Ground-level Aerosol Research Articles

Aerosol composition, air quality, and boundary layer dynamics in the urban background of Stuttgart in winter

Abstract. Aerosol distributions are of great relevance for air quality, especially for cities like Stuttgart, which has limited air exchange due to its location in a basin. We collected a comprehensive set of data from remote sensing and in situ methods including radiosondes for the urban background of downtown Stuttgart to determine the impact of boundary layer mixing processes on local air quality and to evaluate the simulation results of the high-resolution large eddy simulation (LES) model PALM-4U at 10 m grid spacing. Stagnant meteorological conditions caused accumulation of aerosols, and chemical composition analysis shows that ammonium nitrate (37 ± 9 %) and organic aerosol (OA; 34 ± 9 %) dominated during this winter study. Case studies show that clouds during previous nights can weaken temperature inversion and accelerate boundary layer mixing after sunrise by up to 3 h. This is important for ground-level aerosol dilution during the morning rush hour. Furthermore, our observations validate results of the LES model PALM-4U in terms of boundary layer heights and aerosol mixing for 48 h. The simulated aerosol concentrations follow the trend of our observations but are still underestimated by a factor of 4.5 ± 2.1 due to missing secondary aerosol formation processes and uncertainties of emissions and boundary conditions in the model. This paper firstly evaluates the PALM-4U model performance in simulating aerosol spatio-temporal distributions, which can help to improve the LES model and to better understand sources and sinks for air pollution as well as the role of horizontal and vertical transport.

Vertical structure and transport characteristic of aerosol and O3 during the emergency control period in Wuhan, China, using vehicle-lidar observations

During the Lunar New Year of 2020, the Chinese government implemented a strict nationwide lockdown to prevent the spread of the COVID-19, which may have led to changes in the distribution of pollutants. To enhance our understanding of the three-dimensional evolution of pollutants under special control scenarios, this study used vehicle lidar to obtain vertical profiles of aerosols and ozone (O3) in Wuhan during the pandemic. Combined with ground observations and WRF-Chem model simulation results, we explored the evolution characteristics of pollutants. The results showed that the fine particulate matter (PM2.5) concentration near the ground in Wuhan City was significantly reduced (40%) during the emission control period. The aerosol concentration decreased rapidly with an increase in height, and the maximum loading height was consistent with the atmospheric boundary layer height. Conversely, due to factors such as the reduction of the precursor nitrogen dioxide (NO2) and rising temperatures, the near-ground O3 concentration increased by 59%. Despite the significant reduction in emission sources, lidar observations still captured distinct aerosol transport layers on March 7 and March 10, 2020. These transport layers gradually descended, affecting the ground-level aerosol concentration. Additionally, one typical process of O3 external transport was observed in the vertical direction. Our study provides a theoretical basis for a deeper understanding of the spatiotemporal distribution characteristics of pollutants during special control scenarios, aiding policymakers in formulating effective air pollution control strategies.

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

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

Using infrared geostationary remote sensing to determine particulate matter ground-level composition and concentration.

Speciated ground-level aerosol concentrations are required to understand and mitigate health impacts from dust storms, wildfires, and other aerosol emissions. Globally, surface monitoring is limited due to cost and infrastructure demands. While remote sensing can help estimate respirable (i.e. ground level) concentrations, current observations are restricted by inadequate spatiotemporal resolution, uncertainty in aerosol type, particle size, and vertical profile. One key issue with current remote sensing datasets is that they are derived from reflectances observed by polar-orbiting imagers, which means that aerosol is only derived during the daytime and only once or twice per day. Newer quantification methods using geostationary infrared (IR) data have focussed on detecting the presence, or absence, of an event. The determination of aerosol composition or particle size using IR exclusively has received little attention. This manuscript summarizes four scientific papers, published as part of a larger study, and identifies requirements for (a) using infrared radiance observations to obtain continual (i.e. day and night) concentration estimates; (b) increasing temporal resolution by using geostationary satellites; (c) utilizing all infrared channels to maximize spectral differences due to compositional changes; and (d) applying a high-pass filter (brightness temperature differences) to identify compositional variability. Additionally, (e) a preliminary calibration methodology was tested against three severe air quality case study incidents, namely, a dust storm, smoke from prescribed burns, and an ozone smog incident, near Sydney in eastern Australia which highlighted the ability of the method to determine atmospheric stability, clouds, and particle size. Geostationary remote sensing provides near-continuous data at a temporal resolution comparable to monitoring equipment. The spatial resolution (~ 4 km2 at NADIR) is adequate for large sources but coarse for localized sources. The spectral sensitivity of aerosol is limited and appears to be dominated by humidity changes rather than concentration or compositional changes. Geostationary remote sensing can be used to determine the timing, duration, and spatial extent of an air quality event. Brightness temperature differences can assist in qualifying composition with an order of magnitude estimate of concentration.

Impacts of urbanization on atmospheric circulation and aerosol transport in a coastal environment simulated by the WRF-Chem coupled with urban canopy model

This study examines the impacts of urbanization on the local atmospheric circulation and the dispersion of air pollutants over the populated city of Ulsan, South Korea, located in the coastal region. Two experiments are conducted using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) and the urban canopy model (UCM) at 1-km horizontal resolution. The model experiments are conducted in August for three consecutive years of 2016–2018, with an updated land use category and diurnally-varying surface emission and anthropogenic heat flux. The impacts of urbanization are isolated by comparing the control run (CTRL) with realistic land use conditions and the sensitivity run (NOURB) that replaces the urban surfaces with non-urban grasslands. CTRL reproduces the urban heat island (UHI) and associated wind patterns realistically with reliable local land-sea breeze circulations. The positive temperature anomalies develop over the urban area due to enhanced surface heat flux at the urban surfaces, driving low-level convergence and secondary circulation. Enhanced heating by UHI changes the ground-level aerosol concentration differently. While the concentration does not change significantly by UHI in the daytime, it is reduced considerably at night in the urban areas due to enhanced vertical mixing. The dominant process that the urbanization modifies the aerosol concentration is the thermodynamical effects, which are also supported by the observation.

Встречаемость и свойства бактерий цереусной группы в аэрозолях атмосферного воздуха Новосибирской области

Встречаемость и свойства бактерий цереусной группы в аэрозолях атмосферного воздуха Новосибирской области

Investigation of the Successive Ozone Episodes in the El Paso–Juarez Region in the Summer of 2017

The adjacent cities of El Paso in Texas, USA, and Juarez in Mexico commonly experience pollution episodes, especially during the summer months. In the summer of 2017, successive high and low ozone episodes were observed. Aerosol backscattered data from a laser ceilometer was used to monitor and continuously measure the aerosol-layer characteristics. Meteorological parameters together with the aerosol layer height were used to determine the reason behind these successive high and low ozone events. In our research, both modelling and experimental data of the planetary boundary layer height (PBLH) were obtained and related to atmospheric stability. Aerosol backscatter data was used to investigate the structure, evolution, and influence of the top of the aerosol layer, which is a proxy for PBLHs. A shallow aerosol layer height (1164 ± 59 m) was observed during the high ozone episodes, in contrast to a deep aerosol layer (1990 ± 79 m) during the low ozone episodes. The ozone precursors, the ozone, and the ground-level aerosol concentrations were also examined during these episodes. It was observed that when the ozone was high, the PM2.5 was high, and when the ozone was low, the PM10 concentrations were high. Analysis of the wind patterns and synoptic scale meteorology also contributed to a better explanation of the nature of these events.

Six-year observations of aerosol optical properties at a southern African grassland savannah site

Atmospheric aerosols have a significant effect on earth's radiative budget, particularly on regional scales. This paper presents a ~6 year, in situ, ground level aerosol scattering and absorption dataset, measured at a background site strategically positioned to enable differentiation of the effect of anthropogenic, population density and open biomass burning activities on a regional scale. Relatively well-defined seasonal and diurnal patterns were observed for all the aerosol optical properties, i.e. scattering coefficient (σ SP ), absorption coefficient (σ AP ), single scattering albedo (ω 0 ) and Ångström exponent of scattering (α SP ). These patterns were explained by considering southern African specific sources and metrological conditions. Using a receptor modelling method (auto-generated source maps) it was found that air masses that had higher σ SP , σ AP and ω 0 , and lower α SP , if compared with the relatively clean background, passed over source regions with significant industrial or other anthropogenic activities, higher population density, re-circulation of polluted air masses and higher open biomass burning frequency. To quantify differences, four source regions were defined, i.e. Karoo, Kalahari, anti-cyclonic recirculation pattern and the industrial hub of South Africa. Air masses that had passed over the Karoo source region represented the cleanest regional background conditions, while air masses that had passed either over the industrial hub and/or the anti-cyclonic recirculation pattern represented the most significant anthropogenically impacted, as indicated by the aerosol optical properties. The ω 0 medians of air masses that had passed over the Karoo (0.80–0.86) were 9, 12 and 7% lower than in air masses that had passed over source regions with the highest ω 0 median, in the warmest/wettest, coldest, and driest, peak open biomass burning periods, respectively. • A six-year, ground level aerosol optical property dataset is presented. • Well-defined seasonal and diurnal patterns were evident for the optical properties. • Auto-generated source maps were used to explain results within a regional context. • Air masses passing over the Karoo region represented the most natural background. • Anthropogenic influences were evident in air masses passing over all other regions.

Lead Distribution in Ground-Level Atmospheric Aerosol of Zaporizhia City

The results of long-term observations on the geochemical peculiarities of technogenic lead distribution in atmospheric aerosol of Zaporizhia, as well as in soils, bottom sediments, suspended matter from Dnipro river and substance from wet gas filters of open-hearth furnaces PJSC Zaporizhstal are presented. The actual material was obtained on the basis of comprehensive field monitoring during 2015-2019. Simultaneous involvement of data on hydrometeorological conditions (seasonality, precipitation, wind speeds, directions and duration) in the study area provided a integral approach to the processing and generalization of research results. A number of peculiarities of sedimentary substance distribution in air, interrelations of its anthropogenic and natural components are defined. Correlations were revealed between lead concentrations changes and aeolian mineral component distribution, seasonality, and a number of other factors. The highest concentration of lead was found in solid aerosol particles accumulated by the sedimentary trap at the monitoring site. It was revealed that the average concentration of investigated element in river suspension of Dnipro exceeds significantly background values, and for soils exceeds more than 3.5 times the permissible norms. At the same time, the fixed form of lead in atmospheric aerosol, in contrast to other environmental objects, loses its domination, giving the way to easily soluble forms, so the evidence of environmental hazard. Monitoring of seasonal patterns of the element distribution in the city air showed excess concentrations in the cold period of the year, which, according to the authors, is due to both meteorological conditions and redistribution of natural and anthropogenic factors that serve the driving force in processes of arriving and conversion of lead in atmosphere during different seasons of the year.

The Potential Impact of Satellite-Retrieved Cloud Parameters on Ground-Level PM2.5 Mass and Composition.

Satellite-retrieved aerosol optical properties have been extensively used to estimate ground-level fine particulate matter (PM2.5) concentrations in support of air pollution health effects research and air quality assessment at the urban to global scales. However, a large proportion, ~70%, of satellite observations of aerosols are missing as a result of cloud-cover, surface brightness, and snow-cover. The resulting PM2.5 estimates could therefore be biased due to this non-random data missingness. Cloud-cover in particular has the potential to impact ground-level PM2.5 concentrations through complex chemical and physical processes. We developed a series of statistical models using the Multi-Angle Implementation of Atmospheric Correction (MAIAC) aerosol product at 1 km resolution with information from the MODIS cloud product and meteorological information to investigate the extent to which cloud parameters and associated meteorological conditions impact ground-level aerosols at two urban sites in the US: Atlanta and San Francisco. We find that changes in temperature, wind speed, relative humidity, planetary boundary layer height, convective available potential energy, precipitation, cloud effective radius, cloud optical depth, and cloud emissivity are associated with changes in PM2.5 concentration and composition, and the changes differ by overpass time and cloud phase as well as between the San Francisco and Atlanta sites. A case-study at the San Francisco site confirmed that accounting for cloud-cover and associated meteorological conditions could substantially alter the spatial distribution of monthly ground-level PM2.5 concentrations.

Multistatic aerosol–cloud lidar in space: A theoretical perspective

Accurate aerosol and cloud retrievals from space remain quite challenging and typically involve solving a severely ill-posed inverse scattering problem. In this Perspective, we formulate in general terms an aerosol and aerosol–cloud interaction space mission concept intended to provide detailed horizontal and vertical profiles of aerosol physical characteristics as well as identify mutually induced changes in the properties of aerosols and clouds. We argue that a natural and feasible way of addressing the ill-posedness of the inverse scattering problem while having an exquisite vertical-profiling capability is to fly a multistatic (including bistatic) lidar system. We analyze theoretically the capabilities of a formation-flying constellation of a primary satellite equipped with a conventional monostatic (backscattering) lidar and one or more additional platforms each hosting a receiver of the scattered laser light. If successfully implemented, this concept would combine the measurement capabilities of a passive multi-angle multi-spectral polarimeter with the vertical profiling capability of a lidar; address the ill-posedness of the inverse problem caused by the highly limited information content of monostatic lidar measurements; address the ill-posedness of the inverse problem caused by vertical integration and surface reflection in passive photopolarimetric measurements; help relax polarization accuracy requirements; eliminate the need for exquisite radiative-transfer modeling of the atmosphere–surface system in data analyses; yield the day-and-night observation capability; provide direct characterization of ground-level aerosols as atmospheric pollutants; and yield direct measurements of polarized bidirectional surface reflectance. We demonstrate, in particular, that supplementing the conventional backscattering lidar with just one additional receiver flown in formation at a scattering angle close to 170° can dramatically increase the information content of the measurements. Although the specific subject of this Perspective is the multistatic lidar concept, all our conclusions equally apply to a multistatic radar system intended to study from space the global distribution of cloud and precipitation characteristics.

Relationship of ground-level aerosol concentration and atmospheric electric field at three observation sites in the Arctic, Antarctic and Europe

Aerosol number concentrations in the particle size range from ~10nm to 1μm and vertical electric field strength in the surface layer was measured between September 2012 and December 2013 at three observation sites: mid-latitude station Swider, Poland, and, for the first time, in Hornsund in the Arctic, Spitsbergen, and the Antarctic Arctowski station in the South Shetland Islands. The measurements of aerosol concentrations have been performed simultaneously with measurements of the electric field with the aim to assess the local effect of aerosol on the electric field Ez near the ground at the three stations which at present form a network of atmospheric electricity observatories. Measurements have been made regardless of weather conditions at Swider and Arctowski station and mostly on fair-weather days at Hornsund station.The monthly mean particle number concentrations varied between 580 and 2100particlescm−3 at Arctowski, between 90 and 1270particlescm−3 in Hornsund, and between 6700 and 14,000particlescm−3 in the middle latitude station Swider. Average diurnal variations of the ground-level electric field Ez and particle number concentrations in fair-weather conditions were independent of each other for Arctowski and Hornsund stations. At Swider station the diurnal variation is usually characterized by an increase of aerosol concentration in the evening which results in the increased electric field. The assumption of neglecting the influence of varying aerosol concentration on the variation of the electric field in the polar regions, often adopted in studies, is confirmed here by the observations at Arctowski and Hornsund.The results of aerosol observations are also compared with modelled aerosol concentrations for global atmospheric electric circuit models.

Columnar and ground-level aerosol optical properties: sensitivity to the transboundary pollution, daily and weekly patterns, and relationships.

Columnar and ground-level aerosol optical properties co-located in space and time and retrieved from sun/sky photometer and nephelometer measurements, respectively, have been analyzed to investigate the impact of local and transboundary pollution, to analyze their relationships, and hence to contribute to the aerosol load characterization over the Central Mediterranean. The aerosol optical depth (AOD) at 440 nm, the Ångström exponent (Å) calculated from the AOD at 440 and 675 nm, and the asymmetry parameter (g col ) at 440 nm represent the investigated columnar aerosol parameters. The scattering coefficient (σ p) at 450 nm, the scattering Ångström exponent (å) calculated from σ p at 450 and 635 nm, and the asymmetry parameter (g) at 450 nm are the corresponding ground-level parameters. It is shown that the columnar and ground-level aerosol properties were significantly and similarly affected by the main airflows identified with backtrajectory cluster analysis. The yearly averaged daily evolution of σ p, å, and g was fairly correlated to the one of the AOD, Å, and g col , respectively. These results indicate that the aerosol particles were on average characterized by similar yearly averaged optical properties up to the ground level. In particular, the yearly means of columnar and ground-level Ångström exponents, 1.3 ± 0.4 and 1.1 ± 0.4, respectively, which are close to one, reveal a coarse-mode aerosol contribution in addition to the fine-mode particle contribution up to the ground level. Hourly means, day-by-day, and seasonal daily patterns of ground-level parameters were, however, very weakly correlated with the corresponding columnar parameters. The large impact of the local meteorology on the daily evolution of the ground-level aerosol properties, which makes the impact of long-range transported particles less apparent, was mainly responsible for these last results. It has also been found that columnar Ångström exponents much smaller than one may not be linked to å values smaller than 1. This may occurs when coarse-mode particle plumes, advected at high altitudes, do not penetrate inside the planetary boundary layer. Ångström exponents smaller than 1 are due to a significant contribution of coarse-mode particles as dust particles. Therefore, it is shown that å represents one of the best parameters to infer the contribution of coarse-mode particles at the ground level. The daily evolution of the aerosol properties referring to working days (Monday to Friday) and Sunday and the weekly cycle have suggested that the aerosol source contributions varied during the weekends. In particular, the AOD was characterized by a negative weekly cycle (higher AOD values during the weekend than during the weekdays), the Sunday σ p daily mean was 11 % larger than the Monday value, and å reached the highest value on Sunday. The impact up to the ground level of the weekdays' transboundary pollution, which reaches the monitoring site during the weekends, has likely contributed to these results.

How much is particulate matter near the ground influenced by upper-level processes within and above the PBL? A summertime case study in Milan (Italy) evidences the distinctive role of nitrate

Abstract. Chemical and dynamical processes lead to the formation of aerosol layers in the upper planetary boundary layer (PBL) and above it. Through vertical mixing and entrainment into the PBL these layers may contribute to the ground-level particulate matter (PM); however, to date a quantitative assessment of such a contribution has not been carried out. This study investigates this aspect by combining chemical and physical aerosol measurements with WRF/Chem (Weather Research and Forecasting with Chemistry) model simulations. The observations were collected in the Milan urban area (northern Italy) during the summer of 2007. The period coincided with the passage of a meteorological perturbation that cleansed the lower atmosphere, followed by a high-pressure period favouring pollutant accumulation. Lidar observations revealed the formation of elevated aerosol layers and evidence of their entrainment into the PBL. We analysed the budget of ground-level PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 μm) with the help of the online meteorology–chemistry WRF/Chem model, focusing in particular on the contribution of upper-level processes. Our findings show that an important player in determining the upper-PBL aerosol layer is particulate nitrate, which may reach higher values in the upper PBL (up to 30% of the aerosol mass) than in the lower PBL. The nitrate formation process is predicted to be largely driven by the relative-humidity vertical profile, which may trigger efficient aqueous nitrate formation when exceeding the ammonium nitrate deliquescence point. Secondary PM2.5 produced in the upper half of the PBL may contribute up to 7–8 μg m−3 (or 25%) to ground-level concentrations on an hourly basis. The residual aerosol layer above the PBL is also found to potentially play a large role, which may occasionally contribute up to 10–12 μg m−3 (or 40%) to hourly ground-level PM2.5 concentrations during the morning hours. Although the results presented here refer to one relatively short period in one location, this study highlights the importance of considering the interplay between chemical and dynamical processes occurring within and above the PBL when interpreting ground-level aerosol observations.

SPARTAN: a global network to evaluate and enhance satellite-based estimates of ground-level particulate matter for global health applications

Abstract. Ground-based observations have insufficient spatial coverage to assess long-term human exposure to fine particulate matter (PM2.5) at the global scale. Satellite remote sensing offers a promising approach to provide information on both short- and long-term exposure to PM2.5 at local-to-global scales, but there are limitations and outstanding questions about the accuracy and precision with which ground-level aerosol mass concentrations can be inferred from satellite remote sensing alone. A key source of uncertainty is the global distribution of the relationship between annual average PM2.5 and discontinuous satellite observations of columnar aerosol optical depth (AOD). We have initiated a global network of ground-level monitoring stations designed to evaluate and enhance satellite remote sensing estimates for application in health-effects research and risk assessment. This Surface PARTiculate mAtter Network (SPARTAN) includes a global federation of ground-level monitors of hourly PM2.5 situated primarily in highly populated regions and collocated with existing ground-based sun photometers that measure AOD. The instruments, a three-wavelength nephelometer and impaction filter sampler for both PM2.5 and PM10, are highly autonomous. Hourly PM2.5 concentrations are inferred from the combination of weighed filters and nephelometer data. Data from existing networks were used to develop and evaluate network sampling characteristics. SPARTAN filters are analyzed for mass, black carbon, water-soluble ions, and metals. These measurements provide, in a variety of regions around the world, the key data required to evaluate and enhance satellite-based PM2.5 estimates used for assessing the health effects of aerosols. Mean PM2.5 concentrations across sites vary by more than 1 order of magnitude. Our initial measurements indicate that the ratio of AOD to ground-level PM2.5 is driven temporally and spatially by the vertical profile in aerosol scattering. Spatially this ratio is also strongly influenced by the mass scattering efficiency.

Fukushima radionuclides in Vilnius/Lithuania aerosols: Modelling of aerosol transport

Measurements of activity concentrations of 131I; 129mTe, 132Te, 134Cs and; 136Cs, 137Cs in aerosol were carried out in daily samples after the Fukushima accident during the period of March–April, 2011 in Vilnius. The maximum activity concentrations of 131I and 137Cs were found to be 3700μBqm−3 and 1040μBqm−3, respectively. Variations in activity ratios of 132Te/129mTe and 137Cs/136Cs observed after the accident were explained by the decay of the short-lived isotopes, while those in 134Cs/137Cs by the dilution effect of the Chernobyl-derived 137Cs. It was found that different behaviours of highly volatile 131I and 137Cs resulted in enrichment of ground level aerosol particles by 131I with respect to 137Cs. Simulated activity concentrations of 137Cs attached to aerosol particles for the Vilnius site reasonably agreed with measured activities in aerosol samples during the Fukushima accident.

Remarks on representative ground-level air monitoring

The methods applied and the typical influences affecting representative measurements of radionuclides contained in ground-level aerosols are summarized. The activity concentrations of 7Be, 22Na, 40K, 137Cs, and 210Pb were determined in weekly collected samples and the ratio “sample A/sample B” was calculated over a period of 5 years. The results are compared with findings from literature. The estimation of the limits of comparability and reliability were extended to 22Na, 40K, 137Cs, and 210Pb measurements.

Aerosol properties over two urban sites in South Spain during an extended stagnation episode in winter season

Columnar and ground level aerosol properties as well as mass concentrations of some gaseous pollutants were measured at two urban sites (Granada and Málaga, South Spain) before, during and after an extended stagnation episode from 7 to 13 February 2011. This long lasting event was associated with a very strong and persistent blocking high-pressure system over the Iberian Peninsula, together with very intense and persistent temperature inversions near the ground level. The columnar aerosol load at Granada showed a significant increase during this stagnation episode as indicated by aerosol optical depth at 440 nm, reaching values four times higher (0.6) than before and after the event. A significant increase in aerosol load at night time was also evidenced by star photometer measurements. Similarly, pronounced enhancement in columnar aerosol load was observed at Málaga, indicating the regional extension of this event. Analysis of ground level measurements obtained at Granada showed a significant increase in aerosol scattering coefficients and aerosol number concentrations during the stagnation episode. Furthermore, the analysis of aerosol size distribution measurements has evidenced the large contribution of fine particles at ground level as well as in the atmospheric column during the stagnation period. The fine mode radius measured at Granada showed a large displacement towards larger sizes together with a pronounced increase in the geometric standard deviation of the fine mode during the high pollution event in the morning hours on 9 February. This was attributed to the growth of aerosol particles due to coagulation and condensation processes as a result of the high fine aerosol load next to the surface favoured by the high pressure system and thermal inversion on that day. This increases in the radius and width of the fine mode results in more efficient scattering in the 440–1020 nm spectral range which, in combination with nearly constant and low imaginary refractive index (0.002) leads to high single scattering albedo with neutral spectral dependency. During the persistent temperature inversion episode the daily European PM10 mass concentration limit (50 μg m−3) was exceeded in Granada for most days and in Málaga for three consecutive days. Nevertheless, the mass concentrations of NO2, CO and O3 were below the European thresholds with a noticeable decrease in O3 mass concentration at both sites.

Aerosol size distribution from inversion of solar radiances and measured at ground-level during SPALI10 campaign

A comparison of columnar versus ground-level measurements of aerosol size distribution is presented. The purpose was to understand what atmospheric conditions are most suitable for closure between: 1) ground-based remote sensing instruments measuring vertical column-integrated optical and microphysical properties and 2) ground-based in-situ aerosol instruments measuring microphysical properties at the surface scaled to height using aerosol backscatter profiles from lidar measurements. Measurements were taken during the SPALI10 (SPAin Lidar Intercomparison 2010) field campaign at Madrid from 18 October to 5 November 2010 within the frame of the EARLINET-ASOS project. Monitoring included aerosol measurements of size distribution as well as chemical composition. Both combined Scanning Mobility Particle Sizer and Optical Particle Counters were used for determining aerosol size distribution ranging from 15nm to 10μm. Additionally, the column-integrated characterization of the atmospheric aerosol was provided by a sun tracking photometer using a methodology based on non-spherical particles. Two different synoptic situations were analyzed: a stagnation scenario with growing pollution concentrations and a clean atmosphere with low aerosol particle load. The shape of the ground-level aerosol size distribution matches with the column-integrated in both scenarios but absolute values disagree, particularly with stagnation. Better agreement was found for clean atmospheric situations when the mixing layer reaches higher altitudes and the aerosol concentration within the atmosphere follows an exponential decay profile with height.

An update on polar aerosol optical properties using POLAR-AOD and other measurements performed during the International Polar Year

An updated set of time series of derived aerosol optical depth (AOD) and Ångström’s exponent α from a number of Arctic and Antarctic stations was analyzed to determine the long-term variations of these two parameters. The Arctic measurements were performed at Ny-Ålesund (1991–2010), Barrow (1977–2010) and some Siberian sites (1981–1991). The data were integrated with Level 2.0 AERONET sun-photometer measurements recorded at Hornsund, Svalbard, and Barrow for recent years, and at Tiksi for the summer 2010. The Antarctic data-set comprises sun-photometer measurements performed at Mirny (1982–2009), Neumayer (1991–2004), and Terra Nova Bay (1987–2005), and at South Pole (1977–2010). Analyses of daily mean AOD were made in the Arctic by (i) adjusting values to eliminate volcanic effects due to the El Chichón, Pinatubo, Kasatochi and Sarychev eruptions, and (ii) selecting the summer background aerosol data from those affected by forest fire smoke. Nearly null values of the long-term variation of summer background AOD were obtained at Ny-Ålesund (1991–2010) and at Barrow (1977–2010). No evidence of important variations in AOD was found when comparing the monthly mean values of AOD measured at Tiksi in summer 2010 with those derived from multi-filter actinometer measurements performed in the late 1980s at some Siberian sites. The long-term variations of seasonal mean AOD for Arctic Haze (AH) conditions and AH episode seasonal frequency were also evaluated, finding that these parameters underwent large fluctuations over the 35-year period at Ny-Ålesund and Barrow, without presenting well-defined long-term variations. A characterization of chemical composition, complex refractive index and single scattering albedo of ground-level aerosol polydispersions in summer and winter–spring is also presented, based on results mainly found in the literature. The long-term variation in Antarctic AOD was estimated to be stable, within ±0.10% per year, at the three coastal sites, and nearly null at South Pole, where a weak increase was only recently observed, associated with an appreciable decrease in α, plausibly due to the formation of thin stratospheric layers of ageing volcanic particles. The main characteristics of chemical composition, complex refractive index and single scattering albedo of Antarctic aerosols are also presented for coastal particles sampled at Neumayer and Terra Nova Bay, and continental particles at South Pole.