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Simulations are conducted with a cloud-resolving numerical model to examine the transformation of a weak incipient mid-level cyclonic vortex into a tropical cyclone. Results demonstrate that two distinct pathways are possible and that development along a particular pathway is sensitive to model physics and initial conditions. One pathway involves a steady increase of the surface winds to tropical cyclone strength as the radius of maximum winds gradually decreases. A notable feature of this evolution is the creation of small-scale lower tropospheric cyclonic vorticity anomalies by deep convective towers and subsequent merger and convergence by the low-level secondary circulation. The second pathway also begins with a strengthening low-level circulation, but eventually a significantly stronger mid-level circulation develops. Cyclogenesis occurs subsequently when a small-scale surface concentrated vortex forms abruptly near the center of the larger-scale circulation. The small-scale vortex is warm core throughout the troposphere and results in a fall in local surface pressure of a few millibars. It usually develops rapidly, undergoing a modest growth to form a small tropical cyclone. Many of the simulated systems approach or reach tropical cyclone strength prior to development of a prominent mid-level vortex so that the subsequent formation of a strong small-scale surface concentrated vortex in these cases could be considered intensification rather than genesis. <br><br> Experiments are performed to investigate the dependence on the inclusion of the ice phase, radiation, the size and strength of the incipient mid-level vortex, the amount of moisture present in the initial vortex, and the sea surface temperature. Notably, as the sea surface temperature is raised, the likelihood of development along the second pathway is increased. This appears to be related to an increased production of ice. The sensitivity of the pathway taken to model physics and initial conditions revealed by these experiments raise the possibility that the solution to this initial value problem is near a bifurcation point. Future improvements to model parameterizations and more accurate observations of the transformation of disturbances to tropical cyclones should clarify the conditions that favor a particular pathway when starting from a mid-level vortex.
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Aerosol samples (TSP, <i>n</i>=58) were collected on day- and night-time basis at Mangshan in the north of Beijing, China in autumn 2007 to better understand the status of air quality and the influence of urban pollutants in the northern vicinity of Beijing. The samples were analyzed for aerosol mass, total carbon (TC), low molecular weight α, ω-dicarboxylic acids (C<sub>2</sub>-C<sub>12</sub>), ketoacids (ωC<sub>2</sub>-ωC<sub>9</sub>, pyruvic acid), α-dicarbonyls (glyoxal and methylglyoxal), as well as aromatic (phthalic, iso- and tere-phthalic) diacids. Aerosol mass and TC concentrations are higher in daytime than in nighttime. TC/aerosol mass ratios in this study are lower than those reported in megacities in East Asia, but higher than those reported in marine aerosols. Molecular distributions of diacids demonstrated that oxalic (C<sub>2</sub>) acid was the most abundant species, comprising 38–77% of total diacids, followed by succinic (C<sub>4</sub>) and malonic (C<sub>3</sub>) acids. For most compounds, the concentrations were higher in daytime than nighttime, indicating that diacids are produced in daytime by photochemical oxidation of organic precursors emitted from anthropogenic sources in Beijing during the transport to Mangshan area by the northward wind. However, we found that C<sub>2</sub> concentrations are higher in nighttime than in daytime. A positive correlation of C<sub>2</sub> to glyoxylic acid (ωC<sub>2</sub>) was obtained at night when relative humidity increased up to 100%, suggesting that aqueous phase production of C<sub>2</sub> occurs in nighttime via the oxidation of ωC<sub>2</sub>. Depletion of C<sub>2</sub> by photolysis of Fe-oxalato complexes might be another reason for the lower concentrations of C<sub>2</sub> in daytime samples. High phthalic acid/C<sub>4</sub> ratios in the aerosol samples suggest that automobile combustion and coal burning products are important sources, which are subjected to photochemical oxidation during the atmospheric transport of urban aerosols from Beijing. In contrast, higher concentrations of methylglyoxal in nighttime than daytime may suggest that isoprene emitted from the northern forest area is oxidized in daytime and then transported to the sampling area at night by northerly winds. This study demonstrates that secondary organic aerosols are significantly produced and aged in the vicinity of Beijing during atmospheric transport.
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A differential absorption lidar (DIAL) for measurement of atmospheric ozone concentration was operated aboard the Polar 5 research aircraft in order to study the depletion of ozone over Arctic sea ice. The lidar measurements during a flight over the sea ice north of Barrow, Alaska, on 3 April 2011 found a surface boundary layer depletion of ozone over a range of 300 km. The photochemical destruction of surface level ozone was strongest at the most northern point of the flight, and steadily decreased towards land. All the observed ozone-depleted air throughout the flight occurred within 300 m of the sea ice surface. A back-trajectory analysis of the air measured throughout the flight indicated that the ozone-depleted air originated from over the ice. Air at the surface that was not depleted in ozone had originated from over land. An investigation into the altitude history of the ozone-depleted air suggests a strong inverse correlation between measured ozone concentration and the amount of time the air directly interacted with the sea ice.
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Regional and local sources contributing to gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM) at an urban coastal site in Dartmouth, Nova Scotia, Canada were investigated using the Concentration-Weighted Trajectory model (CWT) and Conditional Probability Function. From 2010–2011, GEM, GOM, and PBM concentrations were 1.67 ± 1.01 ng m<sup>−3</sup>, 2.07 ± 3.35 pg m<sup>−3</sup>, and 2.32 ± 3.09 pg m<sup>−3</sup>, respectively. Seasonal variability was observed, with statistically higher GEM and PBM concentrations in winter and spring and higher GOM in spring. In the CWT, concentrations are the weighting factors for the trajectory residence time in modeled grid cells, which results in the identification of source areas based on the CWT values in the grid cells. Potential source areas were identified in regions with known industrial Hg sources particularly in the fall season, but also in regions without these sources (e.g. Atlantic Ocean, northern Ontario and Quebec). CWTs for GOM and PBM that were associated with ≥ 5 kg industrial Hg emissions from 2010–2011 were statistically larger than those with zero Hg emissions, despite a lack of strong correlations. A large proportion of elevated CWTs (85–97%) was in regions with zero industrial Hg sources indicating the potential role of non-point sources, natural emissions, and residential-scale combustion. Analysis of wind data suggests that a commercial harbor and vehicular traffic were potential local sources. Evaluating modeled source areas against Hg emissions inventories was not an ideal method for assessing the CWT model accuracy because of insufficient data on Hg emissions at more precise locations.
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Although the links between stratospheric dynamics, climate and weather have been demonstrated, direct observations of stratospheric winds are lacking, in particular at altitudes above 30 km. We report observations of winds between 8 and 0.01 hPa (~35–80 km) from October 2009 to April 2010 by the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the International Space Station. The altitude range covers the region between 35–60 km where previous space-borne wind instruments show a lack of sensitivity. Both zonal and meridional wind components were obtained, though not simultaneously, in the latitude range from 30° S to 55° N and with a single profile precision of 7–9 m s<sup>–1</sup> between 8 and 0.6 hPa and better than 20 m s<sup>–1</sup> at altitudes above. The vertical resolution is 5–7 km except in the upper part of the retrieval range (10 km at 0.01 hPa). In the region between 1–0.05 hPa, an absolute value of the mean difference < 2 m s<sup>–1</sup> is found between SMILES profiles retrieved from different spectroscopic lines and instrumental settings. Good agreement (absolute value of the mean difference of ~2 m s<sup>–1</sup>) is also found with the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis in most of the stratosphere except for the zonal winds over the equator (difference > 5 m s<sup>−1</sup>). In the mesosphere, SMILES and ECMWF zonal winds exhibit large differences (> 20 m s<sup>–1</sup>), especially in the tropics. We illustrate our results by showing daily and monthly zonal wind variations, namely the semi-annual oscillation in the tropics and reversals of the flow direction between 50–55° N during sudden stratospheric warmings. The daily comparison with ECMWF winds reveals that in the beginning of February, a significantly stronger zonal westward flow is measured in the tropics at 2 hPa compared to the flow computed in the analysis (difference of ~20 m s<sup>–1</sup>). The results show that the comparison between SMILES and ECMWF winds is not only relevant for the quality assessment of the new SMILES winds, but it also provides insights on the quality of the ECMWF winds themselves. Although the instrument was not specifically designed for measuring winds, the results demonstrate that space-borne sub-mm wave radiometers have the potential to provide good quality data for improving the stratospheric winds in atmospheric models.
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Infrared Atmospheric Sounder Interferometer (IASI) observations covering the period from July 2007 to December 2011 are interpreted in terms of monthly mean, 1°×1°, 10 μm dust Aerosol Optical Depth (AOD), mean altitude and coarse mode effective radius. The geographical study area includes the northern tropical Atlantic and the northwest Arabian Sea, both characterised by strong, regular dust events. The method developed relies on the construction of Look-Up-Tables computed for a large selection of atmospheric situations and observing conditions. At a regional scale, a good agreement is found between IASI-retrieved 10 μm AOD and total visible optical depth at 550 nm from either the Moderate resolution Imaging Spectroradiometer (MODIS/Aqua or Terra), or the Multi-angle Imaging SpectroRadiometer (MISR), or the Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar (PARASOL). Taking into account the ratio existing between infrared and visible AODs, the diversity between the different 550 nm AODs is similar to the difference between these and the IASI AODs. The infrared AOD to visible AOD ratio, partly reflecting the varying distribution of the dust layer between the dust coarse mode particles seen by IASI, and the fine mode seen by the other instruments, is found to vary with the region observed with values close to already published values. Comparisons between the climatologies of the 10 μm IASI AOD and of the PARASOL non-spherical coarse mode AOD at 865 nm, both expected to be representative of the dust coarse mode, lead to conclusions differing according to the region considered. These differences are discussed in the light of the MODIS Angström exponent (865–550 nm). At local scale, around six Aerosol Robotic Network (AERONET) sites, close or far from the dust sources, a similar satisfactory agreement is found between IASI and the visible AODs and the differences between these products are shown and analysed. IASI-retrieved dust layer mean altitudes also compare well with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP/CALIPSO) aerosol mean layer altitude, both in terms of climatology and of zonal evolution throughout the Atlantic. Comparisons between the IASI-retrieved dust coarse mode effective radius and retrievals from AERONET at the six sites brings into evidence an almost systematic bias of about +0.35 μm (IASI-AERONET). Removing this bias leads to a satisfactory agreement between the climatologies of these two products. Overall, these results illustrate the dust westward transport characterised by a fast decrease of the dust optical depth, a somewhat slower decrease of the altitude, and an effective radius remaining almost constant during summer throughout the northern tropical Atlantic. They also demonstrate the capability of high resolution infrared sounders to contribute improving our understanding of processes related to the aerosols (transport, sources, cycles, effect of aerosols on the terrestrial radiation, etc.).
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There is growing international interest in mitigating climate change during the early part of this century by reducing emissions of short-lived climate pollutants (SLCPs), in addition to reducing emissions of CO<sub>2</sub>. The SLCPs include methane (CH<sub>4</sub>), black carbon aerosols (BC), tropospheric ozone (O<sub>3</sub>) and hydrofluorocarbons (HFCs). Recent studies have estimated that by mitigating emissions of CH<sub>4</sub>, BC, and O<sub>3</sub> using available technologies, about 0.5 to 0.6 °C warming can be avoided by mid-21st century. Here we show that avoiding production and use of high-GWP (global warming potential) HFCs by using technologically feasible low-GWP substitutes to meet the increasing global demand can avoid as much as another 0.5 °C warming by the end of the century. This combined mitigation of SLCPs would cut the cumulative warming since 2005 by 50% at 2050 and by 60% at 2100 from the CO<sub>2</sub>-only mitigation scenarios, significantly reducing the rate of warming and lowering the probability of exceeding the 2 °C warming threshold during this century.
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Seasonal snow samples obtained at 46 sites in 6 provinces of China in January and February 2010 were analyzed for a suite of chemical species and these data are combined with previously determined concentrations of insoluble light-absorbing particles (ILAP), including all particles that absorb light in the 650–700 nm wavelength interval. The ILAP, together with 14 other analytes, are used as input to a positive matrix factorization (PMF) receptor model to explore the sources of ILAP in the snow. The PMF analysis for ILAP sources is augmented with backward trajectory cluster analysis and the geographic locations of major source areas for the three source types. The two analyses are consistent and indicate that three factors/sources were responsible for the measured light absorption of snow: a soil dust source, an industrial pollution source, and a biomass and / or biofuel burning source. Soil dust was the main source of the ILAP, accounting for ~53% of ILAP on average.
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Gasoline vehicles have recently been pointed out as potentially the main source of anthropogenic secondary organic aerosol (SOA) in megacities. However, there is a lack of laboratory studies to systematically investigate SOA formation in real-world exhaust. In this study, SOA formation from pure aromatic precursors, idling and cold start gasoline exhaust from three passenger vehicles (EURO2–EURO4) were investigated with photo-oxidation experiments in a 6 m<sup>3</sup> smog chamber. The experiments were carried out down to atmospherically relevant organic aerosol mass concentrations. The characterization instruments included a high-resolution aerosol mass spectrometer and a proton transfer mass spectrometer. It was found that gasoline exhaust readily forms SOA with a signature aerosol mass spectrum similar to the oxidized organic aerosol that commonly dominates the organic aerosol mass spectra downwind of urban areas. After a cumulative OH exposure of ~5 × 10<sup>6</sup> cm<sup>−3</sup> h, the formed SOA was 1–2 orders of magnitude higher than the primary OA emissions. The SOA mass spectrum from a relevant mixture of traditional light aromatic precursors gave <i>f</i><sub>43</sub> (<i>mass fraction at m/z = 43</i>), approximately two times higher than to the gasoline SOA. However O : C and H : C ratios were similar for the two cases. Classical C<sub>6</sub>–C<sub>9</sub> light aromatic precursors were responsible for up to 60% of the formed SOA, which is significantly higher than for diesel exhaust. Important candidates for additional precursors are higher-order aromatic compounds such as C<sub>10</sub> and C<sub>11</sub> light aromatics, naphthalene and methyl-naphthalenes. We conclude that approaches using only light aromatic precursors give an incomplete picture of the magnitude of SOA formation and the SOA composition from gasoline exhaust.
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Precise quantification of the cloud condensation nuclei (CCN) number concentration is crucial for understanding aerosol indirect effects and characterizing these effects in models. An evaluation of various methods for CCN parameterization was carried out in this paper based on in situ measurements of aerosol activation properties within HaChi (Haze in China) project. Comparisons were made by closure studies between methods using CCN spectra, bulk activation ratios, cut-off diameters and size-resolved activation ratios. The estimation of CCN number concentrations by the method using aerosol size-resolved activation ratios, either averaged over a day or with diurnal variation, was found to be most satisfying and straightforward. This could be well expected since size-resolved activation ratios include information regarding the effects of size-resolved chemical compositions and mixing states on aerosol activation properties. The method using the averages of critical diameters, which were inferred from measured CCN number concentrations and particle number size distributions, also provided a good prediction of the CCN number concentration. Based on comparisons of all these methods in this paper, it was recommended that the CCN number concentration be predicted using particle number size distributions with inferred critical diameters or size-resolved activation ratios.
