Items

Advanced search

On the representation of immersion and condensation freezing in cloud models using different nucleation schemes

Ice nucleation in clouds is often observed at temperatures >235 K, pointing to heterogeneous freezing as a predominant mechanism. Many models deterministically predict the number concentration of ice particles as a function of temperature and/or supersaturation. Several laboratory experiments, at constant temperature and/or supersaturation, report heterogeneous freezing as a stochastic, time-dependent process that follows classical nucleation theory; this might appear to contradict deterministic models that predict singular freezing behavior. We explore the extent to which the choice of nucleation scheme (deterministic/stochastic, single/multiple contact angles θ) affects the prediction of the fraction of frozen ice nuclei (IN) and cloud evolution for a predetermined maximum IN concentration. A box model with constant temperature and supersaturation is used to mimic published laboratory experiments of immersion freezing of monodisperse (800 nm) kaolinite particles (~243 K), and the fitness of different nucleation schemes. Sensitivity studies show that agreement of all five schemes is restricted to the narrow parameter range (time, temperature, IN diameter) in the original laboratory studies, and that model results diverge for a wider range of conditions. The schemes are implemented in an adiabatic parcel model that includes feedbacks of the formation and growth of drops and ice particles on supersaturation during ascent. Model results for the monodisperse IN population (800 nm) show that these feedbacks limit ice nucleation events, often leading to smaller differences in number concentration of ice particles and ice water content (IWC) between stochastic and deterministic approaches than expected from the box model studies. However, because the different parameterizations of θ distributions and time-dependencies are highly sensitive to IN size, simulations using polydisperse IN result in great differences in predicted ice number concentrations and IWC between the different schemes. The differences in IWC are mostly due to the different temperatures of the onset of freezing in the nucleation schemes that affect the temporal evolution of the ice number concentration. The growth rates of ice particles are not affected by the choice of the nucleation scheme, which leads to very similar particle sizes. Finally, since the choice of nucleation scheme determines the temperature range over which ice nucleation occurs, at habit-prone temperatures (~253 K), there is the potential for variability in the initial inherent growth ratio of ice particles, which can cause amplification or reduction in differences in predicted IWC.
Ice nucleation efficiency of clay minerals in the immersion mode

Emulsion and bulk freezing experiments were performed to investigate immersion ice nucleation on clay minerals in pure water, using various kaolinites, montmorillonites, illites as well as natural dust from the Hoggar Mountains in the Saharan region. Differential scanning calorimeter measurements were performed on three different kaolinites (KGa-1b, KGa-2 and K-SA), two illites (Illite NX and Illite SE) and four natural and acid-treated montmorillonites (SWy-2, STx-1b, KSF and K-10). The emulsion experiments provide information on the average freezing behaviour characterized by the average nucleation sites. These experiments revealed one to sometimes two distinct heterogeneous freezing peaks, which suggest the presence of a low number of qualitatively distinct average nucleation site classes. We refer to the peak at the lowest temperature as "standard peak" and to the one occurring in only some clay mineral types at higher temperatures as "special peak". Conversely, freezing in bulk samples is not initiated by the average nucleation sites, but by a very low number of "best sites". The kaolinites and montmorillonites showed quite narrow standard peaks with onset temperatures 238 K<Tonstd<242 K and best sites with averaged median freezing temperature Tmedbest=257 K, but only some featuring a special peak (i.e. KSF, K-10, K-SA and SWy-2) with freezing onsets in the range 240–248 K. The illites showed broad standard peaks with freezing onsets at 244 K Tonstd<246 K and best sites with averaged median freezing temperature Tmedbest=262 K. The large difference between freezing temperatures of standard and best sites shows that characterizing ice nucleation efficiencies of dust particles on the basis of freezing onset temperatures from bulk experiments, as has been done in some atmospheric studies, is not appropriate. Our investigations demonstrate that immersion freezing temperatures of clay minerals strongly depend on the amount of clay mineral present per droplet and on the exact type (location of collection and pre-treatment) of the clay mineral. We suggest that apparently contradictory results obtained by different groups with different setups are indeed in good agreement when only clay minerals of the same type and amount per droplet are compared. The natural sample from the Hoggar Mountains, a region whose dusts have been shown to be composed mainly of illite, showed very similar freezing characteristics (standard and best) to the illites. Relating the concentration of best IN to the dust concentration in the atmosphere suggested that the best IN in the Hoggar sample would be common enough downwind of their source region to account for ambient IN number densities in the temperature range of 250–260 K at least during dust events.
Aqueous phase processing of secondary organic aerosol from isoprene photooxidation

Transport of reactive air masses into humid and wet areas is highly frequent in the atmosphere, making the study of aqueous phase processing of secondary organic aerosol (SOA) very relevant. We have investigated the aqueous phase processing of SOA generated from gas-phase photooxidation of isoprene using a smog chamber. The SOA collected on filters was extracted by water and subsequently oxidized in the aqueous phase either by H2O2 under dark conditions or by OH radicals in the presence of light, using a photochemical reactor. Online and offline analytical techniques including SMPS, HR-AMS, H-TDMA, TD-API-AMS, were employed for physical and chemical characterization of the chamber SOA and nebulized filter extracts. After aqueous phase processing, the particles were significantly more hygroscopic, and HR-AMS data showed higher signal intensity at m/z 44 and a lower signal intensity at m/z 43, thus showing the impact of aqueous phase processing on SOA aging, in good agreement with a few previous studies. Additional offline measurement techniques (IC-MS, APCI-MS2 and HPLC-APCI-MS) permitted the identification and quantification of sixteen individual chemical compounds before and after aqueous phase processing. Among these compounds, small organic acids (including formic, glyoxylic, glycolic, butyric, oxalic and 2,3-dihydroxymethacrylic acid (i.e. 2-methylglyceric acid)) were detected, and their concentrations significantly increased after aqueous phase processing. In particular, the aqueous phase formation of 2-methylglyceric acid and trihydroxy-3-methylbutanal was correlated with the consumption of 2,3-dihydroxy-2-methyl-propanal, and 2-methylbutane-1,2,3,4-tetrol, respectively, and an aqueous phase mechanism was proposed accordingly. Overall, the aging effect observed here was rather small compared to previous studies, and this limited effect could possibly be explained by the lower liquid phase OH concentrations employed here, and/or the development of oligomers observed during aqueous phase processing.
The lofting of Western Pacific regional aerosol by island thermodynamics as observed around Borneo

Vertical profiles of aerosol chemical composition, number concentration and size were measured throughout the lower troposphere of Borneo, a large tropical island in the western Pacific Ocean. Aerosol composition, size and number concentration measurements (using an Aerodyne Aerosol Mass Spectrometer, Passive Cavity Aerosol Spectrometer Probe and Condensation Particle Counter, respectively) were made both upwind and downwind of Borneo, as well as over the island itself, on board the UK BAe-146 research aircraft as part of the OP3 project. Two meteorological regimes were identified – one dominated by isolated terrestrial convection (ITC) which peaked in the afternoon, and the other characterised by more regionally active mesoscale convective systems (MCS). Upwind profiles show aerosol to be confined to a shallow marine boundary layer below 930 ± 10 hPa (~760 m above sea level, a.s.l.). As this air mass advects over the island with the mean free troposphere synoptic flow during the ITC-dominated regime, it is convectively lofted above the terrestrial surface mixed layer to heights of between 945 ± 22 (~630 m a.s.l.) and 740 ± 44 hPa (~2740 m a.s.l.), consistent with a coupling between the synoptic steering level flow and island sea breeze circulations. Terrestrial aerosol was observed to be lofted into this higher layer through both moist convective uplift and transport through turbulent diurnal sea-breeze cells. At the peak of convective activity in the mid-afternoons, organic aerosol loadings in the lofted layer were observed to be substantially higher than in the morning (by a mean factor of three). This organic matter is dominated by secondary aerosol from processing of biogenic gas phase precursors. Aerosol number concentration profiles suggest formation of new particles aloft in the atmosphere. By the time the air mass reaches the west coast of the island, terrestrial aerosol is enhanced in the lofted layer. Such uplift of aerosol in Borneo is expected to increase aerosol lifetimes in the lower free troposphere downwind, as they are above the boundary layer and therefore less likely to be lost by wet or dry deposition. It is also likely to change the role they play in the semi-direct and direct aerosol effects. The long chain of islands extending from Malaysia to Australia may all similarly be expected to present an orographic barrier to low level mean flow. This would lead to significant transport of aerosol into the tropical free troposphere across the Western Pacific region.
DOAS measurements of formaldehyde and glyoxal above a south-east Asian tropical rainforest

Tropical rainforests act as a huge contributor to the global emissions of biogenic volatile organic compounds (BVOCs). Measurements of their oxidation products, such as formaldehyde (HCHO) and glyoxal (CHOCHO), provide useful indicators of fast photochemistry occurring in the lower troposphere. However, measurements of these species in tropical forest locations are extremely limited. To redress this, HCHO and CHOCHO were measured using the long-path (LP) and multi-axis (MAX) differential optical absorption spectroscopy (DOAS) techniques above the rainforest canopy in Borneo during two campaigns in spring and summer 2008, as part of the Oxidant and Particle Photochemical Processes above a south-east Asian tropical rainforest (OP3) project. The results were compared with concurrent measurements of hydroxyl radical (OH), isoprene (C5H8) (which was the dominant organic species emitted in this forest environment), and various meteorological parameters. Formaldehyde was observed at a maximum concentration of 4.5 ppb and glyoxal at a maximum of 1.6 ppb, significantly higher than previous measurements in rural locations. A 1-D chemistry model was then used to assess the diurnal evolution of formaldehyde and glyoxal throughout the boundary layer. The results, which compare well with the LP-DOAS and MAX-DOAS observations, suggest that the majority of the glyoxal and formaldehyde is confined to the first 500 m of the boundary layer, and that the measured ratio of these species is reproduced using currently accepted product yields for the oxidation of isoprene by OH. An important conclusion is that the measured levels of glyoxal are consistent with the surprisingly high concentrations of OH measured in this environment.
Middle atmosphere response to different descriptions of the 11-yr solar cycle in spectral irradiance in a chemistry-climate model

The 11-yr solar cycle in solar spectral irradiance (SSI) inferred from measurements by the SOlar Radiation & Climate Experiment (SORCE) suggests a much larger variation in the ultraviolet than previously accepted. We present middle atmosphere ozone and temperature responses to the solar cycles in SORCE SSI and the ubiquitous Naval Research Laboratory (NRL) SSI reconstruction using the Goddard Earth Observing System chemistry-climate model (GEOSCCM). The results are largely consistent with other recent modeling studies. The modeled ozone response is positive throughout the stratosphere and lower mesosphere using the NRL SSI, while the SORCE SSI produces a response that is larger in the lower stratosphere but out of phase with respect to total solar irradiance above 45 km. The modeled responses in total ozone are similar to those derived from satellite and ground-based measurements, 3–6 Dobson Units per 100 units of 10.7-cm radio flux (F10.7) in the tropics. The peak zonal mean tropical temperature response using the SORCE SSI is nearly 2 K per 100 units F10.7 – 3 times larger than the simulation using the NRL SSI. The GEOSCCM and the Goddard Space Flight Center (GSFC) 2-D coupled model are used to examine how the SSI solar cycle affects the atmosphere through direct solar heating and photolysis processes individually. Middle atmosphere ozone is affected almost entirely through photolysis, whereas the solar cycle in temperature is caused both through direct heating and photolysis feedbacks, processes that are mostly linearly separable. This is important in that it means that chemistry-transport models should simulate the solar cycle in ozone well, while general circulation models without coupled chemistry will underestimate the temperature response to the solar cycle significantly in the middle atmosphere. Further, the net ozone response results from the balance of ozone production at wavelengths less than 242 nm and destruction at longer wavelengths, coincidentally corresponding to the wavelength regimes of the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) and Spectral Irradiance Monitor (SIM) on SORCE, respectively. A higher wavelength-resolution analysis of the spectral response could allow for a better prediction of the atmospheric response to arbitrary SSI variations.
Tropospheric methanol observations from space: retrieval evaluation and constraints on the seasonality of biogenic emissions

Methanol retrievals from nadir-viewing space-based sensors offer powerful new information for quantifying methanol emissions on a global scale. Here we apply an ensemble of aircraft observations over North America to evaluate new methanol measurements from the Tropospheric Emission Spectrometer (TES) on the Aura satellite, and combine the TES data with observations from the Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp-A satellite to investigate the seasonality of methanol emissions from northern midlatitude ecosystems. Using the GEOS-Chem chemical transport model as an intercomparison platform, we find that the TES retrieval performs well when the degrees of freedom for signal (DOFS) are above 0.5, in which case the model:TES regressions are generally consistent with the model:aircraft comparisons. Including retrievals with DOFS below 0.5 degrades the comparisons, as these are excessively influenced by the a priori. The comparisons suggest DOFS >0.5 as a minimum threshold for interpreting retrievals of trace gases with a weak tropospheric signal. We analyze one full year of satellite observations and find that GEOS-Chem, driven with MEGANv2.1 biogenic emissions, underestimates observed methanol concentrations throughout the midlatitudes in springtime, with the timing of the seasonal peak in model emissions 1–2 months too late. We attribute this discrepancy to an underestimate of emissions from new leaves in MEGAN, and apply the satellite data to better quantify the seasonal change in methanol emissions for midlatitude ecosystems. The derived parameters (relative emission factors of 11.0, 0.26, 0.12 and 3.0 for new, growing, mature, and old leaves, respectively, plus a leaf area index activity factor of 0.5 for expanding canopies with leaf area index <1.2) provide a more realistic simulation of seasonal methanol concentrations in midlatitudes on the basis of both the IASI and TES measurements.
On the quality of MIPAS kinetic temperature in the middle atmosphere

The kinetic temperature and line of sight elevation information are retrieved from the MIPAS Middle Atmosphere (MA), Upper Atmosphere (UA) and NoctiLucent-Cloud (NLC) modes of high spectral resolution limb observations of the CO2 15 μm emission using the dedicated IMK/IAA retrieval algorithm, which considers non-local thermodynamic equilibrium conditions. These variables are accurately derived from about 20 km (MA) and 40 km (UA and NLC) to 105 km globally and both at daytime and nighttime. Typical temperature random errors are smaller than 0.5 K below 50 km, 0.5–2 K at 50–70 km, and 2–7 K above. The systematic error is typically 1 K below 70 km, 1–3 K from 70 to 85 km and 3–11 K from 85 to 100 km. The average vertical resolution is typically 4 km below 35 km, 3 km at 35–50 km, 4–6 km at 50–90 km, and 6–10 km above. We compared our MIPAS temperature retrievals from 2005 to 2009 with co-located ground-based measurements from the lidars located at the Table Mountain Facility and Mauna Loa Observatory, the SATI spectrograph in Granada (Spain) and the Davis station spectrometer, and satellite observations from ACE-FTS, Aura-MLS and TIMED-SABER from 20 km to 100 km. We also compared MIPAS temperatures with the high latitudes climatology from falling sphere measurements. The comparisons show very good agreement, with differences smaller than 3 K below 85–90 km in mid-latitudes. Differences over the poles in this altitude range are larger but can be generally explained in terms of known biases of the other instruments. The comparisons above 90 km worsen and MIPAS retrieved temperatures are always larger than other instrument measurements.
Carbon monoxide and related trace gases and aerosols over the Amazon Basin during the wet and dry seasons

We present the results of airborne measurements of carbon monoxide (CO) and aerosol particle number concentration (CN) made during the Balanço Atmosférico Regional de Carbono na Amazônia (BARCA) program. The primary goal of BARCA is to address the question of basin-scale sources and sinks of CO2 and other atmospheric carbon species, a central issue of the Large-scale Biosphere-Atmosphere (LBA) program. The experiment consisted of two aircraft campaigns during November–December 2008 (BARCA-A) and May–June 2009 (BARCA-B), which covered the altitude range from the surface up to about 4500 m, and spanned most of the Amazon Basin. Based on meteorological analysis and measurements of the tracer, SF6, we found that airmasses over the Amazon Basin during the late dry season (BARCA-A, November 2008) originated predominantly from the Southern Hemisphere, while during the late wet season (BARCA-B, May 2009) low-level airmasses were dominated by northern-hemispheric inflow and mid-tropospheric airmasses were of mixed origin. In BARCA-A we found strong influence of biomass burning emissions on the composition of the atmosphere over much of the Amazon Basin, with CO enhancements up to 300 ppb and CN concentrations approaching 10 000 cm−3; the highest values were in the southern part of the Basin at altitudes of 1–3 km. The ΔCN/ΔCO ratios were diagnostic for biomass burning emissions, and were lower in aged than in fresh smoke. Fresh emissions indicated CO/CO2 and CN/CO emission ratios in good agreement with previous work, but our results also highlight the need to consider the residual smoldering combustion that takes place after the active flaming phase of deforestation fires. During the late wet season, in contrast, there was little evidence for a significant presence of biomass smoke. Low CN concentrations (300–500 cm−3) prevailed basinwide, and CO mixing ratios were enhanced by only ~10 ppb above the mixing line between Northern and Southern Hemisphere air. There was no detectable trend in CO with distance from the coast, but there was a small enhancement of CO in the boundary layer suggesting diffuse biogenic sources from photochemical degradation of biogenic volatile organic compounds or direct biological emission. Simulations of CO distributions during BARCA-A using a range of models yielded general agreement in spatial distribution and confirm the important contribution from biomass burning emissions, but the models evidence some systematic quantitative differences compared to observed CO concentrations. These mismatches appear to be related to problems with the accuracy of the global background fields, the role of vertical transport and biomass smoke injection height, the choice of model resolution, and reliability and temporal resolution of the emissions data base.
A new El Niño-Southern Oscillation forecasting tool based on Southern Oscillation Index

An exploration of the temporal evolution of the El Niño Southern Oscillation (ENSO) during January 1876–November 2011 by means of a new time domain called natural time reveals that the major ENSO events provide precursory signals that are maximized in a time window of almost two years. This finding improves the accuracy of the short-term prediction models of the ENSO extreme events, preventing thus from its disastrous impacts in advance.