I. Allegrini

Annular denuder method for sampling reactive gases and aerosols in the atmosphere

Abstract The annular denuder method (ADM), consisting of a denuder-filter pack assembly, has been developed to measure several gaseous and aerosol species without sampling artifacts. Its application to the determination of gaseous acids and ammonia in the atmosphere indicates that this aim is achieved, even though the ratio of analytes in the gas phase and particulate matter is extremely low. The procedure is particularly suitable for the measurement of trace levels (0.1 μg m −3 ) of HNO 3 and SO 2 (even in the presence of high levels of sulfate and nitrate) as well as of HNO 2 (by making correction for the positive interference of NO 2 and PAN). The ADM sampling and analysis precision for the examined gases does not exceed ±10% in the 0.1–10 μg m −3 range. By sampling at an air flow rate of 151 min −1 , the minimum detectable atmospheric concentrations is ∼0.1 μg m −3 for each species over a 3 h interval.

Measurement of nitrous acid in milan, italy, by doas and diffusion denuders

Abstract Measurements of the atmospheric mixing ratio of nitrous acid have been carried out by means of the differential optical absorption system (DOAS) during a wintertime field study carried out in the urban area of Milan (Italy). Preliminary laboratory calibration of the DOAS was carried out by making use of a standard source of pure HONO which provided an interference-free UV absorption spectrum of HONG and recalculation of the absorption cross-section of this compound. Very high HONO mixing ratios, up to values exceeding 10 ppb, were measured during the campaign. DOAS results were compared with those obtained by using the annular denuder technique and good agreement was observed during the whole measurement period. On the basis of these measurements, a study on the relationship between the dynamic conditions of the lower boundary layer and the temporal trend of HONG mixing ratio can be carried out, in order to obtain indications about the sources of this species in urban atmospheres.

Development of a passive sampling technique for the determination of nitrogen dioxide and sulphur dioxide in ambient air

Abstract Since conventional badge-type samplers depend entirely on an empirical calibration against gas mixtures and use of the constant determined for calculations, a system based on the use of two badge-type passive samplers of different lengths has been studied for the determination of NO 2 or SO 2 . The samplers make use of a filter membrane barrier to damp out eddy motions and promote molecular diffusion from the ambient air to the trapping medium. The badges have a large ratio of crosssectional area to length of diffusion in order to increase sensitivity. Diffusion theory applied to this configuration shows that the concentration results are independent of wind-flow velocity. The samplers have been evaluated at different levels of relative humidity ranging from 0% to 90%. Batches of samplers were exposed for 48 h side-by-side with samplers exposed for consecutive periods of 24 h over the same interval. The results, expressed as a ratio ( R ) between the 48 h and the consecutive 24 h periods showed that the analytes collected (nitrite and sulphate) are stable ( R = 0.98 ± 0.05 for nitrite and R = 1.08 ± 0.09 for sulphate). The detection limit attained (5 ppb of NO 2 and 10 ppb of SO 2 for a 24 h sampling) is adequate for the determination in urban and rural areas. The accuracy of the proposed method, expressed as percent relative error, when compared to a standard denuder technique for NO 2 and SO 2 , was found to be better than about ±10% with coefficient of variation of 5.5% and 6.5%, respectively.

Integrated ambient ozone evaluation by passive samplers and clover biomonitoring mini-stations

An ozone monitoring network was set up using passive samplers and biological mini-stations of two clones of white clover (Trifolium repens L. cv. Regal), NC-R (O(3)-resistant) and NC-S (O(3)-sensitive). This paper reports on a pilot study performed in the period June-October 1999 in the Rome municipal area by using five biomonitoring mini-stations and ozone passive samplers with a new nitrite based design. This combined methodology can be used to obtain information on the biological implications of the injury due to tropospheric ozone. The two techniques can integrate data for the short-medium period and can be placed in different urban and rural sites, proving to be a very useful tool for ozone concentration mapping.

Determination of some atmospheric pollutants inside a museum: relationship with the concentration outside

Indoor and outdoor pollutant concentrations were measured simultaneously in the Galleria degli Uffizi in Florence. SO2, HNO3, HONO and O3 in the gas phase and sulphate, nitrate and ammonium in particulate matter were measured. Indoor and outdoor values were similar. The indoor/outdoor ratios for all the pollutants, with the exception of HONO, were extremely variable ranging from 0.1 to 0.9. For HONO the indoor levels have been found to largely exceed those outdoor. The results show that indoor HONO production could result from heterogenous reactions on the walls and exposed surfaces which could include artworks giving rise to dry acid deposition of nitric acid.

Heterogeneous reactions of SO2 and NO2 on carbonaceous surfaces

Abstract Four different types of carbonaceous material (active carbon, graphitized carbon black, source samples from a power plant and from an urban heating boiler) were exposed to synthetic gas mixtures of SO2 and SO2 + NO2 in sub-ppm ranges at two different relative humidities (60 and 90% r.h.). The interaction of SO2 and NO2 with the carbonaceous materials led to sulfate and to three other products: NO, HONO and NO2−. Formation of NO and NO2− was observed only on active carbon while the other three materials yielded HONO. It was also found that NO2 enhances the deposition of SO2 only on the source sample from the power plant at 60% r.h., whereas the material from the urban heating boiler had no effect both at low and high r.h. The results obtained show that no general statements on the effect of NO2 on the heterogeneous reaction of SO2 on these carbonaceous materials can be made and also that NO2 reactions in the presence of SO2 on carbon-containing surfaces should be considered as a source for the production of HONO in the urban environment.

Measurement of Atmospheric Nitric Acid in Gas Phase and Nitrate in Particulate Matter by means of Annular Denuders

Abstract A denuder technique for the determination of nitric acid and particulate nitrate is described. The technique relies on the use of four annular denuders set in series followed by a downstream filter-pack, and allows the reliable and accurate measurement of gaseous nitric acid, coarse nitrate and fine nitrate, even in the presence of high concentration levels of other nitrogen gaseous species and in case of high particulate to gaseous phase ratio. Field data collected by using the described system in extreme conditions, ranging from very polluted areas to extremely remote sites, are discussed. The data are analyzed in terms of mass distribution of the analyte on the different stages of the set-up; time resolution, minimum detectable concentration and precision of the measurements are also discussed.

Sampling and analysis of ambient air near Los Angeles using an annular denuder system

Abstract The annular denuder system (ADS) was used to measure HNO3, HNO2, NH3, SO2, and fine particle NO3−, SO42− and NH4+ during the 1985 Nitrogen Species Methods Comparison Study. This study was conducted from 11 to 19 September 1985 on the Pomona College Campus at Claremont, California near Los Angeles. This paper describes the performance of the ADS and presents selected results obtained using the system during the study. Most of the variability of the ADS results is associated with sampling rather than analytical operations. Based on collocated operation, the precision depends on the species sampled. Precision estimates (median CVs) for SO2 HNO3 and SO42−, as determined from 4- and 6-h collocated sampling are 8.0,14.3 and 4.1 %. Weighted averages of 4- and 6-h concentrations agree well with the corresponding 10- and 12-h averages for most species. An apparent low bias in the HNO3 results may be associated with interactions between the gas sample and the large Teflon® surface area upstream of the annular denuder.

Field intercomparison exercise on nitric acid and nitrate measurement (Rome, 1988) : a critical approach to the evaluation of the results

Abstract A 5-day intercomparison of measurement techniques for nitric acid and particulate nitrate was carried out at the Area della Ricerca di Roma (Montelibretti, Rome, Italy) during September 18–24, 1988. Sixteen groups from eleven European countries participated in the experiment, intercomparing the performances of several denuder and filter pack systems. In order to obtain a better characterization of the performance of each technique, in addition to HNO 3 and NO 3 − field sampling, the protocol included HNO 3 determinations from a pure source and temporal self-consistency tests. Many ancillary measurements were also undertaken, which proved to be of help in the interpretation of the data. The evaluation of the results is not performed through a simple linear regression of the data, that is by assessing agreement or disagreement between pairs of methods, but by comparing the results obtained in both the additional tests and the field samplings with the predicted deposition pattern. Once the reliability of each single technique has been evaluated in the light of the potential interfering mechanisms, a comparison between the results yielded by groups using the same technique and, finally, different techniques is carried out. The application of these criteria to the data set gathered during the intercomparison shows that the diffusion techniques yield the most reliable results, while teflon-nylon filter packs do not allow a correct discrimination between nitric acid and nitrate, particularly in the presence of high ammonium nitrate concentrations. Filter packs using a cellulose prefilter are only able to measure total nitrate.

Simultaneous determination of nitrogen dioxide and peroxyacetyl nitrate in ambient atmosphere by carbon-coated annular diffusion denuder

Abstract A diffusion denuder for simultaneous collection of ambient nitrogen dioxide and peroxyacetyl nitrate (PAN), based on quantitative collection on an alkaline active carbon surface, has been developed. Nitrogen dioxide yields nitrite anion whereas PAN yields nitrate which are extracted after sampling with water and analysed by ion chromatography. Laboratory and field tests concerning sorption efficiency, effect of humidity and capacity were carried out. The pre-exponential and exponent for the first term of the Gormley-Kennedy equation applied to annular geometry were estimated. It was found that for PAN the agreement between the experimental and calculated values is quite good and compatible with a “perfect sink” behaviour of carbon, whereas, in the case of NO 2 , the experimental collection efficiency is lower than theory but still suitable for ambient sampling. Nitrogen oxide and ozone do not interfere in the determination. SO 2 , HONO and HNO 3 which could interfere can be removed by using two diffusion denuders placed in line before the carbon denuder: one coated with tetrachloromercurate (TCM), for the removal of SO 2 and HNO 3 , and the second, coated with carbonate, for the removal of HONO. The detection limit for 24 h sampling is better than 0.05 and 0.06 ppb for NO 2 and PAN, respectively. The response for NO 2 was compared during a field campaign in Milan with the corresponding integrated response of a chemiluminescence analyser, while data for PAN were compared with the integrated response of a GC-ECD technique. The results obtained are reported and discussed.