S. Gerard Jennings

Marine aerosol formation from biogenic iodine emissions

The formation of marine aerosols and cloud condensation nuclei—from which marine clouds originate—depends ultimately on the availability of new, nanometre-scale particles in the marine boundary layer. Because marine aerosols and clouds scatter incoming radiation and contribute a cooling effect to the Earths radiation budget, new particle production is important in climate regulation. It has been suggested that sulphuric acid—derived from the oxidation of dimethyl sulphide—is responsible for the production of marine aerosols and cloud condensation nuclei. It was accordingly proposed that algae producing dimethyl sulphide play a role in climate regulation, but this has been difficult to prove and, consequently, the processes controlling marine particle formation remains largely undetermined. Here, using smog chamber experiments under coastal atmospheric conditions, we demonstrate that new particles can form from condensable iodine-containing vapours, which are the photolysis products of biogenic iodocarbons emitted from marine algae. Moreover, we illustrate, using aerosol formation models, that concentrations of condensable iodine-containing vapours over the open ocean are sufficient to influence marine particle formation. We suggest therefore that marine iodocarbon emissions have a potentially significant effect on global radiative forcing.

Transport of boreal forest fire emissions from Canada to Europe

In August 1998, severe forest fires occurred in many parts of Canada, especially in the Northwest Territories. In the week from August 5 to 11, more than 1000 different fires burned >1 × 106 ha of boreal forest, the highest 1-week sum ever reported throughout the 1990s. In this study we can unambigously show for the first time that these fires caused pronounced large-scale haze layers above Europe and that they influenced concentrations of carbon monoxide and other trace gases at the surface station Mace Head in Ireland over a period of weeks. Transport took place across several thousands of kilometers. An example of such an event, in which a pronounced aerosol layer was observed at an altitude of 3–6 km over Germany during August 1998, is investigated in detail. Backward trajectories ending at the measured aerosol layer are calculated and shown to have their origin in the forest fire region. Simulations with a particle dispersion model reveal how a substantial amount of forest fire emissions was transported across the Atlantic. The resulting aerosol lamina over Europe is captured well by the model. In addition, the model demonstrates that the forest fire emissions polluted large regions over Europe during the second half of August 1998. Surface measurements at Mace Head are compared to the model results for an anthropogenic and a forest fire carbon monoxide tracer, respectively. While wet deposition removed considerable amounts of aerosol during its transport, forest fire carbon monoxide reached Europe in copious amounts. It is estimated that during August 1998, 32%, 10%, and 58% of the carbon monoxide enhancement over the background level at Mace Head were caused by European and North American anthropogenic emissions and forest fire emissions, respectively.

A dedicated study of new particle formation and fate in the coastal environment (PARFORCE): overview of objectives and achievements

A dedicated study into the formation of new particles, New Particle Formation and Fate in the Coastal Environment (PARFORCE), was conducted over a period from 1998 to 1999 at the Mace Head Atmospheric Research Station on the western coast of Ireland. Continuous measurements of new particle formation were taken over the 2-year period while two intensive field campaigns were also conducted, one in September 1998 and the other in June 1999. New particle events were observed on ∼90% of days and occurred throughout the year and in all air mass types. These events lasted for, typically, a few hours, with some events lasting more than 8 hours, and occurred during daylight hours coinciding with the occurrence of low tide and exposed shorelines. During these events, peak aerosol concentrations often exceeded 106 cm−3 under clean air conditions, while measured formation rates of detectable particle sizes (i.e., d > 3 nm) were of the order of 104–105 cm−3 s−1. Nucleation rates of new particles were estimated to be, at least, of the order of 105–106 cm−3 s−1 and occurred for sulphuric acid concentrations above 2 × 106 molecules cm−3; however, no correlation existed between peak sulphuric acid concentrations, low tide occurrence, or nucleation events. Ternary nucleation theory of the H2SO4-H2O-NH3 system predicts that nucleation rates far in excess of 106 cm−3 s−1 can readily occur for the given sulphuric acid concentrations; however, aerosol growth modeling studies predict that there is insufficient sulphuric acid to grow new particles (of ∼1 nm in size) into detectable sizes of 3 nm. Hygroscopic growth factor analysis of recently formed 8-nm particles illustrate that these particles must comprise some species significantly less soluble than sulphate aerosol. The nucleation-mode hygroscopic data, combined with the lack of detectable VOC emissions from coastal biota, the strong emission of biogenic halocarbon species, and the fingerprinting of iodine in recently formed (7 nm) particles suggest that the most likely species resulting in the growth of new particles to detectable sizes is an iodine oxide as suggested by previous laboratory experiments. It remains an open question whether nucleation is driven by self nucleation of iodine species, a halocarbon derivative, or whether first, stable clusters are formed through ternary nucleation of sulphuric acid, ammonia, and water vapor, followed by condensation growth into detectable sizes by condensation of iodine species. Airborne measurements confirm that nucleation occurs all along the coastline and that the coastal biogenic aerosol plume can extend many hundreds of kilometers away from the source. During the evolution of the coastal plume, particle growth is observed up to radiatively active sizes of 100 nm. Modeling studies of the yield of cloud-condensation nuclei suggest that the cloud condensation nuclei population can increase by ∼100%. Given that the production of new particles from coastal biogenic sources occurs at least all along the western coast of Europe, and possibly many other coastlines, it is suggested that coastal aerosols contribute significantly to the natural background aerosol population.

Coastal new particle formation: Environmental conditions and aerosol physicochemical characteristics during nucleation bursts

Nucleation mode aerosol was characterized during coastal nucleation events at Mace Head during intensive New Particle Formation and Fate in the Coastal Environment (PARFORCE) field campaigns in September 1998 and June 1999. Nucleation events were observed almost on a daily basis during the occurrence of low tide and solar irradiation. In September 1998, average nucleation mode particle concentrations were 8600 cm-3 during clean air events and 2200 cm-3 during polluted events. By comparison, during June 1999, mean nucleation mode concentrations were 27,000 cm-3 during clean events and 3350 cm-3 during polluted conditions. Peak concentrations often reached 500,000-1,000,000 cm-3 during the most intense events and the duration of the events ranged from 2 to 8 hours with a mean of 4.5 hours. Source rates for detectable particle sizes (d > 3 nm) were estimated to be between 104 and 106 cm-3 s-1 and initial growth rates of new particles were as high as 0.1-0.35 nm s-1 at the tidal source region. Recently formed 8 nm particles were subjected to hygroscopic growth and were found to have a growth factor of 1.0-1.1 for humidification at 90% relative humidity. The low growth factors implicate a condensable gas with very low solubility leading to detectable particle formation. It is not clear if this condensable gas also leads to homogeneous nucleation; however, measured sulphuric acid and ammonia concentration suggest that ternary nucleation of thermodynamically stable sulphate clusters is still likely to occur. In clear air, significant particle production (>105 cm-3) was observed with sulphuric acid gas-phase concentration as low as 2 × 10 6 molecules cm-3 and under polluted conditions as high as 1.2 × 108 molecules cm-3. Copyright 2002 by the American Geophysical Union.

European greenhouse gas emissions estimated from continuous atmospheric measurements and radon 222 at Mace Head, Ireland

Flux estimates of CO2, CH4, N2O, and CFCs over western Europe have been inferred from continuous atmospheric records of these species at the atmospheric research station of Mace Head, Ireland. We use radon (222Rn) which has a fairly uniform source over continents as a reference compound to estimate unknown sources of other species. The correlation between each species and 222Rn is calculated for a suite of synoptic events that have been selected in the Mace Head record over the period 1996/97. In the following, we describe the method and its uncertainties, and we establish data selection criteria that minimize the influence of local sources over Ireland, in the vicinity of the station, in order to select synoptic events originating from western Europe. We estimate western European flux densities of 45–30 103 kg C km−2 month−1 during wintertime for CO2, of 4.8–3.5 103 kg CH4 km−2 yr−1, 475–330 kg N2O km−2 yr−1, 2.5–1.8 kg CFC-11 km−2 yr−1 for CFC-11, and 4.2–2.9 kg CFC-12 km−2 yr−1 for CFC-12. Our estimates are independent, although in good agreement with those produced by inventories, except for CFC-11 where our estimate is much lower than the inventory.

Submicron particle, radon, and soot carbon characteristics over the northeast Atlantic

Atmospheric aerosol particles (0.05μm ≤ rdry ≤ 1.5μm), 222Rn, and soot carbon mass were measured on a cruise over the Northeast Atlantic (63°N, 8°W) during October and November 1989. An accumulation mode (AM) was present in all particle data and was characterized by a lognormal size distribution with parameters N cm−3 (total number concentration), rg μm (geometric mean radius) and σg (geometric standard deviation). For aerosol associated with the “cleanest” Northeast Atlantic maritime and Arctic air masses, the AM parameters were found to be 16 cm−3 ≤ N ≤ 48 cm−3, 1.32 ≤ σg ≤ 1.46, and 0.08 μm < rg < 0.09 μm, leading to AM masses of between 0.20 μg m−3 and 0.38 μg m−3. Clean background levels of soot carbon mass for maritime and Arctic air were estimated to be around 20 ng m−3 and were associated with particle size radii r ≤ 0.15 μm. Soot carbon mass showed an excellent correlation with AM number concentration (cc=0.93), demonstrating its usefulness as an air mass tracer and as an indicator of anthropogenic pollutant transport. By comparison, radon, which is often used for this purpose, exhibited a significantly poorer correlation (cc=0.60) for this region. Approximately 9% of the total AM mass was accounted for by soot carbon, regardless of air mass origin, suggesting that early winter marine aerosol in the remote North Atlantic is primarily of anthropogenic origin.

Relative contribution of submicron and supermicron particles to aerosol light scattering in the marine boundary layer

Measurements of the aerosol light scattering coefficient (σsp) at a wavelength of λ - 550 nm were conducted at a coastal atmospheric research station in the east Atlantic Ocean during June 1999. Size distribution measurements between diameters of 3 nm and 40 um (at ambient humidity) were used to derive scattering coefficients from Mie theory. The calculated scattering coefficients were about a factor of 7.4 higher than the measured scattering coefficients. The discrepancy was explained by a reduced cutoff of the sampling system at particle diameters between 6 and 8 μm, dependent on wind speed. The calculated aerosol scattering was about 1 order of magnitude higher than previously reported measurements in the MBL and is attributed to supermicrometer particles at sizes d > 10 μm dominating aerosol scattering. Copyright 2002 by the American Geophysical Union.

Micron-sized droplets irradiated with a pulsed CO 2 laser: measurement of explosion and breakdown thresholds

Measurements of minimum CO(2) laser fluence required to explode or disintegrate 10-60 microm radius droplets of water, ethanol, diesel (hexadecane), CCl(4), bromoform, and ethyl bromide are reported. Threshold fluences range from 0.4 J cm(-2) for 10-microm radius ethanol drops to 20 J cm(-2) for 30microm bromoform drops. Threshold fluences for water droplets are ~3 J cm(-2) independent of drop size. Comparison of the measurements to calculations of laser fluence required for considered absorbing droplets to reach superheat temperature is in good agreement for cases where liquid material properties are known, suggesting that superheating of droplets is the dominant mechanism causing explosion/disintegration. Measured droplet-induced laser breakdown thresholds are considerably higher than explosion thresholds and have less dependence on droplet size and composition. The highest breakdown threshold values are for water drops, which range from 150 to 280 J cm(-2) (0.9-1.7 x 10(9)W cm(-2)) compared with 670 J cm(-2) (4.0 x 10(9) W cm(-2)) for clean air breakdown for the laser pulse length and spot size.

Opendda: a Novel High-Performance Computational Framework for the Discrete Dipole Approximation

This work presents a highly optimized computational framework for the Discrete Dipole Approximation, a numerical method for calculating the optical properties associated with a target of arbitrary geometry that is widely used in atmospheric, astrophysical and industrial simulations. Core optimizations include the bit-fielding of integer data and iterative methods that complement a new Discrete Fourier Transform (DFT) kernel, which efficiently calculates the matrix— vector products required by these iterative solution schemes. The new kernel performs the requisite 3-D DFTs as ensembles of 1-D transforms, and by doing so, is able to reduce the number of constituent 1-D transforms by 60% and the memory by over 80%. The optimizations also facilitate the use of parallel techniques to further enhance the performance. Complete OpenMP-based shared-memory and MPI-based distributed-memory implementations have been created to take full advantage of the various architectures. Several benchmarks of the new framework indicate extremely favorable performance and scalability.

Wind Speed Influences on Marine Aerosol Optical Depth

The Mulcahy (Mulcahy et al., 2008) power-law parameterization, derived at the coastal Atlantic station Mace Head, between clean marine aerosol optical depth (AOD) and wind speed is compared to open ocean MODIS-derived AOD versus wind speed. The reported AOD versus wind speed (U) was a function of ~U2. The open ocean MODIS-derived AOD at 550 nm and 860 nm wavelengths, while in good agreement with the general magnitude of the Mulcahy parameterization, follows a power-law with the exponent ranging from 0.72 to 2.47 for a wind speed range of 2–18 m. For the four cases examined, some MODIS cases underestimated AOD while other cases overestimated AOD relative to the Mulcahy scheme. Overall, the results from MODIS support the general power-law relationship of Mulcahy, although some linear cases were also encountered in the MODIS dataset. Deviations also arise between MODIS and Mulcahy at higher wind speeds (>15 m), where MODIS-derived AOD returns lower values as compared to Mulcahy. The results also support the suggestion than wind generated sea spray, under moderately high winds, can rival anthropogenic pollution plumes advecting out into marine environments with wind driven AOD contributing to AOD values approaching 0.3.