Dry Deposition of Atmospheric Aerosols: Approaches, Observations, and Mechanisms

Aerosols are liquid or solid particles suspended in the atmosphere, typically with diameters on the order of nanometers to microns. These particles impact air quality and the radiative balance of the planet. Dry deposition is a key process for the removal of aerosols from the atmosphere and plays an important role in controlling the lifetime of atmospheric aerosols. Dry deposition is driven by turbulence and shows a strong dependence on particle size. This review summarizes the mechanisms behind aerosol dry deposition, including measurement approaches, field observations, and modeling studies. We identify several gaps in the literature, including deposition over the cryosphere (i.e., snow and ice surfaces) and the ocean; in addition, we highlight new techniques to measure black carbon fluxes. While recent advances in aerosol instrumentation have enhanced our understanding of aerosol sources and chemistry, dry deposition and other loss processes remain poorly investigated. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 72 is April 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Preliminary results for salt aerosol production intended for marine cloud brightening, using effervescent spray atomization

The large-scale production of vast numbers of suitable salt nuclei and their upward launch is one of the main technological barriers to the experimental testing of marine cloud brightening (MCB). Very promising, though not definitive, results have been obtained using an adapted version of effervescent spray atomization. The process is simple, robust and inexpensive. This form of effervescent spraying uses only pressurized water and air sprayed from small nozzles to obtain very fine distributions. While it is far from optimized, and may not be the best method if full deployment is ever desired, we believe that even in its present form the process would lend itself well to preliminary field test investigations of MCB. Measurements obtained using standard aerosol instrumentation show approximately lognormal distributions of salt nuclei with median diameters of approximately 65 nm and geometric standard deviations slightly less than 2. However, these measurements are not in agreement with those based on scanning electron microscopy imaging of collected particles, an observation that has not yet been explained. Assuming the above distribution, 10 15 particles per second could be made with 21 kW of spray power, using approximately 200 nozzles. It is envisioned that existing snow making equipment can be adapted to launch the nuclei 60–100 m into the air, requiring approximately 20 kW of additional power.

Characterization and first results of an ice nucleating particle measurement system based on counterflow virtual impactor technique

A specific instrument combination was developed to achieve a better microphysical and chemical characterization of atmospheric aerosol particles that have the potential to act as ice nucleating particles (INP). For this purpose a pumped counterflow virtual impactor system called IN-PCVI was set up and characterized to separate ice particles that had been activated on INP in the Fast Ice Nucleus Chamber (FINCH) from interstitial, non-activated particles. This coupled setup consisting of FINCH (ice particle activation and counting), IN-PCVI (INP separation and preparation), and further aerosol instrumentation (INP characterization) had been developed for the application in field experiments. The separated INP were characterized on-line with regard to their total number concentration, number size distribution and chemical composition, especially with the Aircraft-based Laser Ablation Aerosol Mass Spectrometer ALABAMA. Moreover, impactor samples for electron microscopy were taken. Due to the coupling the IN-PCVI had to be operated with different flow settings than known from literature, which required a further characterization of its cut-off-behavior. Taking the changed cut-off-behavior into account, the INP number concentration measured by the IN-PCVI system was in good agreement with the one detected by the FINCH optics for water saturation ratios up to 1.01 (ice saturation ratios between 1.21–1.34 and temperatures between −18 and −26 °C). First field results of INP properties are presented which were gained during the INUIT-JFJ/CLACE 2013 campaign at the high altitude research station Jungfraujoch in the Bernese Alps, Switzerland (3580 m a.s.l.).

Ship impacts on the marine atmosphere: insights into the contribution of shipping emissions to the properties of marine aerosol and clouds

We report properties of marine aerosol and clouds measured in the shipping lanes between Monterey Bay and San Francisco off the coast of Central California. Using a suite of aerosol instrumentation onboard the CIRPAS Twin Otter aircraft, these measurements represent a unique set of data contrasting the properties of clean and ship-impacted marine air masses in dry aerosol and cloud droplet residuals. Below-cloud aerosol exhibited average mass and number concentrations of 2 μg m−3 and 510 cm−3, respectively, which are consistent with previous studies performed off the coast of California. Enhancements in vanadium and cloud droplet number concentrations are observed concurrently with a decrease in cloud water pH, suggesting that periods of high aerosol loading are primarily linked to increased ship influence. Mass spectra from a compact time-of-flight Aerodyne aerosol mass spectrometer reveal an enhancement in the fraction of organic at m/z 42 (f42) and 99 (f99) in ship-impacted clouds. These ions are well correlated to each other (R2>0.64) both in and out of cloud and constitute 14% (f44) and 3% (f99) of organic mass during periods of enhanced sulfate. High-resolution mass spectral analysis of these masses from ship measurements suggests that the ions responsible for this variation were oxidized, possibly due to cloud processing. We propose that the organic fractions of these ions be used as a metric for determining the extent to which cloud-processed ship emissions impact the marine atmosphere where (f42 > 0.15; f99 > 0.04) would imply heavy influence from shipping emissions, (0.05 < f42 < 0.15; 0.01 < f99 < 0.04) would imply moderate, but persistent, influences from ships, and (f42 < 0.05; f99 < 0.01) would imply clean, non-ship-influenced air.

Ship impacts on the marine atmosphere: insights into the contribution of shipping emissions to the properties of marine aerosol and clouds

We report properties of marine aerosol and clouds measured in the shipping lanes between Monterey Bay and San Francisco off the coast of Central California. Using a suite of aerosol instrumentation onboard the CIRPAS Twin Otter aircraft, these measurements represent a unique set of data contrasting the properties of clean and ship-impacted marine air masses in dry aerosol and cloud droplet residuals. Average mass and number concentrations of below-cloud aerosol of 2 μg m−3 and 510 cm−3 are consistent with previous studies performed off the coast of California. Enhancement of vanadium and cloud droplet number concentration observed concurrently with a decrease in cloud water pH suggests that periods of high aerosol loading are primarily linked to increased ship influence. Mass spectra from a compact time-of-flight Aerodyne aerosol mass spectrometer reveal an enhancement in the fraction of organic at m/z 42 (f42) and 99 (f99) in ship-impacted clouds. These ions are well correlated to each other (R2 > 0.64) both in and out of cloud and dominate organic mass during periods of enhanced sulfate. High-resolution mass spectral analysis of these masses from ship measurements suggests that the ions responsible for this variation were oxidized, possibly due to cloud processing. We propose that the organic fractions of these ions be used as a metric for determining the extent to which ships impact the marine atmosphere where (f42 > 0.15; f99 > 0.04) would imply heavy influence from shipping emissions, (0.05 < f42 < 0.15; 0.01 < f99 < 0.04) would imply moderate, but persistent, influences from ships, and (f42 < 0.05; f99 < 0.01) would imply clean, non-ship-influenced air.

Liquid flame spray for generating metal and metal oxide nanoparticle test aerosol

A flame-based method for generating nanoparticles with production rate in the order of g/min is presented to be used in a variety of applied studies concerning nanoparticle measurements and toxicological tests. In this study, ferric oxide, titanium dioxide, and silver nanoparticles were produced by this technique, as an example of the variety of producible compounds, and number and surface area were measured by state-of-art aerosol instruments. In the primary experiments of this study, the generator was used in a conventional way, in a fume cupboard, and the aerosol was measured from the exhaust duct of the cupboard. It has been shown that this steady, turbulent flame generator is also suitable for producing high-concentration aerosols in a wider concept. The generated aerosol was measured by variety of aerosol instrumentation to show the applicability of the generator. When using the generator intentionally as a source of aerosol in the flame processing room, mean nanoparticle sizes of 5—60 nm and active surface area concentration ranges of 1—10,000 μm2/cm 3 were covered for the room aerosol.

Gas Phase Oxidation of Elemental Iodine in Containment Conditions

The aim of the study was to verify the possible formation of iodine aerosol when gaseous iodine is exposed to radiation. The effect of oxygen, ozone and iodine concentration as well as that of radiation intensity on IOX aerosol formation was determined. Experimental evidence on kinetics of particle formation, growth and transport was gained. Experiments were carried out using EXSI facility at VTT. Gaseous iodine was produced from I2-KI-water solution and carried into the facility with N2-O2 mixture. Oxygen concentration was varied between 2–50 %-vol. Due to the production technique flow contained always some humidity. UVC lamps were used as source of ionizing radiation. Ozone concentration was also varied with an ozonator. The reactions took place in a flow furnace heated to 120°C. Reaction and aerosol kinetics were studied by varying the residence time between about 2–7 seconds. The formed particles were collected on filters, after which gaseous iodine was trapped in NaOH-water solution. The amount of iodine in filters and in trapping bottles was analysed with ICP-MS. Particle concentration and size distribution were measured online using aerosol instrumentation. Ozone concentration was measured online with FTIR. The morphology as well as the elemental composition of the particles were analysed with SEM-EDX. The conversion of gaseous iodine to aerosol particles was rapid and almost complete even with low ozone concentration. The fraction of gaseous iodine increased though with decreasing residence time. Diameter of the formed primary particles was approximately 10 nm. Particles grew by agglomeration and by surface reaction. Agglomerate size distribution was log-normal with NMD varying between 60–120 nm. Size of the agglomerates increased and number concentration decreased with increasing residence time. Agglomerate size also increased with iodine concentration. Presence of ozone promoted retention of iodine in the facility probably by surface reaction. Some iodine deposited on surfaces evaporated later and formed particles in the gas stream. According to SEM-EDX analysis, particles deposited on carbon foil contained iodine and oxygen. However, the exact speciation is not known. Particles deposited on copper grids had probably reacted to copper iodide.