T.C. O'Connor

Physical characteristics of the ambient aerosol at mace head

This paper describes results of a reasonably extensive series of ambient aerosol measurements conducted for the first time from Mace Head, on the west coast of Ireland. Aerosol size distributions (0.045 < radius, r < 1.5 micrometres (μm)) and basic meteorological parameters were measured under both maritime, modified maritime and continental air mass conditions. It was found that the shape of the size distribution spectra is dependent on air mass source.

PM10 concentration measurements in Dublin City

Mass Concentration of ambient particulate matter with an aerodynamic diameter less than 10µm (PM10) are reported for the first time for a range of sites in Dublin City over a 6 month period from January 1st 1996 to June 30th 1996. PM10 gravimetric mass concentration measurements are made with low flow Partisol 2000 air samplers using an impaction type PM10 inlet and 47mm diameter glass fibre filters. In addition, much finer time resolution measurements (minimum sampling frequency of 30 minutes) are made using a tapered element oscillating microbalance (TEOM) PM10 mass monitor. These PM10 mass concentrations methods are also compared with mass concentration inferred using the standard black smoke method. Analysis of the ambient mass concentration data with reference to traffic density and meteorological influences are presented. Results for the first six months of 1996 show that the average PM10 values range from a high of 49 µg m-3 at the Dublin city centre site to 14 µg m-3 at one of the suburban sites. Intercomparison between PM10 and black smoke mass concentrations show that the relationship is site specific. Statistical analysis between PM10 levels and car traffic number show a positive correlation while a weak negative correlation is found between PM10 levels and rainfall amount, wind speed and air temperature.

Aerosol and trace gas measurements during the mace head experiment

Abstract Measurements obtained between the 9th and 23rd of April 1991 at the Mace Head remote maritime station are presented. These measurements were obtained as part of a EUROTRAC Air Sea Exchange intensive measurement campaign. Analysis of variations in the aerosol and trace concentrations is based on inter-comparison of the measured species and use of local meteorological data and back trajectories for the period. While air masses of principally maritime origin were encountered throughout the campaign, typical background maritime conditions only occurred during short periods. Other periods were highly modified by mainly “local” influences which included local biomass burning and Aitken nuclei (AN) production. The biomass burning was observed to contribute to elevated accumulation mode aerosol (0.1–1.0 μm), black carbon mass concentration, CO, and CH 4 levels. The O 3 concentrations were variable throughout the measurement period. Reasonable correlations were found between the trace gas data except between O 3 and CO during the period influenced by biomass burning when the O 3 levels were generally depleted. Very high AN concentrations (> 20,000 cm −3 ) were measured on a number of sampling days, with a distinct “dawn effect” being observed. The very high AN concentrations occurred during otherwise clean air conditions and also in the presence of combustion aerosols and are principally attributed to photochemical gas-to-particle conversion processes. The conditions under which these events occurred and variations in background conditions are examined.

Mass measurements of aerosol at mace head, on the west coast of Ireland

Abstract Aerosol mass/size measurements have been carried out at the Mace Head research station, on the west coast of Ireland, for a period of over two years. Mass/size distributions were obtained in the size range 0.06–10 μm using Berner low-pressure cascade impactor systems. The distributions obtained reflect the different types of air mass which are encountered at Mace Head. Westerly winds are dominant at the site with the associated air masses being typically maritime. Unimodal mass/size distributions with the peak mass concentration occurring principally in the 2–4 μm size range were characteristic of these conditions. Continental air masses from Europe are typified by mass/size distributions which have peak values in the submicron size range. Distinct bimodal distributions were obtained when aerosols were collected from a mixture of air mass types. Overall, the mass concentration was found to vary between approximately 2 and 45 μg m−3 A wide variation in mass concentration values were measured at similar wind speeds. However, a trend towards higher mass concentrations at higher wind speeds was observed in the coarse mass fraction contribution (> 1 μm) from westerly maritime air masses at higher wind speeds. A drop-off in mass concentration was observed at wind speeds greater than approximately 11 m s−1 This is attributed to the cut-off characteristic of the PM10 inlet used with the impactor system. High mass concentrations during long term pollution events at Mace Head were also observed with the collected aerosol resident principally in the submicron size range during these conditions.

Coastal CCN measurements at Mace Head with enhanced concentrations in strong winds

Surface measurements of cloud condensation nuclei (CCN) number concentration (cm -3 ) are presented for unmodified marine air and for polluted air at Mace Head, for the years 1994 and 1995. The CCN number concentration active at 0.5% supersaturation is found to be approximately log-normal for marine and polluted air at the site. Values of geometric mean, median and arithmetic mean of CCN number concentration (cm -3 ) for marine air are in the range 124-135, 140-150 and 130-157 for the two years of data. Analysis of CCN number concentration for high wind speed, U, up to 20 m s -1 show enhanced CCN production for U in excess of about 10-12 m S - Approximately 7% increase in CCN per 1 m s -1 increase in wind speed is found, up to 17 m s -1 . A relationship of the form log 10 CCN = a+bU is obtained for the periods March 1994 and January, February 1995 for marine air yielding values a of 1.70; 1.90 and b of 0.035 for both periods.

Aerosol climatology measurements with a Nolan-Pollak counter

Abstract The Nolan-Pollak photoelectric nucleus counter was originally developed in the 1940s for aerosol climatology measurements although its versatility led subsequently to many other applications in aerosol science. It has been widely used in manual, automatic and modified forms in many parts of the world to record ambient concentraions of Aitken nuclei. The design, calibration and performance of an automatic counter is described. Measurements of Aitken nuclei at Mace Head, Ireland, are used to discuss its suitability for the purpose of aerosol climatology.

Nolan–Pollak type CN counters in the Vienna aerosol workshop

Abstract Three “standard” Nolan–Pollak (N–P) and a modified N–P design condensation nucleus (CN) counters were included in the Vienna Workshop on Intercomparison of Condensation Nuclei and Aerosol Particle counters. These counters came from diverse backgrounds, namely programs in USA, Europe and Australia. In this work, principles of the operation and previous history of calibration of the N–P expansion counter are briefly reviewed and comparisons between the particular counters used in the workshop are presented and discussed. Counting agreement was found to be very good between the N–P counters, typically better than ±12% for a range of aerosol sizes and compositions from a minimum diameter of 4 nm. The independently calibrated GIV CNC-440 (modified N–P type counter) also agreed well with the N–P counters. The minimum size sensitivity of the N–P counter was examined showing a lower detection limit for insoluble (Ag) particles of around 2.6±0.3 nm diameter.

Aerosol measurements and evidence of gas-to-particle conversion processes at mace head, Ireland

Publisher Summary Some general characteristics of Aitken nuclei (AN) and equivalent black carbon (EBC) during background and polluted conditions have been shown, and estimate of background concentrations are given in this chapter. The characteristics of AN production events have also been considered. Very high AN concentrations are attributed to photochemical gas-to-particle conversion processes. It is likely that these events were due to nucleation following oxidation of biogenic precursor gases. The exact chemical nature of these precursor gases is not clear, however they are thought likely to be sulfur species probably including dimethylsulphide (DMS). Reactive species may also have increased the nucleation potential of precursor gases. The observed decrease in the EBC concentration during AN production may indicate that the absorption characteristics of the ambient aerosol are being modified by coagulative deposition of the AN. Such processes may also affect the hydroscopic properties of the larger aerosol. If such events are typical of coastal boundary areas, then they may significantly alter the number concentrations and physical and chemical characteristics of air masses entering and leaving marine/continental areas.

Surface CCN measurements at mace head, on the west coast of Ireland

Publisher Summary The influence (though indirect) of the cloud condensation nuclei (CCN) population on the radiative properties of cloud, and hence on global climate is a subject of increasing interest and attention since the work of Twomey, who calculated an increase in cloud albedo due to increased CCN number concentration. The currently more accepted climatic role of CCN has resulted in a drive for more worldwide measurements of CCN through programs, such as the World Meteorological Organization (WMO) Global Atmospheric Watch (GAW) aerosol program. This chapter presents surface measurements of cloud condensation nuclei (CCN) number concentration for unmodified marine air and for polluted air at Mace Head, for the years 1994 and 1995. The CCN number concentration is found to be approximately log-normal for marine and polluted air at the site. Values of geometric mean, median, and arithmetic mean of CCN number concentration for marine air are in the range 130-135, 140-150, and 137-157, for the two years of data.