Factors controlling Arctic denitrification in cold winters of the 1990s

International audienceDenitrification of the Arctic winter stratosphere has been calculated using a 3-D microphysical model for the winters 1994/95, 1995/96, 1996/97 and 1999/2000. Denitrification is assumed to occur through the sedimentation of low number concentrations of large nitric acid trihydrate (NAT) particles, as observed extensively in 1999/2000. We examine whether the meteorological conditions that allowed NAT particles to grow to the very large sizes observed in 1999/2000 also occurred in the other cold winters. The results show that winter 1999/2000 had conditions that were optimum for denitrification by large NAT particles, which are a deep concentric cold pool and vortex. Under these conditions, NAT particles can circulate in the cold pool for several days, reaching several micrometres in radius and leading to a high downward flux of nitric acid. The other winters had shorter periods with optimum conditions for denitrification. However, we find that NAT particles could have grown to large sizes in all of these winters and could have caused significant denitrification. We define the quantity "closed flow area'' (the fraction of the cold pool in which air parcel trajectories can form closed loops) and show that it is a very useful indicator of possible denitrification. We find that even with a constant NAT nucleation rate throughout the cold pool, the average NAT number concentration and size can vary by up to a factor of 10 in response to this meteorological quantity. These changes in particle properties account for a high degree of variability in denitrification between the different winters. This large meteorologically induced variability in denitrification rate needs to be compared with that which could arise from a variable nucleation rate of NAT particles, which remains an uncertain quantity in models

Mid-latitude ozone changes: studies with a 3-D CTM forced by ERA-40 analyses

International audienceWe have used an off-line three-dimensional (3-D) chemical transport model (CTM) to study long-term changes in stratospheric O3. The model was run from 1977?2004 and forced by ECMWF ERA-40 and operational analyses. Model runs were performed to examine the impact of increasing halogens and additional stratospheric bromine from short-lived source gases. The analyses capture much of the observed interannual variability in column ozone, but there are also unrealistic features. In particular the ERA-40 analyses cause a large positive anomaly in northern hemisphere (NH) column O3 in the late 1980s. Also, the change from ERA-40 to operational winds at the start of 2002 introduces abrupt changes in some model fields which affect analysis of trends. The model reproduces the observed column increase in NH mid-latitudes from the mid 1990s. Analysis of a run with fixed halogens shows that this increase is not due to a significant decrease in halogen-induced loss, i.e. is not an indication of recovery. The model predicts only a small decrease in halogen-induced loss after 1999. In the upper stratosphere, despite the modelled turnover of chlorine around 1999, O3 does not increase to the effects of increasing ECMWF temperatures, decreasing modelled CH4 at this altitude, and abrupt changes to the SH temperatures at the end of the ERA-40 period. The impact of an additional 5 pptv stratospheric bromine from short-lived species decreases mid-latitude column O3 by about 10 DU. However, the impact on the modelled relative O3 anomaly is generally small except during periods of large volcanic loading

A global off-line model of size-resolved aerosol microphysics: II. Identification of key uncertainties

International audienceWe use the new GLOMAP model of global aerosol microphysics to investigate the sensitivity of modelled sulfate and sea salt aerosol properties to uncertainties in the driving microphysical processes and compare these uncertainties with those associated with aerosol and precursor gas emissions. Overall, we conclude that uncertainties in microphysical processes have a larger effect on global sulfate and sea salt derived condensation nuclei (CN) and cloud condensation nuclei (CCN) concentrations than uncertainties in present-day sulfur emissions. Our simulations suggest that uncertainties in predicted sulfate and sea salt CCN abundances due to poorly constrained microphysical processes are likely to be of a similar magnitude to long-term changes in sulfate and sea salt CCN due to changes in anthropogenic emissions. A microphysical treatment of the global sulfate aerosol allows the uncertainty in climate-relevant aerosol properties to be attributed to specific processes in a way that has not been possible with simpler aerosol schemes. In particular we conclude that: (1) changes in the binary H2SO4-H2O nucleation rate and condensation rate of gaseous H2SO4 cause a shift in the vertical location of the upper tropospheric CN layer by as much as 3 km, while the shape of the CN profile is essentially pre-served (2) uncertainties in the binary H2SO4-H2O nucleation rate have a relatively insignificant effect on marine boundary layer (MBL) aerosol properties; (3) emitting a fraction of anthropogenic SO2 as particulates (to represent production of sulfate particles in power plant plumes below the scale of the model grid (which is of the order of 300 km)) has the potential to change the global mean MBL sulfate-derived CN concentrations by up to 72%, and changes of up to a factor 20 can occur in polluted continental regions; (4) predicted global mean MBL sulfate and sea salt CCN concentrations change by 10 to 60% when several microphysical processes are changed within reasonable uncertainty ranges; (5) sulfate and sea salt derived CCN concentrations are particularly sensitive to primary particle emissions, with global mean MBL sulfate and sea salt CCN changing by up to 27% and local concentrations over continental regions changing by more than 100% when the percentage of anthropogenic SO2 emitted as particulates is changed from 0 to 5%; (6) large changes in sea spray flux have insignificant effects on global sulfate aerosol except when the mass accommodation coefficient of sulfuric acid on the salt particles is set unrealistically low

A global off-line model of size-resolved aerosol microphysics: I. Model development and prediction of aerosol properties

A GLObal Model of Aerosol Processes (GLOMAP) has been developed as an extension to the TOMCAT 3-D Eulerian off-line chemical transport model. GLOMAP simulates the evolution of the global aerosol size distribution using a sectional two-moment scheme and includes the processes of aerosol nucleation, condensation, growth, coagulation, wet and dry deposition and cloud processing. We describe the results of a global simulation of sulfuric acid and sea spray aerosol. The model captures features of the aerosol size distribution that are well established from observations in the marine boundary layer and free troposphere. Modelled condensation nuclei (CN>3nm) vary between about 250&ndash;500 cm^-3 in remote marine boundary layer regions and are generally in good agreement with observations. Modelled continental CN concentrations are lower than observed, which may be due to lack of some primary aerosol sources or the neglect of nucleation mechanisms other than binary homogeneous nucleation of sulfuric acid-water particles. Remote marine CN concentrations increase to around 2000&ndash;10 000 cm<sup-3</sup> (at standard temperature and pressure) in the upper troposphere, which agrees with typical observed vertical profiles. Cloud condensation nuclei (CCN) at 0.2% supersaturation vary between about 1000 cm^-3 in polluted regions and between 10 and 500 cm^-3 in the remote marine boundary layer. New particle formation through sulfuric acid-water binary nucleation occurs predominantly in the upper troposphere, but the model results show that these particles contribute greatly to aerosol concentrations in the marine boundary layer. For this sulfur-sea salt system it is estimated that sea spray emissions account for only ~10% of CCN in the tropical marine boundary layer, but between 20 and 75% in the mid-latitude Southern Ocean. In a run with only natural sulfate and sea salt emissions the global mean surface CN concentration is more than 60% of that from a run with 1985 anthropogenic sulfur emissions, although the natural emissions comprise only 27% of total sulfur emissions. Southern hemisphere marine boundary layer CN are more than 90% natural in origin, while polluted continental CN are more than 90% anthropogenic in origin, although these numbers will change when other anthropogenic CN sources are included in the model

The relationship between aerosol and cloud drop number concentrations in a global aerosol microphysics model

Empirical relationships that link cloud droplet number (CDN) to aerosol number or mass are commonly used to calculate global fields of CDN for climate forcing assessments. In this work we use a sectional global model of sulfate and sea-salt aerosol coupled to a mechanistic aerosol activation scheme to explore the limitations of this approach. We find that a given aerosol number concentration produces a wide range of CDN concentrations due to variations in the shape of the aerosol size distribution. On a global scale, the dependence of CDN on the size distribution results in regional biases in predicted CDN (for a given aerosol number). Empirical relationships between aerosol number and CDN are often derived from regional data but applied to the entire globe. In an analogous process, we derive regional &quot;correlation-relations&quot; between aerosol number and CDN and apply these regional relations to calculations of CDN on the global scale. The global mean percentage error in CDN caused by using regionally derived CDN-aerosol relations is 20 to 26%, which is about half the global mean percentage change in CDN caused by doubling the updraft velocity. However, the error is as much as 25–75% in the Southern Ocean, the Arctic and regions of persistent stratocumulus when an aerosol-CDN correlation relation from the North Atlantic is used. These regions produce much higher CDN concentrations (for a given aerosol number) than predicted by the globally uniform empirical relations. CDN-aerosol number relations from different regions also show very different sensitivity to changing aerosol. The magnitude of the rate of change of CDN with particle number, a measure of the aerosol efficacy, varies by a factor 4. CDN in cloud processed regions of persistent stratocumulus is particularly sensitive to changing aerosol number. It is therefore likely that the indirect effect will be underestimated in these important regions

Bromine in the tropical troposphere and stratosphere as derived from balloon-borne BrO observations

The first tropospheric and stratospheric (4 to 33 km) BrO profile is presented for the inner tropics derived from balloon-borne DOAS (Differential Optical Absorption Spectroscopy) measurements. In combination with photochemical modelling, total stratospheric inorganic bromine (Br&lt;sub&gt;y&lt;/sub&gt;) is deduced to be (21.5&amp;plusmn;2.5) ppt in 4.5-year-old air, probed in 2005. We derive a total contribution of (5.2&amp;plusmn;2.5) ppt from brominated very short-lived substances and inorganic product gases to stratospheric Br&lt;sub&gt;y&lt;/sub&gt; Tropospheric BrO was found to be &lt;1 ppt. Our results are compared to two 3-D CTM SLIMCAT model runs, which differ in the lifetime of the bromine source gases, affecting the vertical distribution of Br&lt;sub&gt;y&lt;/sub&gt; in the lower stratosphere. Bromine source gas measurements performed 10 days earlier Laube et al., 2008, indicate a lower Br&lt;sub&gt;y&lt;/sub&gt; of (17.5&amp;plusmn;0.4) ppt. Potential reasons for this discrepancy are discussed

The impact of the mixing properties within the Antarctic stratospheric vortex on ozone loss in spring

Calculations of equivalent length from an artificial advected tracer provide new insight into the isentropic transport processes occurring within the Antarctic stratospheric vortex. These calculations show two distinct regions of approximately equal area: a strongly mixed vortex core and a broad ring of weakly mixed air extending out to the vortex boundary. This broad ring of vortex air remains isolated from the core between late winter and midspring. Satellite measurements of stratospheric H2O confirm that the isolation lasts until at least mid-October. A three-dimensional chemical transport model simulation of the Antarctic ozone hole quantifies the ozone loss within this ring and demonstrates its isolation. In contrast to the vortex core, ozone loss in the weakly mixed broad ring is not complete. The reasons are twofold. First, warmer temperatures in the broad ring prevent continuous polar stratospheric cloud (PSC) formation and the associated chemical processing (i.e., the conversion of unreactive chlorine into reactive forms). Second, the isolation prevents ozone-rich air from the broad ring mixing with chemically processed air from the vortex core. If the stratosphere continues to cool, this will lead to increased PSC formation and more complete chemical processing in the broad ring. Despite the expected decline in halocarbons, sensitivity studies suggest that this mechanism will lead to enhanced ozone loss in the weakly mixed region, delaying the future recovery of the ozone hole

Design of Lightweight Structural Components for Direct Digital Manufacturing

The rapid growth in direct digital manufacturing technologies has opened the challenge of designing optimal micro-structures for high-performance components. Current topology optimization techniques do not work well for this type of problems and hence in this paper we propose a technique based on an implicit representation of the structural topology. The detailed microstructure is defined by a continuous variable, the size distribution field, defined over the design domain by chosen shape functions. We can optimize the structural topology by optimizing only the weights of the size distribution field and, for any given size distribution, we use standard meshing software to determine the actual detailed micro-structure. We have implemented the optimization loop using commercial CAD and FEA software, running under a genetic algorithm in MATLAB. Application this novel technique to the design of a sandwich beam has produced designs that are superior to any standard solid beam or even optimized truss structure

Description and evaluation of GLOMAP-mode: a modal global aerosol microphysics model for the UKCA composition-climate model

A new version of the Global Model of Aerosol Processes (GLOMAP) is described, which uses a two-moment pseudo-modal aerosol dynamics approach rather than the original two-moment bin scheme. GLOMAP-mode simulates the multi-component global aerosol, resolving sulfate, sea-salt, dust, black carbon (BC) and particulate organic matter (POM), the latter including primary and biogenic secondary POM. Aerosol processes are simulated in a size-resolved manner including primary emissions, secondary particle formation by binary homogeneous nucleation of sulfuric acid and water, particle growth by coagulation, condensation and cloud-processing and removal by dry deposition, in-cloud and below-cloud scavenging. A series of benchmark observational datasets are assembled against which the skill of the model is assessed in terms of normalised mean bias (&lt;i&gt;b&lt;/i&gt;) and correlation coefficient (&lt;i&gt;R&lt;/i&gt;). Overall, the model performs well against the datasets in simulating concentrations of aerosol precursor gases, chemically speciated particle mass, condensation nuclei (CN) and cloud condensation nuclei (CCN). Surface sulfate, sea-salt and dust mass concentrations are all captured well, while BC and POM are biased low (but correlate well). Surface CN concentrations compare reasonably well in free troposphere and marine sites, but are underestimated at continental and coastal sites related to underestimation of either primary particle emissions or new particle formation. The model compares well against a compilation of CCN observations covering a range of environments and against vertical profiles of size-resolved particle concentrations over Europe. The simulated global burden, lifetime and wet removal of each of the simulated aerosol components is also examined and each lies close to multi-model medians from the AEROCOM model intercomparison exercise

Acceleration of global N₂O emissions seen from two decades of atmospheric inversion

Nitrous oxide (N2O) is the third most important long-lived GHG and an important stratospheric ozone depleting substance. Agricultural practices and the use of N-fertilizers have greatly enhanced emissions of N2O. Here, we present estimates of N2O emissions determined from three global atmospheric inversion frameworks during the period 1998–2016. We find that global N2O emissions increased substantially from 2009 and at a faster rate than estimated by the IPCC emission factor approach. The regions of East Asia and South America made the largest contributions to the global increase. From the inversion-based emissions, we estimate a global emission factor of 2.3 ± 0.6%, which is significantly larger than the IPCC Tier-1 default for combined direct and indirect emissions of 1.375%. The larger emission factor and accelerating emission increase found from the inversions suggest that N2O emission may have a nonlinear response at global and regional scales with high levels of N-input