Implementation of a comprehensive ice crystal formation parameterization for cirrus and mixed-phase clouds in the EMAC model (based on MESSy 2.53)

A comprehensive ice nucleation parameterization has been implemented in the global chemistry-climate model EMAC to improve the representation of ice crystal number concentrations (ICNCs). The parameterization of Barahona and Nenes (2009, hereafter BN09) allows for the treatment of ice nucleation taking into account the competition for water vapour between homogeneous and heterogeneous nucleation in cirrus clouds. Furthermore, the influence of chemically heterogeneous, polydisperse aerosols is considered by applying one of the multiple ice nucleating particle parameterizations which are included in BN09 to compute the heterogeneously formed ice crystals. BN09 has been modified in order to consider the pre-existing ice crystal effect and implemented to operate both in the cirrus and in the mixed-phase regimes. Compared to the standard EMAC parameterizations, BN09 produces fewer ice crystals in the upper troposphere but higher ICNCs in the middle troposphere, especially in the Northern Hemisphere where ice nucleating mineral dust particles are relatively abundant. Overall, ICNCs agree well with the observations, especially in cold cirrus clouds (at temperatures below 205&thinsp;K), although they are underestimated between 200 and 220&thinsp;K. As BN09 takes into account processes which were previously neglected by the standard version of the model, it is recommended for future EMAC simulations.</p

Projection of North Atlantic Oscillation and its effect on tracer transport

The North Atlantic Oscillation (NAO) plays an important role in the climate variability of the Northern Hemisphere, with significant consequences on long-range pollutant transport. We investigate the evolution of pollutant transport in the 21st century influenced by the NAO under a global climate change scenario. We use a free-running simulation performed by the ECHAM/MESSy Atmospheric Chemistry (EMAC) model coupled with the ocean general circulation model MPIOM, covering the period from 1950 until 2100. Similarly to other works, the model shows a future northeastward shift of the NAO centres of action and a weak positive trend of the NAO index (over 150 years). Moreover, we find that NAO trends (computed over periods shorter than 30 years) will continue to oscillate between positive and negative values in the future. To investigate the NAO effects on transport we consider carbon monoxide tracers with exponential decay and constant interannual emissions. We find that at the end of the century, the south-western Mediterranean and northern Africa will, during positive NAO phases, see higher pollutant concentrations with respect to the past, while a wider part of northern Europe will, during positive NAO phases, see lower pollutant concentrations. Such results are confirmed by the changes observed in the future for tracer concentration and vertically integrated tracer transport, differentiating the cases of “high NAO” and “low NAO” events

Projection of North Atlantic Oscillation and its effect on tracer transport

The North Atlantic Oscillation (NAO) plays an important role in the climate variability of the Northern Hemisphere, with significant consequences on long-range pollutant transport. We investigate the evolution of pollutant transport in the 21st century influenced by the NAO under a global climate change scenario. We use a free-running simulation performed by the ECHAM/MESSy Atmospheric Chemistry (EMAC) model coupled with the ocean general circulation model MPIOM, covering the period from 1950 until 2100. Similarly to other works, the model shows a future north-eastward shift of the NAO centres of action and a weak positive trend of the NAO index (over 150 years). Moreover, we find that NAO trends (computed over periods shorter than 30 years) will continue to oscillate between positive and negative values in the future. To investigate the NAO effects on transport we consider carbon monoxide tracers with exponential decay and constant interannual emissions. We find that at the end of the century, the south-western Mediterranean and northern Africa will, during positive NAO phases, see higher pollutant concentrations with respect to the past, while a wider part of northern Europe will, during positive NAO phases, see lower pollutant concentrations. Such results are confirmed by the changes observed in the future for tracer concentration and vertically integrated tracer transport, differentiating the cases of "high NAO" and "low NAO" events

North Atlantic Oscillation model projections and influence on tracer transport

The North Atlantic Oscillation (NAO) plays an important role in the climate variability of the Northern Hemisphere with significant consequences on pollutant transport. We study the influence of the NAO on the atmospheric dispersion of pollutants in the near past and in the future by considering simulations performed by the ECHAM/MESSy Atmospheric Chemistry (EMAC) general circulation model. We analyze two model runs: a simulation with circulation dynamics nudged towards ERA-Interim reanalysis data over a period of 35 years (1979–2013) and a simulation with prescribed Sea Surface Temperature (SST) boundary conditions over 150 years (1950–2099). The model is shown to reproduce the NAO spatial and temporal variability and to be comparable with observations. We find that the decadal variability in the NAO, which has been pronounced since 1950s until 1990, will continue to dominate in the future considering decadal periods, although no significant trends are present in the long term projection (100–150 years horizon). We do not find in the model projections any significant temporal trend of the NAO for the future, meaning that neither positive or negative phases will dominate. Tracers with idealised decay and emissions are considered to investigate the NAO effects on transport; it is shown that during the positive phase of the NAO, the transport from North America towards northern Europe is stronger and pollutants are shifted northwards over the Arctic and southwards over the Mediterranean and North Africa, with two distinct areas of removal and stagnation of pollutants

Long-term concentrations of fine particulate matter and impact on human health in Verona, Italy

Verona is an Italian city which experiences low levels of air quality due to its location near the centre of the Po Valley, one of the most polluted areas in Italy and in Europe. High pollutant concentrations, in particular of fine aerosol particles, are associated with detrimental effects on human health. The present study analyses the ground-based measurements of particulate matter with a diameter (PM2.5) and (PM10) registered in Verona and its province since 2002 to 2015. The annual means and the number of days when the European standards were exceeded show that air quality has slightly improved in the analysed period, with statistically significant negative trends present in both PM10 and PM2.5 levels. The annual mortality due to different diseases attributable to PM2.5 has been estimated for the period 2009–2014 by employing concentration-response functions based on epidemiological cohort studies. Results show that, on average, about 299 deaths per year (3 are infants) are caused by PM2.5 related diseases in Verona province. Among these, about 88 deaths per year (1 is infant) occur in Verona municipality. This means that of the total deaths due to diseases of the respiratory and cardiovascular systems is attributable to long-term exposure to PM2.5 pollution

Cold cloud microphysical process rates in a global chemistry-climate model

Microphysical processes in cold clouds which act as sources or sinks of hydrometeors below 0 °C control the ice crystal number concentrations (ICNCs) and in turn the cloud radiative effects. Estimating the relative importance of the cold cloud microphysical process rates is of fundamental importance to underpin the development of cloud parameterizations for weather, atmospheric chemistry and climate models and compare the output with observations at different temporal resolutions. This study quantifies and investigates the cold cloud microphysical process rates by means of the chemistry-climate model EMAC and defines the hierarchy of sources and sinks of ice crystals. The analysis is carried out both at global and at regional scales. We found that globally the freezing of cloud droplets, along with convective detrainment over tropical land masses, are the dominant sources of ice crystals, while aggregation and accretion act as the largest sinks. In general, all processes are characterised by highly skewed distribution. Moreover, the influence of (a) different ice nucleation parameterizations and (b) a future global warming scenario on the rates has been analysed in two sensitivity studies. In the first, we found that the application of different parameterizations for ice nucleation changed only slightly the hierarchy of ice crystal sources. In the second, all microphysical processes followed an upward shift (in altitude) and an increase by up to 10 % in the upper troposphere towards the end of the 21st century. This increase could have important feedbacks, such as leading to enhanced longwave warming of the uppermost atmosphere

Weaker cooling by aerosols due to dust-pollution interactions

The interactions between aeolian dust and anthropogenic air pollution, notably chemical ageing of mineral dust and coagulation of dust and pollution particles, modify the atmospheric aerosol composition and burden. Since the aerosol particles can act as cloud condensation nuclei, this not only affects the radiative transfer directly via aerosol-radiation interactions, but also indirectly through cloud adjustments. We study both radiative effects using the global ECHAM/MESSy atmospheric chemistry-climate model (EMAC) which combines the Modular Earth Submodel System (MESSy) with the European Centre/Hamburg (ECHAM) climate model. Our simulations show that dust-pollution interactions reduce the cloud water path and hence the reflection of solar radiation. The associated climate warming outweighs the cooling which the dust-pollution interactions exert through the direct radiative effect. In total, this results in a net warming by dust-pollution interactions which moderates the negative global anthropogenic aerosol forcing at the top of the atmosphere by (0.2 ± 0.1) W m−2

Global impact of mineral dust on cloud droplet number concentration

The importance of wind-blown mineral dust for cloud droplet formation is studied by considering (i) the adsorption of water on the surface of insoluble particles, (ii) particle coating by soluble material (atmospheric aging) which augments cloud condensation nuclei (CCN) activity, and (iii) the effect of dust on inorganic aerosol concentrations through thermodynamic interactions with mineral cations. The ECHAM5/MESSy Atmospheric Chemistry (EMAC) model is used to simulate the composition of global atmospheric aerosol, while the ISORROPIA-II thermodynamic equilibrium model treats the interactions of K+-Ca2+-Mg2+-NH4+-Na+-SO42−-NO3−-Cl−-H2O aerosol with gas-phase inorganic constituents. Dust is considered a mixture of inert material with reactive minerals and its emissions are calculated online by taking into account the soil particle size distribution and chemical composition of different deserts worldwide. The impact of dust on droplet formation is treated through the "unified dust activation parameterization" that considers the inherent hydrophilicity from adsorption and acquired hygroscopicity from soluble salts during aging. Our simulations suggest that the presence of dust increases cloud droplet number concentration (CDNC) over major deserts (e.g., up to 20 % over the Sahara and the Taklimakan desert) and decreases CDNC over polluted areas (e.g., up to 10 % over southern Europe and 20 % over northeastern Asia). This leads to a global net decrease in CDNC by 11 %. The adsorption activation of insoluble aerosols and the mineral dust chemistry are shown to be equally important for the cloud droplet formation over the main deserts; for example, these effects increase CDNC by 20 % over the Sahara. Remote from deserts the application of adsorption theory is critically important since the increased water uptake by the large aged dust particles (i.e., due to the added hydrophilicity by the soluble coating) reduce the maximum supersaturation and thus cloud droplet formation from the relatively smaller anthropogenic particles (e.g., CDNC decreases by 10 % over southern Europe and 20 % over northeastern Asia by applying adsorption theory). The global average CDNC decreases by 10 % by considering adsorption activation, while changes are negligible when accounting for the mineral dust chemistry. Sensitivity simulations indicate that CDNC is also sensitive to the mineral dust mass and inherent hydrophilicity, and not to the chemical composition of the emitted dust