Index Geophysics

Items

• Enhancements of the refractory submicron aerosol fraction in the Arctic polar vortex: feature or exception?

  In situ measurements with a four-channel stratospheric condensation particle counter (CPC) were conducted at up to 20 km altitude on board the aircraft M-55 <i>Geophysica</i> from Kiruna, Sweden, in January through March (EUPLEX 2003, RECONCILE 2010) and in December (ESSenCe 2011). During all campaigns air masses from the upper stratosphere and mesosphere were subsiding inside the Arctic winter vortex, thus initializing a transport of refractory aerosol into the lower stratosphere (Θ < 500 K). The strength and extent of this downward transport varied between the years depending on the dynamical evolution of the vortex. Inside the vortex and at potential temperatures Θ ≥ 450 K around 11 submicron particles per cm³ were generally detected. Up to 8 of these 11 particles per cm³ were found to contain thermo-stable (at 250 °C) residuals with diameters of 10 nm to about 1 μm. Particle mixing ratios (150 mg⁻¹) and fractions of non-volatile particles (75% of totally detected particles) exhibited highest values in air masses having the lowest content of nitrous oxide (70 nmol mol⁻¹ of N₂O). This indicates that refractory aerosol originates from the upper stratosphere or the mesosphere. Derived from the mixing ratio of the simultaneously measured long-lived tracer N₂O, an empirical index serves to differentiate probed air masses according to their origin: inside the vortex, the vortex edge region, or outside the vortex. Previously observed high fractions of refractory submicron aerosol in the 2003 Arctic vortex were ascribed to unusually strong subsidence during that winter. However, measurements under perturbed vortex conditions in 2010 and during early winter in December 2011 revealed similarly high values. Thus, the abundance of refractory aerosol in the lower stratosphere within the Arctic vortices appears to be a regular feature rather than the exception. During December, the import from aloft into the lower stratosphere appears to be developing; thereafter the abundance of refractory aerosol inside the vortex reaches its highest levels in March. The correlations of refractory aerosol with N₂O suggest that, apart from mean subsidence, diabatic dispersion inside the vortex significantly contributes to the transport of particles to the Arctic lower stratosphere. A measurement-based estimate of the total mass of refractory aerosol inside the vortex is provided for each campaign. Based on the derived increase of particle mass in the lower stratospheric vortex (100–67 hPa pressure altitude) by a factor of 4.5 between early and late winter, we estimate the total mass of mesospheric particles deposited over the winter 2009/2010 in the entire Arctic vortex to range between 77 × 10³ and 375 × 10⁶ kg. This estimate is compared with the expected atmospheric influx of meteoritic material (110 ± 55 × 10³ kg per day). Such estimates at present still hold considerable uncertainties, which are discussed in this article. Nevertheless, the results enable placing constraints on the shape of the so far unknown size distribution of refractory aerosol within the vortex.

• Particle surface area dependence of mineral dust in immersion freezing mode: investigations with freely suspended drops in an acoustic levitator and a vertical wind tunnel

  The heterogeneous freezing temperatures of supercooled drops were measured using an acoustic levitator. This technique allows one to freely suspend single drops in the air without any wall contact. Heterogeneous nucleation by two types of illite (illite IMt1 and illite NX) and a montmorillonite sample was investigated in the immersion mode. Drops of 1 mm in radius were monitored by a video camera while cooled down to −28 °C to simulate freezing within the tropospheric temperature range. The surface temperature of the drops was contact-free, determined with an infrared thermometer; the onset of freezing was indicated by a sudden increase of the drop surface temperature. For comparison, measurements with one particle type (illite NX) were additionally performed in the Mainz vertical wind tunnel with drops of 340 μm radius freely suspended. Immersion freezing was observed in a temperature range between −13 and −26 °C as a function of particle type and particle surface area immersed in the drops. Isothermal experiments in the wind tunnel indicated that after the cooling stage freezing still proceeds, at least during the investigated time period of 30 s. The results were evaluated by applying two descriptions of heterogeneous freezing, the stochastic and the singular model. Although the wind tunnel results do not support the time-independence of the freezing process both models are applicable for comparing the results from the two experimental techniques.

• Measurements of OH and RO2 radicals at Dome C, East Antarctica

  Concentrations of OH radicals and the sum of peroxy radicals, RO₂, were measured in the boundary layer for the first time on the East Antarctic Plateau at the Concordia Station (Dome C, 75.10° S, 123.31° E) during the austral summer 2011/2012. The median concentrations of OH and RO₂ radicals were 3.1 × 10⁶ molecule cm⁻³ and 9.9 × 10⁶ molecule cm⁻³, respectively. These values are comparable to those observed at the South Pole, confirming that the elevated oxidative capacity of the Antarctic atmospheric boundary layer found at the South Pole is not restricted to the South Pole but common over the high Antarctic plateau. At Concordia, the concentration of radicals showed distinct diurnal profiles with the median maximum of 5.2 × 10⁶ molecule cm⁻³ at 11:00 and the median minimum of 1.1 × 10⁶ molecule cm⁻³ at 01:00 for OH radicals and 1.7 × 10⁸ molecule cm⁻³ and 2.5 × 10⁷ molecule cm⁻³ for RO₂ radicals at 13:00 and 23:00, respectively (all times are local times). Concurrent measurements of O₃, HONO, NO, NO₂, HCHO and H₂O₂ demonstrated that the major primary source of OH and RO₂ radicals at Dome C was the photolysis of HONO, HCHO and H₂O₂, with the photolysis of HONO contributing ~75% of total primary radical production. However, photochemical modelling with accounting for all these radical sources overestimates the concentrations of OH and RO₂ radicals by a factor of 2 compared to field observations. Neglecting the net OH production from HONO in the photochemical modelling results in an underestimation of the concentrations of OH and RO₂ radicals by a factor of 2. To explain the observations of radicals in this case an additional source of OH equivalent to about (25–35)% of measured photolysis of HONO is required. Even with a factor of 5 reduction in the concentrations of HONO, the photolysis of HONO represents the major primary radical source at Dome C. To account for a possibility of an overestimation of NO₂ observed at Dome C the calculations were also performed with NO₂ concentrations estimated by assuming steady-state NO₂ / NO ratios. In this case the net radical production from the photolysis of HONO should be reduced by a factor of 5 or completely removed based on the photochemical budget of OH or 0-D modelling, respectively. Another major factor leading to the large concentration of OH radicals measured at Dome C was large concentrations of NO molecules and fast recycling of peroxy radicals to OH radicals.

• Regional climate model assessment of the urban land-surface forcing over central Europe

  For the purpose of qualifying and quantifying the climate impact of cities and urban surfaces in general on climate of central Europe, the surface parameterization in regional climate model RegCM4 has been extended with the Single-layer Urban Canopy Model (SLUCM). A set of experiments was performed over the period of 2005–2009 for central Europe, either without considering urban surfaces or with the SLUCM treatment. Results show a statistically significant impact of urbanized surfaces on temperature (up to 1.5 K increase in summer) as well as on the boundary layer height (increases up to 50 m). Urbanization further influences surface wind with a winter decrease up to −0.6 m s⁻¹, though both increases and decreases were detected in summer depending on the location relative to the cities and daytime (changes up to 0.3 m s⁻¹). Urban surfaces significantly reduce the humidity over the surface. This impacts the simulated summer precipitation rate, showing a decrease over cities of up to −2 mm day⁻¹. Significant temperature increases are simulated over higher altitudes as well, not only within the urban canopy layer. With the urban parameterization, the climate model better describes the diurnal temperature variation, reducing the cold afternoon and evening bias of RegCM4. <br><br> Sensitivity experiments were carried out to quantify the response of the meteorological conditions to changes in the parameters specific to the urban environment, such as street width, building height, albedo of the roofs and anthropogenic heat release. The results proved to be rather robust and the choice of the key SLUCM parameters impacts them only slightly (mainly temperature, boundary layer height and wind velocity). Statistically significant impacts are modelled not only over large urbanized areas, but the influence of the cities is also evident over rural areas without major urban surfaces. It is shown that this is the result of the combined effect of the distant influence of the cities and the influence of the minor local urban surface coverage.

• Competition between water uptake and ice nucleation by glassy organic aerosol particles

  Organic aerosol particles play a key role in climate by serving as nuclei for clouds and precipitation. Their sources and composition are highly variable, and their phase state ranges from liquid to solid under atmospheric conditions, affecting the pathway of activation to cloud droplets and ice crystals. Due to slow diffusion of water in the particle phase, organic particles may deviate in phase and morphology from their thermodynamic equilibrium state, hampering the prediction of their influence on cloud formation. We overcome this problem by combining a novel semi-empirical method for estimation of water diffusivity with a kinetic flux model that explicitly treats water diffusion. We estimate timescales for particle deliquescence as well as various ice nucleation pathways for a wide variety of organic substances, including secondary organic aerosol (SOA) from the oxidation of isoprene, α-pinene, naphthalene, and dodecane. The simulations show that, in typical atmospheric updrafts, glassy states and solid/liquid core-shell morphologies can persist for long enough that heterogeneous ice nucleation in the deposition and immersion mode can dominate over homogeneous ice nucleation. Such competition depends strongly on ambient temperature and relative humidity as well as humidification rate and particle size. Due to differences in glass transition temperature, hygroscopicity and atomic O / C ratio of the different SOA, naphthalene SOA particles have the highest potential to act as heterogeneous ice nuclei. Our findings demonstrate that kinetic limitations of water diffusion into organic aerosol particles are likely to be encountered under atmospheric conditions and can strongly affect ice nucleation pathways. For the incorporation of ice nucleation by organic aerosol particles into atmospheric models, our results demonstrate a demand for model formalisms that account for the effects of molecular diffusion and not only describe ice nucleation onsets as a function of temperature and relative humidity but also include updraft velocity, particle size and composition.

• A new parameterization of particle dry deposition over rough surfaces

  In existing particle dry deposition schemes, the effects of gravity and surface roughness elements on particle motion are often poorly represented. In this study, we propose a new scheme to overcome such deficiencies. Particle deposition velocity is a function of aerodynamic, surface-collection and gravitational resistances. In this study, the effect of gravitation settling is treated analytically. More importantly, the new scheme takes into consideration the impacts of roughness elements on turbulent particle diffusion and surface particle collection. A relationship between aerodynamic and surface-collection processes is established by using an analogy between drag partition and deposition-flux partition. The scheme is then tested against a wind-tunnel data set for four different surfaces and a good agreement between the scheme predictions and the observations is found. The sensitivity of the scheme to the input parameters is tested. Important factors which affect particle deposition in different particle size ranges are identified. The scheme shows good capacity for modeling particle deposition over rough surfaces.

• Stratospheric lifetime ratio of CFC-11 and CFC-12 from satellite and model climatologies

  Chlorofluorocarbons (CFCs) play a key role in stratospheric ozone loss and are strong infrared absorbers that contribute to global warming. The stratospheric lifetimes of CFCs are a measure of their stratospheric loss rates that are needed to determine global warming and ozone depletion potentials. We applied the tracer–tracer correlation approach to zonal mean climatologies from satellite measurements and model data to assess the lifetimes of CFCl₃ (CFC-11) and CF₂Cl₂ (CFC-12). We present estimates of the CFC-11/CFC-12 lifetime ratio and the absolute lifetime of CFC-12, based on a reference lifetime of 52 years for CFC-11. We analyzed climatologies from three satellite missions, the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS), the HIgh Resolution Dynamics Limb Sounder (HIRDLS), and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). We found a CFC-11/CFC-12 lifetime ratio of 0.47±0.08 and a CFC-12 lifetime of 112(96–133) years for ACE-FTS, a ratio of 0.46±0.07 and a lifetime of 113(97–134) years for HIRDLS, and a ratio of 0.46±0.08 and a lifetime of 114(98–136) years for MIPAS. The error-weighted, combined CFC-11/CFC-12 lifetime ratio is 0.46±0.04 and the CFC-12 lifetime estimate is 113(103–124) years. These results agree with the recent Stratosphere-troposphere Processes And their Role in Climate (SPARC) reassessment, which recommends lifetimes of 52(43–67) years and 102(88–122) years, respectively. Having smaller uncertainties than the results from other recent studies, our estimates can help to better constrain CFC-11 and CFC-12 lifetime recommendations in future scientific studies and assessments. Furthermore, the satellite observations were used to validate first simulation results from a new coupled model system, which integrates a Lagrangian chemistry transport model into a climate model. For the coupled model we found a CFC-11/CFC-12 lifetime ratio of 0.48±0.07 and a CFC-12 lifetime of 110(95–129) years, based on a 10-year perpetual run. Closely reproducing the satellite observations, the new model system will likely become a useful tool to assess the impact of advective transport, mixing, and photochemistry as well as climatological variability on the stratospheric lifetimes of long-lived tracers.

• Low temperatures enhance organic nitrate formation: evidence from observations in the 2012 Uintah Basin Winter Ozone Study

  Nitrogen dioxide (NO₂) and total alkyl nitrates (ΣANs) were measured using thermal dissociation laser-induced fluorescence during the 2012 Uintah Basin Winter Ozone Study (UBWOS) in Utah, USA. The observed NO₂ concentration was highest before sunrise and lowest in the late afternoon, suggestive of a persistent local source of NO₂ coupled with turbulent mixing out of the boundary layer. In contrast, ΣANs co-varied with solar radiation with a noontime maximum, indicating that local photochemical production combined with rapid mixing and/or deposition was the dominant factor in determining the ΣAN concentrations. We calculate that ΣANs were a large fraction (~60%) of the HO_x free radical chain termination and show that the temperature dependence of the alkyl nitrate yields enhances the role of ΣANs in local chemistry during winter by comparison to what would occur in the warmer temperatures of summer.

• Modeling of gaseous methylamines in the global atmosphere: impacts of oxidation and aerosol uptake

  Gaseous amines have attracted increasing attention due to their potential role in enhancing particle nucleation and growth and affecting secondary organic aerosol formation. Here we study with a chemistry transport model the global distributions of the most common and abundant amines in the air: monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA). We show that gas phase oxidation and aerosol uptakes are dominant sinks for these methylamines. The oxidation alone (i.e., no aerosol uptake) leads to methylamine lifetimes of 5–10 h in most parts of low and middle latitude regions. The uptake by secondary species with uptake coefficient (γ) of 0.03 (corresponding to the uptake by sulfuric acid particles) reduces the lifetime by ~30% over oceans and much more over the major continents, resulting in a methylamine lifetime of less than 1–2 h over central Europe, eastern Asia, and eastern US. With the estimated global emission flux, from the literature, our simulations indicate that [DMA] in the model surface layer over major continents is generally in the range of 0.1–2 ppt (parts per trillion) when γ = 0.03 and 0.2–10 ppt when γ = 0, and decreases quickly with altitude. [DMA] over oceans is below 0.05 ppt and over polar regions it is below 0.01 ppt. The simulated [MMA] is about a factor of ~2.5 higher while [TMA] is a factor of ~8 higher than [DMA]. The modeled concentrations of methylamines are substantially lower than the limited observed values available, with normalized mean bias ranging from −57 (γ = 0) to −88% (γ = 0.03) for MMA and TMA, and from −78 (γ = 0) to −93% (γ = 0.03) for DMA.

• Modelled black carbon radiative forcing and atmospheric lifetime in AeroCom Phase II constrained by aircraft observations

  Atmospheric black carbon (BC) absorbs solar radiation, and exacerbates global warming through exerting positive radiative forcing (RF). However, the contribution of BC to ongoing changes in global climate is under debate. Anthropogenic BC emissions, and the resulting distribution of BC concentration, are highly uncertain. In particular, long-range transport and processes affecting BC atmospheric lifetime are poorly understood. Here we discuss whether recent assessments may have overestimated present-day BC radiative forcing in remote regions. We compare vertical profiles of BC concentration from four recent aircraft measurement campaigns to simulations by 13 aerosol models participating in the AeroCom Phase II intercomparison. An atmospheric lifetime of BC of less than 5 days is shown to be essential for reproducing observations in remote ocean regions, in line with other recent studies. Adjusting model results to measurements in remote regions, and at high altitudes, leads to a 25% reduction in AeroCom Phase II median direct BC forcing, from fossil fuel and biofuel burning, over the industrial era. The sensitivity of modelled forcing to BC vertical profile and lifetime highlights an urgent need for further flight campaigns, close to sources and in remote regions, to provide improved quantification of BC effects for use in climate policy.