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

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...