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The chemical and physical speciation of atmospheric sulfur was investigated in ambient aerosol samples using a combination of sulfur near-edge x-ray fluorescence spectroscopy (S-NEXFS) and X-ray fluorescence (XRF) microscopy. These techniques were used to determine the composition and oxidation state of sulfur in common primary emission sources and ambient particulate matter collected from the greater Atlanta area. Ambient particulate matter samples contained two oxidation states: S<sup>0</sup> and S<sup>+VI</sup>. Ninety-five percent of the individual aerosol particles (> 1 µm) analyzed contain S<sup>0</sup>. Linear combination fitting revealed that S<sup>+VI</sup> in ambient aerosol was dominated by ammonium sulfate as well as metal sulfates. The finding of metal sulfates provides further evidence for acidic reactions that solubilize metals, such as iron, during atmospheric transport. Emission sources, including biomass burning, coal fly ash, gasoline, diesel, volcanic ash, and aerosolized Atlanta soil, and the commercially available bacterium <i>Bacillus subtilis</i>, contained only S<sup>+VI</sup>. A commercially available <i>Azotobacter vinelandii</i> sample contained approximately equal proportions of S<sup>0</sup> and S<sup>+VI</sup>. S<sup>0</sup> in individual aerosol particles most likely originates from primary emission sources, such as aerosolized bacteria or incomplete combustion.
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The increasing trend of aerosol optical depth in the Middle East and a recent severe dust storm in Syria have raised questions as to whether dust storms will increase and promoted investigations on the dust activities driven by the natural climate variability underlying the ongoing human perturbations such as the Syrian civil war. This study examined the influences of the Pacific Decadal Oscillation (PDO) on dust activities in Syria using an innovative dust optical depth (DOD) dataset derived from Moderate Resolution Imaging Spectroradiometer (MODIS) Deep Blue aerosol products. A significantly negative correlation is found between the Syrian DOD and the PDO in spring from 2003 to 2015. High DOD in spring is associated with lower geopotential height over the Middle East, Europe, and North Africa, accompanied by near-surface anomalous westerly winds over the Mediterranean basin and southerly winds over the eastern Arabian Peninsula. These large-scale patterns promote the formation of the cyclones over the Middle East to trigger dust storms and also facilitate the transport of dust from North Africa, Iraq, and Saudi Arabia to Syria, where the transported dust dominates the seasonal mean DOD in spring. A negative PDO not only creates circulation anomalies favorable to high DOD in Syria but also suppresses precipitation in dust source regions over the eastern and southern Arabian Peninsula and northeastern Africa.<br><br>On the daily scale, in addition to the favorable large-scale condition associated with a negative PDO, enhanced atmospheric instability in Syria (associated with increased precipitation in Turkey and northern Syria) is also critical for the development of strong springtime dust storms in Syria.
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Formaldehyde (HCHO) column data from satellites are widely used as a proxy for emissions of volatile organic compounds (VOCs), but validation of the data has been extremely limited. Here we use highly accurate HCHO aircraft observations from the NASA SEAC<sup>4</sup>RS (Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys) campaign over the southeast US in August–September 2013 to validate and intercompare six retrievals of HCHO columns from four different satellite instruments (OMI, GOME2A, GOME2B and OMPS; for clarification of these and other abbreviations used in the paper, please refer to Appendix A) and three different research groups. The GEOS-Chem chemical transport model is used as a common intercomparison platform. All retrievals feature a HCHO maximum over Arkansas and Louisiana, consistent with the aircraft observations and reflecting high emissions of biogenic isoprene. The retrievals are also interconsistent in their spatial variability over the southeast US (<i>r</i> = 0.4–0.8 on a 0.5° × 0.5° grid) and in their day-to-day variability (<i>r</i> = 0.5–0.8). However, all retrievals are biased low in the mean by 20–51 %, which would lead to corresponding bias in estimates of isoprene emissions from the satellite data. The smallest bias is for OMI-BIRA, which has high corrected slant columns relative to the other retrievals and low scattering weights in its air mass factor (AMF) calculation. OMI-BIRA has systematic error in its assumed vertical HCHO shape profiles for the AMF calculation, and correcting this would eliminate its bias relative to the SEAC<sup>4</sup>RS data. Our results support the use of satellite HCHO data as a quantitative proxy for isoprene emission after correction of the low mean bias. There is no evident pattern in the bias, suggesting that a uniform correction factor may be applied to the data until better understanding is achieved.
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Biomass burning (BB) aerosols have a significant effect on regional climate, and represent a significant uncertainty in our understanding of climate change. Using a combination of cavity ring-down spectroscopy and integrating nephelometry, the single scattering albedo (SSA) and Ångstrom absorption exponent (AAE) were measured for several North American biomass fuels. This was done for several particle diameters for the smoldering and flaming stage of white pine, red oak, and cedar combustion. Measurements were done over a wider wavelength range than any previous direct measurement of BB particles. While the offline sampling system used in this work shows promise, some changes in particle size distribution were observed, and a thorough evaluation of this method is required. The uncertainty of SSA was 6 %, with the truncation angle correction of the nephelometer being the largest contributor to error. While scattering and extinction did show wavelength dependence, SSA did not. SSA values ranged from 0.46 to 0.74, and were not uniformly greater for the smoldering stage than the flaming stage. SSA values changed with particle size, and not systematically so, suggesting the proportion of tar balls to fractal black carbon change with fuel type/state and particle size. SSA differences of 0.15–0.4 or greater can be attributed to fuel type or fuel state for fresh soot. AAE values were quite high (1.59–5.57), despite SSA being lower than is typically observed in wildfires. The SSA and AAE values in this work do not fit well with current schemes that relate these factors to the modified combustion efficiency of a burn. Combustion stage, particle size, fuel type, and fuel condition were found to have the most significant effects on the intrinsic optical properties of fresh soot, though additional factors influence aged soot.
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Peroxyacetyl nitrate (PAN) is an important chemical species in the troposphere as it aids the long-range transport of NO<sub><i>x</i></sub> and subsequent formation of O<sub>3</sub> in relatively clean remote regions. Over the past few decades observations from aircraft campaigns and surface sites have been used to better understand the regional distribution of PAN. However, recent measurements made by satellites allow for a global assessment of PAN in the upper troposphere–lower stratosphere (UTLS). In this study, we investigate global PAN distributions from two independent retrieval methodologies, based on measurements from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument, on board Envisat from the Institute of Meteorology and Climate Research (IMK), Karlsruhe Institute of Technology, and the Department of Physics and Astronomy, University of Leicester (UoL). Retrieving PAN from MIPAS is challenging due to the weak signal in the measurements and contamination from other species. Therefore, we compare the two MIPAS datasets with observations from the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS), in situ aircraft data and the 3-D chemical transport model TOMCAT. MIPAS shows peak UTLS PAN concentrations over the biomass burning regions (e.g. ranging from 150 to > 200 pptv at 150 hPa) and during the summertime Asian monsoon as enhanced convection aids the vertical transport of PAN from the lower atmosphere. At 150 hPa, we find significant differences between the two MIPAS datasets in the tropics, where IMK PAN concentrations are larger by 50–100 pptv. Comparisons between MIPAS and ACE-FTS show better agreement with the UoL MIPAS PAN concentrations at 200 hPa, but with mixed results above this altitude. TOMCAT generally captures the magnitude and structure of climatological aircraft PAN profiles within the observational variability allowing it to be used to investigate the MIPAS PAN differences. TOMCAT–MIPAS comparisons show that the model is both positively (UoL) and negatively (IMK) biased against the satellite products. These results indicate that satellite PAN observations are able to detect realistic spatial variations in PAN in the UTLS, but further work is needed to resolve differences in existing retrievals to allow quantitative use of the products.
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A short introduction is given to some wave-heating mechanisms applicable to the solar corona. Their relevance to the capabilities of the SoHO mission is briefly discussed.
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The observational systems utilised during the TRACT experimental campaign (September 1992) can roughly be divided into: existing climate stations and towers: background chemical monitoring stations; aerological measurements; instrumented aircraft; and special micrometeorological stations operated only during the experimental period. Database 4, which contains the latter surface measurements, includes station descriptions and data from 21 sites operated by nine different groups during the campaign period. These data will be described briefly. There are a number of advantages in using a relational database for analysis of this data. Without creating an unnecessarily large database which is awkward to view and slow to process, specific measurements can be selected easily since data from each site have been standardized using a common time index. This allows comparison of the same or different variables from one or more sites for investigating data quality or the effects of site location. Data can readily be combined into hourly or daily averages or time series for comparison with other data sets (e.g. model or aircraft data). Spatial analysis is also straightforward, although the clustering of the sites in the centre of the investigation area means that care must be taken. Some preliminary comparisons are made here in order to demonstrate the availability and reliability of the data.
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In this short paper the exclusion circles and vertical phase locities for gravity waves launched from the ground into a time-varying wind are studied using a ray-tracing technique. It is shown that waves with initial observed phase speeds that should place them within the local temporally varying exclusion circle, are often Doppler shifted outside of the circle. This, and the finite lifetime of some critical levels, allow waves to survive the critical layer and reach higher altitudes. Also, for slower phase-speed waves, the temporally varying wind can shift the observed frequency to negative values, so that the observed phase motions will be opposite (i.e. horizontally reversed and vertically upward), even though the energy still propagates upward. This effect can also cause the phase velocity to move inside the local exclusion circle. Due to the directional filtering of wave sources by the stratospheric wind, the percentage of such reverse-propagating waves will change systematically with local time and height in our simplified but realistic model. These results are related to ground-based systems, optical and radar, which sample the wind field and gravity waves in the middle atmosphere.
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The scattering behaviour of bullet-rosette and bullet-shaped ice particles is investigated using model calculations (ray tracing method) with special emphasis on the asymmetry factor <i>g</i>. Because the variability of the geometrical features of these particles is very large, some representative shapes are used in the calculations. The model is based on geometrical optics, and particles are assumed to be oriented randomly; a wavelength of 0.56 μm is considered; absorption is neglected. The scattering behaviour of bullet rosettes is compared to that of single branches out of the bullet rosette. It turns out that there are slight differences in the asymmetry factor values, depending on the lengths of the branches (∆<i>g</i>~0.02) and on the angles between the branches (∆<i>g</i>~0.01). Bullets show some special features in their phase functions due to the pyramid. The length of the particle influences the asymmetry factor (∆<i>g</i>~0.10), as does the shape of the pyramid (∆<i>g</i>~0.07). The influence of the pyramidal shape decreases with increasing particle length. Bullets were compared to hexagonally shaped columns. This was done for two columns, one as long as the columnar part of the bullet (length without pyramid), and one for a column as long as the bullet including the pyramid. Asymmetry factor values of bullets with a pyramidal angle of 28° deviate less than ∆<i>g</i>~0.01 from the range given by the two values of the columns.
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In order to investigate the causal relationship between magnetic storms and substorms, variations of the mid-latitude geomagnetic indices, ASY (asymmetric part) and SYM (symmetric part), at substorm onsets are examined. Substorm onsets are defined by three different phenomena; (1) a rapid increase in the mid-latitude asymmetric-disturbance indices, ASY-D and ASY-H, with a shape of so-called `mid-latitude positive bay'; (2) a sharp decrease in the AL index; (3) an onset of Pi2 geomagnetic pulsation. The positive bays are selected using eye inspection and a pattern-matching technique. The 1-min-resolution SYM-H index, which is essentially the same as the hourly Dst index except in terms of the time resolution, does not show any statistically significant development after the onset of substorms; it tends to decay after the onset rather than to develop. It is suggested by a simple model calculation that the decay of the magnetospheric tail current after substorm onset is responsible for the decay of the Dst field. The relation between the IMF southward turning and the development of the Dst field is re-examined. The results support the idea that the geomagnetic storms and substorms are independent processes; that is, the ring-current development is not the result of the frequent occurrence of substorms, but that of enhanced convection caused by the large southward IMF. A substorm is the process of energy dissipation in the magnetosphere, and its contribution to the storm-time ring-current formation seems to be negligible. The decay of the Dst field after a substorm onset is explained by a magnetospheric energy theorem.
