High number concentrations of transparent exopolymer particles (TEP) in ambient aerosol particles and cloud water – A case study at the tropical Atlantic Ocean

Transparent exopolymer particles (TEP) exhibit the properties of gels and are ubiquitously found in the world oceans. Possibly, TEP may enter the atmosphere as part of sea spray aerosol. Here, we report number concentrations of TEP (diameter > 4.5 µm) in ambient aerosol and cloud water samples from the tropical Atlantic Ocean as well as in generated aerosol particles using a plunging waterfall tank that was filled with the ambient sea water. The ambient TEP concentrations ranged between 7 × 102 and 3 × 104 #TEP m−3 in supermicron aerosol particles and correlations to sodium (Na+) and calcium (Ca2+) (R2 = 0.5) suggested some contribution via bubble bursting. Cloud water TEP concentrations were between 4 × 106 and 9 × 106 #TEP L−1 corresponding to equivalent air concentrations of 2–4 × 103 #TEP m−3. The TEP concentrations in the tank-generated aerosol particles, produced from the same waters and sampled with an equivalent system, were significantly lower (4 × 102–2 × 103 #TEP m−3) compared to the ambient concentrations. Based on Na+ concentrations in seawater and in the atmosphere, the enrichment factor for TEP in the atmosphere was calculated. The tank-generated TEP were enriched by a factor of 50 compared to sea water and, therefore, in-line with published enrichment factors for supermicron organic matter in general and TEP specifically. TEP enrichment in the ambient atmosphere was on average 1 × 103 in cloud water and 9 × 103 in ambient aerosol particles and therefore about two orders of magnitude higher than the corresponding enrichment from the tank study. Such high enrichment of supermicron particulate organic constituents in the atmosphere is uncommon and we propose that atmospheric TEP concentrations resulted from a combination of enrichment during bubble bursting transfer from the ocean and TEP in-situ formation in atmospheric phases. Abiotic in-situ formation might have occurred from aqueous reactions of dissolved organic precursors that were present in particle and cloud water samples, while biotic formation involves bacteria, which were abundant in the cloud water samples. The ambient TEP number concentrations were two orders of magnitude higher than recently reported ice nucleating particle (INP) concentrations measured at the same location. As TEP likely possess good properties to act as INP, in future experiments it is worth studying if a certain part of TEP contributes a fraction of the biogenic INP population.

Time-dependence of Heterogeneous Ice Nucleation by Ambient Aerosols: Laboratory Observations and a Formulation for Models

The time dependence of ice-nucleating particle (INP) activity is known to exist, yet for simplicity it is often omitted in atmospheric models as an approximation. Hitherto only limited experimental work has been done to quantify this time dependency, for which published data are especially scarce regarding ambient aerosol samples and longer time scales. In this study, the time dependence of INP activity is quantified experimentally for ambient environmental samples. The experimental approach includes a series of hybrid experiments with alternating constant cooling and isothermal experiments using a recently developed cold-stage setup called the Lund University Cold-Stage (LUCS). This approach of observing ambient aerosol samples provides the optimum realism for representing their time dependence in any model. Six ambient aerosol samples were collected representing aerosol conditions likely influenced by these types of INPs: marine, mineral dust, continental pristine, continental polluted, combustion-related and rural continental aerosol. Active INP concentrations were seen to be augmented by about 40 % to 100 % (or 70 % to 200 %), depending on the sample, over 2 (or 10) hours. This degree of time dependence observed was comparable to that seen in previous published works. Our observations show that the minority of active ice nuclei (IN) with strong time dependency on hourly time scales display only weak time dependence on short time scales of a few minutes. A general tendency was observed for the natural time scale of the freezing to dilate increasingly with time. The fractional freezing rate was observed to steadily declines exponentially with the order of magnitude (logarithm) of the time since the start of isothermal conditions. A representation of time dependence for incorporation into schemes of heterogeneous ice nucleation that currently omit time dependence is proposed.

Efficacy of a portable, moderate-resolution, fast-scanning differential mobility analyzer for ambient aerosol size distribution measurements

Ambient aerosol size distributions obtained with a compact scanning mobility analyzer, the “Spider” differential mobility analyzer (DMA), are compared to those obtained with a conventional mobility analyzer, with specific attention to the effect of mobility resolution on the measured size distribution parameters. The Spider is a 12 cm diameter radial differential mobility analyzer that spans the 10–500 nm size range with 30 s mobility scans. It achieves its compact size by operating at a nominal mobility resolution R=3 (sheath flow = 0.9 L min−1; aerosol flow = 0.3 L min−1) in place of the higher ratio of sheath flow to aerosol flow commonly used. The question addressed here is whether the lower resolution is sufficient to capture key characteristics of ambient aerosol size distributions. The Spider, operated at R=3 with 30 s up- and downscans, was co-located with a TSI 3081 long-column mobility analyzer, operated at R=10 with a 360 s sampling duty cycle. Ambient aerosol data were collected over 26 consecutive days of continuous operation, in Pasadena, CA. Over the 17–500 nm size range, the two instruments exhibit excellent correlation in the total particle number concentrations and geometric mean diameters, with regression slopes of 1.13 and 1.00, respectively. Our results suggest that particle sizing at a lower resolution than typically employed may be sufficient to obtain key properties of ambient size distributions, at least for these two moments of the size distribution. Moreover, it enables better counting statistics, as the wider transfer function for a given aerosol flow rate results in a higher counting rate.

Aerosol dynamics and dispersion of radioactive particles

In the event of a failure of a nuclear power plant with release of radioactive material into the atmosphere, dispersion modelling is used to understand how the released radioactivity is spread. For the dispersion of particles, Lagrangian particle dispersion models (LPDMs) are commonly used, in which model particles, representing the released material, are transported through the atmosphere. These model particles are usually inert and undergo only first-order processes such as dry deposition and simplified wet deposition along the path through the atmosphere. Aerosol dynamic processes including coagulation, condensational growth, chemical interactions, formation of new particles and interaction with new aerosol sources are usually neglected in such models. The objective of this study is to analyse the impact of these advanced aerosol dynamic processes if they were to be included in LPDM simulations for use in radioactive preparedness. In this investigation, a fictitious failure of a nuclear power plant is studied for three geographically and atmospherically different sites. The incident was simulated with a Lagrangian single-trajectory box model with a new simulation for each hour throughout a year to capture seasonal variability of meteorology and variation in the ambient aerosol. (a) We conclude that modelling of wet deposition by incorporating an advanced cloud parameterization is advisable, since it significantly influence simulated levels of airborne and deposited activity including radioactive hotspots, and (b) we show that inclusion of detailed ambient-aerosol dynamics can play a large role in the model result in simulations that adopt a more detailed representation of aerosol–cloud interactions. The results highlight a potential necessity for implementation of more detailed representation of general aerosol dynamic processes into LPDMs in order to cover the full range of possible environmental characteristics that can apply during a release of radionuclides into the atmosphere.

Ambient Aerosol Measurements in Different Environments

Particulate matter (PM) in the atmosphere has diverse natural and anthropogenic sources, and is a complex, heterogeneous mixture [...]

Efficacy of a portable, moderate-resolution, fast-scanning DMA for ambient aerosol size distribution measurements

Ambient aerosol size distributions obtained with a compact, scanning mobility analyzer, the Spider DMA, are compared to those obtained with a conventional mobility analyzer, with specific attention to the effect of mobility resolution on the measured size distribution parameters. The Spider is a 12-cm diameter radial differential mobility analyzer that spans the 10–500 nm size range with 30s mobility scans. It achieves its compact size by operating at a nominal mobility resolution R = 3 (sheath flow = 0.9 L/min, aerosol flow = 0.3 L/min), in place of the higher sheath-to-aerosol flow commonly used. The question addressed here is whether the lower resolution is sufficient to capture the dynamics and key characteristics of ambient aerosol size distributions. The Spider, operated at R = 3 with 30s up and down scans, was collocated with a TSI 3081 long-column mobility analyzer, operated at R = 10 with a 360s sampling duty cycle. Ambient aerosol data were collected over 26 consecutive days of continuous operation, in Pasadena, CA. Over the 20–500 nm size range, the two instruments exhibit excellent correlation in the total particle number concentrations and geometric mean diameters, with regression slopes of 1.13 and 1.00, respectively. Our results suggest that particle sizing at a lower resolution than typically employed is sufficient in obtaining the key properties of ambient size distributions.