Abstract
Abstract. The ammonia cycle and nitrate particle formation are introduced into the LMDz-INCA (Laboratoire de Météorologie Dynamique, version 4 – INteraction with Chemistry and Aerosols, version 3) global model. An important aspect of this new model is that both fine nitrate particle formation in the accumulation mode and coarse nitrate forming on existing dust and sea-salt particles are considered. The model simulates distributions of nitrates and related species in agreement with previous studies and observations. The calculated present-day total nitrate direct radiative forcing since the pre-industrial is −0.056 W m−2. This forcing corresponds to 18% of the sulfate forcing. Fine particles largely dominate the nitrate forcing, representing close to 90% of this value. The model has been used to investigate the future changes in nitrates and direct radiative forcing of climate based on snapshot simulations for the four representative concentration pathway (RCP) scenarios and for the 2030, 2050, and 2100 time horizons. Due to a decrease in fossil fuel emissions in the future, the concentration of most of the species involved in the nitrate–ammonium–sulfate system drop by 2100 except for ammonia, which originates from agricultural practices and for which emissions significantly increase in the future. Despite the decrease of nitrate surface levels in Europe and North America, the global burden of accumulation mode nitrates increases by up to a factor of 2.6 in 2100. This increase in ammonium nitrate in the future arises despite decreasing NOx emissions due to increased availability of ammonia to form ammonium nitrate. The total aerosol direct forcing decreases from its present-day value of −0.234 W m−2 to a range of −0.070 to −0.130 W m−2 in 2100 based on the considered scenario. The direct forcing decreases for all aerosols except for nitrates, for which the direct negative forcing increases to a range of −0.060 to −0.115 W m−2 in 2100. Including nitrates in the radiative forcing calculations increases the total direct forcing of aerosols by a factor of 1.3 in 2000, by a factor of 1.7–2.6 in 2030, by 1.9–4.8 in 2050, and by 6.4–8.6 in 2100. These results show that the agricultural emissions of ammonia will play a key role in the future mitigation of climate change, with nitrates becoming the dominant contributor to the anthropogenic aerosol optical depth during the second half of the 21st century and significantly increasing the calculated aerosol direct forcing. This significant increase in the influence that nitrate exerts on climate in the future will at the same time affect regional air quality and nitrogen deposition to the ecosystem.
Highlights
The formation of ammonium nitrate particles in the atmosphere is a secondary process initiated by the oxidation of NO2 to nitric acid (HNO3) and by its further reaction with ammonia (NH3)
Further work is needed to better characterize the individual nitrate measurement error to see where modeled nitrate is consistent with measurements
An important feature of this new model is that both fine nitrate particle formation in the accumulation mode from nitric acid and ammonia reaction, and coarse nitrate particles forming on existing dust and seasalt particles are considered
Summary
The formation of ammonium nitrate particles in the atmosphere is a secondary process initiated by the oxidation of NO2 to nitric acid (HNO3) and by its further reaction with ammonia (NH3). Since the LMDz-INCA general circulation model is designed for long-term simulations, we use a simplified approach, and the phase equilibrium of the ammonium– sulfate–nitrate aerosol system is introduced for fine particles based on the simple thermodynamical formulation used initially in regional acid deposition chemical transport models (Hov et al, 1988; Ackermann et al, 1995), in global chemical transport models (Tie et al, 2005) and more recently in an earth system model (Bellouin et al, 2011) In addition to this formation of fine nitrate particles in the accumulation mode, the role of nitric acid uptake on mineral dust and sea-salt particles to form coated coarse nitrate particles can play an important role in the total nitrate and nitric acid budget in the atmosphere and on the radiative forcing (e.g., Liao and Seinfeld, 2005; Myhre et al, 2006; Bauer et al, 2007; Feng and Penner, 2007; Xu and Penner, 2012).
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