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Atmospheric HONO acts as a major source for OH radicals in polluted areas, playing an important role in formation of secondary pollutants. However, the atmospheric HONO sources remain unclear. Here we propose that the heterogeneous reaction of NO2 on aerosols during aging processes acts as the dominant source for nocturnal HONO. Based on the nocturnal variations of HONO and related species in Tai'an city of China, we firstly developed new method to estimate the localized HONO dry deposition velocity (v(HONO)). The estimated v(HONO) of 0-0.077 m/s was in a good agreement with the reported ranges. Additionally, we set up a parametrization to reflect the HONO formation from the aged air parcels based on the variation of HONO/NO2 ratio. The detailed variation of nocturnal HONO could be well reproduced by a complete budget calculation coupled with above parameterizations, with the difference between the observed and calculated HONO levels being <5 %. The results also revealed the average contribution of HONO formation from aged air parcels to atmospheric HONO could achieve to be ~63 % in average.

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The dry deposition process refers to the flux loss of an atmospheric pollutant due to uptake of the pollutant by the earth's surfaces. Dry deposition flux of a chemical species is typically calculated as the product of its surface-layer concentration and its dry deposition velocity (Vd). Field measurement based Vd data are very scarce or do not exist for many chemical species considered in chemistry transport models. In the present study, gaseous and particulate dry deposition schemes were applied to generate a database of hourly Vd for 45 gaseous species and three particle size ranges for two years (2016-2017) at a 15 km by 15 km horizontal resolution across North America. Hourly Vd of the 45 gaseous species ranged from < 0.001 to 4.6 cm/sec across the whole domain, with chemical species-dependent median (mean) values being in the range of 0.018-1.37 cm/sec (0.05-1.43 cm/sec). The spatial distributions of the two-year average Vd showed values higher than 1-3 cm/sec for those soluble and reactive species over certain land types. Soluble species have the highest Vd over water surfaces, while insoluble but reactive species have the highest Vd over forests. Hourly Vd of PM2.5 across the whole domain ranged from 0.039 to 0.75 cm/sec with median (mean) value of 0.18 (0.20) cm s-1, while the mean Vd for PM2.5-10 is twice that of PM2.5. Uncertainties in the modeled Vd are typically on the order of a factor of 2.0 or larger, which needs to be considered when applying the dataset in other studies.

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Human health risks brought by fine atmospheric particles raise scholarly and policy awareness about the role of urban trees as bio-filters of air pollution. While a large number of empirical studies have focused on the characteristics of vegetation leaves and their effects on atmospheric particle retention, the dry deposition of particles on branches, which plays a significant role in capturing and retaining particles during the defoliation period and contributes substantially to total removal of atmospheric particles, is under-investigated. To fill in this knowledge gap, this case study examined the dry deposition velocities (Vd) of submicron particulate matters (PM1) on the branches of six common deciduous species in Shanghai (China) using laboratory experiments. And the association between Vd and key branch anatomical traits (including surface roughness, perimeter, rind width proportion, lenticel density, peeling, and groove/ridge characteristics) was explored. It was found that surface roughness would increase Vd, as a rougher surface significantly increases turbulence, which is conducive to particle diffusion. By contrast, peeling, branch perimeter, and lenticel density would decrease Vd. Peeling represents the exfoliated remains on the branch surfaces which may flutter considerably with airflow, leading to particle resuspension and low Vd. When branch perimeter increases, the boundary layer of branches thickens and a wake area appears, increasing the difficulty of particles to reach branch surface, and reducing Vd. While lenticels can increase the roughness of branch surface, their pointy shape would uplift airflow and cause a leeward wake area, lowering Vd. This finely wrought study contributes to a better understanding of branch dry deposition during leaf-off seasons and potential of deciduous trees serving as nature-based air filters all year round in urban environments.

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This study aimed at estimating the transport dynamics of a single severe acute respiratory syndrome corona-virus 2 (SARS-CoV-2)-laden droplet of 1 to 500 µm in diameter at a wind speed from 1 to 4 m/s. Motion dynamics of SARS-CoV-2-laden respiratory droplets under calm or turbulent air conditions were quantified using a combined model. Dalton's law was implemented to estimate their evaporation. One-factor-at-a-time procedure was applied for the sensitivity analysis of model of deposition velocity. The transport distance of the single virus ranged from 167 to 1120 m as a function of the droplet size, wind speed, and falling time. The evaporation times of the droplets ≤ 3 and ≤ 14 µm in diameter were shorter than their settling times from 1.7 m in height at midnight and midday, respectively. Such droplets remained in the air for about 5 min as the droplet nuclei with SARS-CoV-2. The minimum transport distance of the respiratory droplets of 1-15 µm varied between 8.99 and 142 m at a wind speed range of 1-4 m/s, based on their deposition velocity. With their short transport distance, the larger droplet (30 to 500 µm) was not suspended in the air even under the windy conditions. The deposition velocity was found most sensitive to the droplet diameter. The droplets < 15 µm in diameter completely evaporated at midday and the droplet nuclei with the single virus can travel a minimum distance of 500 m under a horizontal wind speed of 3 m/s.

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Deposition of atmospheric pollution as particulate matter (PM) has become a serious issue in many urban areas. This study measured and estimated the amount of atmospheric PM deposition onto oriental plane (Platanus orientalis L.) trees located in Tehran Megapolis, Iran. PM deposited on the leaves of urban trees during spring and summer was estimated using leaf wash measurements. In addition to direct measurements, the dry deposition velocity and the yearly whole-tree PM deposition were estimated using both field measurements and a theoretical model of deposition flux. We estimated air quality improvement as a result of the trees at respiratory height (1.5 m), tree height (10 m), and boundary layer height (1719 m). Foliar PM deposition during spring and summer was estimated to average 0.05 g/leaf and 41.39 g/tree using direct measurements. The annual PM deposited on the leaves, trunk, and branches of an average urban tree was calculated to be 78.60 g/tree. Trees were estimated to improve air quality at 1.5 m, 10 m, and 1719 m from ground level by 25.8%, 5.8%, and 0.1%, respectively. Hence, oriental plane trees substantially reduce PM at respiratory height.

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Urban green infrastructure is closely linked to the alleviation of pollution from atmospheric particulate matter. Although particle deposition has been shown to depend on leaf characteristics, the findings from earlier studies are sometimes ambiguous due to the lack of controlling variables. In this study, we investigated the impact of leaf morphological characteristics on PM2.5 dry deposition velocity by employing a control-variable approach. We focused on four indices: trichome density, petiole length, aspect ratio (width-to-length ratio), and fractal deviation. For each index, tree species were chosen from the same family or genus to minimize the influence of other factors and make a group of treatments for an individual index. The dry deposition velocities of PM2.5 were determined through application of an indirect method. The results revealed that the presence of leaf trichomes had a positive effect on PM2.5 dry deposition velocity, and a higher trichome density also led to a greater particle deposition velocity. Lower leaf aspect ratio, shorter petioles, and higher leaf fractal deviation were associated with greater PM2.5 dry deposition velocity. The control-variable approach allows to investigate the correlation between deposition velocity and a certain leaf characteristic independently while minimizing the effects of others. Thus, our study can clarify how a single leaf characteristic affects particle deposition velocity, and expound its potential mechanism more scientifically than the published studies. Our research points out the importance of controlling variables, and also provides ideas for future researches on related factors to be found. Meanwhile the results would help provide insight into design improvements or adaptive management for the alleviation of air pollution.

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Many studies have estimated particulate matter (PM) removal by urban trees using dry deposition models; however, few studies have quantified the accuracy of their results. Thus, this study investigated the dry deposition of PM and its associated soluble ions in five broadleaved species in three districts of Taichung, central Taiwan, through field experiments. The total suspended particulate (TSP) dry deposition flux on leaf surfaces varied with sampling time, site, and tree species. By contrast, single-factor effects were observed for PM10 and PM2.5. The average dry deposition velocities of TSPs, PM10, and PM2.5 were 0.63, 0.062, and 0.028 cm s-1, respectively. Moreover, the dry deposition velocities of sulfate and nitrate were estimated to be 0.186 and 0.194 cm s-1, respectively. A significant relationship was observed between the ambient concentration and the dry deposition flux for all size fractions of PM. By contrast, weak and negative correlations were found between particle deposition velocity and wind speed. The measured PM2.5 dry deposition velocity was approximately equal to the dry deposition velocity obtained with the i-Tree model (0.03 cm s-1), which indicated the promising application potential of i-Tree in Taiwan. Compound and rough leaves, such as leaves of the Taiwan golden-rain tree, intercepted a high amount of PM2.5, whereas the pongam tree, which has thin leaves and wax surfaces, exhibited the lowest TSP interception. Species difference mostly occurred in the dry deposition flux of nitrate rather than sulfate; however, the interception of sulfate by trees revealed the possibility of the long-range transport of air pollutants. The results of this study elucidate the dry deposition of PM and its associated soluble ions in real-world situations.

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Assessing the impact of radioactive iodine on humans subsequent to a nuclear accident requires a better understanding of its behaviour in the environment. An original approach aimed at developing a model constrained by data collected during experimental campaigns has been developed. These experimental campaigns, named MIOSEC 2 and MIOSEC 3 respectively, were conducted in the middle of grassland. They are based on emissions of gaseous elemental iodine (I2) into the atmosphere above the grassland to determine the dry deposition velocities of iodine on the grass and to model these velocities as a function of the environmental conditions, particularly wind friction velocity, sensible heat flux, and stomatal resistance. The measured dry deposition velocities were between 0.02 and 0.49 cm s-1 during MIOSEC 2, varying by more than one order of magnitude, and between 0.48 and 1.25 cm s-1 during MIOSEC 3. The dry deposition model for iodine developed as a result of these experiments relies on the micrometeorological characteristics of the atmospheric surface layer, the pertinent physical and chemical properties of the iodine and the surface properties of the grass; all these parameters were measured at the time of the experiments. Given the experimental conditions, the modelled dry deposition velocities varied between 0.11 and 0.51 cm s-1 during MIOSEC 2 and between 0.31 and 1.6 cm s-1 during MIOSEC 3. The dry deposition model for iodine indicates that the variations in deposition velocity are induced by the mechanical turbulence, since there is significant correlation between the dry deposition velocities of iodine and the wind friction velocities on grass. The model also shows that the higher deposition velocity values during MIOSEC 3 are due to the fact that the stomata were more open during the experiments. There is also significant correlation between the experimental results and modelled values both for MIOSEC 2 (R2 = 0.61) and for MIOSEC 3 (R2 = 0.71).

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Atmospheric dry deposition is a major pathway for removal of polycyclic aromatic hydrocarbons (PAHs) from the atmosphere. Despite its significance in the atmospheric environment, measurements of the dry deposition velocity (VDD) and deposition fluxes (FDD) of PAHs are relatively limited. In this study, a passive dry deposition (PAS-DD) collector was co-deployed with passive air sampler polyurethane foam (PAS-PUF) from November 2015 to November 2016 in two major cities (Kathmandu and Pokhara), Nepal, to investigate the VDD and FDD of PAHs. The VDD of PAHs ranged from 0.25 to 0.5 cm s-1 and the annual average was recorded as 0.37 ± 0.08 cm s-1. On the basis of measured VDD, the FDD of ∑15PAHs in Kathmandu and Pokhara were estimated as 66 and 5 kg yr-1 respectively. According to the measured VDD of Kathmandu and Pokhara in this study, and the previously published VDD data of Toronto, Canada, where the same PAS-DD collector was used, a significant multi-linear correlation (r2 = 0.79, p < 0.05) was found between VDD of higher molecular weight (HMW with MW ≥ 228.3 and ≥ 4 rings) PAHs and meteorological parameters (precipitation and wind speed) and vapor pressure of PAHs. To the best of our knowledge, this enabled the development of an empirical model that can exhibit the combined effects of meteorological conditions on the VDD of HMW PAHs. The model was used to estimate the VDD values for major cities in the Indo-Gangetic Plain (IGP) region and the maximum estimated proportion of HMW PAHs deposited by dry deposition reached up to 60% of total emissions. Although PAH emissions in the IGP region pose global risks, the results of this study highlight the considerable risk for local IGP residents, due to the large dry deposition proportion of HMW PAHs.

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As a typical semi-closed bay, Jiaozhou Bay, is remarkably affected by human activities. Biogenic elements transported into the oceans by the atmospheric deposition of anthropogenic particles can lead to profound impacts on the ecosystem of Jiaozhou Bay. In this paper, the researches of atmospheric dry and wet depositions in the Jiaozhou Bay were systematically summarized from the following three aspects: study methods, variation characteristics of the fluxes and their influencing factors and the ecological effects of atmospheric deposition. The concentrations and fluxes of nitrogen species in atmospheric dry and wet depositions were very high in the Jiaozhou Bay compared with other bays, estuaries and marginal seas around the world. The dissolved inorganic nitrogen (DIN) was the main component of the total dissolved nitrogen (TDN) and the dissolved organic nitrogen (DON) accounted for 22%-31% of TDN. However, the concentrations and fluxes of phosphate and silicate species were very low. The atmospheric deposition fluxes of (NO3--N+NO2--N) were slightly higher than terrestrial inputs, while the fluxes of NH4+-N, PO43--P and SiO32--Si were very low compared with terrestrial inputs. The concentration of total suspended particulates (TSP) in the air, the intensity of the emission sources, precipitation amount and meteorological conditions are the major factors influencing the atmospheric depositions of biogenic elements, which can increase the primary productivity, change the structure of nutrients and the structure of phytoplankton communities in surface seawater and further promote the succession of phytoplankton dominant species from diatom to dinoflagellate in the Jiaozhou Bay. On that basis, the future research should be focused on constructing the monitoring network for atmospheric dry and wet depositions, accurately quantifying the deposition rates of aerosol particles of different forms and sizes, recognizing the ecological effects and biogeochemical mechanisms of atmospheric depositions of biogenic elements and discriminating the mechanisms, fluxes and influencing factors of atmospheric deposition biogenic elements via indirect approaches such as overland runoff, sheetflood and permeation etc. It is significant for illuminating the effects of anthropogenic activities on the coastal waters as well as understanding the bio-geochemical processes of biogenic elements in marginal seas by elucidating atmospheric depositions of biogenic elements in the Jiaozhou Bay.

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The atmospheric dry deposition flux pollution levels were determined and the sources apportioned for 15 US EPA priority polycyclic aromatic hydrocarbons (PAHs) collected at 13 sampling sites in the Lanzhou valley using passive sampling techniques. The results are as follows. The annual atmospheric dry deposition fluxes ranged from 7.48 to 53.94 µg·(m2·d)-1, with a mean of 18.65 µg·(m2·d)-1. The highest flux levels for both the heating and non-heating seasons, 60.85 µg·(m2·d)-1 and 47.03 µg·(m2·d)-1, respectively, appeared at the traffic site (Donggang Bridge), where the heaviest traffic in the Lanzhou valley occurs. The lowest flux levels of 8.16 µg·(m2·d)-1 and 6.80 µg·(m2·d)-1 for the heating and non-heating seasons, respectively, were found at Baita Mountain, which has a higher percentage of vegetation cover. Meanwhile, the flux levels at the background site (Guantangou Mountain) were significantly lower than those of the other sampling sites. For dry deposition, the PAH components Phe, Flua, Flu, and Pyr were dominant in the heating and non-heating periods, and the sums of the percentages of 3- and 4-ring PAHs were 87.53% for the heating season and 82.73% for the non-heating season. Moreover, the percentage of 5- and 6-ring PAHs for the non-heating period was higher than that for the heating period, which may be because the lighter PAHs were easily volatilized, and thus escaped. Furthermore, the sources of atmospheric dry deposition were identified by principal component analysis (PCA). The results of source apportionment showed that the main atmospheric PAH dry deposition in the Lanzhou valley were from vehicle exhaust emissions, coal combustion, and the coking industry. In further detail, coal combustion and the coking industry contributed most of the PAH dry deposition emissions during the heating period except for the traffic area site of Donggang Bridge, whereas vehicle exhaust was the dominant contributor during the non-heating season. In addition, the annual average dry deposition velocities of atmospheric PAHs were calculated using the dry deposition model for three sites: 0.20 cm·s-1 downtown (Environmental Protection Agency of Lanzhou; JCZ), 0.15 cm·s-1 in an industrial area (Lanyuan Hotel of Xigu; LLH), and 0.17 cm·s-1 in a traffic area (the Staff Hospital of Gansu Province; ZGH), of which the latter two were relatively lower because of comprehensive meteorological factors such as wind speed, temperature, and land use categories. Regarding the dry deposition flux values of the 3- and 4-ring PAHs, the simulated values were a bit larger than the observed values, but all were at the same level of magnitude. However, the simulated flux values were closer to their observed values for 4-ring PAHs than for 3-ring PAHs, which indicated that 3-ring PAHs were lost more easily than 4-ring PAHs were during monitoring.