Abstract. Surface concentrations of secondary inorganic particle components over the UK have been analysed for 2001-2010 using the EMEP4UK regional atmospheric chemistry transport model and evaluated against measurements. Gas/particle partitioning in the EMEP4UK model simulations used a bulk approach, which may lead to uncertainties in simulated secondary inorganic aerosol. However, model simulations were able to accurately represent both the long-term decadal surface concentrations of particle sulfate and nitrate and an episode in early 2003 of substantially elevated nitrate measured across the UK by the AGANet network. The latter was identified as consisting of three separate episodes, each of less than 1 month duration, in February, March and April. The primary cause of the elevated nitrate levels across the UK was meteorological: a persistent high-pressure system, whose varying location impacted the relative importance of transboundary versus domestic emissions. Whilst long-range transport dominated the elevated nitrate in February, in contrast it was domestic emissions that mainly contributed to the March episode, and for the April episode both domestic emissions and long-range transport contributed. A prolonged episode such as the one in early 2003 can have substantial impact on annual average concentrations. The episode led to annual concentration differences at the regional scale of similar magnitude to those driven by long-term changes in precursor emissions over the full decade investigated here. The results demonstrate that a substantial part of the UK, particularly the south and southeast, may be close to or exceeding annual mean limit values because of import of inorganic aerosol components from continental Europe under specific conditions. The results reinforce the importance of employing multiple year simulations in the assessment of emissions reduction scenarios on particulate matter concentrations and the need for international agreements to address the transboundary component of air pollution.

No abstract

Abstract. The UK Acid Gases and Aerosol Monitoring Network (AGANet) was established in 1999 (12 sites, increased to 30 sites from 2006), to provide long-term national monitoring of acid gases (HNO3, SO2, HCl) and aerosol components (NO3−, SO42−, Cl−, Na+, Ca2+, Mg2+). An extension of a low-cost denuder-filter pack system (DELTA) that is used to measure NH3 and NH4+ in the UK National Ammonia Monitoring Network (NAMN) provides additional monthly speciated measurements for the AGANet. A comparison of the monthly DELTA measurement with averaged daily results from an annular denuder system showed close agreement, while the sum of HNO3 and NO3− and the sum of NH3 and NH4+ from the DELTA are also consistent with previous filter pack determination of total inorganic nitrogen and total inorganic ammonium, respectively. With the exception of SO2 and SO42−, the AGANet provides, for the first time, the UK concentration fields and seasonal cycles for each of the other measured species. The largest concentrations of HNO3, SO2, and aerosol NO3− and SO42− are found in southern and eastern England and smallest in western Scotland and Northern Ireland, whereas HCl are highest in south-eastern, south-western, and central England, that may be attributed to dual contribution from anthropogenic (coal combustion) and marine sources (reaction of sea salt with acid gases to form HCl). Na+ and Cl− are spatially correlated, with largest concentrations at coastal sites, reflecting a contribution from sea salt. Temporally, peak concentrations in HNO3 occurred in late winter and early spring attributed to photochemical processes. NO3− and SO42− have a spring maxima that coincides with the peak in concentrations of NH3 and NH4+, and are therefore likely attributable to formation of NH4NO3 and (NH4)2SO4 from reaction with higher concentrations of NH3 in spring. By contrast, peak concentrations of SO2, Na+, and Cl− during winter are consistent with combustion sources for SO2 and marine sources in winter for sea salt aerosol. Key pollutant events were captured by the AGANet. In 2003, a spring episode with elevated concentrations of HNO3 and NO3− was driven by meteorology and transboundary transport of NH4NO3 from Europe. A second, but smaller episode occurred in September 2014, with elevated concentrations of SO2, HNO3, SO42−, NO3−, and NH4+ that was shown to be from the Icelandic Holuhraun volcanic eruptions. Since 1999, AGANet has shown substantial decrease in SO2 concentrations relative to HNO3 and NH3, consistent with estimated decline in UK emissions. At the same time, large reductions and changes in the aerosol components provide evidence of a shift in the particulate phase from (NH4)2SO4 to NH4NO3. The potential for NH4NO3 to release NH3 and HNO3 in warm weather, together with the surfeit of NH3 also means that a larger fraction of the reduced and oxidized N is remaining in the gas phase as NH3 and HNO3 as indicated by the increasing trend in ratios of NH3 : NH4+ and HNO3 : NO3− over the 16-year period. Due to different removal rates of the component species by wet and dry deposition, this change is expected to affect spatial patterns of pollutant deposition with consequences for sensitive habitats with exceedance of critical loads of acidity and eutrophication. The changes are also relevant for human health effects assessment, particularly in urban areas as NH4NO3 constitutes a significant fraction of fine particulate matter ( < 2.5 µm) that are linked to increased mortality from respiratory and cardiopulmonary diseases.

Global Change Biology, 20 (2). 566-580. 10.1111/gcb.12357 Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner.

Total gaseous mercury was collected at ten sites which comprise part of the UK rural . Using data from nine of the sites, we were able to show seasonality within the data and through kriging we were able to interpolate the TGM concentrations over the UK, revealing a south-east to north-west declining concentration gradient. Using continuous speciated mercury measurements from one of the network sites, we show through wind sector analysis and air-mass back trajectories that this spatial trend is likely to be due to air masses moving over the UK from continental Europe on easterly winds. The levels of TGM recorded in the south-east of the UK also more closely match observed background TGM levels on the continent, which could indicate that the TGM concentrations from the north of the UK are a better reflection of the true North Atlantic atmospheric mercury background level.

Science-based approaches in addressing future risks and challenges for forests require close collaboration among the communities operating different monitoring and research networks as well as experts in process and large-scale modelling. Results of the COST FP0903 conference which took place in October 2010 in Rome, reveal valuable results from different European forest monitoring and research networks. However, the need for closer integration of these activities is obvious. In this paper, representatives from major European networks recommend a new approach for forest monitoring and research in Europe, based on a reasonable number of highly instrumented “supersites” and a larger number of intensive monitoring plots linked to these. This system needs to be built on existing infrastructures but requires increased coordination, harmonisation and a joint long term platform for data exchange and modelling

Abstract. Surface concentrations of secondary inorganic particle components over the UK have been analysed for 2001–2010 using the EMEP4UK regional atmospheric chemistry transport model. In early 2003 an episode of substantially elevated surface concentrations of ammonium nitrate was measured across the UK by the AGANET network. The EMEP4UK model was able accurately to represent both the long-term decadal surface concentrations and the episode in 2003. The latter was identified as consisting of three separate episodes, each of less than 1 month duration, in February, March and April. The primary cause of the elevated nitrate levels across the UK was meteorological, a persistent high pressure system, but whose varying location impacted the relative importance of transboundary vs. domestic emissions. Whilst long-range transport dominated the elevated nitrate in February, in contrast it was domestic emissions that mainly contributed to the March episode, and for the April episode both domestic emissions and long-range transport contributed. A prolonged episode such as the one in early 2003 can have substantial impact on annual average concentrations. The episode led to annual concentration differences at the regional scale of similar magnitude to those driven by long-term changes in precursor emissions over the full decade investigated here. The results demonstrate that a substantial part of the UK, particularly the south and south-east, may be close to or actually exceeding annual mean limit values because of import of inorganic aerosol components from continental Europe under specific conditions. The results reinforce the importance of employing multiple year simulations in the assessment of emissions reduction scenarios on PM concentrations and the need for international agreements to address the transboundary component of air pollution.

Abstract. A comprehensive European dataset on monthly atmospheric NH3, acid gases (HNO3, SO2, HCl), and aerosols (NH4+, NO3-, SO42-, Cl−, Na+, Ca2+, Mg2+) is presented and analysed. Speciated measurements were made with a low-volume denuder and filter pack method (DEnuder for Long-Term Atmospheric sampling, DELTA®) as part of the EU NitroEurope (NEU) integrated project. Altogether, there were 64 sites in 20 countries (2006–2010), coordinated between seven European laboratories. Bulk wet-deposition measurements were carried out at 16 co-located sites (2008–2010). Inter-comparisons of chemical analysis and DELTA® measurements allowed an assessment of comparability between laboratories. The form and concentrations of the different gas and aerosol components measured varied between individual sites and grouped sites according to country, European regions, and four main ecosystem types (crops, grassland, forests, and semi-natural). The smallest concentrations (with the exception of SO42- and Na+) were in northern Europe (Scandinavia), with broad elevations of all components across other regions. SO2 concentrations were highest in central and eastern Europe, with larger SO2 emissions, but particulate SO42- concentrations were more homogeneous between regions. Gas-phase NH3 was the most abundant single measured component at the majority of sites, with the largest variability in concentrations across the network. The largest concentrations of NH3, NH4+, and NO3- were at cropland sites in intensively managed agricultural areas (e.g. Borgo Cioffi in Italy), and the smallest were at remote semi-natural and forest sites (e.g. Lompolojänkkä, Finland), highlighting the potential for NH3 to drive the formation of both NH4+ and NO3- aerosol. In the aerosol phase, NH4+ was highly correlated with both NO3- and SO42-, with a near-1:1 relationship between the equivalent concentrations of NH4+ and sum (NO3-+ SO42-), of which around 60 % was as NH4NO3. Distinct seasonality was also observed in the data, influenced by changes in emissions, chemical interactions, and the influence of meteorology on partitioning between the main inorganic gases and aerosol species. Springtime maxima in NH3 were attributed to the main period of manure spreading, while the peak in summer and trough in winter were linked to the influence of temperature and rainfall on emissions, deposition, and gas–aerosol-phase equilibrium. Seasonality in SO2 was mainly driven by emissions (combustion), with concentrations peaking in winter, except in southern Europe, where the peak occurred in summer. Particulate SO42- showed large peaks in concentrations in summer in southern and eastern Europe, contrasting with much smaller peaks occurring in early spring in other regions. The peaks in particulate SO42- coincided with peaks in NH3 concentrations, attributed to the formation of the stable (NH4)2SO4. HNO3 concentrations were more complex, related to traffic and industrial emissions, photochemistry, and HNO3:NH4NO3 partitioning. While HNO3 concentrations were seen to peak in the summer in eastern and southern Europe (increased photochemistry), the absence of a spring peak in HNO3 in all regions may be explained by the depletion of HNO3 through reaction with surplus NH3 to form the semi-volatile aerosol NH4NO3. Cooler, wetter conditions in early spring favour the formation and persistence of NH4NO3 in the aerosol phase, consistent with the higher springtime concentrations of NH4+ and NO3-. The seasonal profile of NO3- was mirrored by NH4+, illustrating the influence of gas–aerosol partitioning of NH4NO3 in the seasonality of these components. Gas-phase NH3 and aerosol NH4NO3 were the dominant species in the total inorganic gas and aerosol species measured in the NEU network. With the current and projected trends in SO2, NOx, and NH3 emissions, concentrations of NH3 and NH4NO3 can be expected to continue to dominate the inorganic pollution load over the next decades, especially NH3, which is linked to substantial exceedances of ecological thresholds across Europe. The shift from (NH4)2SO4 to an atmosphere more abundant in NH4NO3 is expected to maintain a larger fraction of reactive N in the gas phase by partitioning to NH3 and HNO3 in warm weather, while NH4NO3 continues to contribute to exceedances of air quality limits for PM2.5.