Mixing Layer Height Research Articles

Correlation of aerosol mass near the ground with aerosol optical depth during two seasons in Munich

Relations of the aerosol optical depth (AOD) with aerosol mass concentration near the ground, particulate matter (PM), have been studied on the basis of measurements. The objective is with respect to possible remote sensing methods to get information on the spatial and temporal variation of aerosols which is important for human health effects. Worldwide the AOD of the atmospheric column is routinely monitored by sun-photometers and accessible from satellite measurements also. It is implied here that the AOD is caused mainly by attenuation processes within the mixing layer because this layer includes nearly all atmospheric aerosols. Thus the mixing layer height (MLH) is required together with the AOD, measured by ground-based sun-photometers (around 560nm), to get information about aerosols near the ground. MLH is determined here from surface-based remote sensing. Investigations were performed during two measurement campaigns in and near Munich in May and November/December 2003 on the basis of daily mean values. Using AOD and MLH measurements the aerosol extinction coefficient of the mixing layer has been calculated. This quantity was correlated with the measured PM10, PM2.5 and PM1 mass concentrations near the ground by performing a linear regression and thus providing a mass extinction efficiency giving squares of the correlation coefficients (R2) between 0.48 (PM1 during summer campaign) and 0.90 (PM2.5 during winter campaign). These correlations suggest that the derived mass extinction efficiencies represent a statistically significant relation between the aerosol extinction coefficients and the surface-based PM mass concentrations mainly during winter conditions.

Atmospheric surface layer characteristics of turbulence above the Pantanal wetland regarding the similarity theory

In this manuscript, some micrometeorological characteristics of the Pantanal are presented. This region, one of the greatest wetlands in the world, is located in the central western part of South America. It has very peculiar environmental and ecological characteristics, a great biodiversity and an irregular hydrological cycle, which often presents floods during the wet season and droughts and fires during the dry season. The experimental data were collected in a meteorological tower during the Interdisciplinary Pantanal Experiment (IPE-1), on April–May 1998. This was a transition period between the wet and the dry season, which a shallow water layer was present at several parts of the experimental field, even around the 25 m-height extended meteorological tower. The surface energy budget components associated with the existence of the shallow water layer, its diurnal variability and some turbulent variables related to the surface–atmosphere turbulent exchange processes were investigated. Eddy-correlation and variance methods have been used to estimate turbulent fluxes and to study the validity of the Monin–Obukhov similarity theory for the Pantanal region. A mixed layer slab model has been used to estimate the height of the turbulent mixing-layer and to provide useful information to test the validity of general relationships concerning horizontal wind velocity variances above Pantanal. In general, the dimensionless relationships between turbulent variables and scaling parameters agree well with the ones found in the literature. Some aspects regarding the heat storage in the shallow water layer present interesting information about its role in the surface–atmosphere energy exchanges processes along the day.

Multisensor Estimation of Mixing Heights over a Coastal City

AbstractAn airborne microwave temperature profiler (MTP) was deployed during the Texas 2000 Air Quality Study (TexAQS-2000) to make measurements of boundary layer thermal structure. An objective technique was developed and tested for estimating the mixed layer (ML) height from the MTP vertical temperature profiles. The technique identifies the ML height as a threshold increase of potential temperature from its minimum value within the boundary layer. To calibrate the technique and evaluate the usefulness of this approach, coincident estimates from radiosondes, radar wind profilers, an aerosol backscatter lidar, and in situ aircraft measurements were compared with each other and with the MTP. Relative biases among all instruments were generally less than 50 m, and the agreement between MTP ML height estimates and other estimates was at least as good as the agreement among the other estimates. The ML height estimates from the MTP and other instruments are utilized to determine the spatial and temporal evolution of ML height in the Houston, Texas, area on 1 September 2000. An elevated temperature inversion was present, so ML growth was inhibited until early afternoon. In the afternoon, large spatial variations in ML height developed across the Houston area. The highest ML heights, well over 2 km, were observed to the north of Houston, while downwind of Galveston Bay and within the late afternoon sea breeze ML heights were much lower. The spatial variations that were found away from the immediate influence of coastal circulations were unexpected, and multiple independent ML height estimates were essential for documenting this feature.

A Modelling Study of Flux Imbalance and the Influence of Entrainment in the Convective Boundary Layer

It is frequently observed in field experiments that the eddy covariance heat fluxes are systematically underestimated as compared to the available energy. The flux imbalance problem is investigated using the NCAR’s large-eddy simulation (LES) model imbedded with an online scheme to calculate Reynolds-averaged fluxes. A top–down and a bottom–up tracer are implemented into the LES model to quantify the influence of entrainment and bottom–up diffusion processes on flux imbalance. The results show that the flux imbalance follows a set of universal functions that capture the exponential decreasing dependence on u */w *, where u * and w * are friction velocity and the convective velocity scale, respectively, and an elliptic relationship to z/z i , where z i is the mixing-layer height. The source location in the boundary layer is an important factor controlling the imbalance magnitude and its horizontal and vertical distributions. The flux imbalance of heat and the bottom–up tracer is tightly related to turbulent coherent structures, whereas for the top–down diffusion, such relations are weak to nonexistent. Our results are broadly consistent with previous studies on the flux imbalance problem, suggesting that the published results are robust and are not artefacts of numerical schemes.

Vehicle-related Fine Particulate Air Pollution in Seoul, Korea

Vehicle exhaust is a dominant source of air pollutants in urban areas. Since people are easily exposed to vehicle exhaust particles while driving a car and/or traveling via public transportation, air pollution near traffic has been extensively studied in developed countries. In this paper, investigations on vehicle-related fine particulate air pollution at roadsides and on roads in Seoul, Korea were reviewed to understand air pollution near traffic. Comparison of <TEX>$PM_{10}$</TEX> concentrations in Seoul showed that roadside air is more contaminated than urban air, implying that exposure levels near vehicular emissions are more critical to sensitive persons. Concentrations of ultrafine particles and BC (black carbon) at roadsides of Seoul fluctuate highly for short durations, responding to traffic situations. Diurnal variations of ultrafine particles and BC concentrations at roadsides seem to be affected by traffic volume, mixing layer height, and wind speed. Concentrations of ultrafine particles and BC decrease as distance from the road increases due to dilution during transport. On-road air pollution seems to be more severe than roadside air pollution in Seoul. Since nearby traffic air pollution has not been well understood in Seoul, further studies including various vehicular air pollutants and representative locations are needed.

Aerosol physical properties and Radiative forcing at the outflow region from the Indo‐Gangetic plains during typical clear and hazy periods of wintertime

Results of a campaign mode measurements of column integrated aerosol optical depths, near surface mass concentrations and LIDAR profiles during winter 2004 at Kharagpur located at the vent of the out flow region from the Indo‐Gangetic plains are presented with a view to characterize the atmospheric aerosol radiative forcing during hazy and clear sky conditions. The multispectral optical depths, surface mass concentrations and black carbon are high, while a decrease in Angstrom exponent value was observed during the hazy sky days. During hazy conditions, the LIDAR profiles show a decrease in the mixed layer height leading to confinement and subsidence of aerosols. Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model along with Optical Properties of Aerosols and Clouds (OPAC) are used to perform composite aerosol forcing estimation, incorporating LIDAR derived mixed layer heights. A considerable decrease in estimated single scattering albedo has been observed during hazy sky days. The atmospheric absorption during hazy sky days increased by 75% compared to clear days. Negative forcing was observed both at the top of the atmosphere and at the surface. The forcing estimates of the present study are compared with the similar studies over other locations in the Indo‐Gangetic plains during the campaign period.

Multiple atmospheric layering and mixing-layer height in the Inn valley observed by remote sensing

Automatic mixing layer height monitoring was performed by continuous sodar and ceilometer measurements in the Inn valley east of Innsbruck, Austria during a winter measuring campaign on air and noise pollution. The ceilometer, an eye-safe commercial lidar originally designed to detect cloud base heights and vertical visibility for aviation safety purposes, was operated for about two months; the sodar was operated for more than four months. Special software for this ceilometer provides routine retrievals of multiple aerosol layers and mixing layer height from the optical backscatter data. An existing retrieval method for the mixing layer height from sodar data has also been enhanced in order to detect multiple atmospheric layering. Particular emphasis is given to the detection of thermally stable layers and inversions within the lower valley atmosphere and their temporal development. Such layers influence significantly the diurnal variations of air pollution and traffic noise impact. A comparison is performed with parallel mixing layer height retrievals from the sodar and the ceilometer. In clear and cold winter nights sometimes several layers one above the other can be detected with both instruments. These multiple layers form due to an interaction between the mountain wind and the down-slope winds. In the absence of low clouds and precipitation ceilometers can estimate the mixing-layer-height fairly well. Ceilometer and sodar partly complement each other.

Impact of turbulence, land surface, and radiation parameterizations on simulated boundary layer properties in a coastal environment

The impact of planetary boundary layer (PBL) turbulence, land surface, and radiation parameterizations on mesoscale simulations of boundary layer properties in a coastal environment are examined using observations from different platforms and numerical simulations using the mesoscale model MM5 during a 10‐day period in July 2004. The parameterization schemes examined are the MRF and Eta PBL schemes, the simple soil model and the more sophisticated NOAH land surface model, and the Dudhia and RRTM longwave radiation parameterizations. Comparisons are made between simulated and observed near surface mean variables, radiation, turbulence fluxes, mixed layer heights and morning inversion strengths, low‐level jets, and land‐sea breeze circulations. The comparisons indicate that for the Gulf Coast environment and typical summertime conditions, the Eta PBL scheme clearly outperforms the MRF PBL scheme in nearly all aspects. The results reveal that the popular Dudhia radiation scheme tends to overpredict the downward longwave radiation, which consequently results in a warm bias at night and a weaker nocturnal low level jet. Although the NOAH land surface model is much more sophisticated than the simple soil model, it failed to deliver significantly improved simulations of boundary layer properties for the conditions considered in this study.

Wintertime aerosol characteristics over the Indo‐Gangetic Plain (IGP): Impacts of local boundary layer processes and long‐range transport

The Indo‐Gangetic Plain (IGP) encompasses a vast area, (accounting for ∼21% of the land area of India), which is densely populated (accommodating ∼40% of the Indian population). Highly growing economy and population over this region results in a wide range of anthropogenic activities. A large number of thermal power plants (most of them coal fed) are clustered along this region. Despite its importance, detailed investigation of aerosols over this region is sparse. During an intense field campaign of winter 2004, extensive aerosol and atmospheric boundary layer measurements were made from three locations: Kharagpur (KGP), Allahabad (ALB), and Kanpur (KNP), within the IGP. These data are used (1) to understand the regional features of aerosols and BC over the IGP and their interdependencies, (2) to compare it with features at locations lying at far away from the IGP where the conditions are totally different, (3) to delineate the effects of mesoscale processes associated with changes in the local atmospheric boundary layer (ABL), (4) to investigate the effects of long‐range transport or moving weather phenomena in modulating the aerosol properties as well as the ABL characteristics, and (5) to examine the changes as the season changes over to spring and summer. Our investigations have revealed very high concentrations of aerosols along the IGP, the average mass concentrations (MT) of total aerosols being in the range 260 to 300 μg m−3 and BC mass concentrations (MB) in the range 20 to 30 μg m−3 (both ∼5 to 8 times higher than the values observed at off‐IGP stations) during December 2004. Despite, BC constituted about 10% to the total aerosol mass concentration, a value quite comparable to those observed elsewhere over India for this season. The dynamics of the local atmospheric boundary layer (ABL) as well as changes in local emissions strongly influence the diurnal variations of MT and MB, both being inversely correlated with the mixed layer height (Zi) and the ventilation coefficient (Vc). The share of BC to total aerosols is highest (∼12%) during early night and lowest (∼4%) in the early morning hours. While an increase in the Vc results in a reduction in the concentration almost simultaneously, an increase in Zimax has its most impact on the concentration after ∼1 day. Accumulation mode aerosols contributed ∼90% to the aerosol concentration at ALB, ∼77 % at KGP and 74% at KNP. The BC mass mixing ratio was ∼10% over all three locations and is comparable to the value reported for Trivandrum, a tropical coastal location in southern India. This indicates presence of submicron aerosols species other than BC (such as sulfate) over KGP and KNP. A cross‐correlation analysis showed that the changes in MB at KGP is significantly correlated with those at KNP, located ∼850 km upwind, and ALB after a delay of ∼7 days, while no such delay was seen between ALB and KNP. Back trajectory analyses show an enhancement in MB associated with trajectories arriving from west, the farther from to the west they arrive, the more is the increase. This, along with the ABL characteristics, indicate two possibilities: (1) advection of aerosols from the west Asia and northwest India and (2) movement of a weather phenomena (such as cold air mass) conducive for build up of aerosols from the west to east. As the winter gives way to summer, the change in the wind direction and increased convective mixing lead to a rapid decrease in MB.

Aerosol optical depth, aerosol composition and air pollution during summer and winter conditions in Budapest

The dependence of aerosol optical depth (AOD) on air particulate concentrations in the mixing layer height (MLH) was studied in Budapest in July 2003 and January 2004. During the campaigns gaseous (CO, SO(2), NO(x), O(3)), solid components (PM(2.5), PM(10)), as well as ionic species (ammonium, sulfate and nitrate) were measured at several urban and suburban sites. Additional data were collected from the Budapest air quality monitoring network. AOD was measured by a ground-based sun photometer. The mixing layer height and other common meteorological parameters were recorded. A linear relationship was found between the AOD and the columnar aerosol burden; the best linear fit (R(2)=0.96) was obtained for the secondary sulfate aerosol due to its mostly homogeneous spatial distribution and its optically active size range. The linear relationship is less pronounced for the PM(2.5) and PM(10) fractions since local emissions are very heterogeneous in time and space. The results indicate the importance of the mixing layer height in determining pollutant concentrations. During the winter campaign, when the boundary layer decreases to levels in between the altitudes of the sampling stations, measured concentrations showed significant differences due to different local sources and long-range transport. In the MLH time series unexpected nocturnal peaks were observed. The nocturnal increase of the MLH coincided with decreasing concentrations of all pollutants except for ozone; the ozone concentration increase indicates nocturnal vertical mixing between different air layers.

Injection height of biomass burning aerosols as seen from a spaceborne lidar

This paper analyzes new lidar measurements from space over regions of biomass burning activity. The height of the aerosol layers deduced from the lidar observations is compared to the mixing layer top diagnosed from numerical weather forecasts, to identify whether or not the aerosols are directly injected in the free troposphere. During July and August 2006, the best cases (limited cloudiness, high density of fires) are found over South Africa and Northern Australia. Over these regions, the top of the aerosol layer is close to the mixing layer height, which is a strong indication that the aerosols are injected within the mixing layer. Other tropical areas with biomass burning activity are more difficult to interpret but the valid data support the same conclusion. For higher latitudes regions with biomass burning activity, although several aerosol plumes are identified above the mixing layer, most of the load is within the mixing layer. These observations made over a limited period and set of regions indicate that cases with pyro‐convection and/or direct injection to the free troposphere are not frequent.

Urban air pollution modelling and measurements of boundary layer height

An urban field trial has been undertaken with the aim of assessing the performance of the boundary layer height (BLH) determination of two models: the Met Office Unified Model (UM) and a Gaussian-type plume model, ADMS. Pulsed Doppler lidar data were used to measure mixing layer height and cloud base heights for a variety of meteorological conditions over a 3 week period in July 2003. In this work, the daily growth and decay of the BLH from the lidar data and model simulations for 5 days are compared. The results show that although the UM can do a good job of reproducing the boundary layer growth, there are occasions where the BLH is overestimated by 30–100%. Within dispersion models it is the BLH that effectively limits the height to which pollution disperses, so these results have very important implications for pollution dispersion modelling. The results show that correct development of the boundary layer in the UM is critically dependant on morning cloud cover. The ADMS model is used routinely by local authorities in the UK for local air-quality forecasting. The ADMS model was run under three settings; an ‘urban’ roughness, a ‘rural’ roughness and a ‘transition’ roughness. In all cases, the ‘urban’ setting over estimated the BLH and is clearly a poor predictor of urban BLH. The ‘transition’ setting, which distinguishes between the meteorological data input site and the dispersion modelling site, gave the best results under the well mixed conditions of the trial.

A numerical study of daily transitions in the convective boundary layer

We examine daily (morning–afternoon) transitions in the atmospheric boundary layer based on large-eddy simulations. Under consideration are the effects of the stratification at the top of the mixed layer and of the wind shear. The results describe the transitory behaviour of temperature and wind velocity, their second moments, the boundary-layer height Z m (defined by the maximum of the potential temperature gradient) and its standard deviation σ m , the mixed-layer height z i (defined by the minimum of the potential temperature flux), entrainment velocity W e, and the entrainment flux H i . The entrainment flux and the entrainment velocity are found to lag slightly in time with respect to the surface temperature flux. The simulations imply that the atmospheric values of velocity variances, measured at various instants during the daytime, and normalized in terms of the actual convective scale w*, are not expected to collapse to a single curve, but to produce a significant scatter of observational points. The measured values of the temperature variance, normalized in terms of the actual convective scale Θ*, are expected to form a single curve in the mixed layer, and to exhibit a considerable scatter in the interfacial layer.

Boundary-layer dynamics and its influence on atmospheric chemistry at Summit, Greenland

Sonic anemometer turbulence measurements were made at Summit, Greenland during summer 2004 and spring 2005. These measurements allow for the characterization of the variability of the atmospheric boundary layer at this site by describing seasonal and diurnal changes in sensible heat flux and boundary layer stability as well as providing estimates of mixing layer height. Diurnal sensible heat fluxes at Summit ranged from −18 to −2 W m −2 in the spring and from −7 to +10 W m −2 in the summer. Sustained stable surface layer conditions and low wind speeds occured during the spring but not during the summer months. Unstable conditions were not observed at Summit until late April. Diurnal cycles of convective conditions during the daytime (0700–1700 h local time) were observed throughout July and August. Boundary layer heights, which were estimated for neutral to stable conditions, averaged 156 m for the spring 2005 observations. Comparisons of the boundary layer characteristics of Summit with those from South Pole, Antarctica, provide possible explanations for the significant differences in snowpack and surface-layer chemistry between the two sites.

Influence of mixing layer height upon air pollution in urban and sub-urban areas

The mixing layer height is an important parameter characterising the potential of the atmospheric boundary layer to take up emitted air pollutants. During continuous measurements in Hanover, Germany, from 2001 until 2003 and around Munich, Germany, in summer and winter 2003 mixing layer heights (MLH) were determined by different remote sensing systems mainly from the thermal structure and turbulence of the air (SODAR), for some time from the aerosol layering of the air (ceilometer), and for a short period directly from the temperature profile (RASS). The temporal variations of the concentrations of PM 10 and PM 2.5 as well as of CO and NO x simultaneously measured near the surface were investigated and correlated with the MLH derived from SODAR data. The pollution measurements were performed inside a street canyon and at an urban background station close to Hanover and at three measurement locations inside and outside of Munich complementing the available monitoring networks. The analyses show that the correlations of pollutant concentrations with MLH are smallest inside street canyons. Correlations at the urban background stations are larger in winter than in summer, and they are larger for the urban stations than for the rural stations. It turns out further that the correlation of NO X concentrations with MLH is larger than the correlation of particles concentrations. Explanations for these findings must consider the varying emission source strengths for NO X and particles and the influence of gas-to-particle conversion within air masses especially during daytime in summer.

A study of the variation of urban mixed layer heights

The AERMET model is used to estimate hourly mixing heights during the Joint URBAN (2003) experiment in Oklahoma City, Oklahoma. AERMET is a simple 2-D model that requires only routine meteorological observations and an early morning atmospheric sounding to estimate convective boundary layer (CBL) growth. Estimated mixing heights are compared with observed mixing heights measured during Joint URBAN 2003. Observed CBL heights are derived from profiler data using a peak signal-to-noise ratio method. The method of deriving mixing heights from profiler data is validated using daily atmospheric sounding data. Estimated mixing heights using AERMET show good agreement with observations on days of varying temperature and cloud cover. AERMET was able to estimate the rapid boundary layer growth observed in the late morning and early afternoon hours during highly convective conditions. CBL heights of over 3000 m are observed in sounding data during the late afternoon. Estimated CBL heights of over 3000 m during the late afternoon agreed well with observations from the sounding and profiler data.

Estimates of the potential temperature profile from lidar measurements of boundary layer evolution

The Soil Moisture‐Atmosphere Coupling Experiment (SMACEX) was conducted in the Walnut Creek Watershed near Ames, Iowa, over the period from 15 June to 11 July 2002. A main focus of SMACEX is the investigation of the interactions between the atmospheric boundary layer, surface moisture, and canopy. A vertically staring elastic lidar was used to provide a high time resolution, continuous record of the mixed layer height at the edge between a soybean and a corn field. The height and thickness of the entrainment zone are used to estimate the vertical potential temperature profile in the boundary layer using surface energy measurements in the Batchvarova‐Gryning mixed layer model. Calculated values of potential temperature compared well to radiosonde measurements taken simultaneously with the lidar measurements. The root‐mean‐square difference between the lidar‐derived values and the balloon‐based values is 1.20°C.

Mixing layer height over Munich, Germany: Variability and comparisons of different methodologies

On 4 days in summer and winter the mixing layer height over the municipal area of Munich, Germany, was determined by several remote sensing instruments and in situ probes. The main motivation was to obtain information on aerosols, and therefore we decided to understand the mixing layer as that layer where most of the locally produced aerosols are concentrated. In this paper we wanted to investigate the potential of the quite different methodologies which depend on measurements of aerosol properties and those which do not. The operation of two lidars, a ceilometer, a wind‐temperature‐radar, a sodar, radiosondes, and aerosol probes onboard of a microlight aircraft allowed such a thorough intercomparison. As the instruments were located at different sites, the horizontal homogeneity of the mixing layer could also be observed. It was found that the agreement between the different methodologies is very good as long as the mixing layer height does not exceed approximately 1 km, which is the common measurement range of all instruments. In summer, however, the mixing layer can reach 2 km and more, so that the lidar turns out to be the most capable remote sensing technique. Another advantage of the lidar is the possibility to clearly derive the internal structure of the mixing layer. The latter is important in cases when simple parameterizations assume vertical homogeneity of aerosol properties within the mixing layer. On the other hand, lidars are quite expensive and require a trained operator. As a conclusion, the development of unattended working lidars including automated data evaluation should be fostered. From the limited data set it was found that the mixing layer height in Munich did not change more than approximately 100 m over a horizontal distance of around 50 km. If this finding can be confirmed by further measurements, the area of Munich is a good test bed for the validation of aerosol retrievals from satellite data with medium spatial resolution and for the validation of the numerical treatment of aerosols in mesoscale chemistry transport models.

Influences of relative humidity on aerosol optical properties and aerosol radiative forcing during ACE-Asia

In situ measurements at Gosan, South Korea, and onboard C-130 aircraft during ACE-Asia were analyzed to investigate the influence of relative humidity (RH) on aerosol optical properties and radiative forcing. The temporal variation of aerosol chemical composition at the Gosan super-site was highly dependent on the air mass transport pathways and source region. RH in the springtime over East Asia were distributed with very high spatial and temporal variation. The RH profile onboard C-130 aircraft measurements exhibits a mixed layer height of about 2 km. Aerosol scattering coefficient ( σ sp) under ambient RH was greatly enhanced as compared with that at dry RH (RH<40%). From the aerosol optical and radiative transfer modeling studies, we found that the extinction and scattering coefficients are greatly enhanced with RH. Single scattering albedo with RH is also sensitively changed in the longer wavelength. Asymmetry parameter ( g) is gradually increased with RH although g decreases with wavelength at a given RH. Aerosol optical depth (AOD) at 550 nm and RH of 50% increased to factors 1.24, 1.51, 2.16, and 3.20 at different RH levels 70, 80, 90, and 95%, respectively. Diurnal-averaged aerosol radiative forcings for surface, TOA, and atmosphere were increased with RH because AOD was increased with RH due to hygroscopic growth of aerosol particles. This result implies that the hygroscopic growth due to water-soluble or hydrophilic particles in the lower troposphere may significantly modify the magnitude of aerosol radiative forcing both at the surface and TOA. However, the diurnal-averaged radiative forcing efficiencies at the surface, TOA, and atmosphere were decreased with increasing RH. The decrease of the forcing efficiency with RH results from the fact that increasing rate of aerosol optical depth with RH is greater than the increasing rate of aerosol radiative forcing with RH.

Variation of column‐integrated aerosol properties in a Chinese urban region

Thirty‐three months of aerosol data in Beijing are presented in this paper. Aerosol optical thickness (AOT) increases from January to June and then decreases gradually. However, airborne particulate matter with diameter less than 10 μm (PM10) concentration exhibits higher values in winter and spring and lower concentration in summer. For the same PM10 concentration, AOT in summer is approximately two, three, and four times that in autumn, winter, and spring, respectively. AOT increases persistently during daytime, and the diurnal variation varies from about 15% in summer to about 45% in winter. The seasonal and diurnal variation of AOT is quite different from that of surface particle concentration. This is partly attributed to the variation of atmospheric mixing layer height. Aerosol volume concentrations increase with AOT by nearly identical magnitude for fine and coarse mode except in spring. The volume concentration of coarse mode in spring increases by a magnitude of more than two times that derived in remaining seasons. Aerosol fine mode radius increases with AOT, whereas coarse mode radius keeps relatively invariable with AOT. Mean aerosol single‐scattering albedo at 440 nm is about 0.90 and decreases slightly with wavelength. Aerosol single‐scattering albedos increase and their spectral dependence reverses during dust periods. Aerosol size and absorption in Beijing are close to results derived in Mexico City and Kanpur, but they are quite different from those in Maryland and Paris. Therefore different urban aerosol models should be created and used in satellite remote sensing in different urban regions.