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.

Determining the fluctuation of PM2.5 mass concentration and its applicability to Monin-Obukhov similarity

In this manuscript, a method based on the nonlinear relationship between mass concentration of fine-particle and visibility is introduced to obtain the high-frequency mass concentration and vertical mass flux of PM_2.5 during a haze pollution process. The high-frequency sampling was obtained using a visibility meter upgraded by our research group. To make the results more reliable, we restricted the service conditions of this method, that is, RH < 80% and the limited visibility value (15 km herein). The method was applied to a period of pollution from 27 December 2018 to 7 January 2019 at the Pingyuan site, located in the North China Plain. A 1-Hz fluctuation of the mass concentration of PM_2.5 was obtained, as was the vertical turbulent flux of the mass concentration of PM_2.5. The mean flux of the mass concentration of PM_2.5 for the entire 13 days was 0.022 μg m^-2 s^-1. The turbulent fluxes in five short-term pollution cases lasting for several hours or several days were -0.015 μg m^-2 s^-1, -0.03 μg m^-2 s^-1, 0.053 μg m^-2 s^-1, 0.023 μg m^-2 s^-1 and 0.075 μg m^-2 s^-1. Then, the relationship between the absolute value of the vertical flux of PM_2.5 mass concentration w^'c^'¯ and many turbulent variables showed that the vertical flux decreases exponentially with the increase of variables such as turbulent kinetic energy, standard deviation σ_u, σ_w, horizontal mean wind U, momentum flux τ and heat flux H. However, the absolute value of the vertical flux w^'c^'¯ was not affected by the value of the mass concentration of PM_2.5. The turbulent characteristics of PM_2.5 were also analysed. Under unstable conditions, the normalized standard deviation of PM_2.5σ_c/C_∗=20.07-zL^-1/3, where c is mass concentration of PM_2.5, C_∗ is scale of PM_2.5. The normalized spectra (nS_PM2.5(n)/σ_c²) followed the Kolmogorov form, n^-2/3, where n is frequency, and the co-spectra of deviation of vertical wind speed and mass concentration of PM_2.5 (nC_wPM2.5(n)/σ_wσ_c) was n^-4/3. Finally, the universal function of the mass concentration of PM_2.5 was calculated. The fitted curves for unstable and stable conditions were φ_czL=4.01-9.8zL^-1/2 and φ_czL=4.01+1.2zL. Although the observed values of the universal function of the mass concentration of PM_2.5 were widely scattered with respect to z/L, the trend is obvious. The measurements show that PM_2.5 has similar properties to regular scalars such as potential temperature or water vapor. Particularly, the measurement shows a good trend to satisfy the traditional Monin-Obukhov similarity theory (MOST). However, further experiments should be implemented for verification and to provide a more accurate coefficient.

Bulk atmospheric deposition of persistent organic pollutants and polycyclic aromatic hydrocarbons in Central Europe

Polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) are ubiquitous and toxic contaminants. Their atmospheric deposition fluxes on the regional scale were quantified based on simultaneous sampling during 1 to 5 years at 1 to 6 background/rural sites in the Czech Republic and Austria. The samples were extracted and analysed by means of gas chromatography coupled to mass spectrometry. For all seasons and sites, total deposition fluxes for Σ₁₅PAHs ranged 23-1100 ng m^-2 d^-1, while those for Σ6PCBs and Σ12OCPs ranged 64-4400 and 410-7800 pg m^-2 d^-1, respectively. Fluoranthene and pyrene were the main contributors to the PAH deposition fluxes, accounting on average for 19% each, while deposition fluxes of PCBs and OCPs were dominated by PCB153 (26%) and γ-hexachlorobenzene (30%), respectively. The highest deposition flux of Σ₁₅PAHs was generally found in spring, while no seasonality was found for PCB deposition. For deposition fluxes for Σ₁₂OCPs, no clear spatial trend was found, confirming the perception of long-lived regional pollutants. Although most OCPs and PCBs hardly partition to the particulate phase in ambient air, on average, 42% of their deposition fluxes were found on filters, confirming the perception that particle deposition is more efficient than dry gaseous deposition. Due to methodological constraints, fluxes derived from bulk deposition samplers should be understood as lower estimates, in particular with regard to those substances which in ambient aerosols mostly partition to the particulate phase.

Continuous flow hygroscopicity-resolved relaxed eddy accumulation (Hy-Res REA) method of measuring size-resolved sodium chloride particle fluxes

The accurate representation of aerosols in climate models requires direct ambient measurement of the size- and composition-dependent particle production fluxes. Here, we present the design, testing, and analysis of data collected through the first instrument capable of measuring hygroscopicity-based, size-resolved particle fluxes using a continuous-flow Hygroscopicity-Resolved Relaxed Eddy Accumulation (Hy-Res REA) technique. The Hy-Res REA system used in this study includes a 3D sonic anemometer, two fast-response solenoid valves, two condensation particle counters, a scanning mobility particle sizer, and a hygroscopicity tandem differential mobility analyzer. The different components of the instrument were tested inside the US Environmental Protection Agency's Aerosol Test Facility for sodium chloride and ammonium sulfate particle fluxes. The new REA system design does not require particle accumulation, and therefore avoids the diffusional wall losses associated with long residence times of particles inside the air collectors of traditional REA devices. A linear relationship was found between the sodium chloride particle fluxes measured by eddy covariance and REA techniques. The particle detection limit of the Hy-Res REA flux system is estimated to be ~3 × 10⁵ m^-2 s^-1. The estimated sodium chloride particle classification limit, for the mixture of sodium chloride and ammonium sulfate particles of comparable concentrations, is ~6 × 10⁶ m^-2 s^-1.

Spatio-temporal variations in PM leaf deposition: A meta-analysis

Particulate matter (PM) pollution in urban cities is of great concern for public health due to its global and adverse effect of human health while ecosystems function and vegetation control is an effective and eco-friendly way to alleviate PM pollution. We reviewed 150 studies conducted in 15 countries that were published between 1960 and 2016 and used a meta-analysis to examine the time trends and regional differences in leaf deposited PM of urban greening plants. The results suggested that the weekly PM leaf deposition varied markedly with both plant species and space-time and the average value was 1.71 ± 0.05 g m^-2·wk^-1, and the variations occurred because of vegetation factors, characteristics of the PM source and meteorological factors. Moreover, fine particulate matter accounts for the minimum proportion of the total PM mass but its number ratio is maximum, more than 90% of the total number of particles. This meta-analysis illustrated the spatio-temporal trends and variations in PM leaf deposition and the influencing factors, which provides a scientific basis for the mechanism of PM deposition on leaf surface as well as plant selection and configuration in urban greening.