Abstract. Nitrate aerosols are expected to become more important in the future atmosphere due to the expected increase in nitrate precursor emissions and the decline of ammoniumsulphate aerosols in wide regions of this planet. The GISS climate model is used in this study, including atmospheric gas-and aerosol phase chemistry to investigate current and future (2030, following the SRES A1B emission scenario) atmospheric compositions. A set of sensitivity experiments was carried out to quantify the individual impact of emissionand physical climate change on nitrate aerosol formation. We found that future nitrate aerosol loads depend most strongly on changes that may occur in the ammonia sources. Furthermore, microphysical processes that lead to aerosol mixing play a very important role in sulphate and nitrate aerosol formation. The role of nitrate aerosols as climate change driver is analyzed and set in perspective to other aerosol and ozone forcings under pre-industrial, present day and future conditions. In the near future, year 2030, ammonium nitrate radiative forcing is about −0.14 W/m 2 and contributes roughly 10% of the net aerosol and ozone forcing. The present day nitrate and pre-industrial nitrate forcings are −0.11 and −0.05 W/m 2 , respectively. The steady increase of nitrate aerosols since industrialization increases its role as a non greenhouse gas forcing agent. However, this impact is still small compared to greenhouse gas forcings, therefore the main role nitrate will play in the future atmosphere is as an air pollutant, with annual mean near surface air concentrations, in the fine particle mode, rising above 3 µg/m 3 in China and therefore reaching pollution levels, like sulphate aerosols.
[1] A computationally efficient model to calculate gas/aerosol partitioning of semivolatile inorganic aerosol components has been developed for use in global atmospheric chemistry and climate models. We introduce an approximate method for the activity coefficient calculation that directly relates aerosol activity coefficients to the ambient relative humidity, assuming chemical equilibrium. We demonstrate that this method provides an alternative for the computationally expensive iterative activity coefficient calculation methods presently used in thermodynamic gas/aerosol models. The gain of our method is that the entire system of the gas/aerosol equilibrium partitioning can be solved noniteratively, a substantial advantage in global modeling. We show that our equilibrium simplified aerosol model (EQSAM) yields results similar to those of current state-of-theart equilibrium models.
Abstract. To elucidate human induced changes of aerosol load and composition in the atmosphere, a coupled aerosol and gas-phase chemistry transport model of the troposphere and lower stratosphere has been used. The present 3-D modeling study focuses on aerosol chemical composition change since preindustrial times considering the secondary organic aerosol formation together with all other main aerosol components including nitrate. In particular, we evaluate non-seasalt sulfate (nss-SO = 4 ), ammonium (NH + 4 ), nitrate (NO − 3 ), black carbon (BC), sea-salt, dust, primary and secondary organics (POA and SOA) with a focus on the importance of secondary organic aerosols. Our calculations show that the aerosol optical depth (AOD) has increased by about 21% since preindustrial times. This enhancement of AOD is attributed to a rise in the atmospheric load of BC, nss-SO = 4 , NO − 3 , POA and SOA by factors of 3.3, 2.6, 2.7, 2.3 and 1.2, respectively, whereas we assumed that the natural dust and sea-salt sources remained constant. The nowadays increase in carbonaceous aerosol loading is dampened by a 34-42% faster conversion of hydrophobic to hydrophilic carbonaceous aerosol leading to higher removal rates. These changes between the various aerosol components resulted in significant modifications of the aerosol chemical composition. The relative importance of the various aerosol components is critical for the aerosol climatic effect, since atmospheric aerosols behave differently when their chemical composition changes. According to this study, the aerosol composition changed significantly over the different contiCorrespondence to: M. Kanakidou (mariak@chemistry.uoc.gr) nents and with height since preindustrial times. The presence of anthropogenically emitted primary particles in the atmosphere facilitates the condensation of the semi-volatile species that form SOA onto the aerosol phase, particularly in the boundary layer. The SOA burden that is dominated by the natural component has increased by 24% while its contribution to the AOD has increased by 11%. The increase in oxidant levels and preexisting aerosol mass since preindustrial times is the reason of the burden change, since emissions have not changed significantly. The computed aerosol composition changes translate into about 2.5 times more water associated with non sea-salt aerosol. Additionally, aerosols contain 2.7 times more inorganic components nowadays than during the preindustrial times. We find that the increase in emissions of inorganic aerosol precursors is much larger than the corresponding aerosol increase, reflecting a non-linear atmospheric response.
[1] Real-time measurements of ammonia, nitric acid, hydrochloric acid, sulfur dioxide and the water-soluble inorganic aerosol species, ammonium, nitrate, chloride, and sulfate were performed at a pasture site in the Amazon Basin (Rondônia, Brazil). The measurements were made during the late dry season (biomass burning), the transition period, and the onset of the wet season (clean conditions) using a wet-annular denuder (WAD) in combination with a Steam-Jet Aerosol Collector (SJAC). Measurements were conducted from 12 September to 14 November 2002 within the framework of LBA-SMOCC (Large-Scale Biosphere Atmosphere Experiment in Amazonia -Smoke Aerosols, Clouds, Rainfall, and Climate: Aerosols From Biomass Burning Perturb Global and Regional Climate). Real-time data were combined with measurements of sodium, potassium, calcium, magnesium, and low-molecular weight (LMW) polar organic acids determined on 12-, 24-, and 48-hours integrated filter samples. The contribution of inorganic species to the fine particulate mass (D p 2.5 mm) was frequently below 20% by mass, indicating the preponderance of organic matter. The measured concentration products of NH 3 Â HNO 3 and NH 3 Â HCl persistently remained below the theoretical equilibrium dissociation constants of the NH 3 /HNO 3 /NH 4 NO 3 and NH 3 /HCl/NH 4 Cl systems during daytime (RH < 90%). The application of four thermodynamic equilibrium models (EQMs) indicates that the fine mode aerosol anions NO 3 À , Cl À , and SO 4 2À were balanced predominantly by mineral cations (particularly pyrogenic K + ) during daytime. At nighttime (RH > 90%) fine-mode NH 4 NO 3 and NH 4 Cl are predicted to be formed in the aqueous aerosol phase. Probably, Cl À was driven out of the aerosol phase largely by reaction of pyrogenic KCl with HNO 3 and H 2 SO 4 . As shown by an updated version of the equilibrium simplified aerosol model (EQSAM2), which incorporates mineral aerosol species and lumped LMW polar organic acids, daytime aerosol NH 4 + was mainly balanced by organic compounds. -H 2 O aerosol system and its gas phase precursors at a pasture site in the Amazon Basin: How relevant are mineral cations and soluble organic acids?,
We present gas/aerosol partitioning calculations of multicomponent aerosols and aerosol associated water on a global scale. We have coupled a computationally efficient gas‐aerosol scheme (EQSAM) to a global atmospheric chemistry‐transport model (TM3). Our results show that gas/aerosol partitioning strongly affects the gas‐phase concentrations at relatively low temperatures. During winter and at night during all seasons the calculated aerosol load, including water, is considerably higher than without accounting for gas/aerosol partitioning. The reason is that gaseous nitric acid near the surface is often neutralized by ammonia and therefore partitions almost completely into the aerosol phase to yield ammonium nitrate (NH4NO3). The aerosol NH4NO3 has a longer atmospheric residence time compared to the corresponding precursor gases (NH3 and HNO3) and can therefore be transported over larger distances, for instance from India to Africa and Europe. These modeling results are intriguing; however, verification requires in situ measurements. A comparison with a limited set of ground‐based measurements indicates that our model yields realistic results for the ammonium‐sulfate‐nitrate‐water aerosol system in relatively polluted locations where ammonium nitrate is important. For remote locations for which we underestimate the total aerosol load, however, it will be necessary to also account for other aerosol species such as sea salt, mineral dust and organic compounds. We further show that assumptions on turbulent mixing and model resolution have a much stronger effect on aerosol calculations than the uncertainties resulting from the simplifications made in EQSAM.
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