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Journal of Geophysical Research 08/2012; 117(D15202). · 3.02 Impact Factor
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ABSTRACT: The Convective and Orographically-induced Precipitation Study (COPS) campaign was conducted during the summer of 2007. A suite of instruments housed at the top of the Hornisgrinde Mountain (1156 m) in the Black Forest region of south-west Germany provided datasets that allow an investigation into the physical, chemical and hygroscopic properties of the aerosol particles sampled during COPS. Organic mass loadings were found to dominate the aerosol composition for the majority of the project, exceeding 8 µg m−3 during a period of high pressure, high temperature, and low wind speed. The ratio of organic:sulphate sub-micron mass concentration exceeds 10:1 during the same time period. Back trajectories show air from this time-frame passing slowly over the local forest and not passing over any local anthropogenic sources. Occasional peaks in nitrate mass loadings were associated with changes in the typical wind direction from south-westerly to north-westerly where air had passed over the Stuttgart region. Size distribution data shows a dominant accumulation-mode when the measurement site was free from precipitation events. A sharp increase in ultrafine particle number concentration was seen during most days commencing around noon. The apparent growth of these particles is associated with an increase in organic mass loading, suggesting condensational growth. For the most part, with the exception of the high pressure period, the aerosol properties recorded during COPS were comparable to previous studies of continental aerosol properties. Copyright © 2011 Royal Meteorological Society
Quarterly Journal of the Royal Meteorological Society 12/2010; 137(S1):252 - 266. · 2.91 Impact Factor
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K. L. Furneaux,
L. K. Whalley,
D. E. Heard,
H. M. Atkinson,
W. J. Bloss, M. J. Flynn,
M. W. Gallagher,
Ingham T,
Kramer L,
J. D. Lee,
Leigh R,
G. B. McFiggans,
A. S. Mahajan,
P. S. Monks,
Oetjen H,
J. M. C. Plane,
J. D. Whitehead
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ABSTRACT: Point source measurements of IO by laser induced fluorescence spectroscopy were made at a semi-polluted coastal location during the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) campaign in September 2006. The site, on the NW French coast in Roscoff, was characterised by extensive intertidal macroalgae beds which were exposed at low tide. The closest known iodine active macroalgae beds were at least 300 m from the measurement point. From 20 days of measurements, IO was observed above the instrument limit of detection on 14 days, of which a clear diurnal profile was observed on 11 days. The maximum IO mixing ratio was 30.0 pptv (10 s integration period) during the day, amongst the highest concentrations ever observed in the atmosphere, and 1–2 pptv during the night. IO concentrations were strongly dependent on tidal height, the intensity of solar irradiation and meteorological conditions. An intercomparison of IO measurements made using point source and spatially averaged DOAS instruments confirms the presence of hot-spots of IO caused by an inhomogeneous distribution of macroalgae. The co-incident, point source measurement of IO and ultra fine particles (2.5 nm≥d≥10 nm) displayed a strong correlation, providing evidence that IO is involved in the production pathway of ultra fine particles at coastal locations. Finally, a modelling study shows that high IO concentrations which are likely to be produced in a macrolagae rich environment can significantly perturb the concentrations of OH and HO2 radicals. The effect of IO on HOx is reduced as NOx concentrations increase.
Atmospheric Chemistry and Physics. 01/2010;
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J. D. Whitehead,
M. W. Gallagher,
J. R. Dorsey,
Robinson N,
A. M. Gabey,
Coe H,
McFiggans G, M. J. Flynn,
Ryder J,
Nemitz E,
Davies F
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ABSTRACT: Atmospheric aerosol measurements were conducted near Danum Valley, in the Malaysian state of Sabah, North-East Borneo, as part of the OP3 and ACES projects, in April and June/July 2008. Here, aerosol fluxes and diurnal variability in and above the rainforest canopy were examined in order to gain an understanding of their behaviour in the surface layer of the South-East Asian rainforest. Aerosol fluxes were calculated by eddy covariance from measurements above the rainforest canopy on the Global Atmosphere Watch (GAW) tower. Upward fluxes were seen on most mornings between 09:00 and 11:00 local time and this could be attributed to venting of the nocturnal boundary layer as it broke up in the morning. Measurements were also conducted within the canopy and trunk space at a nearby site. Profiles in aerosol number concentrations were investigated using GRIMM Optical Particle Counters (OPCs) at various levels within the rainforest canopy and trunk space, as well as a single OPC on a vertically moving platform. These showed an overnight increase in larger particles (1–20 μm) at all levels, but much more prominently near the top of the canopy, which could be attributed to fog formation. At ground level, number concentrations in this size range correlated with enhancements in biological aerosol concentrations, measured using a Wide Issue Bioaerosol Spectrometer (WIBS) located near the forest floor, suggesting that coarse particle number concentrations were dominated by biological aerosols. A comparison of particle number concentrations (in the size range 0.5–1.0 μm) between above canopy and the trunk space showed correlations, despite turbulence data suggesting persistent decoupling between the two measurement sites. These correlations often relied on a shift of the particle time-series against each other, implying a time delay in observations between the sites, which varied according to time of day. This lag time was shortest during the middle of the day by a significant margin. This was not observed for aerosols larger than 1.0 μm. Further evidence of daytime coupling between above canopy and the trunk space in terms of aerosol measurements is implied by comparison of measurements from an Aerosol Mass Spectrometer (AMS) at the GAW tower and simultaneous bag sampling at the in-canopy site, subsequently analysed with the AMS. Transport of particles through the canopy seems to occur through large-scale, sporadic turbulent events, suggesting that the coupling between the canopy space and the air above is due to these ventilation events.
Atmospheric Chemistry and Physics. 01/2010;
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Atmospheric Chemistry and Physics. 01/2010; 10(2):647-668.
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H M Jones,
J Crosier,
A Russell, M J Flynn,
M Irwin,
T W Choularton,
H Coe,
G Mcfiggans,
Jones Hm,
Flynn Mj,
Choularton Tw
[show abstract]
[hide abstract]
ABSTRACT: The Convective and Orographically-induced Precipitation Study (COPS) campaign was conducted during the summer of 2007. A suite of instruments housed at the top of the Hornisgrinde Mountain (1156 m) in the Black Forest region of south-west Germany provided datasets that allow an investigation into the physical, chemical and hygroscopic properties of the aerosol particles sampled during COPS. Organic mass loadings were found to dominate the aerosol composition for the majority of the project, exceeding 8 µg m −3 during a period of high pressure, high temperature, and low wind speed. The ratio of organic:sulphate sub-micron mass concentration exceeds 10:1 during the same time period. Back trajectories show air from this time-frame passing slowly over the local forest and not passing over any local anthropogenic sources. Occasional peaks in nitrate mass loadings were associated with changes in the typical wind direction from south-westerly to north-westerly where air had passed over the Stuttgart region. Size distribution data shows a dominant accumulation-mode when the measurement site was free from precipitation events. A sharp increase in ultrafine particle number concentration was seen during most days commencing around noon. The apparent growth of these particles is associated with an increase in organic mass loading, suggesting condensational growth. For the most part, with the exception of the high pressure period, the aerosol properties recorded during COPS were comparable to previous studies of continental aerosol properties.. 2010. In situ aerosol measurements taken during the 2007 COPS field campaign at the Hornisgrinde ground site.
Quarterly Journal of the Royal Meteorological Society 01/2010; 137:252–266. · 2.91 Impact Factor
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Geophysical Research Letters 02/2009; 36:L04806. · 3.79 Impact Factor
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ABSTRACT: Organic matter frequently represents the single largest fraction of fine particulates in urban environments and yet the exact contributions from different sources and processes remain uncertain, owing in part to its substantial chemical complexity. Positive Matrix Factorisation (PMF) has recently proved to be a powerful tool for the purposes of source attribution and profiling when applied to ambient organic aerosol data from the Aerodyne Aerosol Mass Spectrometer (AMS). Here we present PMF analysis applied to AMS data from UK cities for the first time. Three datasets are analysed, with the focus on objectivity and consistency. The data were collected in London during the Regent's Park and Tower Environmental Experiment (REPARTEE) intensives and Manchester. These occurred during the autumn and wintertime, such that the primary fraction would be prominent. Ambiguities associated with rotationality within sets of potential solutions are explored and the most appropriate solution sets selected based on comparisons with external data. In addition to secondary organic aerosols, three candidate sources of primary organic aerosol (POA) were identified according to mass spectral and diurnal profiles; traffic emissions, cooking and solid fuel burning. Traffic represented, on average, 40% of POA during colder conditions and exhibited a hydrocarbon-like mass spectrum similar to those previously reported. Cooking aerosols represented 34% of POA and through laboratory work, their profile was matched with that sampled from the heating of seed oils, rather than previously-published spectra derived from charbroiling. This suggests that in these locations, oil from frying may have contributed more to the particulate than the meat itself. Solid fuel aerosols represented 26% of POA during cold weather conditions but were not discernable during the first REPARTEE experiment, when conditions were warmer than the other campaigns. This factor showed features associated with biomass burning and occurred mainly at night. Grid-scale emission factors of the combustion aerosols suitable for use in chemical transport models were derived relative to CO and NOx. The traffic aerosols were found to be 14.4 μg m−3 ppm−1 relative to CO for Manchester and 28 μg m−3 ppm−1 relative to NOx for London. Solid fuel emissions were derived as 17.3 μg m−3 ppm−1 relative to CO for Manchester. These correspond to mass emission ratios of 0.012, 0.021 (as NO) and 0.014 respectively and are of a similar order to previously published estimates, derived from other regions or using other approaches.
Atmospheric Chemistry and Physics Discussions. 01/2009;
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ABSTRACT: Here we present the first observations of simultaneous ozone deposition fluxes and ultrafine particle emission fluxes over an extensive infra-littoral zone. Fluxes were measured by the eddy covariance technique at the Station Biologique de Roscoff, on the coast of Brittany, north-west France. This site overlooks a very wide (3 km) littoral zone controlled by very deep tides (9.6 m) exposing extensive macroalgae beds available for significant iodine mediated photochemical production of ultrafine particles. The aspect at the Station Biologique de Roscoff provides an extensive and relatively flat, uniform fetch within which micrometeorological techniques may be utilized to study links between ozone deposition to macroalgae (and sea water) and ultrafine particle production. Ozone deposition to seawater at high tide was significantly slower (vd[O3]=0.302±0.095 mm s−1) than low tidal deposition. A statistically significant difference in the deposition velocities to macroalgae at low tide was observed between night time (vd[O3]=1.00±0.10 mm s−1) and daytime (vd[O3]=2.05±0.16s−1) when ultrafine particle formation results in apparent particle emission. Very high emission fluxes of ultrafine particles were observed during daytime periods at low tides ranging from 50 000 particles cm−2 s−1 to greater than 200 000 particles cm−2 s−1 during some of the lowest tides. These emission fluxes exhibited a significant relationship with particle number concentrations comparable with previous observations at another location. Apparent particle growth rates were estimated to be in the range 17–150 nm h−1 for particles in the size range 3–10 nm. Under certain conditions, particle growth may be inferred to continue to greater than 120 nm over tens of hours; sizes at which they may readily behave as cloud condensation nuclei (CCN) under reasonable supersaturations that may be expected to pertain at the top of the marine boundary layer. These results link direct depositional loss and photochemical destruction of ozone to the formation of particles and hence CCN from macroalgal emissions at a coastal location.
Atmospheric Chemistry and Physics Discussions. 01/2009;
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ABSTRACT: Particle deposition velocities (11–3000 nm diameter) measured above grassland by eddy covariance during the EU GRAMINAE experiment in June 2000 averaged 0.24 and 0.03 mm s−1 to long (0.75 m) and short (0.07 m) grass, respectively. After fertilisation with 108 kg N ha−1 as calcium ammonium nitrate, sustained apparent upward fluxes of particles were observed. Analysis of concentrations and fluxes of potential precursor gases, including NH3, HNO3, HCl and selected VOCs, shows that condensation of HNO3 and NH3 on the surface of existing particles is responsible for this effect. A novel approach is developed to derive particle growth rates at the field scale, from a combination of measurements of vertical fluxes and particle size-distributions. For the first 9 days after fertilization, growth rates of 11 nm particles of 7.04 nm hr−1 and 1.68 nm hr−1 were derived for day and night-time conditions, respectively. This implies total NH4NO3 production rates of 1.11 and 0.44 μg m−3 h−1, respectively. The effect translates into a small error in measured ammonia fluxes (0.06% day, 0.56% night) and a large error in NH4+ and NO3− aerosol fluxes of 3.6% and 10%, respectively. By converting rapidly exchanged NH3 and HNO3 into slowly depositing NH4NO3, the reaction modifies the total N budget, though this effect is small (3 emission dominates the net flux. It is estimated that 3.8% of the fertilizer N was volatilised as NH3, of which 0.05% re-condensed to form NH4NO3 particles within the lowest 2 m of the surface layer. This surface induced process would at least scale up to a global NH4NO3 formation of ca. 0.21 kt N yr−1 from NH4NO3 fertilisers and potentially 45 kt N yr−1 from NH3 emissions in general.
Biogeosciences. 01/2009;
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McFiggans G,
C. S. E. Bale,
S. M. Ball,
J. M. Beames,
W. J. Bloss,
L. J. Carpenter,
Dorsey J,
Dunk R, M. J. Flynn,
K. L. Furneaux, [......],
Oetjen H,
A. J. Orr-Ewing,
J. M. C. Plane,
Potin P,
A. J. L. Shillings,
Thomas F,
R. von Glasow,
Wada R,
L. K. Whalley,
J. D. Whitehead
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ABSTRACT: This paper presents a summary of the measurements that were made during the heavily-instrumented Reactive Halogens in the Marine Boundary Layer (RHaMBLe) coastal study in Roscoff on the North West coast of France. It was clearly demonstrated that iodine-mediated coastal particle formation occurs, driven by daytime low tide emission of molecular iodine, I2, by macroalgal species fully or partially exposed by the receding waterline. Ultrafine particle concentrations strongly correlate with the rapidly recycled reactive iodine species, IO, produced at high concentrations following photolysis of I2. The heterogeneous macroalgal I2 sources lead to variable relative concentrations of iodine species observed by path-integrated and in situ measurement techniques. Apparent particle emission fluxes were associated with an enhanced apparent depositional flux of ozone, consistent with both a direct O3 deposition to macroalgae and involvement of O3 in iodine photochemistry and subsequent particle formation below the measurement height. The magnitude of the particle formation events was observed to be greatest at the lowest tides with higher concentrations of ultrafine particles growing to much larger sizes, probably by the condensation of anthropogenically-formed condensable material. At such sizes the particles should be able to act as cloud condensation nuclei at reasonable atmospheric supersaturations.
Atmospheric Chemistry and Physics Discussions. 01/2009;
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ABSTRACT: Very short lived halocarbons (VSLH) play an important role in the transport of halogen atoms to the troposphere and lower stratosphere. Here we describe the development of a relaxed eddy accumulation system using gas chromatography coupled to a mass spectrometer (REA-GC/MS) to determine surface fluxes of VSLH with a time resolution of about 0.5 h. Laboratory tests showed that use of a common inlet for upward, downward and deadband flow paths resulted in mixing of upward and downward moving air masses, therefore we recommend the use of separate inlets. The modified system underwent field trials at Mace Head on the west coast of Ireland, where there are dense kelp beds known to emit a range of halocarbons. Over a 16 h period in mid-September 2007, the mean fluxes obtained were 16.1±1.8, 4.0±0.54 and 1.2±0.2 nmol m−2 day−1 for CH2Br2 CH2ICl and CH2IBr, respectively.
Atmospheric Measurement Techniques. 01/2009;
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Hensen A,
Nemitz E, M. J. Flynn,
Blatter A,
S. K. Jones,
L. L. Sørensen,
Hensen B,
Pryor S,
Jensen B,
R. P. Otjes,
Cobussen J,
Loubet B,
J.W. Erisman,
M. W. Gallagher,
Neftel A,
M. A. Sutton
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ABSTRACT: The exchange of NH3 between grassland and the atmosphere was determined using Relaxed Eddy Accumulation (REA) measurements. The use of REA is of special interest for NH3, since the determination of fluxes at one height permits multiple systems to be deployed to quantify vertical flux divergence (either due to effects of chemical production or advection). During the Braunschweig integrated experiment four different continuous-sampling REA systems were operated during a period of about 10 days and were compared against a reference provided by independent application of the Aerodynamic Gradient Method (AGM). The experiment covered episodes before, after cutting and fertilising and provided a wide range of fluxes −60–3600 ng NH3 m−2 s−1 for testing the REA systems. The REA systems showed moderate to good correlation with the AGM estimates, with r2 values for the linear regressions between 0.3 and 0.82. For the period immediately after fertilization, the REA systems showed average fluxes 20% to 70% lower than the reference. At periods with low fluxes REA and AGM can agree within a few %. Overall, the results show that the continuous REA technique can now be used to measure NH3 surface exchange fluxes. While REA requires greater analytical precision in NH3 measurement than the AGM, a key advantage of REA is that auto-referencing periods can be introduced to remove bias between sampling inlets. However, while the data here indicate differences consistent with advection effects, further improvements in sampling precision are needed to allow measurement of flux divergence. Wet chemical techniques will be developed further since they use the sticky and reactive properties of NH3 that impedes development of cheaper optical systems.
Biogeosciences Discussions. 01/2008;
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ABSTRACT: Measurements of aerosol properties were made in aged polluted and clean background air masses encountered at the North Norfolk (UK) coastline as part of the TORCH2 field campaign in May 2004. Hygroscopic growth factors (GF) at 90% relative humidity (RH) for D0=27–217 nm particles and size-resolved chemical composition were simultaneously measured using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) and an Aerodyne aerosol mass spectrometer (Q-AMS), respectively. Both hygroscopic properties and chemical composition showed pronounced variability in time and with particles size. With this data set we could demonstrate that the Zdanovskii-Stokes-Robinson (ZSR) mixing rule combined with chemical composition data from the AMS makes accurate quantitative predictions of the mean GF of mixed atmospheric aerosol particles possible. In doing so it is crucial that chemical composition data are acquired with high resolution in both particle size and time, at least matching the actual variability of particle properties. The closure results indicate an ensemble GF of the organic fraction of ~1.20±0.10 at 90% water activity. Thus the organics contribute somewhat to hygroscopic growth, particularly at small sizes, however the inorganic salts still dominate.
Furthermore it has been found that most likely substantial evaporation losses of NH4NO3 occurred within the HTDMA instrument, exacerbated by a long residence time of ~1 min. Such an artefact is in agreement with our laboratory experiments and literature data for pure NH4NO3, both showing similar evaporation losses within HTDMAs with residence times of ~1 min. Short residence times and low temperatures are hence recommended for HTDMAs in order to minimise such evaporation artefacts.
Atmospheric Chemistry and Physics. 12/2007; 7(23):6131-6144.
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ABSTRACT: The Hector thunderstorm is studied with a numerical cloud-resolving model. Special attention is given to modelling the mixed-phase and glaciated cloud microphysical processes (along with the implications of aerosols) and their influence on the resulting microphysical and dynamical storm structure. Radiative impacts are also calculated. Simulations are performed for a typical storm case from the EMERALD-II convective cloud experiment in November and December 2002.It is found that, for intense thunderstorms, aerosol indirect effects are generally modified from recently proposed theoretical considerations. Specifically, the proposed ‘glaciation’ indirect effect, resulting from increasing ice nuclei concentrations, is small for intense convection. More importantly, increasing ice number concentrations results in a ‘collection’ indirect effect (where aggregation and accretion processes lead to precipitation) rather than the ‘glaciation’, Bergeron–Findeisen process. There is a ‘thermodynamic’ indirect effect for Hector, as increasing the cloud droplet number concentration from maritime to continental values resulted in a suppression of the heterogeneous freezing process. However, for extreme continental cases, liquid and raindrop freezing by collection processes acquires higher importance; hence there is an optimal value for strong cumulonimbus development. The ‘glaciation’ indirect effect is found to be similar to increasing the rate of ice production by the Hallett–Mossop process. Another aspect of this study shows that there is a significant impact of microphysics on cloud dynamics, and so studying aerosol–cloud effects must also consider dynamical feedback, a strong component of which arises from the latent heat released during homogeneous freezing.The important indirect effects may be well described by recent theory for smaller, more common stratiform and cumulus clouds; however, in the tropics, the importance of Hector-type storms cannot be ignored as they, and other similar storms, provide a mechanism for the production of widespread cirrus and the release of a large amount of precipitation. Copyright © 2006 Royal Meteorological Society
Quarterly Journal of the Royal Meteorological Society 04/2007; 132(621C):3079 - 3106. · 2.91 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Measurements of aerosol properties were made in aged polluted and clean background air masses encountered at the North Norfolk (UK) coastline as part of the TORCH2 field campaign in May 2004. Hygroscopic growth factors (GF) at 90% relative humidity (RH) for D0=27–217 nm particles and size-resolved chemical composition were simultaneously measured using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) and an Aerodyne aerosol mass spectrometer (Q-AMS), respectively. Both hygroscopic properties and chemical composition showed pronounced variability in time and with particles size. With this data set we could demonstrate that the Zdanovskii-Stokes-Robinson (ZSR) mixing rule combined with chemical composition data from the AMS makes accurate quantitative predictions of the mean GF of mixed atmospheric aerosol particles possible. In doing so it is crucial that chemical composition data are acquired with high resolution in both particle size and time, at least matching the actual variability of particle properties. The closure results indicate an ensemble GF of the organic fraction of ~1.20±0.10 at 90% water activity. Thus the organics contribute somewhat to hygroscopic growth, particularly at small sizes, however the inorganic salts still dominate. Furthermore it has been found that most likely substantial evaporation losses of NH4NO3 occurred within the HTDMA instrument, exacerbated by a long residence time of ~1 min. Such an artefact is in agreement with our laboratory experiments and literature data for pure NH4NO3, both showing similar evaporation losses within HTDMAs with residence times of ~1 min. Short residence times and low temperatures are hence recommended for HTDMAs in order to minimise such evaporation artefacts.
Atmospheric Chemistry and Physics. 01/2007;
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ABSTRACT: Aircraft observations of ice-crystal size and habit distributions in the cirrus outflow from deep convection at several geographic locations are reported. In situ measurements were made in the outflow from maritime thunderstorms near Kwajalein, part of the Marshall Islands and of thunderstorms with more continental aerosol concentrations both in the United States and near Darwin, Australia over the Tiwi Islands. Images of chain-like aggregates of small ice crystals, some with plate-like shapes were observed with a state-of-the-art microphysics probe in the outflow regions of continental storms that were typically highly electrified, displaying lightning. The ‘chains’ were not found in the outflow regions of maritime storms that are typically less electrically active. The striking similarity between these images and previous laboratory measurements of ice aggregation in electric fields are remarked upon. This evidence is used to support the theory that chain aggregates of ice crystals may be common in fully glaciated regions of continental thunderstorms, where ice-particle number densities are high, and their presence is due to the electric field alignment of ice crystals with subsequent enhancement of the aggregation process by dipole induction resulting in short-range attractive inter-particle forces. It is not confirmed where in the storm the aggregates were typically formed; however, in the Darwin thunderstorms they were noted to occur with the highest frequency towards the cirrus outflow base when the cirrus base altitude was high, and generally decreased in frequency with increasing distance from the storm. The potential consequences of electrically enhanced aggregation in continental storms and related electric field mechanisms along with the role of homogeneous freezing in intense thunderclouds are discussed. Copyright © 2005 Royal Meteorological Society
Quarterly Journal of the Royal Meteorological Society 12/2006; 131(608):1695 - 1712. · 2.91 Impact Factor
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ABSTRACT: Measurements of aerosol properties were made in aged polluted and clean background air masses encountered at the North Norfolk (UK) coastline during the second field campaign of the Tropospheric ORganic CHemistry project (TORCH2) in May 2004. Hygroscopic growth factor (GF) measurements were performed at 90% relative humidity (RH) for D0=27–217 nm particles using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA), while the aerosol composition was simultaneously measured with an Aerodyne aerosol mass spectrometer (Q-AMS). During the clean background events the aerosol was characterised by little size dependence of properties with generally large GFs and inorganic sulphate being the dominant compound. In aged polluted air masses the particles were dominated by inorganic sulphate and nitrate at larger sizes, whereas organics were the largest fraction in smaller particles, thus explaining the trend of smaller GFs at smaller sizes. Organics do contribute to the hygroscopic growth, particularly at small sizes, but generally the dominant contribution to growth at 90% RH comes from inorganic salts. The ZSR mixing rule was used to predict GFs based on the chemical composition, theoretical GFs of pure inorganic salts and a "bulk" GF of ~1.20 for the organics. Good quantitative closure with HTDMA measurements as a function of both particle size and time was achieved in the absence of nitrate. However, GFs were clearly overpredicted at times when a significant fraction of nitrate was present. After careful considerations we attribute the overprediction to substantial evaporation losses of ammonium nitrate in the HTDMA instrument. If true, this implies that the ZSR predictions based on composition might be more representative of the actual "bulk" behaviour of undisturbed ambient particles than the HTDMA measurements. The simplified model approach using the ZSR rule and a constant organic growth factor made high size and time resolution possible, which has proven to be essential for a valid closure study. The ZSR mixing rule appears to be sufficiently accurate, as the GF predictions are more sensitive to the exact GFs of the inorganic compounds than to the growth factor of the moderately hygroscopic organics. Therefore a more detailed analysis and modelling of the organic fraction at the expense of time and size resolution is not worth the effort for an aged aerosol and discrepancies in either direction might even be cancelled out by averaging.
Atmospheric Chemistry and Physics Discussions. 01/2006;
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Coe H,
J. D. Allan,
M. R. Alfarra,
K. N. Bower, M. J. Flynn,
G. B. McFiggans,
D. O. Topping,
P. I. Williams,
C. D. O&apos,
Dowd,
Dall&apos,
Osto M,
D. C. S. Beddows,
R M Harrison
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ABSTRACT: A suite of aerosol physical and chemical measurements were made at the Mace Head Atmospheric Research Station, Co. Galway, Ireland, a coastal site on the eastern seaboard of the north Atlantic Ocean during NAMBLEX. The data have been used in this paper to show that over a wide range of aerosol sizes there is no impact of the inter-tidal zone or the surf zone on measurements made at 7 m above ground level or higher. During the measurement period a range of air mass types were observed. During anticyclonic periods and conditions of continental outflow Aitken and accumulation mode were enhanced by a factor of 5 compared to the marine sector, whilst coarse mode particles were enhanced during westerly conditions. Baseline marine conditions were rarely met at Mace Head during NAMBLEX and high wind speeds were observed for brief periods only. The NAMBLEX experiment focussed on a detailed assessment of photochemistry in the marine environment, investigating the linkage between the HO<sub>x</sub> and the halogen radical cycles. Heterogeneous losses are important in both these cycles. In this paper loss rates of gaseous species to aerosol surfaces were calculated for a range of uptake coefficients. Even when the accommodation coefficient is unity, lifetimes due to heterogeneous loss of less than 10 s were never observed and rarely were they less than 500 s. Diffusional limitation to mass transfer is important in most conditions as the coarse mode is always significant. We calculate a minimum overestimate of 50% in the loss rate if this is neglected and so it should always be considered when calculating loss rates of gaseous species to particle surfaces. HO<sub>2</sub> and HOI have accommodation coefficients of around 0.03 and hence we calculate lifetimes due to loss to particle surfaces of 2000 s or greater under the conditions experienced during NAMBLEX. Aerosol composition data collected during this experiment provide representative information on the input aerosol characteristics to western Europe. During NAMBLEX the submicron aerosol was predominately acidified sulphate and organic material, which was most likely internally mixed. The remaining accumulation mode aerosol was sea salt. The organic and sulphate fractions were approximately equally important, though the mass ratio varies considerably between air masses. Mass spectral fingerprints of the organic fraction in polluted conditions are similar to those observed at other locations that are characterised by aged continental aerosol. In marine conditions, the background input of both sulphate and organic aerosol into Europe was observed to be between 0.5 and 1 µg m<sup>−3</sup>. Key differences in the mass spectra were observed during the few clean periods but were insufficient to ascertain whether these changes reflect differences in the source fingerprint of the organic aerosol. The coarse mode was composed of sea salt and showed significant displacement of chloride by nitrate and to a lesser extent sulphate in polluted conditions.
Atmospheric Chemistry and Physics. 01/2006;
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Coe H,
J. D. Allan,
M. R. Alfarra,
K. N. Bower, M. J. Flynn,
G. B. McFiggans,
D. O. Topping,
P. I. Williams,
C. D. O’Dowd,
Dall’Osto M,
D. C. S. Beddows,
R M Harrison
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ABSTRACT: Aerosol number concentrations and size distributions from 3 nm to 20 µm diameter were measured at the Mace Head Atmospheric Research Station, Co. Galway, Ireland, a coastal site on the eastern seaboard of the north Atlantic Ocean. Both on and offline size resolved aerosol composition measurements were also made using an Aerodyne Aerosol Mass Spectrometer (AMS) and ion chromatographic analysis of daily samples collected using a Micro-Orifice Uniform Deposit Impactor (MOUDI). Particle number concentrations, size distributions and AMS measurements were determined at 7 and 22 m above ground level to investigate local effects on the aerosol size distribution induced by the tidal zone. During periods of new particle formation ultrafine particle number concentrations are large and variable, however, outside these periods no variability in particle number was observed at any size, nor was the particle composition variable. Analysis of particle size distributions show that within each air mass observed particle number concentrations were very consistent. During anticyclonic periods and conditions of continental outflow Aitken and accumulation mode were enhanced by a factor of 5 compared to the marine sector, whilst coarse mode particles were enhanced during westerly conditions. Baseline marine conditions were rarely met at Mace Head during NAMBLEX and high wind speeds were observed for brief periods only. Loss rates of gaseous species to aerosol surfaces were calculated for a range of uptake coefficients. Even when the accommodation coefficient is unity, lifetimes of less than 100 s were never observed and rarely were lifetimes less than 500 s. Diffusional limitation to mass transfer is important in most conditions as the coarse mode is always significant, we calculate a minimum overestimate of 50% in the loss rate if this is neglected and so it should always be considered when calculating loss rates of gaseous species to particle surfaces. HO2 and HOI have accommodation coefficients of around 0.03 and hence we calculate lifetimes due to loss to particle surfaces of 2000 s or greater. Aerosol composition measurements using the AMS show accumulation mass modes of acidified sulphate and organic material, both of which have the same shape and are centred at around 350 nm vacuum aerodynamic diameter, implying an internal mixture. The organic and sulphate are approximately equally important, though the mass ratio varies considerably between air masses. Mass spectral fingerprints of the organic fraction in polluted conditions are similar to those observed at other locations that are characterised by aged continental aerosol. Even in marine conditions a background concentration of between 0.5 and 1 µg m−3 of both organic and sulphate was observed. Key differences in the mass spectra were observed during the few clean periods but were insufficient to ascertain whether these changes reflect differences in the source fingerprint of the organic aerosol. However, in an accompanying paper (Dall'Osto et al., 2005) periods of organic dominated aerosol particles were also observed and could be separated from the aged continental aerosol. The coarse mode was composed of sea salt and showed significant displacement of chloride by nitrate and to a lesser extent sulphate in polluted conditions.
Atmospheric Chemistry and Physics Discussions. 01/2005;
