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The size distribution of mineral dust aerosols partially determines their interactions with clouds, radiation, ecosystems, and other components of the Earth system. Several theoretical models predict that the dust size distribution depends on the wind speed at emission, with larger wind speeds predicted to produce smaller aerosols. The present study investigates this prediction using a compilation of published measurements of the size-resolved vertical dust flux emitted by eroding soils. Surprisingly, these measurements indicate that the size distribution of naturally emitted dust aerosols is independent of the wind speed. The recently formulated brittle fragmentation theory of dust emission is consistent with this finding, whereas other theoretical models are not. The independence of the emitted dust size distribution with wind speed simplifies both the interpretation of geological records of dust deposition and the parameterization of dust emission in atmospheric circulation models.
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This paper presents the results of laboratory studies on the condensational uptake of gaseous organic compounds in the exhaust of a light-duty gasoline engine onto preexisting sulfate and nitrate seed particles. Significant condensation of the gaseous organic compounds in the exhaust occurs onto these inorganic particles on a time scale of 2–5 min. The amount of condensed organic mass (COM) is proportional to the seed particle mass, suggesting that the uptake is due to dissolution determined by the equilibrium partitioning between gas phase and particles, not adsorption. The amount of dissolution in unit seed mass, <i>S</i>, decreases as a power function with increased dilution of the exhaust, ranging from 0.23 g g<sup>−1</sup> at a dilution ratio of 81, to 0.025 g g<sup>−1</sup> at a dilution ratio of 2230. It increases nonlinearly with increasing concentration of the total hydrocarbons in the gas phase (THC), rising from 0.12 g g<sup>−1</sup> to 0.26 g g<sup>−1</sup> for a <i>C</i><sub>THC</sub> increase of 1 to 18 μg m<sup>−3</sup>, suggesting that more organics are partitioned into the particles at higher gas phase concentrations. In terms of gas-particle partitioning, the condensational uptake of THC gases in gasoline engine exhaust can account for up to 30% of the total gas + particle THC. The organic mass spectrum of COM has the largest fragment at <i>m/z</i> 44, with mass ratios of mass fragments 43/44 and 57/44 at 0.59 and 2.91, much lower than those reported for gasoline engine primary organic aerosols. The mass fragment 44/total organic mass ratio of 0.097 indicates that COM contains large oxygenated components. By incorporating the present findings, regional air quality modelling results suggest that the condensational uptake of THC onto sulfate particles alone can be comparable to the primary particle mass under moderately polluted ambient conditions. These findings are important for modelling and regulating the air quality impacts of gasoline vehicular emissions.
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In this work, three different thermal protocols were tested on untreated and water-washed aerosol samples to study the influence of soluble organic and inorganic compounds on EC measurements. Moreover, analyses on the water soluble extracts were carried out. The aim was to find out the most suitable protocol to analyse samples collected in a heavily polluted area. Indeed, the tests were performed on real samples collected at an urban background station in the Po Valley, which is one of the main pollution hot-spots in Europe. <br><br> The main differences among the tested protocols were the maximum temperature of the He step (i.e. 870 °C, 650 °C, and 580 °C) and the duration of the plateaus during the heating procedure. Our measurements evidenced the presence of a significant amount of weakly light-absorbing carbonaceous aerosol evolving during the highest temperature step in He (i.e. 870 °C), which makes lower temperature protocols not suitable for EC determination in samples collected in heavily polluted areas like Milan.
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Collision-induced water drop breakup in a vertical wind tunnel was observed using a high speed camera for interactions between larger drop sizes (up to 7 mm diameter) than have previously been experimentally observed. Three distinct collisional breakup types were observed and the drop size distributions from each were analysed for comparison with predictions of fragment distributions from larger drops by two sets of established breakup parameterisations. The observations showed some similarities with both parameterisations but also some marked differences for the breakup types that could be compared, particularly for fragments 1 mm and smaller. Modifications to the parameterisations are suggested and examined. Presented is also currently the largest dataset of bag breakup distributions observed. Differences between this and other experimental research studies and modelling parameterisations, and the associated implications for interpreting results are discussed. Additionally, the stochastic coalescence and breakup equation was solved computationally using a breakup parameterisation, and the evolving drop-size distribution for a range of initial conditions was examined. Initial cloud liquid water content was found to have the greatest influence on the resulting distribution, whereas initial drop number was found to have relatively little influence. This may have implications when considering the effect of aerosol on cloud evolution, raindrop formation and resulting drop size distributions. Calculations presented show that, using an ideal initial cloud drop-size distribution, ~1–3% of the total fragments are contributed from collisional breakup between drops of 4 and 6 mm.
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In this paper we report chemically resolved measurements of organic aerosol (OA) and related tracers during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at the Blodgett Forest Research Station, California from 15 August–10 October 2007. OA contributed the majority of the mass to the fine atmospheric particles and was predominately oxygenated (OOA). The highest concentrations of OA were during sporadic wildfire influence when aged plumes were impacting the site. In situ measurements of particle phase molecular markers were dominated by secondary compounds and along with gas phase compounds could be categorized into six factors or sources: (1) aged biomass burning emissions and oxidized urban emissions, (2) oxidized urban emissions (3) oxidation products of monoterpene emissions, (4) monoterpene emissions, (5) anthropogenic emissions and (6) local methyl chavicol emissions and oxidation products. There were multiple biogenic components that contributed to OA at this site whose contributions varied diurnally, seasonally and in response to changing meteorological conditions, e.g. temperature and precipitation events. Concentrations of isoprene oxidation products were larger when temperatures were higher during the first half of the campaign (15 August–12 September) due to more substantial emissions of isoprene and enhanced photochemistry. The oxidation of methyl chavicol, an oxygenated terpene emitted by ponderosa pine trees, contributed similarly to OA throughout the campaign. In contrast, the abundances of monoterpene oxidation products in the particle phase were greater during the cooler conditions in the latter half of the campaign (13 September–10 October), even though emissions of the precursors were lower, although the mechanism is not known. OA was correlated with the anthropogenic tracers 2-propyl nitrate and carbon monoxide (CO), consistent with previous observations, while being comprised of mostly non-fossil carbon (>75%). The correlation between OA and an anthropogenic tracer does not necessarily identify the source of the carbon as being anthropogenic but instead suggests a coupling between the anthropogenic and biogenic components in the air mass that might be related to the source of the oxidant and/or the aerosol sulfate. Observations of organosulfates of isoprene and α-pinene provided evidence for the likely importance of aerosol sulfate in spite of neutralized aerosol although acidic plumes might have played a role upwind of the site. This is in contrast to laboratory studies where strongly acidic seed aerosols were needed in order to form these compounds. These compounds together represented only a minor fraction (<1%) of the total OA mass, which may be the result of the neutralized aerosol at the site or because only a small number of organosulfates were quantified. The low contribution of organosulfates to total OA suggests that other mechanisms, e.g. NO<sub>x</sub> enhancement of oxidant levels, are likely responsible for the majority of the anthropogenic enhancement of biogenic secondary organic aerosol observed at this site.
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Acetylene (C<sub>2</sub>H<sub>2</sub>) volume mixing ratios (VMRs) have been successfully retrieved from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) Level 1B radiances during August 2003, providing the first global map of such data and ratios to CO in the literature. The data presented here contain most information between 300 hPa and 100 hPa with systematic errors less than 10% at the upper levels. Random errors per point are less than 15% at lower levels and are closer to 30% at 100 hPa. <br></br> Global distributions of the C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>H<sub>2</sub>/CO ratio confirm significant features associated with both the Asian monsoon anticyclone and biomass burning for this important hydrocarbon in a characteristic summer month (August 2003), showing tight correlations regionally, particularly at lower to medium values, but globally emphasising the differences between sources and lifetimes of CO and C<sub>2</sub>H<sub>2</sub>. The correlations are seen to be particularly disturbed in the regions of highest C<sub>2</sub>H<sub>2</sub> concentrations, indicating variability in the surface emissions or fast processing. <br></br> A strong isolation of C<sub>2</sub>H<sub>2</sub> within the Asian monsoon anticyclone is observed, evidencing convective transport into the upper troposphere, horizontal advection within the anticyclone at 200 hPa, distinct gradients at the westward edge of the vortex and formation of a secondary dynamical feature from the eastward extension of the anticyclone outflow over the Asian Pacific. Ratios of C<sub>2</sub>H<sub>2</sub>/CO are consistent with the evidence from the cross-sections that the C<sub>2</sub>H<sub>2</sub> is uplifted rapidly in convection. <br></br> Observations are presented of enhanced C<sub>2</sub>H<sub>2</sub> associated with the injection from biomass burning into the upper troposphere and the outflow from Africa at 200 hPa into both the Atlantic and Indian Oceans. In the biomass burning regions, C<sub>2</sub>H<sub>2</sub> and CO are well correlated, but the uplift is less marked and peaks at lower altitudes compared to the strong effects observed in the Asian monsoon anticyclone. Ratios of C<sub>2</sub>H<sub>2</sub>/CO clearly decay along transport pathways for the outflow, indicating photochemical ageing of the plumes. <br></br> Overall, the data show the distinctive nature of C<sub>2</sub>H<sub>2</sub> distributions, confirm in greater detail than previously possible features of hydrocarbon enhancements in the upper troposphere and highlight the future use of MIPAS hydrocarbon data for testing model transport and OH decay regimes in the middle to upper troposphere.
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It is well known that lightning produces NO<sub>x</sub> as a result of the high temperatures in discharge channels. Since most viable proposed electrification mechanisms involve ice crystals, it is reasonable to assume that lightning discharge channels frequently pass through fields of ice particles of various kinds. We address the question of whether ice crystals may serve as catalysts for the production of NO<sub>x</sub> by lightning discharges. If so, and if the effect is large, it would need to be taken into account in estimates of global NO<sub>x</sub> production by lightning. In this theoretical study, we make a series of plausible assumptions about the temperature and concentration of reactant species in the environment of discharges and we postulate a mechanism by which ice crystals are able to adsorb nitrogen atoms. We then compare production rates between uncatalyzed and catalyzed reactions at 2000 K, 3000 K, and 4000 K, which are reasonable temperatures in lightning channels as they cool down. Ice crystal catalysis is expected to produce 2.7 times more NO than if ice crystals were not present. Catalyzed NO production rates are greater at 2000 K, whereas uncatalyzed production rates are greater at 4000 K. Thus, temperatures that favor rapid NO production without ice crystals adsorbing nitrogen atoms are unfavorable for NO production in the presence of ice crystals, and vice versa. The density of atmospheric ice crystals is much larger at 10 km where intracloud (IC) flashes peak than at 5 km where cloud to ground (CG) flashes peak, thus catalytic processes are expected to be more important for IC flashes than CG flashes, perhaps explaining a portion of the discrepancy in IC and CG production rates.
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We report measurements of the ultraviolet absorption cross sections of OC<sup>32</sup>S, OC<sup>33</sup>S, OC<sup>34</sup>S and O<sup>13</sup>CS from 195 to 260 nm. The OCS isotopologues were synthesized from isotopically-enriched elemental sulfur by reaction with carbon monoxide. The measured cross section of OC<sup>32</sup>S is consistent with literature spectra recorded using natural abundance samples. Relative to the spectrum of the most abundant isotopologue, substitution of heavier rare isotopes has two effects. First, as predicted by the reflection principle, the Gaussian-based absorption envelope becomes slightly narrower and blue-shifted. Second, as predicted by Franck-Condon considerations, the weak vibrational structure is red-shifted. Sulfur isotopic fractionation constants (<sup>33</sup>ε, <sup>34</sup>ε) as a function of wavelength are not highly structured, and tend to be close to zero on average on the high energy side and negative on the low energy side. The integrated photolysis rate of each isotopologue at 20 km, the approximate altitude at which most OCS photolysis occurs, was calculated. Sulfur isotopic fractionation constants at 20 km altitude are (−3.7 ± 4.5)‰ and (1.1 ± 4.2)‰ for <sup>33</sup>ε and <sup>34</sup>ε, respectively, which is inconsistent with the previously estimated large fractionation of over 73‰ in <sup>34</sup>ε. This demonstrates that OCS photolysis does not produce sulfur isotopic fractionation of more than ca. 5‰, suggesting OCS may indeed be a significant source of background stratospheric sulfate aerosols. Finally, the predicted isotopic fractionation constant for <sup>33</sup>S excess (<sup>33</sup><i>E</i>) in OCS photolysis is (−4.2 ± 6.6)‰, and thus photolysis of OCS is not expected to be the source of the non-mass-dependent signature observed in modern and Archaean samples.
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The Advanced Regional Prediction System, a mesoscale atmospheric model, is applied to simulate the month of June 2006 with a focus on the near surface air temperatures around Paris. To improve the simulated temperatures which show errors up to 10 K during a day on which a cold front passed Paris, a data assimilation procedure to calculate 3-D analysis fields of specific cloud liquid and ice water content is presented. The method is based on the assimilation of observed cloud optical thickness fields into the Advanced Regional Prediction System model and operates on 1-D vertical columns, assuming that the horizontal background error covariance is infinite, i.e. an independent pixel approximation. The rationale behind it is to find vertical profiles of cloud liquid and ice water content that yield the observed cloud optical thickness values and are consistent with the simulated profile. Afterwards, a latent heat adjustment is applied to the temperature in the vertical column. Data from several meteorological stations in the study area are used to verify the model simulations. The results show that the presented assimilation procedure is able to improve the simulated 2 m air temperatures and incoming shortwave radiation significantly during cloudy days. The scheme is able to alter the position of the cloud fields significantly and brings the simulated cloud pattern closer to the observations. As the scheme is rather simple and computationally inexpensive, it is a promising new technique to improve the surface fields of retrospective model simulations for variables that are affected by the position of the clouds.
