Deposition Velocity Research Articles

Constant flux layers with gravitational settling: links to aerosols, fog and deposition velocities

Abstract. Turbulent boundary layer concepts of constant flux layers and surface roughness lengths are extended to include aerosols and the effects of gravitational settling. Interactions between aerosols and the Earth's surface are represented via a roughness length for aerosol which will generally be different from the roughness lengths for momentum, heat or water vapour. Gravitational settling will impact vertical profiles and the surface deposition of aerosols, including fog droplets. Simple profile solutions are possible in neutral and stably stratified atmospheric surface boundary layers. These profiles can be used to predict deposition velocities and to illustrate the dependence of deposition velocity on reference height, friction velocity and gravitational settling velocity.

Urban road greenbelt configuration: The perspective of PM2.5 removal and air quality regulation.

The establishment of the road green belt (RGB) is an effective means to reduce particle matter (PM2.5) emissions from road traffic. This study tested the ability of 23 common tree species in Shenzhen to reduce PM2.5 concentrations using field investigations and wind tunnel tests. The association between leaf microstructure and individual reduction ability was also analyzed. Finally, the impact of three RGB configurations (i.e., arbor, shrub, arbor+shrub) on road PM2.5 dispersion and deposition was simulated using the ENVI-met three-dimensional aerodynamic model, based on which an optimal RGB configuration was proposed. There were three key findings of the tests. First, the wind speed was the main factor affecting the PM2.5 concentration (54.2%), followed by vehicle flow (27.7%), temperature (14.2%), and time factor (7.6%). Second, the range of dry deposition velocity (Vd) was 0.04-6.4m/s, and the dominant dust-retaining plant species were the evergreen trees, Ficus microcarpa and Ficus altissima, and the evergreen shrubs, Codiaeum variegatum and Fagraea ceilanica. A higher proportion of grooves or larger stomata would increase the probability that the blade would capture PM2.5. Third, the shrub RGB demonstrated the best performance in terms of pollutant dispersion; its PM2.5 concentration at the respiratory height (RH, 1.5m) on the pedestrian crossing was 15-20% lower than the other RGB configurations. In terms of pollutant deposition, the arbor+shrub composite RGB was two-fold better than the other RGB configurations. Moreover, it was more advantageous to plant shrub RGBs in street canyons to achieve a balance between the lowest concentration and the largest deposition of PM2.5 pollutants. The findings of this study will facilitate the RGB configurations with good dust retention ability.

The effectiveness of urban trees in reducing airborne particulate matter by dry deposition in Tehran, Iran.

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 (1.5m), tree height (10m), and boundary layer height (1719m). Foliar PM deposition during spring and summer was estimated to average 0.05g/leaf and 41.39g/tree using direct measurements. The annual PM deposited on the leaves, trunk, and branches of an average urban tree was calculated to be 78.60g/tree. Trees were estimated to improve air quality at 1.5m, 10m, and 1719m from ground level by 25.8%, 5.8%, and 0.1%, respectively. Hence, oriental plane trees substantially reduce PM at respiratory height.

Cold spray deposition characteristic and bonding of CrMnCoFeNi high entropy alloy

Particle bonding physics remains an important area of study in cold spray (CS) deposition. It has been argued that for pure metals, large interfacial strains in particles triggered either by adiabatic shear instability (ASI) or hydrostatic plasticity promote bonding while the bonding mechanisms of high entropy alloys (HEA) have not been explored. HEAs are an emerging class of materials that have superior work-hardening ability and are usually resistant to softening, a behavior that is different from pure elements and conventional alloys. In this study, equiatomic CrMnCoFeNi HEA feedstock powder is produced and CS experiments are conducted to explore the deformation evolution of these HEA particles and their bonding characteristics. The shear localization strain of the HEA is theoretically estimated and compared to conventional alloys and elements to explain the deposition behavior of the HEA. Results show that the particle impact morphology is strain rate dependent in addition to being material and microstructure dependent. Electron channelling contrast imaging reveals severe plastic deformation at the lower half of the particles due to the dynamic inertia effect. Deformation nano-twins with a thickness of about 200 nm were observed. HEA/HEA pair has a higher critical velocity compared to HEA/Nickel, HEA/Inconel625 and HEA/Stainless steel 304 pairs. This is attributed to the excellent strain hardening and intermediate softening of the HEA alloy used, which potentially postpones shear localization and impedes bonding, requiring a higher deposition velocity. Finally, it is shown that the ASI based criterion for bonding can predict the deposition mechanism of HEAs more inclusively.

Anisotropic wetting and final shape of droplets impacting on micropillars with non-vertical lateral walls

The control of the droplet shape during the impact on a solid surface is of relevance for several practical applications, such as inkjet printing technologies. Although several studies have reported factors affecting the final droplet shape, understanding of the liquid spreading process is still lacking. In this work, droplets of different velocities are deposited on surfaces patterned with conical and cylindrical pillars. It is shown that for the structures of the same height and pitch, the shape of the lateral wall of the micro-structures affects the droplet shape. In addition, at higher deposition velocity, the initial polygonal/square shape of the droplets evolves into a more circular shape. The change in the shape due to the lateral wall of the structures is the result of the solid–liquid contact both above and below the structures and the way in which the liquid is able to move in between the structures for different passage areas.

Development of Site Classification System and Seismic Site Coefficients for Korea

ABSTRACT The contemporary two-parameter-based site classification system of Korea, which is depth to bedrock and averaged shear wave velocity of soil deposits overlying the bedrock, has been reported to be ineffective in representing the unique amplification characteristics of shallow bedrock profiles. A new site classification scheme conditioned on site natural period and corresponding intensity-dependent seismic coefficients for Korean site conditions is proposed based on extensive one-dimensional non-linear site response analysis outputs. It is highlighted that the proposed site classification system and site coefficients coupled with a new elastic response spectrum considerably reduce the residuals with the computed site response analysis outputs.

Gravitational effect on particle deposition in narrow channels with porous walls

Convective transport of particles in a narrow horizontal channel with porous walls is analyzed by considering the gravitational effect. Particle concentration, penetration and deposition velocity in three different channels are obtained by using the theoretical solutions. The results show that the differences between the concentrations in the three channels become evident with the increase of the permeability coefficient and slip factor of porous materials, especially for fine particles. The wall structure of the channel has significant influences on particle deposition velocity. When both the upper and lower walls are porous, the deposition in the channel is weakened. In addition, particle deposition velocities on the porous walls are consistent with each other, and the velocities are higher than those on the solid walls.

Uncertainty analysis of modeled ozone changes due to anthropogenic emission reductions in Eastern Texas

We investigate whether an air quality model can predict a concentration change between two related scenarios with greater certainty than either scenario alone and also estimate the uncertainty in Relative Reduction Factors (RRFs). RRFs are ratios of future-year to base-year model results that are used by the U.S. Environmental Protection Agency and the States to predict future ozone (O3) and thus are critical to developing emission control strategies to reduce O3 and achieve compliance with the O3 standard. We conducted simulations for 2012 and 2020 using the Comprehensive Air Quality Model with Extensions and calculated the uncertainties for O3, O3 change (ΔO3), and RRFs using O3 sensitivities. These uncertainties were derived from estimated uncertainties in an extensive set of model inputs comprising the boundary concentrations, dry deposition velocities, emissions, and chemistry. We developed new equations to determine the uncertainties in ΔO3 and RRFs. We evaluated the RRFs obtained from our model by comparing them to recent ozone trends in Eastern Texas (the trends omit 2020 due to the effects of the pandemic.) Uncertainties in ΔO3 and RRFs depend strongly on the degree of correlation between errors in the anthropogenic emissions in the two years. We considered Case A, full correlation of the errors, and Case B, no correlation of errors, as limiting cases. The uncertainty in ΔO3 in Case A is 10–20% of the uncertainty in O3 over most of eastern Texas, and in Case B it is 45–75%. For monitoring sites in three Texas cities, the uncertainty in the RRFs is 0.2–1.0% of the RRFs (Case A) or 4.8–8.5% (Case B). Using ΔO3 or RRFs with base-year design values to predict O3 is likely to produce a more accurate prediction than the modeled O3 for the future year regardless of whether Case A or Case B holds. However, determining the degree of correlation of emission errors between years is necessary to refine further the uncertainty in predicted O3 and merits future study.

Size distributions and dry deposition fluxes of water-soluble inorganic nitrogen in atmospheric aerosols in Xiamen Bay, China.

Size-segregated aerosol particles were collected using a high volume MOUDI sampler at a coastal urban site in Xiamen Bay, China, from March 2018 to June 2020 to examine the seasonal characteristics of aerosol and water-soluble inorganic ions (WSIIs) and the dry deposition of nitrogen species. During the study period, the annual average concentrations of PM1, PM2.5, PM10, and TSP were 14.8 ± 5.6, 21.1 ± 9.0, 35.4 ± 14.2 μg m−3, and 45.2 ± 21.3 μg m−3, respectively. The seasonal variations of aerosol concentrations were impacted by the monsoon with the lowest value in summer and the higher values in other seasons. For WSIIs, the annual average concentrations were 6.3 ± 3.3, 2.1 ± 1.2, 3.3 ± 1.5, and 1.6 ± 0.8 μg m−3 in PM1, PM1-2.5, PM2.5–10, and PM>10, respectively. In addition, pronounced seasonal variations of WSIIs in PM1 and PM1-2.5 were observed, with the highest concentration in spring-winter and the lowest in summer. The size distribution showed that SO42−, NH4+ and K+ were consistently present in the submicron particles while Ca2+, Mg2+, Na+ and Cl− mainly accumulated in the size range of 2.5–10 μm, reflecting their different dominant sources. In spring, fall and winter, a bimodal distribution of NO3− was observed with one peak at 2.5–10 μm and another peak at 0.44–1 μm. In summer, however, the fine mode peak disappeared, likely due to the unfavorable conditions for the formation of NH4NO3. For NH4+ and SO42−, their dominant peak at 0.25–0.44 μm in summer and fall shifted to 0.44–1 μm in spring and winter. Although the concentration of NO3–N was lower than NH4–N, the dry deposition flux of NO3–N (35.77 ± 24.49 μmol N m−2 d−1) was much higher than that of NH4–N (10.95 ± 11.89 μmol N m−2 d−1), mainly due to the larger deposition velocities of NO3–N. The contribution of sea-salt particles to the total particulate inorganic N deposition was estimated to be 23.9—52.8%. Dry deposition of particulate inorganic N accounted for 0.95% of other terrestrial N influxes. The annual total N deposition can create a new productivity of 3.55 mgC m−2 d−1, accounting for 1.3–4.7% of the primary productivity in Xiamen Bay. In light of these results, atmospheric N deposition could have a significant influence on biogeochemistry cycle of nutrients with respect to projected increase of anthropogenic emissions from mobile sources in coastal region.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10874-021-09427-8.

Historical dry deposition of air pollution in the urban background in Oslo, Norway, compared to Western European data

The historical (1835–2020) dry deposition of major air pollutants (SO2, NOx, O3 and PM2.5) in the urban background in Oslo, Norway, in a situation that could represent the building facades, was approximated from reported fuel combustion, emission factors, air concentrations since 1960, and dry deposition velocities. The annual accumulated dry deposition (and thus not considering the removal processes) of the pollutants, together, was found to have varied from about 2.3 to 27 g m−2, with the maximum in the 1960s caused by high SO2 emissions from the combustion of fuel oils, and with 1.6 kg m−2 having deposited over all the years. The deposition of PM2.5 was found to have dominated from 1835, have increased to a maximum in 1875 and then slowly decreased. The SO2 deposition decreased to a low value around 1990. The NOx deposition was also at its highest in the 1960s to about 1970, it became the largest from the 1980s, and then showed a clear decrease from about 2010. The O3 deposition was lower in the years of the maximum total and NOx deposition. The dry deposition of O3 and NOx were found to be about similar in 2020, more than two times that of PM2.5 and more than four times that of SO2. The trends of the NOx emissions were found to reflect the relative (1975) and absolute (∼2000) turning points of the environmental Kuznets curves (EKC) that has been suggested for Norway, whereas the trend of the SO2 emissions seems to have “shortcut” this development by the strong regulations in the emissions from 1970 that lead to near simultaneous relative and absolute reductions. The gradual decrease of the PM2.5 emissions from about 1945 seems to correspond with the decrease in combustion energy intensity in the economy as wood was substituted with more energy efficient fuels and then with the continued reduction in the wood burning.

Effects of flow regime and conjugate heat transfer on particle deposition in heat exchange duct

Deposition of micron particles in heat exchange duct is commonly encountered in many thermal and HVAC engineering applications. Previous studies on this important issue always considered turbulent air flow regime without conjugate heat transfer (CHT). However, the flow regimes and the CHT may have significant influences on particle deposition behaviors. Therefore, this paper aims to study particle deposition characteristics in laminar flow (LF), developing turbulent flow (DTF) and fully developed turbulent flow (FDTF) regimes with and without CHT. The v′2‾−fturbulence model and discrete particle model (DPM) were used to predict gas phase and solid phase, respectively. Moreover, a modified discrete random walk method was adopted to model turbulent particle dispersion. The results showed that particle deposition velocities greatly increase for small particles (dp<20μm) but keep invariable for large particles (dp>20μm) due to thermophoretic deposition. WhenΔT=100Kanddp=1μm, the peak deposition velocity enhancement ratio reaches 3.8, 9.7 and 7.5 for LF, DTF and FDTF regimes respectively. Different flow regimes have obvious effects on the deposition characteristics. Particle deposition velocity for DTF regime is the highest, followed by FDTF case and LF case. Thermophoresis effect is reduced by the CHT, as the temperature difference decreases due to thermal conduction in solid domain. The CHT in DTF or FDTF regime has obviously larger effect on deposition characteristics, compared with the LF regime.

Surface deposition of marine fog and its treatment in the Weather Research and Forecasting (WRF) model

Abstract. There have been many studies of marine fog, some using Weather Research and Forecasting (WRF) and other models. Several model studies report overpredictions of near-surface liquid water content (Qc), leading to visibility estimates that are too low. This study has found the same. One possible cause of this overestimation could be the treatment of a surface deposition rate of fog droplets at the underlying water surface. Most models, including the Advanced Research Weather Research and Forecasting (WRF-ARW) Model, available from the National Center for Atmospheric Research (NCAR), take account of gravitational settling of cloud droplets throughout the domain and at the surface. However, there should be an additional deposition as turbulence causes fog droplets to collide and coalesce with the water surface. A water surface, or any wet surface, can then be an effective sink for fog water droplets. This process can be parameterized as an additional deposition velocity with a model that could be based on a roughness length for water droplets, z0c, that may be significantly larger than the roughness length for water vapour, z0q. This can be implemented in WRF either as a variant of the Katata scheme for deposition to vegetation or via direct modifications in boundary-layer modules.

Nitrous acid production and uptake by Zea mays plants in growth chambers in the presence of nitrogen dioxide

Nitrous acid (HONO) photolysis is an important atmospheric reaction that leads to the formation of hydroxyl radicals (OH), the main diurnal atmospheric oxidants. The process of HONO formation remains unclear, and comparisons between field measurements and model results have highlighted the presence of unknown HONO sources. HONO production on plant surfaces was recently suggested to contribute to atmospheric HONO formation, but there is limited information on the quantification of HONO production and uptake by plants. To address this gap in the existing knowledge, the current study investigated HONO exchange on living Zea mays plants. Experiments were conducted in growth chambers under controlled experimental conditions (temperature, relative humidity, NO2 mixing ratio, light intensity, CO2 mixing ratio) at temperatures ranging between 283 and 299 K. To investigate the effect of drought on HONO plant-atmosphere exchanges, experiments were carried out on two sets of Zea mays plants exposed to two different water supply conditions during their growth: optimal watering (70% of the field capacity) and water stress (30% of the field capacity). Results indicated that the uptake of HONO by control Zea mays plants increased linearly with ambient temperature, and was correlated with CO2 assimilation for temperatures ranging from 283 to 299 K. At 299 K, HONO production on the leaves offset this uptake and Zea mays plants were a source of HONO, with a net production rate of 27 ± 7 ppt h-1. Deposition velocities were higher for HONO than CO2, suggesting a higher mesophyll resistance for CO2 than HONO. As water stress reduced the stomatal opening, it also decreased plant-atmosphere gas exchange. Thus, climate change, which may limit the availability of water, will have an impact on HONO exchange between plants and the atmosphere.

Efficient ensemble generation for uncertain correlated parameters in atmospheric chemical models: a case study for biogenic emissions from EURAD-IM version 5

Abstract. Atmospheric chemical forecasts heavily rely on various model parameters, which are often insufficiently known, such as emission rates and deposition velocities. However, a reliable estimation of resulting uncertainties with an ensemble of forecasts is impaired by the high dimensionality of the system. This study presents a novel approach, which substitutes the problem into a low-dimensional subspace spanned by the leading uncertainties. It is based on the idea that the forecast model acts as a dynamical system inducing multivariate correlations of model uncertainties. This enables an efficient perturbation of high-dimensional model parameters according to their leading coupled uncertainties. The specific algorithm presented in this study is designed for parameters that depend on local environmental conditions and consists of three major steps: (1) an efficient assessment of various sources of model uncertainties spanned by independent sensitivities, (2) an efficient extraction of leading coupled uncertainties using eigenmode decomposition, and (3) an efficient generation of perturbations for high-dimensional parameter fields by the Karhunen–Loéve expansion. Due to their perceived simulation challenge, the method has been applied to biogenic emissions of five trace gases, considering state-dependent sensitivities to local atmospheric and terrestrial conditions. Rapidly decreasing eigenvalues state that highly correlated uncertainties of regional biogenic emissions can be represented by a low number of dominant components. Depending on the required level of detail, leading parameter uncertainties with dimensions of 𝒪(106) can be represented by a low number of about 10 ensemble members. This demonstrates the suitability of the algorithm for efficient ensemble generation for high-dimensional atmospheric chemical parameters.

Evaluation of heat and mass transfer in ferromagnetic fluid flow over a stretching sheet with combined effects of thermophoretic particle deposition and magnetic dipole

The features of ferromagnetic fluids make it supportive for an extensive range of usages in directing magnetic drugs and magnetic hyperthermia. Owing to all such potential applications, the current study is concerned with the ferromagnetic liquid flow past a stretching sheet with thermophoretic particle deposition and a magnetic dipole. The equations of described flow are transformed into ordinary differential equations (ODEs) by selecting suitable similarity variables. These ODEs are later solved numerically by the fourth and fifth-order Runge–Kutta–Fehlberg (RKF-45) technique by using a shooting scheme. The influence of sundry parameters on velocity, thermophoretic diffusive deposition velocity, skin-friction, Nusselt number, Sherwood number, thermal and concentration profiles are illustrated graphically. The results reveal that the increase in ferromagnetic interaction parameter improves the thermal profile but declines the velocity and concentration profiles. The imposition of thermophoretic particle deposition results in declination of the mass transfer. The escalating values of the ferromagnetic interaction parameter decline the rate of heat transfer. Finally, the rise in values of the thermophoretic parameter and thermophoretic coefficient decays thermophoretic diffusive deposition velocity.

FORCAsT‐gs: Importance of Stomatal Conductance Parameterization to Estimated Ozone Deposition Velocity

AbstractThe role of stomata in regulating photosynthesis and transpiration, and hence governing global biogeochemical cycles and climate, is well‐known. Less well‐understood, however, is the importance of stomatal control to the exchange of other trace gases between terrestrial vegetation and the atmosphere. Yet these gases determine atmospheric composition, and hence air quality and climate, on scales ranging from local to global, and seconds to decades. Vegetation is a major sink for ground‐level ozone via the process of dry deposition and the primary source of many biogenic volatile organic compounds (BVOCs). The rate of dry deposition is largely controlled by the rate of diffusion of a gas through the stomata, and this also governs the emission rate of some key BVOCs. It is critical therefore that canopy‐atmosphere exchange models capture the physiological processes controlling stomatal conductance and the transfer of trace gases other than carbon dioxide and water vapor. We incorporate three of the most widely used coupled stomatal conductance‐photosynthesis models into the one‐dimensional multi‐layer FORest Canopy‐Atmosphere Transfer (FORCAsT1.0) model to assess the importance of choice of parameterization on simulated ozone deposition rates. Modeled GPP and stomatal conductance across a broad range of ecosystems differ by up to a factor of two between the best and worst performing model configurations. This leads to divergences in seasonal and diel profiles of ozone deposition velocity of up to 30% and deposition rate of up to 13%, demonstrating that the choice of stomatal conductance parameterization is critical in accurate quantification of ozone deposition.

Indirect method for determining the dry deposition velocity of submicron particulate matter on leaves

Plant leaves, as natural receptors of airborne particles, can retain particles onto their surfaces, and absorb gaseous pollutants, thus mitigating air pollution and improving air quality. Dry deposition is considered the main process for particle removal from the atmosphere, and its velocity (Vd) is a crucial parameter for describing the process. Wind tunnels, a conventional approach to determine Vd, are costly and require substantial space, and regular inspections to maintain their systematic integrity. Hence, this study established a simpler and more straightforward method based on relevant research to obtain the submicron particulate matter (aerodynamic diameter ≤1 μm; PM1) Vd on plant leaves. This method involves determining the attenuation pattern of particle concentration in a smog chamber. The Vd values of six tree species (three broadleaved and three coniferous) were obtained through the indirect method. In addition, we determined the Vd of the same tree species with a wind tunnel and compared the values from both methods. Through the paired-samples t-test, it's found that there is no significant difference (sig. = 0.59) between these two methods, which means that the indirect method is feasible to determine Vd. We also compared our results with those from other researches, and we found that the Vd values in our study might be lower because leaves and shoots were used in this research, while branches or seedings were selected in the literature. Overall, when applying such values to predictive models or in concrete studies, researchers must consider factors such as real-time meteorological conditions (humidity, temperature, etc.) and pollutant concentration. The indirect method requires less space and is less costly than the wind tunnel method; therefore, it can easily be used to conduct experiments under controlled conditions, which is helpful for simulating various scenarios.

Is there a causal relationship between Particulate Matter ([formula omitted]) and air Temperature data? An analysis based on the Liang–Kleeman information transfer theory

In the literature, it is well known that mineral dust play a key role in the atmospheric radiation budget. How to identify the cause–effect relationship between mineral dust and climatic parameters remains a crucial issue in atmospheric science and environment. In this study, the causal relation between particulate matter that have an aerodynamic diameter less than 10 μm diameter (PM10) and air Temperature (T) is investigated for different time scales. For this purpose, two normalization schemes based on San Liang (2014)’s information flow formula and the classical convergent cross mapping introduced by Sugihara et al. (2012) were applied to eleven years of daily time series recorded in Guadeloupe archipelago. Both methods showed there is a bidirectional causality between the studied parameters. Indeed, we noticed that PM10 concentrations tend to stabilize T values. This phenomenon has been attributed to a greenhouse effect which is strongly linked to African dust seasonality. During the high dust season, this effect is 13.2 times greater than in the low season. On the other hand, we found that T values tend to make PM10 concentrations more uncertain in the low dust season while they homogenize PM10 fluctuations in the high season. All these behaviors could be assigned to the impact of T values on PM10 dry deposition velocity. To conclude, our results showed that information flow approach is an efficient tool to extract the cause–effect relationship between two dynamical events in atmospheric science, i.e. a field where several parameters interact simultaneously.

Towards an experimental approach for measuring the removal of urban air pollutants by green roofs

Green roofs are a promising approach to mitigate air pollution in urban environments, but limited experimental data is yet available. In this study, we developed a laboratory-scale setup to measure the removal nitrogen dioxide (NO2) and ozone (O3) using a variety of common green roof species. Experiments were conducted on detached leaves and whole plants using two chambers (0.6 L and 12 L), visible lighting, NOx/O3 sources and online analyzers. Three species were the best performant (Thymus vulgaris, Sedum sexangulare and Heuchera Americana L.) with deposition velocities (vd) ranging from 1.6 to 4.82 m/h for NO2, and 1.7–5.56 m/h for O3. In both cases, thyme was the most effective plant likely due to its higher stomatal area and the reactivity of its volatile metabolites with O3 leading to several oxidized by-products. Furthermore, NO2 uptake was found to be enhanced by surface water released by leaf transpiration leading to the production of nitrous acid (HONO). Similar values of (vd) were observed (3.84–4.65 m/h) when whole thyme plant was used. The soil was also found to be competitive in removing O3 but less performant in capturing NO2. Using a dry deposition model, we estimated that the three plant species can uptake up to 9 kg/ha/year of NO2 and 13.6 kg/ha/year, which fall in agreement with previously reported modeling data. Our experimental approach can be a rapid tool for screening the depollution performances of green roof species enabling an effective prioritization for deployment in urban environments.

Extension of a gaseous dry deposition algorithm to oxidized volatile organic compounds and hydrogen cyanide for application in chemistry transport models.

The dry deposition process refers to flux loss of an atmospheric pollutant due to uptake of the pollutant by the Earth’s surfaces, including vegetation, underlying soil, and any other surface types. In chemistry transport models (CTMs), the dry deposition flux of a chemical species is typically calculated as the product of its surface layer concentration and its dry deposition velocity (Vd); the latter is a variable that needs to be highly empirically parameterized due to too many meteorological, biological, and chemical factors affecting this process. The gaseous dry deposition scheme of Zhang et al. (2003) parameterizes Vd for 31 inorganic and organic gaseous species. The present study extends the scheme of Zhang et al. (2003) to include an additional 12 oxidized volatile organic compounds (oVOCs) and hydrogen cyanide (HCN), while keeping the original model structure and formulas, to meet the demand of CTMs with increasing complexity. Model parameters for these additional chemical species are empirically chosen based on their physicochemical properties, namely the effective Henry’s law constants and oxidizing capacities. Modeled Vd values are compared against field flux measurements over a mixed forest in the southeastern US during June 2013. The model captures the basic features of the diel cycles of the observed Vd. Modeled Vd values are comparable to the measurements for most of the oVOCs at night. However, modeled Vd values are mostly around 1 cm s−1 during daytime, which is much smaller than the observed daytime maxima of 2–5 cm s−1. Analysis of the individual resistance terms and uptake pathways suggests that flux divergence due to fast atmospheric chemical reactions near the canopy was likely the main cause of the large model–measurement discrepancies during daytime. The extended dry deposition scheme likely provides conservative Vd values for many oVOCs. While higher Vd values and bidirectional fluxes can be simulated by coupling key atmospheric chemical processes into the dry deposition scheme, we suggest that more experimental evidence of high oVOC Vd values at additional sites is required to confirm the broader applicability of the high values studied here. The underlying processes leading to high measured oVOC Vd values require further investigation.