Journal of Environmental Monitoring

Local and regional air pollution in Ireland during an intensive aerosol measurement campaign

D. Ceburnis*^a, J. Yin^b, A. G. Allen^b, S. G. Jennings^a, R. M. Harrison^b, E. Wright^c, M. Fitzpatrick^c, T. Healy^c and E. Barry^d
aAtmospheric Research Group, Department of Experimental Physics, National University of Ireland Galway, University Road, Galway, Ireland. E-Mail: darius.ceburnis@nuigalway.ie; Fax: +35391494584; Tel: +35391493280
^bSchool of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK B15 2TT
^cAtmospheric and Noise Unit, Community and Environment Department, Dublin City Council, Dublin, Ireland
^dEnvironmental Laboratory, Cork City Council, City Hall, Cork, Ireland

An intensive two month measurement campaign has been performed during a two year study of major component composition of urban PM₁₀ and PM_2.5 in Ireland (J. Yin, A. G. Allen, R. M. Harrison, S. G. Jennings, E. Wright, M. Fitzpatrick, T. Healy, E. Barry, D. Ceburnis and D. McCusker, Atmos. Res., 2005, 78(3–4), 149–165). Measurements included size-segregated mass, soluble ions, elemental carbon (EC) distributions, fine and coarse fraction organic carbon (OC) and major gases along with standard meteorological measurements. The study revealed that urban emissions in Ireland had mainly a local character and therefore were confined within a limited area of 20–30 km radius, without significantly affecting regional air quality. Gaseous measurements have shown that urban emissions in Ireland had clear, but fairly limited influence on the regional air quality due to favorable mixing conditions at higher wind speeds, in particular from the western sector. Size-segregated mass and chemical measurements revealed a clear demarcation size between accumulation and coarse modes at about 0.8 µm which was constant at all sites. Carbonaceous compounds at the urban site accounted for up to 90% of the particle mass in a size range of 0.066–0.61 µm. Nss SO₄^2– concentrations in PM_2.5 were only slightly higher at the urban site compared to the rural or coastal sites, while NO₃^– and NH₄⁺ concentrations were similar at the urban and coastal sites, but were a factor of 2 to 3 higher than at the rural site. OC was highly variable between the sites and revealed clear seasonal differences. Natural or biogenic OC component accounted for <10% in winter and up to 30% in summer of the PM_2.5 OC at urban sites. A contribution of biogenic OC component to PM_2.5 OC mass at rural site was dominant.

Airborne particles are produced from a variety of sources, including incomplete combustion processes, industry and construction, as well as naturally as a result of re-suspension of surface soil material, sea spray, volcanic activity, biomass burning, organic debris and reactions leading to condensation of volatile precursors. Fine and coarse particles possess distinct as well as similar mechanisms of formation, with a variable degree of source overlap.^1,2 The potential impact of airborne particles on human health effects ^3,4 and economic productivity is of greatest concern within the European context, and drives changes in legislation designed to reduce concentrations of particulates arising from anthropogenic activities. Air quality is one of the major environmental issues facing Ireland due to the countrys rapid development, particularly in the transport, energy and building/road construction sectors.^5,6 Emissions from road traffic, such as NO_x, benzene and fine particulate matter have become the greatest potential threat to air quality, particularly in urban areas. At remote locations, only small spatial differences in chemical composition are found (although variations are found in total mass), showing the consistency of the regional background in the absence of strong local sources.⁷ These features are in contrast to the typical urban environment, where a multiplicity of sources lead to a heterogeneous aerosol population and chemical composition. Very limited available information shows that these pollutants will present a difficult challenge if the future EU limits are to be met. A study of the nature and origin of PM₁₀ and PM_2.5 particles at five sites including urban roadside, urban background/centre, rural and coastal environments has been performed in Ireland for the first time during 2001–2003.⁸ Receptor modeling techniques used in this study successfully discriminated major source categories with unexplained materials accounting for about 7–28% of the particle mass. It has been shown that sea-salt was a significant chemical component both in fine and coarse particles at all urban, coastal and rural sites. It was shown that ammonium sulfate/ammonium nitrate and organic materials dominated fine particles.

The main objectives of the intensive campaign in 2002 were (1) to obtain a more comprehensive understanding of pollution sources in Ireland, their strength and impact on air quality by higher time resolution of the measurements; (2) to obtain size-resolved aerosol particle chemical composition measurements in order to assign accurate size distributions to the major source categories and (3) to study the impact of organic carbon on the chemical mass balance.

The intensive campaign took place during four weeks in February–March 2002 (19.02.02–21.03.02) at three stations (Fig. 1): Ahascragh, Ballinasloe, Co. Galway (rural site D), Coleraine Street (Dublin urban site B) and Wicklow Head, Co. Wicklow (coastal site C). The measurements included meteorological parameters (temperature, wind speed and direction, relative humidity, air mass trajectories), gases (SO₂, NO, NO₂, NO_x, O₃, CO), total particle number concentration and chemical components of size-resolved aerosols (CH₃SO₃^–, Cl^–, NO₃^–, SO₄^2–, Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, elemental carbon).

	Fig. 1 Sampling sites.

Wind speed and direction sensors, supplied with Dichotomous Partisol Sampler (R&P), were mounted 5 m above ground, while the temperature was measured at 2 m level by a standard sensor (R&P). Total particle number concentration was measured by a TSI 3010 counter at the rural site and TSI 3022A counters at the coastal and urban background sites. The TSI counters were calibrated against each other and counts varied within 5%. At the coastal site (C) trace gases (NO_x, SO₂, CO, O₃) were determined using Scintrex (Ontario, Canada) Model LMA-3 chemiluminescence (NO_x), Thermo Environmental Model 43C pulsed fluorescence (SO₂), Thermo Environmental Model 48C infrared absorption (CO) and Monitor Labs (San Diego, USA) UV absorption (O₃) analysers. At the rural site (D) and urban site B trace gases (NO_x, SO₂ and CO (only at site B)) were measured using Advanced Pollution Instrumentation Inc. (San Diego, USA) models M-200 chemiluminescence, M-100 fluorescence and M300 gas filter correlation analysers, respectively. Instrument calibrations were undertaken using certified standards immediately before and after the intensive campaign.

Meteorological parameters and gaseous measurements were made with a 5 min time resolution (except SO₂ at the coastal site, which was run with 1 h time resolution), total particle number concentration was made with 1 min time resolution, while MOUDI (Micro-Orifice Uniform Deposit Impactor) samples were changed weekly.

MOUDI impactors were mounted with PTFE (PolyTetraFluoroEthylene) filters as collection substrates. The MOUDI impactors were capable of collecting particles in up to 12 size fractions within the range 0.054–18 µm. A 12-stage MOUDI was used at the urban site B (Dublin) and at the rural background site D (west of Ireland), whereas at the coastal background site C (east coast of Ireland) a 10-stage MOUDI was employed. The 10-stage MOUDI was essentially the same as the other two MOUDI impactors, but was missing the two smallest particle stages. Those smallest particles below 0.18 µm (10-stage MOUDI) or 0.054 µm (12-stage MOUDI) were collected on a backup filter.

MOUDI substrates (PTFE filters) were first weighed gravimetrically after preconditioning at 35–45% relative humidity and 20 °C and then analysed for chemical species using ion chromatography (Dionex DX500 ion chromatograph for anions and a Dionex DX100 for cations). More analytical details can be found in Yin et al.⁸

The elemental carbon (EC) content of the exposed PTFE MOUDI filters was determined using an EEL reflectometer by measuring their blackness (i.e. light absorbance). These absorption data were calibrated against a thermal method (LECO Instruments Model RC412 Carbon Determinator), based on the assumption that EC is the principal light-absorbing species in ambient air. OC content was not measured on PTFE MOUDI filters. More analytical details can be found in Yin et al.⁸

Organic (OC) and elemental (EC) carbon were studied during two separate sub-campaigns in January and August 2003, representing winter and summer seasons, using Quartz filters. In January 2003, samples were collected at five sites, including the aforementioned three sites (B, C and D), as well as at a Dublin roadside site A (College Road) and an urban site in the city of Cork (E). In August 2003, samples were collected only at sites D and E. Coarse (PM_2.5–10) and fine (PM_2.5) aerosol fractions were collected on quartz filters on a daily basis by a Dichotomous Partisol Sampler (R&P). In total, 120 samples were collected on quartz filters. The thermal method was used for the analysis of organic carbon (OC). Two main phases, of 100 to 350 °C and 350 to 800 °C, were used for OC and EC, respectively. More analytical details can be found in Yin et al.⁸ As OC and EC was studied during different sub-campaigns than soluble ions, chemical or gravimetric mass balance was not attempted, but estimated indirectly. Hence, OC concentration was presented as carbon mass and no multiplication was applied to convert carbon mass into mass of organic matter.

Regional meteorology during the intensive campaign was characterized by westerly air flow from the Atlantic Ocean during the first two weeks and by easterly flow at the end of the campaign. Westerly air flow brings the cleanest air into Ireland, while the easterly flow brings polluted air masses from the United Kingdom and continental Europe. Backward trajectories were calculated using the NOAA HYSPLIT model.⁹ 120 hour backward trajectories, which originated over the North Atlantic without contact with land prior to arrival in Ireland, were referred to as Atlantic airflow. The meteorological situation was rather stable during the individual four weeks and therefore enabled estimation of the influence of pollution sources in Ireland, when air masses were passing over Ireland either from the west or from the east (Fig. 2). Calm periods with wind speed of less than 0.5 m s^–1 accounted for less than 5% of the recorded time, were uniformly distributed among different sectors and hence were not excluded from the frequency distribution wind rose (Fig. 2). Other meteorological parameters (temperature, relative humidity and wind speed) were mostly affected by regional meteorology. Average temperature at the rural inland site (6 °C) was about a factor of 2 lower than at the coastal site (12 °C). Temperature at the rural site was lowest in northerly air masses (2 to 4 °C) and highest in westerly and south-easterly air masses (5 to 10 °C). Relative humidity was directly related to the temperature, between 60 and 80% at the rural site and between 40 and 60% at the coastal site. Lower relative humidity at the coastal site, despite its proximity to the ocean, can be explained by lower precipitation rates in eastern Ireland compared to the west of the country. Also westerly Atlantic air masses are generally wetter than easterly air masses originating over the European continent. Wind speed was typically variable between 1 and 6 m s^–1, with higher variability at the rural site. Strongest winds were from the western sector, while the weakest winds generally came from the eastern sector. Meteorology at the Dublin site B had strong local character, affected by street canyon. Here wind speed was very low, generally below 1 m s^–1, and therefore measured wind direction was at least partially not reliable. Due to the low wind speed combined with urban heating effects, local temperature was above 15 °C on average, and relative humidity was around 40%.

	Fig. 2 Frequency distribution pattern of local wind direction during the intensive campaign measured at the rural site D (left) and coastal site C (right).

High resolution trace gas measurements enabled us to estimate the strength and impact of local and regional pollution sources at the study sites. Fig. 3 presents concentration distribution patterns of SO₂, NO, NO₂ and particle number concentrations at site D during the intensive campaign. NO is a short-lived gas and its concentration reflects the influence of nearby sources. Elevated NO concentrations were measured in air arriving from the 100 degrees direction, where Ballinasloe Town (6500 inhabitants) is situated at a distance of 5–6 km from the rural site. NO₂ concentration did not follow the same pattern, as NO₂ is a photochemical product of NO oxidation, and exhibited a more uniform concentration pattern characteristic of distant regional sources. The SO₂ concentration pattern revealed both local and regional sources, with elevated concentrations from the northeastern and eastern sectors. SO₂ derives mainly from fossil fuel combustion, here from nearby towns (Ballinasloe and Athlone) and distant sources (in Ireland, the United Kingdom and continental Europe). The gas phase measurements show that site D is clearly rural and that nearby local pollution sources had little impact on air quality. Despite clearly higher concentration levels in some cases, gaseous concentrations were generally very low (Table 1). CPC counts showed a similar pattern as that of SO₂ with elevated counts from the northeastern sector. The CPC pattern did not show a clear influence from nearby Ballinasloe Town, but that was likely caused by slight differences in wind speed as CPC concentrations are more dependent on wind speed than gaseous concentrations. However, it should be pointed out, that elevated CPC counts were also recorded in the northwestern sector. A close examination of the data revealed that this was mostly caused by a single local meteorological event during a rapid change in wind direction, which caused a polluted air mass from the northeastern sector to affect the relatively clean northwestern sector. This change in wind direction was accompanied by a slack wind, low ambient temperature and a relatively small number of data points from that otherwise relatively clean sector. Pollutant concentrations in air masses from the western sector were generally low, with CPC counts of about 1700 particles cm^–3. By comparison, in clean air masses advecting into Ireland from the Atlantic, CPC counts are below 700 particles cm^–3, ¹⁰ what is a factor of 2.5 lower than recorded in this study. Therefore, it can be concluded that emissions from Galway City (a distance of 65 km to the west from the rural site), had a clear, but fairly limited influence on the regional air quality, with favorable mixing conditions at higher wind speeds from the western sector leading to dilution of Galway City emissions.

	Fig. 3 Concentration distribution patterns of gases: NO, NO2, SO2 and condensation particle counts at rural site D during the intensive campaign.

Gaseous concentration patterns at the coastal site C were similar in their nature to those at the rural site. Generally, concentrations were very low, with the lowest in the western sector. Variation of SO₂ concentration at the coastal site exhibited local character, with multiple concentration spikes associated with local emissions from either burned domestic fuel or from ships. The latter emissions dominated, advecting from the Irish Sea sector, with duration less than one hour, and accompanied by high CPC counts. Particle number concentrations in the western sector were very similar though slightly higher than at the rural site, implying that emissions during air mass passage over Ireland enhanced particle concentration by about 20–25%. However, NO_x and SO₂ concentrations remained at very low levels in air from the western sector.

Dublin site B represented a background urban site with a local meteorology determined by a street canyon. Gaseous concentrations were not particularly high, as the 95 percentile concentration did not come close to any statutory air quality limit value (Table 1). However, concentration distribution patterns revealed clear pollution sources. NO, NO₂ and CO emissions were advecting mainly from a southern direction and the busy North King Street. SO₂ concentrations did not follow that pattern, suggesting that urban emissions came mainly from burnt domestic fuel. Elevated SO₂ concentrations from the south-eastern sector pointed to ship and port emissions. There was high (r = 0.75–0.80) and statistically significant correlation (P 0.01) between pollutants: CO and NO_x, NO₂, NO and particle number counts. Also moderate correlation (r = 0.41, P 0.01) was observed between SO₂ and particle number, pointing to the fact that domestic fuel emissions also contributed to particle emissions.

Overall, gaseous measurements performed during the intensive campaign showed that emissions sources in Ireland had mainly a local character and therefore were confined within a limited area of about 20–30 km radius, without affecting regional air quality. Even large emission sources like the city of Dublin did not reveal any noticeable influence on concentration levels measured at the coastal site in Wicklow, just 40–50 km distant. This phenomenon could be explained by unstable meteorological conditions, favouring good mixing in the boundary layer, a high precipitation rate and a good state of the car fleet producing moderate emissions.

Size resolved particle distribution and particle chemical composition was used to assess the influence of pollutant emissions in Ireland on aerosol chemistry. A comparison of gravimetric mass size distribution with size distribution of chemical species enabled an estimation of the importance of different pollution sources contributing to particulate chemical species. Stable regional meteorology during some of the weeks of the intensive campaign was also a favorable factor. As already mentioned above, there was a clear westerly flow during the first week of the campaign, bringing clean marine air masses into Ireland along a west-east transect.

3.3.1. Particle mass distribution.

Size resolved gravimetric mass distribution showed clear differences between particle modes at the three sites as shown in Fig. 4. A demarcation size between accumulation and coarse modes was at about 0.8 µm and was constant at all sites (Fig. 4). This fact puts a new light on the justification of the size of 2.5 µm, which is frequently used to separate fine and coarse particle modes. Apparently, having a cut-off size of 2.5 µm, fine particles are partly mixed with the coarse ones and thus elevated fine particle mass is unlikely to be due to anthropogenic pollution alone. Note, that the limit size of 0.8 µm represents a wet diameter, as MOUDI impactors were run at ambient temperature. Therefore, these measurements suggest that dichotomous samplers should separate particles at a standard 1.0 µm size, at least under Irish conditions with a strong primary sea salt source. An accumulation or fine particle mode diameter and mass was significantly different at the rural and the coastal or urban sites with an ultra-fine mode being visible at the urban site. The difference in accumulation mode size between the rural sample and the coastal or urban sample was about 1.3 (a shift from 0.35 to 0.46 µm in diameter) in westerly air masses (Fig. 4). The increase in accumulation mode diameter broadly corresponds to a difference in mass of over 2.0 (Fig. 4) as the mass is proportional to the diameter cubed. The accumulation mode concentration was very different depending on air mass history, and was a factor of 2 to 3 higher in easterly compared to westerly air masses. The exception was the Coleraine Street site (site B), where the accumulation mode concentration was stable and fairly independent of the air mass sector. This pointed to the fact that here particle mass was dominated by urban emissions, with only a relatively small influence of air mass history. Size shift of the accumulation mode (to larger size compared to the rural site) along with increased mass points to aerosol growth, possibly due to cloud processing of particles during air mass transport over land.

	Fig. 4 Size resolved gravimetric mass distribution measured at the rural, urban and coastal sites during February 28–March 7, 2002.

The coarse particle mode was also characteristic of the different sites. The coarse particle mode at the rural site D was dominated by sea salt particles of 2 µm in size, while larger particles were washed out or deposited before reaching the site. At the coastal site C, coarse mode particle mass was dominated by 3–8 µm particles and was quite dependent on wind direction, with a factor of about 2 to 3 higher mass during the week of easterly air flow compared to westerly flow. Coarse mode particle mass at the Coleraine Street site exhibited moderate variation depending on the wind direction, but much less than at the coastal site.

During the intensive campaign, ordinary PM₁₀ samples were collected on alternate days, as during the long-term study.⁸ Even though MOUDI impactor samples were deployed continuously during one week periods, gravimetric PM₁₀ mass determined by sampling on alternate days by dichotomous sampler can be compared with gravimetric mass sampled by MOUDI impactor. Therefore, one week-long MOUDI samples were compared with the average of 3 to 4 individual daily Partisol samples over the same week. The comparison is presented in Fig. 5. There was fairly close agreement between the values, owing to the fact that the corresponding samples did not represent the exact same days. Largest disagreement was found at the rural site. It is fair to say that concentrations at the rural site were the most sensitive as they were the lowest among the sites and, therefore, any slight changes in particle load on a day-to-day basis had significant influence on the average value. However, during the first week of the campaign, which was characterized by stable westerly air flow, agreement was very good at all three sites.

	Fig. 5 Comparison of PM10 gravimetric mass sampled by a dichotomous Partisol sampler and a MOUDI impactor during the intensive campaign.

3.3.2. Composition of soluble ions.

Chemical species contributing to the measured aerosol mass are presented in Fig. 6 for rural site D and urban site B. Potassium is not presented in the Fig. 6 due to its low contribution to the total mass and to make the graph more easily readable. There is a clear size demarcation where particle chemical composition changes its distribution pattern. Note, that organic carbon (OC) was not analysed in size-segregated samples. Particles up to 1.0 µm in size are predominantly composed of EC, nss SO₄^2– (non-sea salt sulfate), NO₃^– and NH₄⁺. Particles above 1.0 µm are almost entirely composed of sea salt (NaCl, sea salt SO₄^2–, Ca and Mg). There were only two exceptions: (1) at the rural site when due to the strong westerly winds during the first week of the campaign sea salt significantly contributed to the 0.56–1.0 µm size range; and (2) at the urban background site, where Ca significantly contributed to the coarse mode due to re-suspended road dust. The clear difference in chemical composition, which occurred at 1.0 µm size, supports the critics of the 2.5 µm cut-off size utilized in dichotomous samplers and confirms, therefore, that separation between fine and coarse particles using a 2.5 µm cut point is not particularly meaningful.

	Fig. 6 Size resolved chemical composition (a) at the urban site B and (b) at the rural site D during February 21–28, 2002.

On average, all chemical species concentrations measured in the size resolved samples were very similar to the ones measured in PM₁₀ and PM_2.5 samples.⁸ Due to the effect of road traffic, substantial elevations of EC concentrations up to a factor of 10 were recorded at the Coleraine Street site B (Dublin) in comparison with the rural or coastal background sites. EC concentration at the Coleraine Street site accounted for up to 90% of the chemically resolved particle mass in the size range of 0.1–0.2 µm. As EC comes primarily from fossil fuel combustion it has a strong local character. Note, that OC was not analysed in size resolved samples, but OC data from two sub-campaigns revealed that it contributed almost equally to PM_2.5 mass at urban sites, despite different emission sources (as discussed below). There was much less of a difference between other chemical species at the different sites. Nss SO₄^2– concentrations were only slightly higher at the urban site compared to the rural or coastal site, while NO₃^– and NH₄⁺ concentrations were similar at the urban and coastal sites, but significantly higher than at the rural site. The only unusual finding was a noticeable contribution of calcium to the smallest sized particles at all sites. The source of such particles has not been identified, but may be originating from either lubricating oils ¹¹ or tire wear.¹²

Constant westerly air flow during the first week of the intensive campaign (February 21–28, 2002) provided us with insight of the influence of pollution sources in Ireland on aerosol chemical composition. Despite lower aerosol number concentrations from the western sector, the chemical composition of particles is different from that expected for clean marine air masses. Fig. 7 shows the increase of all species concentrations during air mass passage over Ireland from west to the east during the last week of February 2002. Nss SO₄^2– concentration increase was modest (from 0.27 to 0.32 µg m^–3), however NO₃^– concentration increased about two times (from 0.081 to 0.183 µg m^–3) and NH₄⁺ concentration increased three times (from 0.064 to 0.181 µg m^–3). Note that NO₃^– and NH₄⁺ concentrations were similar at the coastal and urban sites while nss SO₄^2– concentrations were different at the coastal and urban sites. Such a pattern implies that gaseous precursors to NO₃^– and NH₄⁺ are spread across the country, where the agriculture industry is the most likely candidate of NH₃ emissions (precursor of NH₄⁺), while vehicles dominate NO_x emissions (precursor of NO₃^–). SO₂ emissions across the country contributed to the nss SO₄^2– during air mass transport across Ireland, however imported sulfate was high, relative to the imported fractions of nitrate and ammonium, while urban emissions contributed to its increase at the urban site. This suggests that gas-to-particle conversion of sulfur was quite rapid, occurring during the short time period of air mass residence in the city (about 2 hours in this case). It was, however, not the case for NO₃^–, despite substantial urban NO₂ concentrations. The difference probably stems from the fact that conversion of SO₂ into SO₄^2– can occur rapidly in the aqueous phase, while NO₂ is mainly produced from primary NO emissions via photochemical reaction before being further converted into NO₃^–. ¹³

	Fig. 7 Increase of chemical species concentration during west–east air mass passage over Ireland during February 21–28, 2002.

There was a fairly constant easterly air flow during the last week of the intensive campaign. During that week, all chemical species showed concentration decrease during air mass transport over the country. Therefore, wet removal of pollutants was very efficient even during short time periods of 5 to 10 hours (wind speed 5–10 m s^–1, distance 200 km) and reduced concentrations by roughly 40% of their initial value. In this case Ireland was generally a sink of long-range transported atmospheric pollutants.

3.3.3. Elemental and organic carbon.

	Fig. 8 Winter (January 15–31, 2003) OC and EC concentrations in Atlantic air mass over Ireland at all five sites.

However, OC exhibited quite a different pattern with a larger increase in OC concentration during transport over land (between sites D and C in Fig. 8) and a smaller difference when comparing coastal (C) and urban sites (A, B, and E). This finding clearly demonstrates that various emission sources over land contribute to the OC concentration increase. However, not all emissions over land are of anthropogenic origin, as some emissions can be of a biogenic origin, for example, from organic soils.¹⁴ In fact, Fig. 8 implies that during winter most of the OC in the rural atmosphere could indeed be of biogenic origin, as EC concentration increases about two times during air mass transport over land (from 0.29 to 0.60 µg m^–3), while OC concentration increases about 5 times (0.38 to 1.83 µg m^–3) during air mass transport (sites D and C in Fig. 8). There was further increase in OC concentration at urban sites, but proportionally lower than was observed for EC. Assuming that anthropogenic sources produce internally mixed EC and OC particles, neither wet nor dry removal processes can explain such a difference in relative increases of EC and OC. Even secondary OC is at least partially mixed with pre-existing particles due to a condensational sink. At the same time, urban emissions had limited influence on OC content in urban aerosol, because short air mass residence time over the city is normally not long enough for secondary processes to take effect and only primary OC emissions will have contributed to the small increase. However, extreme primary emissions, such as at the Dublin roadside site (A), are capable of increasing OC concentration substantially (Fig. 8). This increase was due to primary emissions as EC concentration increased even more dramatically.

Another important observation of OC is that it showed seasonal differences very different from that of EC (Fig. 9). Only Atlantic air masses were selected for comparison to clearly reveal seasonal differences in OC concentrations. EC exhibited a well-documented pattern with lower concentrations during summer (August 1–15, 2003) when compared to winter (January 15–31, 2003), which is caused by lower anthropogenic emissions during summer. It would normally be expected that OC emissions would also be lower during summer. However, OC showed an opposite pattern with significantly higher concentrations during summer months. It is thought that biogenic OC from marine sources contributes significantly to the OC concentration increase during summer. Increased organic carbon content in marine aerosols over Ireland has already been linked to plankton blooms in the north Atlantic.¹⁵ On an absolute scale, the difference in OC concentration between winter (0.38 µg m^–3) and summer (1.43 µg m^–3) months at the rural site (1.05 µg m^–3) is similar to the difference (0.80 µg m^–3) in OC at urban site E in Atlantic air masses (3.08 and 3.88 µg m^–3, respectively). It was already discussed that OC at the rural site can be entirely of biogenic origin or at least dominated by biogenic OC component. Then Fig. 9 implies that about 12% of OC in winter and about 37% in summer in the urban areas in Ireland can be of biogenic origin. However, it must be considered that contribution of secondary anthropogenic OC will likely be larger during summer due to higher temperature. Therefore, the more realistic contribution of marine biogenic OC to the total organic mass in the urban areas would probably be less than 10% in winter and up to 30% in summer. The contribution of the marine biogenic component of OC is likely to be less but still important in continental air masses as well. However, terrestrial biogenic sources of OC may contribute more significantly in continental air masses. The conceptual model of the direct transfers of OC in rainfall,¹⁶ concluded that the net transfer of continental OC to the ocean could be close to zero due to a significant marine to continental flux. Indeed, the data have shown that difference in OC concentration in different air masses (marine, modified marine or continental) during summer varied within 50% at the rural site and no significant difference was observed at the urban site (possibly due to high absolute concentration). The difference in EC concentration among different air masses varied within 400% at the rural site and within 50% at the urban site. Again, assuming internal mixture of EC and OC in polluted air masses, such a pattern can only be explained by a significant amount of OC being produced by biogenic sources regardless of air mass history.

	Fig. 9 Seasonal differences in OC and EC concentrations in Atlantic air masses at the rural site (D) and urban site (E).

Organic carbon exhibited clear seasonal differences in the concentration ratio of OC/EC and varied from 0.4 to 5.5 during winter and from 1.0 to 30.0 during summer among the sites. The OC/EC concentration ratio of coarse particles increased more significantly during summer than that of fine particles and was much more variable, but was broadly in-line with the seasonal pattern of fine particle OC (as discussed above) where the ratio increased in summer due to both lower EC and higher OC concentrations. Such highly variable OC/EC ratios make so-called estimated or averaged ratios at least 100% in error and of little value in achieving a mass balance.

3.3.4 Mass balance analysis.

	Fig. 10 Chemical mass balance showing unresolved mass (%) of MOUDI samples collected during the intensive measurement campaign, (a) at the urban site B and (b) at the rural site D.

The ion balance method, which is the ratio of positive and negative ions, can give further insight about missing chemical species. Fig. 11 shows chemical ion balance at rural site D and urban site B. Deficiency in negative ions across small particle sizes suggests that non-analysed organic acids could be responsible for the missing mass. There was also missing mass across larger particle sizes. In this case, the mass balance and the ion balance methods suggest that the missing candidate species could be crustal material, particularly silicates and carbonates, providing large sources of negative ions. At the coastal site (not illustrated) there was little missing mass across larger particle sizes and there was rather good ion balance due to the dominant contribution of sea salt.

	Fig. 11 Ion balance of MOUDI samples (a) at the urban site B and (b) at the rural site D.

Measurements performed during an intensive campaign showed that anthropogenic trace gas emission sources in Ireland had mainly a local influence and were therefore largely confined within a limited area of 20–30 km radius, without greatly affecting regional air quality.

Meteorological conditions during particular periods enabled estimation of the influence of aerosol sources in Ireland, when air masses are passing over Ireland from the west and the east. Increase in aerosol species concentrations during the air mass passage was most likely due to the transformation of gaseous precursors into aerosol components via aqueous phase processes.

Sea salt was a dominant component of particles larger than 0.8 µm at all sites, with a smaller re-suspension component associated with frequent precipitation events in Ireland. Nss SO₄^2–, NO₃^– and NH₄⁺ were present in long-range transported material, and their concentration increase during air mass transport over Ireland was relatively low. Nss SO₄^2– concentrations were only slightly higher at the urban site compared to the rural or coastal sites, while NO₃^– and NH₄⁺ concentrations were fairly similar at the urban and coastal sites, but were a factor of 2 to 3 higher than at the rural site.

Substantial elevations of up to 10 times in EC concentrations were recorded at the Coleraine Street urban site (Dublin) in comparison to the rural or coastal background sites, mainly due to the effect of road traffic. EC concentration at the Coleraine Street site accounted for up to 90% of the chemically resolved particle mass in the size range of 0.1–0.2 µm. As EC comes primarily from burnt fossil fuel it has a strong local character. In general, EC and OC dominated fine particle content, independent of site.

Organic carbon exhibited clear seasonal differences and the concentration ratio of OC/EC varied from 0.4 to 5.5 during winter and from 1.0 to 30.0 during summer among the sites. OC seasonal pattern was generally opposite to that of EC, with higher concentrations during summer. The concentration ratio of OC/EC during summer clearly revealed a significant biogenic or natural component of OC, which contributed from less than 10% up to 30% even at the relatively polluted urban site, while at the rural site the biogenic OC contribution was probably dominant.

Irelands geographical position, which ensures frequent inflow of fresh North Atlantic air and unstable meteorological conditions, favouring good mixing in the boundary layer, frequent precipitation events, and a good state of the car fleet are the main factors contributing to relatively clean air in Ireland. For airborne particulates the only concern are some of the most polluted urban sites, where concentrations can be increased due to primary vehicle emissions. The contribution of sea salt to PM₁₀ and PM_2.5 mass should be taken into account when considering possible exceedences of PM mass limit values.

Acknowledgements

This work was funded by the Environmental Protection Agency under the National Development Plan 2000–2006 (Project 2000-LS-6.1-M1). The authors also thank Dr Brian Davison of Lancaster University for assistance with provision of field instrumentation.

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