Abstract
Abstract. Complementary approaches have been taken to better understand the sources and their spatial distribution for secondary inorganic (nitrate and sulphate) and secondary organic aerosol sampled at a rural site (Harwell) in the southern United Kingdom. A concentration field map method was applied to 1581 daily samples of chloride, nitrate and sulphate from 2006 to 2010, and 982 samples for organic carbon and elemental carbon from 2007 to 2010. This revealed a rather similar pattern of sources for nitrate, sulphate and secondary organic carbon within western/central Europe, which in the case of nitrate, sulphate, organic carbon and secondary organic carbon, correlated significantly with EMEP emissions maps of NOx, SO2, and VOC respectively. A slightly more southerly source emphasis for secondary organic carbon may reflect the contribution of biogenic sources. Trajectory clusters confirm this pattern of behaviour with a major contribution from mainland European sources. Similar behaviours of, on the one hand, sulphate and organic carbon and, on the other hand, EC and nitrate showed that the former are more subject to regional influence than the latter in agreement with the slower atmospheric formation of sulphate and secondary organic aerosol than for nitrate, and the local/mesoscale influences upon primary EC. However, careful analysis of back trajectories and Concentration Field Maps indicates a strong contribution of mainland European sites to EC concentrations at Harwell. In a separate study, measurements of sulphate, nitrate, elemental and organic carbon were made in 100 simultaneously collected samples at Harwell and at a suburban site in Birmingham (UK). This showed a significant correlation in concentrations between the two sites for all of the secondary constituents, further indicating secondary organic aerosol to be a regional pollutant behaving similarly to sulphate and nitrate.
Highlights
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Nitrate and elemental carbon (EC) are the two particulate pollutants with concentrations that show the greatest variability as shown by the standard deviations which are larger than mean concentrations
Higher organic carbon (OC) and secondary OC (SOC) concentrations attributed to the Atlantic Ocean during the warm season suggest the more important photochemical activity during this season associated with southerly air masses assuming no emission from the ocean (Fig. S7, results not shown for SOC because they are very similar to OC)
Summary
Particulate nitrate, sulphate, chloride, elemental and organic carbon and gravimetric PM10 and PM2.5 collected from January 2006 to December 2010 are included in this study (see Table 1 for a summary of the available data). The site is located 80 km almost due west of central London It is in the grounds of the Harwell Science Centre in Oxfordshire and is on open land with agricultural fields. Anthropogenic emissions are mostly located in these directions, and European continental air masses may pass over the London area receiving additional anthropogenic emissions before reaching the site. The nearest anthropogenic sources are a nearby railway, some moderately trafficked B roads (highways of intermediate traffic density) and other surrounding activities from the university and local residents. It is about 115 km to the NNW of Harwell
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