CBL turbulence statistics. One-dimensional velocity spectra and velocity–temperature cospectra are invoked to characterize the turbulence flow in the CBL with varying aerosol pollution conditions. The results show that aerosol heating makes the profiles of turbulent heat flux curvilinear, while the total (turbulent plus radiative) heat flux profile retains the linear relationship with height throughout the CBL. The horizontal and vertical velocity variances are reduced significantly throughout the radiatively heated CBL with increased aerosol optical depth (AOD). The potential temperature variance is also reduced, especially in the entrainment zone and near the surface. The velocity spectral density tends to be smaller overall, and the peak of the velocity spectra is shifted toward larger wavenumbers as AOD increases. This shift reveals that the energy-containing turbulent eddies become smaller, which is also supported by visual inspection of the vertical velocity pattern over horizontal planes. The modified CBL turbulence scales for velocity and temperature are found to be applicable for normalizing the corresponding profiles, indicating that a correction factor for aerosol radiative heating is needed for capturing the general features of the CBL structure in the presence of aerosol radiative heating.

New positive feedback mechanism between boundary layer meteorology and secondary aerosol formation during severe haze events

Severe haze events during which particulate matter (PM) increases quickly from tens to hundreds of microgram per cubic meter in 1–2 days frequently occur in China. Although it has been known that PM is influenced by complex interplays among emissions, meteorology, and physical and chemical processes, specific mechanisms remain elusive. Here, a new positive feedback mechanism between planetary boundary layer (PBL), relative humidity (RH), and secondary PM (SPM) formation is proposed based on a comprehensive field experiment and model simulation. The decreased PBL associated with increased PM increases RH by weakening the vertical transport of water vapor; the increased RH in turn enhances the SPM formation through heterogeneous aqueous reactions, which further enhances PM, weakens solar radiation, and decreases PBL height. This positive feedback, together with the PM-Radiation-PBL feedback, constitutes a key mechanism that links PM, radiation, PBL properties (e.g. PBL height and RH), and SPM formation, This mechanism is self-amplifying, leading to faster PM production, accumulation, and more severe haze pollution.

Modeling the radiative effects of biomass burning aerosols on carbon fluxes in the Amazon region

Every year, a dense smoke haze covers a large portion of South America originating from fires in the Amazon Basin and central parts of Brazil during the dry biomass burning season between August and October. Over a large portion of South America, the average aerosol optical depth at 550 nm exceeds 1.0 during the fire season, while the background value during the rainy season is below 0.2. Biomass burning aerosol particles increase scattering and absorption of the incident solar radiation. The regional-scale aerosol layer reduces the amount of solar energy reaching the surface, cools the near-surface air, and increases the diffuse radiation fraction over a large disturbed area of the Amazon rainforest. These factors affect the energy and CO2 fluxes at the surface. In this work, we applied a fully integrated atmospheric model to assess the impact of biomass burning aerosols in CO2 fluxes in the Amazon region during 2010. We address the effects of the attenuation of global solar radiation and the enhancement of the diffuse solar radiation flux inside the vegetation canopy. Our results indicate that biomass burning aerosols led to increases of about 27% in the gross primary productivity of Amazonia and 10% in plant respiration as well as a decline in soil respiration of 3%. Consequently, in our model Amazonia became a net carbon sink; net ecosystem exchange during September 2010 dropped from +101 to -104 TgC when the aerosol effects are considered, mainly due to the aerosol diffuse radiation effect. For the forest biome, our results point to a dominance of the diffuse radiation effect on CO2 fluxes, reaching a balance of 50-50% between the diffuse and direct aerosol effects for high aerosol loads. For C3 grasses and savanna (cerrado), as expected, the contribution of the diffuse radiation effect is much lower, tending to zero with the increase in aerosol load. Taking all biomes together, our model shows the Amazon during the dry season, in the presence of high biomass burning aerosol loads, changing from being a source to being a sink of CO2 to the atmosphere.

Aerosol and boundary-layer interactions and impact on air quality

Air quality is concerned with pollutants in both the gas phase and solid or liquid phases. The latter are referred to as aerosols, which are multifaceted agents affecting air quality, weather and climate through many mechanisms. Unlike gas pollutants, aerosols interact strongly with meteorological variables with the strongest interactions taking place in the planetary boundary layer (PBL). The PBL hosting the bulk of aerosols in the lower atmosphere is affected by aerosol radiative effects. Both aerosol scattering and absorption reduce the amount of solar radiation reaching the ground and thus reduce the sensible heat fluxes that drive the diurnal evolution of the PBL. Moreover, aerosols can increase atmospheric stability by inducing a temperature inversion as a result of both scattering and absorption of solar radiation, which suppresses dispersion of pollutants and leads to further increases in aerosol concentration in the lower PBL. Such positive feedback is especially strong during severe pollution events. Knowledge of the PBL is thus crucial for understanding the interactions between air pollution and meteorology. A key question is how the diurnal evolution of the PBL interacts with aerosols, especially in vertical directions, and affects air quality. We review the major advances in aerosol measurements, PBL processes and their interactions with each other through complex feedback mechanisms, and highlight the priorities for future studies.

Effects of meteorology and secondary particle formation on visibility during heavy haze events in Beijing, China

The causes of haze formation in Beijing, China were analyzed based on a comprehensive measurement, including PBL (planetary boundary layer), aerosol composition and concentrations, and several important meteorological parameters such as visibility, RH (relative humidity), and wind speed/direction. The measurement was conducted in an urban location from Nov. 16, 2012 to Jan. 15, 2013. During the period, the visibility varied from >20 km to less than a kilometer, with a minimum visibility of 667 m, causing 16 haze occurrences. During the haze occurrences, the wind speeds were less than 1m/s, and the concentrations of PM2.5 (particle matter with radius less than 2.5 μm) were often exceeded 200 μg/m(3). The correlation between PM2.5 concentration and visibility under different RH values shows that visibility was exponentially decreased with the increase of PM2.5 concentrations when RH was less than 80%. However, when RH was higher than 80%, the relationship was no longer to follow the exponentially decreasing trend, and the visibility maintained in very low values, even with low PM2.5 concentrations. Under this condition, the hygroscopic growth of particles played important roles, and a large amount of water vapor acted as particle matter (PM) for the reduction of visibility. The variations of meteorological parameters (RH, PBL heights, and WS (wind speed)), chemical species in gas-phase (CO, O3, SO2, and NOx), and gas-phase to particle-phase conversions under different visibility ranges were analyzed. The results show that from high visibility (>20 km) to low visibility (<2 km), the averaged PBL decreased from 1.24 km to 0.53 km; wind speeds reduced from 1m/s to 0.5m/s; and CO increased from 0.5 ppmv to 4.0 ppmv, suggesting that weaker transport/diffusion caused the haze occurrences. This study also found that the formation of SPM (secondary particle matter) was accelerated in the haze events. The conversions between SO2 and SO4 as well as NOx to NO3(-) increased, especially under high humidity conditions. When the averaged RH was 70%, the conversions between SO2 and SO4 accounted for about 20% concentration of PM2.5, indicating that formation of secondary particle matter had important contribution for the haze occurrences in Beijing.