Ambient aerosol sampling: Problems,goals and wind tunnel test results of hi-volume samplers

The area of ambient air sampling and its inherent problems and current goals are discussed in general. In particular, recently completed tests of the collection effectiveness of the Rocky Flats Hi-Volume Sampler are compared to previously completed tests of the Standard EPA Hi-Volume Sampler for a variety of field realistic conditions. Collection effectiveness is defined as the ratio of the aerosol collected on the collection substrates of the sampler to that collected by an isokinetic sampling system.The collection effectiveness of the Standard EPA Hi-Volume Sampler was determined as a function of particle size (5–50 μm) and sampler orientation (0° and 45°) at a base condition wind speed of 4.6 m/s and 8% relative turbulence intensity. The results indicated a strong effect of orientation on collection effectiveness at a sampling rate of 1416 l/min. Wind speed over the range of 1.5–4.6 m/s does not greatly influence the collection effectiveness of 15 μm particles. Free stream turbulence levels of 1 and 8% relative intensity has no effect upon collection characteristics.The collection effectiveness of the Rocky Flats Hi-Volume Sampler was determined as a function of particle size (1–34 μm), wind speed (1.52–12.19 m/s) and sampler orientation to the mean flow (0°, 45°, 180°). The results show the sampler, with an inlet flow rate of 880 l/min, has an inlet effectiveness that was a slight function of orientation angle for particles 1–10 μm with a larger effect seen for 20–34 μm; a strong effect of velocity was seen up to 5 m/s where a further increase showed only a slight decrease in effectiveness.The Microsorban-98 filter that is presently used in the Rocky Flats Sampler was tested for efficiency over the size range of particles from 0.01–1 μm and with three different face velocities using the sampler flow rates of 600, 800 and 1 000 l/min corresponding to pressure drops of 20–24 in. of water (3.74–4.49 cm HaG). The filter paper, which was of the fiber type, was found to be 99.9% efficient over the range of particle sizes and pressure drops tested.     

Validation of the new passive monitor without a membrane in indoor air

A series of experiments were conducted to investigate the suitability of activated carbon cloth (ACC) strips for diffusive sampling of volatile organic compounds in air without using a membrane. Experiments were carried out in an environmental chamber and in an indoor air environment. The monitors were analyzed qualitatively by liquid extraction and gas chromatography-mass spectrometry and quantitatively by gas chromatography with a flame ionization detector. It was shown that ACC monitors can provide satisfactory quantitative data for a wide variety of volatile organic compounds in indoor air ranging in concentration from 1 mg/m³ to 0.02 g/m³. It was also observed that the absence of a membrane does not affect the linearity of the sampling rate. In addition, a packaging system has been developed that allows samples to be sent out and returned to the laboratory for analysis by mail. Recycling, a convenient procedure of exposure, delivery, and return by postal service, as well as simple analysis all show that this passive sampler substantially reduces the cost of monitoring, thus demonstrating its benefits in the determination of contamination sources within both large industrial sites or any other building where such an analysis may be required.     

A new method for assessing the contribution of Primary Biological Atmospheric Particles to the mass concentration of the atmospheric aerosol

Primary Biologic Atmospheric Particles (PBAPs) constitute an interesting and poorly investigated component of the atmospheric aerosol. We have developed and validated a method for evaluating the contribution of overall PBAPs to the mass concentration of atmospheric particulate matter (PM). The method is based on PM sampling on polycarbonate filters, staining of the collected particles with propidium iodide, observation at epifluorescence microscope and calculation of the bioaerosol mass using a digital image analysis software. The method has been also adapted to the observation and quantification of size-segregated aerosol samples collected by multi-stage impactors.Each step of the procedure has been individually validated. The relative repeatability of the method, calculated on 10 pairs of atmospheric PM samples collected side-by-side, was 16%.The method has been applied to real atmospheric samples collected in the vicinity of Rome, Italy. Size distribution measurements revealed that PBAPs was mainly in the coarse fraction of PM, with maxima in the range 5.6–10 μm. 24-h samples collected during different period of the year have shown that the concentration of bioaerosol was in the range 0.18–5.3 μg m^− 3 (N = 20), with a contribution to the organic matter in PM₁₀ in the range 0.5–31% and to the total mass concentration of PM₁₀ in the range 0.3–18%.The possibility to determine the concentration of total PBAPs in PM opens up interesting perspectives in terms of studying the health effects of these components and of increasing our knowledge about the composition of the organic fraction of the atmospheric aerosol.     

Comprehensive study of endocrine disrupting compounds using grab and passive sampling at selected wastewater treatment plants in South East Queensland, Australia

Chemical (gas chromatography-mass spectrometry, GC-MS) and biological (E-Screen assay) analyses were used to determine the concentrations of 15 endocrine disrupting compounds (EDCs) and estrogen equivalent (EEq) in grab and passive samples from five municipal wastewater treatment plants (WWTPs) in South East Queensland, Australia. EEq concentrations derived by E-Screen assays for the grab samples were between 108-356 ng/L for the influents and < 1-14.8 ng/L for the effluents with the exception of one effluent sample which was at 67.8 ng/L EEq. The EDC concentrations and EEq values for the passive samples were several times lower than those of the grab samples: a decrease probably caused by, but not limited to biofouling, low flow rate, biodegradation and temperature which can progressively reduce the uptake of compounds into the sampler. At this stage, grab sampling is the most reliable method for field monitoring; nevertheless, passive sampler is a useful sampling tool but the method requires more research to ensure that the information obtained can be interpreted appropriately. Although alkylphenols and phthalates were detected at higher concentrations in the wastewater samples as compared to natural hormones, the environmental risk may be negligible as their estrogenic potencies are several orders of magnitude lower than that of the natural estrogens. In most wastewater samples, the natural estrogens contributed to 60% or more of the EEq value. Removal efficacy of most estrogenic and xenoestrogenic compounds from the conventional activated sludge or biological nutrient removal (BNR) WWTPs monitored in this study was in the range of 80-> 99%. The efficiency of the WWTPs in removing estrogenic activity was > 95%. The EEqs of the E-Screen and those calculated from the results of extensive chemical analyses using the estradiol equivalency factors were comparable for most of the WWTPs samples.     

Plutonium isotopes in surface air of Prague in 1986–2006

Monitoring of ^239,240Pu in surface air of Prague started in 1986 in connection with the Chernobyl accident. Measurable activities of 10–28 μBq m⁻³ were found from 29 April 1986 to 5 May 1986. In the most of the monitoring periods of 1987–1996, activities of ^239,240Pu in air were not measurable. Positive values for ^239,240Pu and ²³⁸Pu in air could be obtained after installation of an aerosol sampler with higher flow-rate in 1997. Activity concentrations of ^239,240Pu and ²³⁸Pu in Prague air in the most of quarters of 1997–2006 were in the range 0.53–5.06 and <0.16–1.10 nBq m⁻³, respectively. Seasonal fluctuations can be found in content of ^239,240Pu in air. Activity ratios of ²³⁸Pu/^239,240Pu in air are higher than those in top soil, so it can be supposed that ²³⁸Pu is coming to air of Prague also from other sources than resuspension of fallout from atmospheric nuclear tests.     

A study of effects of ambient urban air pollution using personal samplers; a preliminary report

Air pollution has been associated with an increased incidence of respiratory disease. However, significant differences may exist between air pollution levels measured at conventional fixed monitoring stations and actual levels inhaled by a subject. Furthermore, studies of effects of air pollution might best be done using asthmatics as study subjects, since they have irritable airways. This is a preliminary report of a study using a control and asthmatic group in which effects of air pollution are assessed by sympton and medication diaries and simple pulmonary function tests. Air pollution exposure is measured using a small portable sampler for particulates, SO₂ and NO₂; these samplers are carried by the subject (“personal”) and are situated inside and outside the home. Levels obtained are compared to data obtained from the same type of sampler located at a fixed monitoring station. Preliminary analysis of the data shows that the levels of pollutants are low and there are significant differences between the four air pollution measurements, with weak correlations among the various measurements. In this preliminary report, change in pulmonary function during the day correlates only with personal NO₂ measurements. This suggests the need for estimating air pollution exposure using “personal” samplers, when investigating health effects.     

Experimental measurements of aerosol concentrations in offices

A new automated version of the piezoelectric microbalance measures the mass concentration of airborne particles smaller than a preselected aerodynamic cutoff diameter. It is designed for near-real-time, unattended, round-the-clock measurements of nearly any aerosol environment inhabited by humans. The instrument uses an electrostatic precipitator to deposit particles with greater than 95% efficiency onto a piezoelectric quartz crystal sensor which is able to detect a deposit of 0.005 μg. The precipitator and sensor are nearly identical to those in the portable instrument reported previously. Measurements comparing within ± 15% with gravimetrically measured filter samples are documented for a wide variety of aerosols in the 50 μg/m³ to 5.5 mg/m³ range. The instrument measures particles from 10 μm down to 0.01 μm in diameter, including submicron combustion smokes and metallic fumes. The piezoelectric microbalance technique senses the mass concentration of the aerosol, rather than light scattering properties as is done by photometers and nephelometers. The piezobalance, with 1 L/min sample flow, is more sensitive than any other mass-sensing instrument, making it especially suited for low concentration indoor measurements, even below 50 μg/m³. An automatic piezobalance recently measured respirable aerosol mass concentrations in several offices. Each measurement was the average concentration during a 30-min period. The 24-h/day measurements continued for several days. Especially interesting is the diurnal pattern, both for offices with and without smokers. The effect of a single nearby smoker was clearly illustrated when the smoker was absent one day in the middle of a week. Normal daytime peak concentrations in that office reached 80–110 μg/m³ with a smoker present, but only 50–60 μg/m³ when the smoker was absent. Curious smokers who briefly stopped byt o see the instrument caused single half-hour averages to triple, to values as high as 294 μg/m³ in one office.     

Development of a personal exposure monitor for two sizes of inhalable particulates

Measurement of personal exposure to ambient level particulate concentrations is often extremely difficult because of a lack of personal exposure monitors capable of collecting measurable quantities within a meaningful sampling period. A new personal exposure monitor for two fractions of inhalable particulates (i.e., the 3–15 μm aerodynamic diameter and the < 3 μm or respirable fraction) has been developed and characterized. This monitor is capable of collecting a sample of each fraction that is quantifiable with ambient concentrations of inhalable/respirable particulates as low as 25 μg/m³ in a 24-h sampling period. Wind tunnel tests have been made on the particulate personal exposure monitor to determine sampling efficiency as a function of relative wind speed and orientation with respect to the sampler.     

Energy and exergy analysis of a PCM-based solar powered winter air conditioning using desiccant wheel during nocturnal

In this paper, heating and humidification of air for space have been carried out by using a phase change material (PCM)-based solar-powered desiccant wheel air conditioning (SPDWAC) in winter. The analysis of the setup has been done at different air flow rates. At low and high air flow rates, system has mean thermal coefficient of performance of 0.121 and 0.172, respectively, and mean exergy efficiency of 0.0787 and 0.0846, respectively. The mean thermal coefficient of performance of the system at high air flow rate (127.23 kg h-1) is 1.42 times the low air flow rate (63.62 kg h-1) and average exergy efficiency of the system at high air flow rate is 1.07 times the low air flow rate. It is observed that with an increase in air flow rate, efficiency of the evacuated tube solar air collector (ETSAC) increases. The average efficiency of the ETSAC at high air flow rate is 15.60%. The maximum average energy efficiency (17.80%) and exergy efficiency (17.08%) of the PCM storage system have been obtained at high air flow rate. The overall performance of the system showed that the use of PCM storage is feasible to run the system in winter during the hours of darkness.     

Mineralogical,chemical and toxicological characterization of urban air particles

Systematic characterization of morphological, mineralogical, chemical and toxicological properties of various size fractions of the atmospheric particulate matter was a main focus of this study together with an assessment of the human health risks they pose. Even though near-ground atmospheric aerosols have been a subject of intensive research in recent years, data integrating chemical composition of particles and health risks are still scarce and the particle size aspect has not been properly addressed yet. Filling this gap, however, is necessary for reliable risk assessment. A high volume ambient air sampler equipped with a multi-stage cascade impactor was used for size specific particle collection, and all 6 fractions were a subject of detailed characterization of chemical (PAHs) and mineralogical composition of the particles, their mass size distribution and genotoxic potential of organic extracts. Finally, the risk level for inhalation exposure associated to the carcinogenic character of the studied PAHs has been assessed. The finest fraction (< 0.45 μm) exhibited the highest mass, highest active surface, highest amount of associated PAHs and also highest direct and indirect genotoxic potentials in our model air sample. Risk assessment of inhalation scenario indicates the significant cancer risk values in PM 1.5 size fraction. This presented new approach proved to be a useful tool for human health risk assessment in the areas with significant levels of air dust concentration.     

Evaluation of selected monitoring methods for formaldehyde in domestic environments

Varioys analytical methodologies for the monitoring of formaldehyde (CH₂O) concentrations in domestic environments have been developed and evaluated. A modified CEA Instruments, Inc., analyzer has near-real-time CH₃O-specific analysis capability with an 0.01 mg/m³ detection limit. A solid sorbent, 13X molecular sieve has been utilized in a pumped collection unit with a demonstrated 0.03–12.5 mg/m³ linear dynamic range using sampling periods of ≤ 15 min. The development of screening-type techniques has included (1) a semipermeable-membrane passive sampler for measurements of average CH₂O concentrations over 8–24-h periods, and (2) a visual colorimetric analysis method for semiquantitative CH₂O determinations using solid chemical reagents. A preliminary field evaluation has been completed. The results show excellent agreement between the new CH₂O monitoring methods and a reference sampling and analysis technique. A generation apparatus for the production of CH₂O vapor is also reported with a demonstrated linear dynamic range between 0.003 and 12.5 mg/m³.     

Surface air concentration and deposition of lead-210 in French Guiana: two years of continuous monitoring

To make up for the lack of data on (210)Pb aerosol deposition in tropical regions and to use this radionuclide as an aerosol tracer, a monitoring station was run for two years at Petit-Saut, French Guiana. Lead-210 concentration in air at ground level was monitored continuously together with atmospheric total deposition. The air concentration has a mean value of 0.23+/-0.02 mBq m(-3) during both wet and dry seasons, and it is only weakly affected by the precipitation mechanism. This result was unexpected in a wet tropical region, with a high precipitation rate. In contrast, deposition clearly correlates with precipitation for low/moderate rainfall (<15 cm per 15-day), while this correlation is masked by strong fluctuations at high rainfall. The estimated mean annual deposition over the last ten years is 163+/-75 Bq m(-2) y(-1). This provides a procedure for estimating this mean flux at other sites in French Guiana.     

A passive sampling device for determining formaldehyde in indoor air

A passive sampling device based on the principle of diffusion has been developed for the determination of formaldehyde in ambient air. The sampler consists of a capped glass tube (with approximate dimensions of 2.4 × 9 cm) containing a glass-fiber filter treated with NaHSO₃. In the field, the device collects a sample by being uncapped for a specified sampling time. After being recapped and returned to the laboratory, the filter is analyzed by the chromotropic acid (CTA) method. Laboratory validation studies were conducted by exposing the sampling devices for 1 week to dry formaldehyde gas generated by passing trioxane vapor over an acid catalyst bed. In these tests, formaldehyde concentrations ranged from 0.05 to 0.80 mL/m³. Reproducibility was excellent, with relative standard deviations averaging 5.4% for five constant concentrations. The lower detection limit was determined to be 3.6 mL/m³ h. In an occupational environment an 8-h sample would be sufficient to detect compliance with the OSHA permissible exposure limit of 3 mL/m³; in a residential environment a 1-week sample would allow detection of 0.025 mL/m³ for indoor air quality audits.     

Urban green-cover and the environmental performance of Chennai city

Chennai city the capital of Tamil Nadu is located in southeastern India. Its average population growth rate is 25% per decade, which recurrently alters the city’s land-cover particularly the receding green-cover distressed the city’s self-renewal capacity, in terms of groundwater recharge, pollution sequestration and microclimatic amelioration. This has been appraised by means of a GIS model. This model was developed using three sets of green-cover associated parameters, namely air quality amelioration, hydrological process regulation and microclimatic amelioration. The outcome confirms the difference in the city’s environmental performance between the 1997 and 2001. At some parts of the city, due to the green-cover change, the extent of modification was 38%, in terms of mean percent change in all three sets of parameters mentioned earlier. Through coefficient of correlation (r) method, relationship between green-cover change and environmental performance change are checked. It confirms positive relationship (r = 1) in all parts, except at few places.     

Relationship between variations of Be, Pb and Pb concentrations and sub-regional atmospheric transport: Simultaneous observation at distant locations

In order to investigate the applicability of ²¹²Pb as a tracer for atmospheric transport in the sub-regional scale (few hundred kilometers in horizontal direction and up to ∼1 km by height), we measured the air concentrations of the short-lived radionuclide ²¹²Pb along with the long-lived ⁷Be and ²¹⁰Pb near the ground surface. For this purpose, simultaneous observations were continued for several days at three locations: a reference point representative for standard land surface atmosphere conditions, a second location at an altitude 650 m near the reference point, and on a solitary island ∼180 km from the reference point. Measurements of radioactivity in aerosol particle samples collected at intervals of 2-3 h with a high-volume air sampler were performed by extremely low background gamma-ray spectrometry with the use of Ge detectors located at the Ogoya Underground Laboratory. Concentration of ⁷Be or ²¹⁰Pb and their variation patterns was found to be similar among the three points during the whole observation period except for moment of the passage of a cold front. The results indicate that distributions of concentrations of the long-lived nuclides were uniform in this range. On the other hand, concentration levels and the variation patterns of the short-lived ²¹²Pb differed greatly from one location to another, reflecting differences in geographical location and altitude of the observation points. Additionally, there were certain indications that observed concentration of ²¹²Pb contained two components: an autogenous component from sources nearby and a heterogenous one from faraway sources carried by atmospheric horizontal transport. Results of this study provide experimental proof that ²¹²Pb can be used as a tracer of sub-regional atmospheric transport.     

Can car air filters be useful as a sampling medium for air pollution monitoring purposes?

Urban air quality and real human exposure to chemical environmental stressors is an issue of high scientific and political interest. In an effort to find innovative and inexpensive means for air quality monitoring, the ability of car engine air filters (CAFs) to act as efficient samplers collecting street level air, to which people are exposed to, was tested. In particular, in the case of taxis, air filters are replaced after regular distances, the itineraries are almost exclusively urban, cruising mode is similar and, thus, knowledge of the air flow can provide with an integrated city air sample. The present pilot study focused on polycyclic aromatic hydrocarbons (PAHs), the most important category of organic pollutants associated with traffic emissions. Concentrations of ΣPAHs in CAFs ranged between 650 and 2900 μg CAF^− 1, with benzo[b]fluoranthene, benzo[k]fluoranthene and indeno[123-cd]pyrene being the most abundant PAHs. Benzo[a]pyrene (BaP) ranged between 110 and 250 μg CAF^− 1, accounting regularly for 5–15% of the total carcinogenic PAHs. The CAF PAH loads were used to derive road-level atmospheric PAH concentrations from a standard formula relating to the CAF air flow. Important parameters/assumptions for these estimates are the cruising speed and the exposure duration of each CAF. Based on information obtained from the garage experts, an average ‘sampled air volume’ of 48,750 m³ per CAF was estimated, with uncertainty in this calculation estimated to be about a factor of 4 between the two extreme scenarios. Based on this air volume, ΣPAHs ranged between 13 and 56 ng m^− 3 and BaP between 2.1 and 5.0 ng m^− 3, suggesting that in-traffic BaP concentrations can be many times higher than the limit values set by the UK (0.25 ng m^− 3) and the European Union (1.0 ng m^− 3), or from active sampling stations normally cited on building roof tops or far from city centres.Notwithstanding the limitations of this approach, the very low cost, the continuous availability of very high amounts of “sample”, and the “retroactivity” render it very useful and complementary to existing passive sampling techniques. This approach yields estimated air concentrations that reflect the pollutant concentrations to which taxi drivers, pedestrians, cyclists and road-related professionals are exposed.     

Investigation of source apportioning for α-HCH using enantioselective analysis

Enantiomeric analysis can be used as a complementary tool for source apportionment of chiral compounds, particularly for α-HCH. In this study we used archived samples from studies related to the distribution of POPs in air–water and air–soil–grass systems. Such approach is based on the behaviour of chiral compounds released into the atmosphere from a primary source, when they are expected to show racemic or close to racemic composition. Contrarily, when chiral compounds have been reemitted from secondary sources (e.g. water or soil), their enantiomeric signatures are frequently non-racemic and are similar to the signature of the secondary source. To show such evidence, extracts from passive air samples deployed throughout Europe were analyzed for the enantiomers of α-HCH. The proximity to a large water body showed a high impact on the enantiomeric signatures: Baltic air had enantiomeric fractions (EFs) < 0.500, while Mediterranean air had predominantly EFs > 0.500. Similarly, Atlantic air showed a latitude influence: above 50°N most EFs < 0.500, whereas at latitudes below 50°N, EFs were > 0.500. A similar trend was also observed for EFs of α-HCH measured in air samples from a latitudinal transect during an Atlantic cruise. This transect shows that samples from higher latitudes (above 40°N) have EF < 0.500, whereas in the more southern samples (African coast and Southern Atlantic), there is no clear trend for EFs. Inland air samples showed a large range in EF values, with racemic signatures for samples with the highest α-HCH concentrations and an increasing spread in the EFs for lower α-HCH concentrations. As expected, the EF values of α-HCH in air, soils and grass were also impacted by latitude. Correlations between EFs and geographic characteristics of the sampling locations, as well as α-HCH concentrations, α-/γ-isomer ratios, or temperature suggest that enantioselective analysis can give additional information on the distribution and sources of α-HCH in the environment.     

Human exposure to environmental pollutants after a tire landfill fire in Spain: Health risks

In May 2016, a fire occurred in one of the largest landfills in Europe (Seseña, Toledo, Spain), where 70,000–90,000 tons of tires had been illegally accumulated for > 15 years. Because of the proximity of population nuclei and the duration of the episode (> 20 days), we conducted a preliminary human health risk assessment study just after the tire fire. Samples of air and soil were collected in 3 areas surrounding the landfill (El Quiñón, at only 500 m, and Seseña Nuevo and Seseña Viejo, both at 4 km), as well as in background sites. In addition, samples of crops (barley, wheat, cabbage and lettuce) were also obtained from local farmers. The concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and a number of trace elements (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Sn, Tl, and V) were analyzed in all the samples. The concentrations of all the target pollutants, excepting PAHs, were relatively similar at the different sampling zones, irrespective of the distance to the landfill. In turn, a significant increase of PAHs was noted near the tire landfill, with air levels up to 6-times higher than those found at 4 km (134 vs. 19.5–22.7 ng/m³). Similarly, PAH concentrations in lettuce were relatively higher than those typically found in monitoring programs of food safety. Because of the increase of airborne PAHs, cancer risks due to exposure to environmental pollutants for the population living at El Quiñón, near the landfill, were between 3- and 5-times higher than those estimated for the inhabitants of Seseña. After this preliminary study, further investigations, focused only on PAHs, but more extensive in terms of number of samples, should be conducted to assure that PAHs have been progressively degraded through time.     

Plutonium isotopes in surface air of Prague in 1986-2006

Monitoring of (239,240)Pu in surface air of Prague started in 1986 in connection with the Chernobyl accident. Measurable activities of 10-28muBqm(-3) were found from 29 April 1986 to 5 May 1986. In the most of the monitoring periods of 1987-1996, activities of (239,240)Pu in air were not measurable. Positive values for (239,240)Pu and (238)Pu in air could be obtained after installation of an aerosol sampler with higher flow-rate in 1997. Activity concentrations of (239,240)Pu and (238)Pu in Prague air in the most of quarters of 1997-2006 were in the range 0.53-5.06 and <0.16-1.10nBqm(-3), respectively. Seasonal fluctuations can be found in content of (239,240)Pu in air. Activity ratios of (238)Pu/(239,240)Pu in air are higher than those in top soil, so it can be supposed that (238)Pu is coming to air of Prague also from other sources than resuspension of fallout from atmospheric nuclear tests.     

Three years of operational experience from Schauinsland CTBT monitoring station

Data from three years of operation of a low-level aerosol sampler and analyzer (RASA) at Schauinsland monitoring station are reported. The system is part of the International Monitoring System (IMS) for verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The fully automatic system is capable to measure aerosol borne gamma emitters with high sensitivity and routinely quantifies 7Be and 212Pb. The system had a high level of data availability of 90% within the reporting period. A daily screening process rendered 66 tentative identifications of verification relevant radionuclides since the system entered IMS operation in February 2004. Two of these were real events and associated to a plausible source. The remaining 64 cases can consistently be explained by detector background and statistical phenomena. Inter-comparison with data from a weekly sampler operated at the same station shows instabilities of the calibration during the test phase and a good agreement since certification of the system.