Existing descriptions of bi-directional ammonia (NH ₃ ) 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 NH ₃ 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 NH ₃ 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 NH ₃ 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 NH ₃ 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 NH ₃ emissions may increase from 65 (45–85) Tg N in 2008 to reach 132 (89–179) Tg by 2100.

An analysis of the 22-yr ozone (O₃) series (1988–2009) at the subtropical high mountain Izaña~station (IZO; 2373 m a.s.l.), representative of free troposphere (FT) conditions, is presented. Diurnal and seasonal O₃ variations as well as the O₃ trend (0.19 ± 0.05 % yr⁻¹ or 0.09 ppbv yr⁻¹), are assessed. A climatology of O₃ transport pathways using backward trajectories shows that higher O₃ values are associated with air masses travelling above 4 km altitude from North America and North Atlantic Ocean, while low O₃ is transported from the Saharan continental boundary layer (CBL). O₃ data have been compared with PM₁₀, ²¹⁰Pb, ⁷Be, potential vorticity (PV) and carbon monoxide (CO). A clear negative logarithmic relationship was observed between PM₁₀ and surface O₃ for all seasons. A similar relationship was found between O₃ and ²¹⁰Pb. The highest daily O₃ values (90th percentile) are observed in spring and in the first half of summer time. A positive correlation between O₃ and PV, and between O₃ and ⁷Be is found throughout the year, indicating that relatively high surface O₃ values at IZO originate from the middle and upper troposphere. We find a good correlation between O₃ and CO in winter, supporting the hypothesis of long-range transport of photochemically generated O₃ from North America. Aged air masses, in combination with sporadic inputs from the upper troposphere, are observed in spring, summer and autumn. In summer time high O₃ values seem to be the result of stratosphere-to-troposphere (STT) exchange processes in regions neighbouring the Canary Islands. Since 1995–1996, the North Atlantic Oscillation has changed from a predominantly high positive phase to alternating between negative, neutral or positive phases. This change results in an increased flow of the westerlies in the mid-latitude and subtropical North Atlantic, thus favouring the transport of O₃ and its precursors from North America, and a higher frequency of storms over North Atlantic, with a likely higher incidence of STT processes in mid-latitudes. These processes lead to an increase of tropospheric O₃ in the subtropical North Atlantic region after 1996 that has been reflected in surface O₃ records at IZO.