Nephelometer derived and directly measured aerosol optical depth of the atmospheric boundary layer

The aerosol optical depth of the atmospheric boundary layer was determined both from direct solar irradiance measurements and from vertical extrapolation of ground-based nephelometry, during a period with cloudless skies and high aerosol mass loadings in the Netherlands. The vertical profile of the aerosol was obtained from lidar measurements. From humidity controlled nephelometry at the ground and humidity profiles from soundings, the scattering aerosol extinction as a function of height was assessed. Integration of the extinction over the aerosol layer gave the aerosol optical depth of the atmospheric boundary layer. This optical depth at the narrow band of the nephelometer was translated to a spectrally integrated value, assuming an Angstrom wavelength exponent of 1.5, a typical value for The Netherlands.It was found that scattering by the boundary layer aerosol contributed on average 80% to the total atmospheric aerosol optical depth. The uncertainty in this value is estimated to be of the order of 13%. Ammonium nitrate dominated the light scattering. This is an anthropogenic aerosol component.The radiative forcing caused by the light scattering of the anthropogenic aerosol was calculated assuming an upward scattered fraction of 0.3. An average value of − 12 W m ⁻² was found (with an estimated uncertainty of 20%). This corresponds to an absolute increase in the planetary albedo of 0.03, which is equivalent to a 15% increase in the local planetary albedo of 0.2.     

One year measurements of aerosol optical properties over an urban coastal site: Effect on local direct radiative forcing

We present results of direct aerosol radiative forcing over a French Mediterranean coastal zone based on one year of continuous observations of aerosol optical properties during 2005–2006. Monthly-mean aerosol optical depth at 440 nm ranged between 0.1 and 0.34, with high Angstrom coefficient (α > 1.2). The single scattering albedo (at 525 nm) estimated at the surface ranged between 0.7 and 0.8, indicating significant absorption. The presence of aerosols over the Mediterranean zone during summer decreases the shortwave radiation reaching the surface by as much as 26 ± 3.9 W m^− 2, and increases the top of the atmosphere reflected radiation by as much as 5.2 ± 1.0 W m^− 2. The shortwave atmospheric absorption translates to an atmospheric heating of 2.5 to 4.6 K day^− 1. Concerted efforts are needed for investigating the possible impact of the increase in heating rate on the maintenance of heat-waves frequently occurring over this coastal region during summer time.     

THE IMPACT OF RELATIVE HUMIDITY ON THE RADIATIVE PROPERTY AND RADIATIVE FORCING OF SULFATE AEROSOL*

With the data of complex refractive index of sulfate aerosol,the radiative properties of the aerosol under 8 relative humidity conditions are calculated in this paper.By using the concentration distribution from two CTM models and LASG GOALS/AGCM,the radiative forcing due to hygroscopic sulfate aerosol is simulated.The results show that:(1)With the increase of relative humidity,the mass extinction coefficiency factor decreases in the shortwave spectrum;single scattering albedo keeps unchanged except for a little increase in longwave spectrum,and asymmetry factor increases in whole spectrum.(2)Larger differences occur in radiative forcingsimulated by using two CTM data,and the global mean forcing is -0.268 and -0.816 W/m²,respectively.(3)When the impact of relative humidity on radiative property is taken into account,the distribution pattern of radiative forcing due to the wet particles is very similar to that of dry sulfate,but the forcing value decreases by 60%.     

Condensed water in tropical cyclone “Oliver”, 8 February 1993

On February 8, 1993, the NASA DC-8 aircraft profiled from 10,000 to 37,000 feet (3.1–11.3 km) pressure altitude in a stratified section of tropical cyclone “Oliver” over the Coral Sea northeast of Australia. Size, shape and phase of cloud and precipitation particles were measured with a 2-D Greyscale probe. Cloud/ precipitation particles changed from liquid to ice as soon as the freezing level was reached near 17,000 feet (5.2 km) pressure altitude. The cloud was completely glaciated at −5°C. There was no correlation between ice particle habit and ambient temperature. In the liquid phase, the precipitation-cloud drop concentration was 4.0 × 10³ m⁻³, the geometric mean diameter D_g=0.5−0.7 mm, and the liquid water content 0.7−1.9 g m⁻³. The largest particles anywhere in the cloud, dominated by fused dendrites at concentrations similar to that of raindrops (2.5 × 10³ m⁻³) but a higher condensed water content (5.4 g m⁻³ estimated) were found in the mixed phase; condensed water is removed very effectively from the mixed layer due to high settling velocities of the large mixed particles. The highest number concentration (4.9 × 10⁴ m⁻³), smallest size (D_g=0.3−0.4 mm), largest surface area (up to 2.6 × 10² cm² m⁻³ at 0.4−1.0 g m⁻³ of condensate) existed in the ice phase at the coldest temperature (−40°C) at 35,000 feet (10.7 km). Each cloud contained aerosol (haze particles) in addition to cloud particles. The aerosol total surface area exceeded that of the cirrus particles at the coldest temperature. Thus, aerosols must play a significant role in the upscattering of solar radiation. Light extinction (6.2 km⁻¹) and backscatter (0.8 sr⁻¹ km⁻¹) was highest in the coldest portion of the cirrus cloud at the highest altitude.     

Clear-sky aerosol radiative forcing effects based on multi-site AERONET observations over Europe

Summary One of the great unknowns in climate research is the contribution of aerosols to climate forcing and climate perturbation. In this study, retrievals from AERONET are used to estimate the direct clear-sky aerosol top-of-atmosphere and surface radiative forcing effects for 12 multi-site observing stations in Europe. The radiative transfer code sdisort in the libRadtran environment is applied to accomplish these estimations. Most of the calculations in this study rely on observations which have been made for the years 1999, 2000, and 2001. Some stations do have observations dating back to the year of 1995. The calculations rely on a pre-compiled aerosol optical properties database for Europe. Aerosol radiative forcing effects are calculated with monthly mean aerosol optical properties retrievals and calculations are presented for three different surface albedo scenarios. Two of the surface albedo scenarios are generic by nature bare soil and green vegetation and the third relies on the ISCCP (International Satellite Cloud Climatology Project) data product. The ISCCP database has also been used to obtain clear-sky weighting fractions over AERONET stations. The AERONET stations cover the area 0° to 30° E and 42° to 52° N. AERONET retrievals are column integrated and this study does not make any seperation between the contribution of natural and anthropogenic components. For the 12 AERONET stations, median clear-sky top-of-atmosphere aerosol radiative forcing effect values for different surface albedo scenarios are calculated to be in the range of −4 to −2 W/m². High median radiative forcing effect values of about −6 W/m² were found to occur mainly in the summer months while lower values of about −1 W/m² occur in the winter months. The aerosol surface forcing also increases in summer months and can reach values of −8 W/m². Individual stations often have much higher values by a factor of 2. The median top-of-atmosphere aerosol radiative forcing effect efficiency is estimated to be about −25 W/m² and their respective surface efficiency is around −35 W/m². The fractional absorption coefficient is estimated to be 1.7, but deviates significantly from station to station. In addition, it is found that the well known peak of the aerosol radiative forcing effect at a solar zenith angle of about 75° is in fact the average of the peaks occurring at shorter and longer wavelengths. According to estimations for Central Europe, based on mean aerosol optical properties retrievals from 12 stations, the critical threshold of the aerosol single scattering albedo, between cooling and heating in the presence of an aerosol layer, is close between 0.6 and 0.76.     

On radiative forcing of sulphate aerosol produced from ion-promoted nucleation mechanisms in an atmospheric global model

A significant fraction of the total number of particles present in the atmosphere is formed by nucleation in the gas phase. Nucleation and the subsequent growth process influence both number concentration of particles and their size distribution besides chemical and optical properties of atmospheric aerosols. Sulphate aerosol nucleation mechanisms promoted by ions have been evaluated here in a tropospheric interactive chemistry-aerosol module for mass and number concentration in a global atmospheric model. The indirect radiative forcing of sulphate particles is assessed in this model; indirect radiative forcing is different for ion-induced (IIN) and ion-mediated (IMN) mechanisms. The indirect radiative forcing in 10-year simulation runs has been calculated as ?1.42?W/m² (IIN) and ?1.54?W/m² (IMN). The 5% emission of primary sulphate particles in simulations changes the indirect radiative forcing from ?1.42 to ?1.44?W/m² for IIN case, and from ?1.54 to ?1.55 W/m² for the IMN case. More precisely, owing to greater nucleation rates, IMN mechanisms produces greater cooling than the IIN mechanisms in the backdrop that both mechanisms produce almost identical distribution of CDNC in their pre-industrial runs. The inclusion of primary particles in simulations with IIN and IMN mechanisms increases both CDNC and the indirect radiative forcing.     

Deposition of atmospheric particulate PCBs in suburban site of Turkey

Dry deposition and air concentration samples were collected from July 2004 to May 2005 at a suburban site in Turkey. A water surface sampler (WSS) was used to measure directly the dry deposition flux of particulate polychlorinated biphenyls (PCBs) while a high volume air sampler (HVAS) was employed to collect air samples. Particulate PCB concentrations accounted for 15% of total PCBs (gas + particle phase) at the site. The overall particulate phase PCB flux ranged from 2 to 160 ng m^− 2 d^− 1 with an average of 46.3 ± 40.6 ng m^− 2 d^− 1. Forty one PCB congeners were targeted in the samples while twenty one congeners were found to be higher than detection limits in deposition samples. Fluxes for homolog groups ranged between 0.9 (7-CBs) and 21.0 (3-CBs) ng m^− 2 d^− 1. Measured dry deposition fluxes were lower than the ones usually reported for urban sites. Average PCB dry deposition velocity, calculated using flux values and concurrently measured atmospheric concentrations, was 1.26 ± 1.86 cm s^− 1 depended on size distribution of particles, atmospheric PCB concentrations and meteorological conditions.     

Fog and rain water chemistry at Mt. Fuji: A case study during the September 2002 campaign

Measurements of fog and rain water chemistry at the summit of Mt. Fuji, the highest peak in Japan, as well as at Tarobo, the ESE slope of Mt. Fuji in September 2002. The pH of fog and rain water sampled at Mt. Fuji varied over a range of 4.0–6.8. Acidic fogs (pH < 5.0) were observed at the summit when the air mass came from the industrial regions on the Asian continent. The ratio of [SO₄²⁻]/[NO₃⁻] in the fog water was lower at Tarobo than at the summit. High concentrations of Na⁺ and Cl⁻ were determined in the rain water sampled at the summit, possibly because of the long-range transport of sea-salt particles raised by a typhoon through the middle troposphere. The vertical transport of sea-salt particles would influence the cloud microphysical properties in the middle troposphere. Significant loss of Mg²⁺ was seen in the rain water at the summit. The concentrations of peroxides in the fog and rain water were relatively large (10–105 μM). The potential capacity for SO₂ oxidation seems to be strong from summer to early autumn at Mt. Fuji. The fog water peroxide concentrations displayed diurnal variability. The peroxide concentrations in the nighttime were significantly higher than those in the daytime.     

The microstructure of selected, small, isolated, cumulus clouds near Red Deer, Alberta

Physical experiments designed to explore the potential of rain augmentation through airborne glaciogenic seeding on small, isolated non-precipitating cumuliform clouds near Red Deer, Alberta were carried out during the period 1982–1985. The microstructure of 90 cumulus congestus clouds have been documented through repeated in-situ sampling using a cloud physics instrumented aircraft platform. Observations from the inspection passes of 57 clouds seeded with either dry ice pellets or silver iodide pyrotechnics, and all the passes of 33 natural clouds are presented.Measurements of the cloud droplet concentration indicate that Alberta cumulus clouds are typically continental in nature, with an average droplet concentration of 535 cm⁻³ and an average droplet diameter of 10.6 μm. Alberta clouds have average liquid water contents of 0.57 g m⁻³, with a peak 1-sec value of 3.17 g m⁻³. The 1-km average liquid water contents are 0.83 g m⁻³, with a peak value of 2.81 g m⁻³. Cloud lifetimes vary between 11 and 20 minutes. Concentrations of naturally occurring ice crystals are found to be low. The average maximum 1-km ice concentration was 31⁻¹, and the peak 1-km concentration was 73.11⁻¹ in the natural cloud dataset. Evidence of precipitation-sized particles was detected in 21% (7 of 33) of the clouds, and precipitation below cloud base was detected in 6% (2 of 33) of the clouds.A comparison of the Alberta cloud characteristics to the cumulus clouds from different locations showed that there are some distinct differences between Alberta clouds and the clouds from the other regions.     

Influence of the total atmospheric optical depth and cloud cover on solar irradiance components

Broadband solar irradiance data obtained in the spectral range 400–940 nm at Kwangju, South Korea from 1999–2000 have been analyzed to investigate the effects of cloud cover and atmospheric optical depth on solar radiation components. Results from measurements indicate that the percentage of direct and diffuse horizontal components of solar irradiance depend largely on total optical depth (TOD) and cloud cover. During summer and spring, the percentages of diffuse solar irradiance relative to the global irradiance were 5.0% and 4.9% as compared to 2.2% and 3.0% during winter and autumn. The diffuse solar irradiance is higher than the direct in spring and summer by 24.2%, and 40.6%, respectively, which may largely be attributed to the attenuation (scattering) of radiation by heavy dust pollution and large cloud amount. In cloud-free conditions with cloud cover ≤2/10, the fraction of the direct and diffuse components were 66.0% and 34.0%, respectively, with a mean daily global irradiance value of 7.92±2.91 MJ m⁻² day⁻¹. However, under cloudy conditions (with cloud cover ≥8/10), the diffuse and direct fractions were 97.9% and 2.2% of the global component, respectively. The annual mean TOD under cloudless conditions (cloud cover≤2/10) yields 0.74±0.33 and increased to as much as 3.15±0.67 under cloudy conditions with cloud amount ≥8/10. An empirical formula is derived for estimating the diffuse and direct components of horizontal solar irradiance by considering the total atmospheric optical depth (TOD). Results from statistical models are shown for the estimation of solar irradiance components as a function of TOD with sufficient accuracy as indicated by low standard error for each solar zenith angle (SZA).     

The effects of in-cloud mass production on atmospheric light scatter

Direct physical measurements of particle mass and number concentration indicate an increase in overall aerosol mass resulting from cloud processing, most likely through aqueous-phase chemistry (e.g., SO₂ oxidation). Measurements conducted in the Pennines of Northern England reveal an average increase of 14 to 20% in dry aerosol mass (0.003<particle diameter<0.9 μm) after aerosol passage through an orographic cloud. The rate of in-cloud mass production is most sensitive to changes in upwind particle size distributions, SO₂ concentration, and cloud water acidity. Newly-formed mass appears in size range between 200 and 600 nm and enhances the bimodality of the particle number distribution after cloud processing. Furthermore, the cloud-produced mass is estimated to increase total light scattering, b_sp, by 18 to 24%. The scattering efficiency of the dry, cloud-generated aerosol is 5.0±0.3 m² g⁻¹ and increases to 7.4±0.7 m² g⁻¹ when adjusted to 90% relative humidity by incorporating particle hygroscopicity data.     

A Study on Sulfate Optical Properties and Direct Radiative Forcing Using LASG-IAP General Circulation Model

The direct radiative forcing(DRF) of sulfate aerosols depends highly on the atmospheric sulfate loading and the meteorology,both of which undergo strong regional and seasonal variations.Because the optical properties of sulfate aerosols are also sensitive to atmospheric relative humidity,in this study we first examine the scheme for optical properties that considers hydroscopic growth.Next,we investigate the seasonal and regional distributions of sulfate DRF using the sulfate loading simulated from NCAR CAM-Chem together with the meteorology modeled from a spectral atmospheric general circulation model(AGCM) developed by LASG-IAP.The global annual-mean sulfate loading of 3.44 mg m 2 is calculated to yield the DRF of 1.03 and 0.57 W m 2 for clear-sky and all-sky conditions,respectively.However,much larger values occur on regional bases.For example,the maximum all-sky sulfate DRF over Europe,East Asia,and North America can be up to 4.0 W m 2.The strongest all-sky sulfate DRF occurs in the Northern Hemispheric July,with a hemispheric average of 1.26 W m 2.The study results also indicate that the regional DRF are strongly affected by cloud and relative humidity,which vary considerably among the regions during different seasons.This certainly raises the issue that the biases in model-simulated regional meteorology can introduce biases into the sulfate DRF.Hence,the model processes associated with atmospheric humidity and cloud physics should be modified in great depth to improve the simulations of the LASG-IAP AGCM and to reduce the uncertainty of sulfate direct effects on global and regional climate in these simulations.     

Influence of atmospheric relative humidity on ultraviolet flux and aerosol direct radiative forcing: Observation and simulation

The atmospheric aerosols can absorb moisture from the environment due to their hydrophilicity and thus affect atmospheric radiation fluxes. In this article, the ultraviolet radiation and relative humidity (RH) data from ground observations and a radiative transfer model were used to examine the influence of RH on ultraviolet radiation flux and aerosol direct radiative forcing under the clear-sky conditions. The results show that RH has a significant influence on ultraviolet radiation because of aerosol hygroscopicity. The relationship between attenuation rate and RH can be fitted logarithmically and all of the R² of the 4 sets of samples are high, i.e. 0.87, 0.96, 0.9, and 0.9, respectively. When the RH is 60%, 70%, 80% and 90%, the mean aerosol direct radiative forcing in ultraviolet is ?4.22W m^?2, ?4.5W m^?2, ?4.82W m^?2 and ?5.4W m^?2, respectively. For the selected polluted air samples the growth factor for computing aerosol direct radiative forcing in the ultraviolet for the RH of 80% varies from 1.19 to 1.53, with an average of 1.31.     

Sensitivity Tests of an Energy Balance Model to Choice of Stability Functions and Measurement Accuracy

Sensible (H) and latent (L_vE) heat fluxes are obtained by a combined energy budget – similarity model applied to observations from Melle in Belgium and Cabauw in The Netherlands. The sensitivity to both the stability functions and the accuracy of input data is investigated. In a first step, fluxes are calculated for a selection of stability functions and compared to values obtained with pre-defined (reference) functions. For the diurnal fluxes higher than 10 W m⁻² in 1996 at Melle, the root-mean-square rmsreaches 9 W m⁻² for H and 6 W m⁻² for L_vE, depending on the chosen functions. A lesser sensitivity is obtained at Cabauw and can be explained by lower absolute values of the stability parameter ζ (L involving the Obukhov length) mainly induced by higher mean wind speeds. Different stability bins are also considered. It is concluded that a more accurate assessment of the stability functions is already desirable for absolute values of L above a few metres. These values are not so scarce at Melle and should be captured in the future by an increasing number of new developing long-term measurement stations. In a second step, a statistical approach is proposed with errors depicted by both systematic biases and random fluctuations represented by means of Gaussian distributions. The results show that very accurate measurements are needed in order to maintain the mean annual value of the bias and rms below 5–10 W m⁻², and thus to allow the discrimination between the sensitivity to errors on input data and to the stability functions selection.     

Albedo enhancement over land to counteract global warming: impacts on hydrological cycle

A recent modelling study has shown that precipitation and runoff over land would increase when the reflectivity of marine clouds is increased to counter global warming. This implies that large scale albedo enhancement over land could lead to a decrease in runoff over land. In this study, we perform simulations using NCAR CAM3.1 that have implications for Solar Radiation Management geoengineering schemes that increase the albedo over land. We find that an increase in reflectivity over land that mitigates the global mean warming from a doubling of CO₂ leads to a large residual warming in the southern hemisphere and cooling in the northern hemisphere since most of the land is located in northern hemisphere. Precipitation and runoff over land decrease by 13.4 and 22.3%, respectively, because of a large residual sinking motion over land triggered by albedo enhancement over land. Soil water content also declines when albedo over land is enhanced. The simulated magnitude of hydrological changes over land are much larger when compared to changes over oceans in the recent marine cloud albedo enhancement study since the radiative forcing over land needed (?8.2?W?m^?2) to counter global mean radiative forcing from a doubling of CO₂ (3.3?W?m^?2) is approximately twice the forcing needed over the oceans (?4.2?W?m^?2). Our results imply that albedo enhancement over oceans produce climates closer to the unperturbed climate state than do albedo changes on land when the consequences on land hydrology are considered. Our study also has important implications for any intentional or unintentional large scale changes in land surface albedo such as deforestation/afforestation/reforestation, air pollution, and desert and urban albedo modification.     

Role of organic acids (formic, acetic, pyruvic and oxalic) in the formation of cloud condensation nuclei (CCN): a review

Although it is believed that organic aerosols play a key role in cloud nucleation and make an important contribution to the cloud condensation nuclei (CCN) population, their specific species remain poorly characterized. This paper reviews the current knowledge of organic acids (mainly formic, acetic, pyruvic and oxalic acids). Without specification, organic acids in this paper refer to these four organic acids in the gas and aerosol phases. This paper analyzes the extent to which organic acids act as CCN and compares the physical and chemical properties of organic acids with those of CCN. The results show that aerosol formate and acetate concentrations range from 0.02 to 5.3 nmol m⁻³ and from 0.03 to 12.4 nmol m⁻³, respectively, and that between 34 to 77% of formate and between 21 to 66% of acetate are present in the fine fraction of aerosols. It was found that although most (98–99%) of these volatile organic acids are present in the gas phase, their concentrations in the aerosol particles are sufficient to make them a good candidate for CCN. The results also show that organic acids may make an important contribution to the formation of CCN in some special sources such as vegetation emissions and biomass-burning. Organic acids are expected to contribute significantly to the estimates of indirect (cloud-mediated) forcing due to aerosols.     

Remote sensing of aerosols over the Solar Village, Saudi Arabia

Aerosol optical properties over Solar Village, Saudi Arabia have been studied using ground-based remote sensing observations through the Aerosol Robotic Network (AERONET). Our analysis covered 8 recorded years of aerosol measurements, starting from February 1999 through January 2007. The seasonal mean values of aerosol optical thickness (AOT), the Ångström wavelength exponent α and the surface wind speed (V), exhibit a one year cyclical pattern. Seasonal variations are clearly found in the shape and magnitude of the volume size distribution (VSD) of the coarse size mode due to dust emission. The Spring is characterized by dusty aerosols as the modal value of the exponent α was low ~ 0.25 while that of AOT was high ~ 0.3. The modal value of wind speed was the highest ~ 3.6 m/s in spring. The increase in wind speed is responsible for increasing the concentration of dust particles during Spring. Spring of 2003 has the highest mean values of AOT, V and VSD and the lowest mean value for the exponent α. The seasonal mean values of the exponent α are anticorrelated with those of the wind speed (r = − 0.63). The annual mean values of the exponent α are well correlated (r = 0.77) with those of the difference between the maximum and minimum values of temperature ΔT. They are anticorrelated (r = − 0.74) with the annual mean values of the relative humidity. Large aerosol particles and high relative humidity increase the radiative forcing. This results in reduction of the values of the temperature difference ΔT.     

Importance of sulfate emission to sulfur deposition at urban and rural sites in China

Throughfall (TF) and wet only (WO) deposition along with SO₂ and sulfate (SO₄²⁻) concentration in air at 4 urban and rural sites in southwestern China were monitored in order to understand the role of different forms of sulfur (S) emission to the S deposition and its effect in China. The sites were located in Chongqing, Hunan, and Guizhou provinces. S deposition at the most polluted site reached 15 g S m^− 2 yr^− 1. At three of the sites, located in the vicinity of several emission sources, dry S deposition is 2.1–4.2 times that of wet deposition, which is significantly higher than what is found in most other parts of the world.Main components in airborne particles (PM₁₀) are (NH₄)₂SO₄ and CaSO₄ at the highly polluted Tie Shan Ping (TSP) site. Dust particles of gypsum (CaSO₄) in the air are partly due to direct emission and partly from the reaction of calcium oxides and carbonates with sulfuric acid in the air. To illustrate the importance of sulfate emission to total S deposition we analyzed the source of S deposition based on both measurements and models. Results indicated that direct emission of SO₄²⁻ particles could account for high proportion in total S deposition at the three most polluted sites.     

Optical and radiative properties of aerosols during a severe haze episode over the North China Plain in December 2016

The optical and radiative properties of aerosols during a severe haze episode from 15 to 22 December 2016 over Beijing, Shijiazhuang, and Jiaozuo in the North China Plain were analyzed based on the ground-based and satellite data, meteorological observations, and atmospheric environmental monitoring data. The aerosol optical depth at 500 nm was < 0.30 and increased to > 1.4 as the haze pollution developed. The Ångström exponent was > 0.80 for most of the study period. The daily single-scattering albedo was > 0.85 over all of the North China Plain on the most polluted days and was > 0.97 on some particular days. The volumes of fine and coarse mode particles during the haze event were approximately 0.05–0.21 and 0.01–0.43 μm³, respectively—that is, larger than those in the time without haze. The daily absorption aerosol optical depth was about 0.01–0.11 in Beijing, 0.01–0.13 in Shijiazhuang, and 0.01–0.04 in Jiaozuo, and the average absorption Ångström exponent varied between 0.6 and 2.0. The aerosol radiative forcing at the bottom of the atmosphere varied from –23 to –227,–34 to –199, and –29 to –191 W m^–2 for the whole haze period, while the aerosol radiative forcing at the top of the atmosphere varied from –4 to –98, –10 to –51, and –21 to –143 W m^–2 in Beijing, Shijiazhuang, and Jiaozuo, respectively. Satellite observations showed that smoke, polluted dust, and polluted continental components of aerosols may aggravate air pollution during haze episodes. The analysis of the potential source contribution function and concentration-weighted trajectory showed that the contribution from local emissions and pollutants transport from upstream areas were 190–450 and 100–410 μg m^–3, respectively.     

Atmospheric deposition of nitrogen, sulfur and chloride in Thessaloniki, Greece

In the present study, the wet and dry depositions of particulate NO₃⁻, SO₄²⁻, Cl⁻ and NH₄⁺ were measured using a wet/dry sampler as a surrogate surface. Gas phase compounds of nitrogen, sulfur and chloride (HNO₃, NH₃, SO₂ and HCl) were measured by an annular denuder system (ADS) equipped with a back up filter for the collection of particles with diameter ≤ 5 μm. Ambient concentrations of NO, NO₂ and SO₂ were also taken into consideration. Sampling was conducted at an urban site in the center of the city of Thessaloniki, northern Greece. The presence of the aerosol species was examined by cold/warm period and the possible compounds in dry deposits were also considered. Dry deposition fluxes were found to be well correlated with ambient particle concentrations in order to be used for the calculation of particle deposition velocity. Average particulate deposition velocities calculated were 0.36, 0.20, 0.20 and 0.10 cm s^− 1 for Cl⁻, NO₃⁻, SO₄²⁻ and NH₄⁺, respectively. Total dry deposition fluxes (gas and particles) were estimated at 3.24 kg ha^− 1 year^− 1 for chloride (HCl + p-Cl), 9.97 kg ha^− 1 year^− 1 for nitrogen oxidized (NO + NO₂ + HNO₃ + p-NO₃), 5.32 kg ha^− 1 year^− 1 for nitrogen reduced (NH₃ + p-NH₄) and 15.77 kg ha^− 1 year^− 1 for sulfur (SO₂ + p-SO₄). 70–90% total dry deposition was due to gaseous species deposition. The contribution of dry deposition to the total (wet + dry) was at the level of 60–70% for sulfur and nitrogen (oxidized and reduced), whereas dry chloride deposition contributed 35% to the total. The dry-to-wet deposition ratio of all the studied species was found to be significantly associated with the precipitation amount, with nitrogen species being better and higher correlated. Wet, dry and total depositions measured in Thessaloniki, were compared with other countries of Europe, US and Asia.