International audienceExisting descriptions of bi-directional ammonia (NH3) land-atmosphere exchange incorporate temperature and moisture controls, and are beginning to be used in regional chemical transport models. However, such models have typically applied simpler emission factors to upscale the main NH3 emission terms. While this approach has successfully simulated the main spatial patterns on local to global scales, it fails to address the environment- and climate-dependence of emissions. To handle these issues, we outline the basis for a new modelling paradigm where both NH3 emissions and deposition are calculated online according to diurnal, seasonal and spatial differences in meteorology. We show how measurements reveal a strong, but complex pattern of climatic dependence, which is increasingly being characterized using ground-based NH3 monitoring and satellite observations, while advances in process-based modelling are illustrated for agricultural and natural sources, including a global application for seabird colonies. A future architecture for NH3 emission-deposition modelling is proposed that integrates the spatio-temporal interactions, and provides the necessary foundation to assess the consequences of climate change. Based on available measurements, a first empirical estimate suggests that 5°C warming would increase emissions by 42 per cent (28-67%). Together with increased anthropogenic activity, global NH3 emissions may increase from 65 (45-85) Tg N in 2008 to reach 132 (89-179) Tg by 2100

Abstract. The EMEP4UK modelling system is a high resolution (5×5 km 2 ) application of the EMEP chemistrytransport model, designed for scientific and policy studies in the UK. We demonstrate the use and performance of the EMEP4UK system through the study of ground-level ozone (O 3 ) during the extreme August 2003 heat-wave. Meteorology is generated by the Weather Research and Forecast (WRF) model, nudged every six hours with reanalysis data. We focus on SE England, where hourly average O 3 reached up to 140 ppb during the heat-wave. EMEP4UK accurately reproduces elevated O 3 and much of its day-to-day variability during the heat-wave. Key O 3 precursors, nitrogen dioxide and isoprene, are less well simulated, but show generally accurate diurnal cycles and concentrations to within a factor of ∼2-3 of observations. The modelled surface O 3 distribution has an intricate spatio-temporal structure, governed by a combination of meteorology, emissions and photochemistry. A series of sensitivity runs with the model are used to explore the factors that influenced O 3 levels during the heat-wave. Various factors appear to be important on different days and at different sites. Ozone imported from outside the model domain, especially the south, is very important on several days during the heat-wave, contributing up to 85 ppb. The effect of dry deposition is also important on several days. Modelled isoprene concentrations are generally best simulated if isoprene emissions are changed from the base emissions: typically doubled, but elevated by up to a factor of five on one hotCorrespondence to: M. Vieno (vieno.massimo@gmail.com) day. We found that accurate modelling of the exact positions of nitrogen oxide and volatile organic compound plumes is crucial for the successful simulation of O 3 at a particular time and location. Variations in temperature of ±5 K were found to have impacts on O 3 of typically less than ±10 ppb.

Little things matter Particulate air pollution 2.5 micrometers or smaller in size (PM2.5) is a major cause of human mortality, and controlling its production is a health policy priority. Nitrogen oxides are an important precursor of PM2.5 and have been a focus of pollution control programs. However, Gu et al . now show that abating ammonia emissions is also an important component of PM2.5 reduction, and the societal benefits of abatement greatly outweigh the costs (see the Perspective by Erisman). Reducing ammonia emissions thus would be a cost-effective complement to nitrogen oxides and sulfur dioxide controls. —HJS

A statistical Lagrangian atmospheric transport model was used to generate annual maps of deposition of sulphur and oxidised and reduced nitrogen for the United Kingdom at a 5 x 5 km 2 resolution. The model was run using emissions for the year 2002. The model was compared with measurements of gas concentrations (SO 2 , NO x , HNO 3 and NH 3 ) and of wet deposition and aerosol concentrations of SO 4 2-, NO 3 -and NH 4 + from national monitoring networks. Good correlation was obtained, demonstrating that the model is capable of accurately estimating the mass balance and spatial distribution of sulphur and nitrogen compounds in the atmosphere. A future emissions scenario for the year 2020 was used to test the influence of shipping emissions on sulphur deposition in the United Kingdom. The results show that, if shipping emissions are assumed to increase at a rate of 2.5% per year, their relative contribution to sulphur deposition is expected to increase from 9% to 28% between 2002 and 2020. The model was compared to both a European scale and a global scale chemical transport model and found to give broad agreement with the magnitude and location of sulphur deposition associated with shipping emissions. Enforcement of the MARPOL convention to reduce the sulphur content in marine fuel to 1% was estimated to result in a 6% reduction in total sulphur deposition to the UK for the year 2020. The percentage area of sensitive habitats with exceedance of critical loads for acidity in the UK was predicted to decrease by 1% with the implementation of the MARPOL convention.

Abstract. The global nitrogen (N) cycle at the beginning of the 21st century has been shown to be strongly influenced by the inputs of reactive nitrogen (N r ) from human activities, including combustion-related NO x , industrial and agricultural N fixation, estimated to be 220 Tg N yr −1 in 2010, which is approximately equal to the sum of biological N fixation in unmanaged terrestrial and marine ecosystems. According to current projections, changes in climate and land use during the 21st century will increase both biological and anthropogenic fixation, bringing the total to approximately 600 Tg N yr −1 by around 2100. The fraction contributed directly by human activities is unlikely to increase substantially if increases in nitrogen use efficiency in agriculture are achieved and control measures on combustion-related emissions implemented.Some N-cycling processes emerge as particularly sensitive to climate change. One of the largest responses to climate in the processing of N r is the emission to the atmosphere of NH 3 , which is estimated to increase from 65 Tg N yr −1 in 2008 to 93 Tg N yr −1 in 2100 assuming a change in global surface temperature of 5 • C in the absence of increased anthropogenic activity. With changes in emissions in response to increased demand for animal products the combined effect would be to increase NH 3 emissions to 135 Tg N yr −1 . Another major change is the effect of climate changes on aerosol composition and specifically the increased sublimation of NH 4 NO 3 close to the ground to form HNO 3 and NH 3 in a warmer climate, which deposit more rapidly to terrestrial surfaces than aerosols. Inorganic aerosols over the polluted regions especially in Europe and North America were dominated by (NH 4 ) 2 SO 4 in the 1970s to 1980s, and large reductions in emissions of SO 2 have removed most of the SO 2− 4 from the atmosphere in these regions. Inorganic aerosols from anthropogenic emissions are now dominated by NH 4 NO 3 , a volatile aerosol which contributes substantially to PM 10 and human health effects globally as well as eutrophication and climate effects. The volatility of NH 4 NO 3 and rapid dry deposition of the vapour phase dissociation Published by Copernicus Publications on behalf of the European Geosciences Union. D. Fowler et al.: Effects of global change during the 21st century on the nitrogen cycleproducts, HNO 3 and NH 3 , is estimated to be reducing the transport distances, deposition footprints and inter-country exchange of N r in these regions.There have been important policy initiatives on components of the global N cycle. These have been regional or country-based and have delivered substantial reductions of inputs of N r to sensitive soils, waters and the atmosphere. To date there have been no attempts to develop a global strategy to regulate human inputs to the nitrogen cycle. However, considering the magnitude of global N r use, potential future increases, and the very large leakage of N r in many forms to soils, waters and the atmosphere, international action is re...

Abstract. We present high-resolution (5 km × 5 km) atmospheric chemical transport model (ACTM) simulations of the impact of newly estimated traffic-related emissions on secondary organic aerosol (SOA) formation over the UK for 2012. Our simulations include additional diesel-related intermediate-volatility organic compound (IVOC) emissions derived directly from comprehensive field measurements at an urban background site in London during the 2012 Clean Air for London (ClearfLo) campaign. Our IVOC emissions are added proportionally to VOC emissions, as opposed to proportionally to primary organic aerosol (POA) as has been done by previous ACTM studies seeking to simulate the effects of these missing emissions. Modelled concentrations are evaluated against hourly and daily measurements of organic aerosol (OA) components derived from aerosol mass spectrometer (AMS) measurements also made during the ClearfLo campaign at three sites in the London area. According to the model simulations, diesel-related IVOCs can explain on average ∼ 30 % of the annual SOA in and around London. Furthermore, the 90th percentile of modelled daily SOA concentrations for the whole year is 3.8 µg m −3 , constituting a notable addition to total particulate matter. More measurements of these precursors (currently not included in official emissions inventories) is recommended. During the period of concurrent measurements, SOA concentrations at the Detling rural background location east of London were greater than at the central London location. The model shows that this was caused by an intense pollution plume with aPublished by Copernicus Publications on behalf of the European Geosciences Union. 6454 R. Ots et al.: Simulating SOA from missing diesel-related IVOC emissions in London strong gradient of imported SOA passing over the rural location. This demonstrates the value of modelling for supporting the interpretation of measurements taken at different sites or for short durations.

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.

Abstract. The reduction of ambient concentrations of fine particulate matter (PM 2.5 ) is a key objective for air pollution control policies in the UK and elsewhere. Long-term exposure to PM 2.5 has been identified as a major contributor to adverse human health effects in epidemiological studies and underpins ambient PM 2.5 legislation. As a range of emission sources and atmospheric chemistry transport processes contribute to PM 2.5 concentrations, atmospheric chemistry transport models are an essential tool to assess emissions control effectiveness. The EMEP4UK atmospheric chemistry transport model was used to investigate the impact of reductions in UK anthropogenic emissions of primary PM 2.5 , NH 3 , NO x , SO x or non-methane VOC on surface concentrations of PM 2.5 in the UK for a recent year (2010) and for a future current legislation emission (CLE) scenario (2030). In general, the sensitivity to UK mitigation is rather small. A 30 % reduction in UK emissions of any one of the above components yields (for the 2010 simulation) a maximum reduction in PM 2.5 in any given location of ∼ 0.6 µg m −3 (equivalent to ∼ 6 % of the modelled PM 2.5 ). On average across the UK, the sensitivity of PM 2.5 concentrations to a 30 % reduction in UK emissions of individual contributing components, for both the 2010 and 2030 CLE baselines, increases in the order NMVOC, NO x , SO x , NH 3 and primary PM 2.5 ; however there are strong spatial differences in the PM 2.5 sensitivities across the UK. Consequently, the sensitivity of PM 2.5 to individual component emissions reductions varies between area and population weighting. Reductions in NH 3 have the greatest effect on area-weighted PM 2.5 . A full UK population weighting places greater emphasis on reductions of primary PM 2.5 emissions, which is simulated to be the most effective single-component control on PM 2.5 for the 2030 scenario. An important conclusion is that weighting corresponding to the average exposure indicator metric (using data from the 45 model grids containing a monitor whose measurements are used to calculate the UK AEI) further increases the emphasis on the effectiveness of primary PM 2.5 emissions reductions (and of NO x emissions reductions) relative to the effectiveness of NH 3 emissions reductions. Reductions in primary PM 2.5 have the largest impact on the AEI in both 2010 and the 2030 CLE scenario. The summation of the modelled reductions to the UK PM 2.5 AEI from 30 % reductions in UK emissions of primary PM 2.5 , NH 3 , SO x , NO x and VOC totals 1.17 and 0.82 µg m −3 for the 2010 and 2030 CLE simulations, respectively (not accounting for non-linearity).