Deposition Of Semivolatile Organic Compounds Research Articles

Atmospheric gas-particle partitioning versus gaseous/particle-bound deposition of SVOCs: Why they are not equivalent

Semi-volatile organic compounds (SVOCs) can be particle-bound or in the gas phase in the atmosphere, depending on the (temperature dependent) gas-particle partitioning of the chemicals and the fraction of particles in air. Several studies linked gas-particle partitioning of SVOCs in the atmosphere directly to the gaseous/particle-bound deposition of these chemicals, i.e. in cases of compounds occurring mainly in the gas phase, the deposition was also assumed to be mainly in gaseous form. In this study, we apply a multi-media fate model to point out that gas-particle partitioning of SVOCs in air and gaseous/particle-bound deposition of SVOCs are driven by different mechanism and, thus, cannot be deduced from each other. We apply our calculations to polychlorinated biphenyls (PCBs), as model SVOCs. We show that the fraction of particle-bound deposition to deciduous forest is 1.5–190 times higher in winter and between 5 and 1000 times higher in summer than the particle-bound fraction of these chemicals in air. The fraction of particle-bound deposition to coniferous forest is 1.5–172 times higher in winter and between 5 and 1000 times higher in summer than the particle-bound fraction of PCBs in air. In addition to the fractions of particle-bound SVOCs in air and particle-bound deposition, we recalculated particle-bound and gaseous deposition velocities to coniferous and deciduous forest for PCBs. The deposition velocities obtained for dry gaseous deposition (<1 m/h) are much lower than the existing values in the literature (10–200 m/h) because earlier studies assumed that for PCBs occurring predominantly in the gas phase, interception was also completely due to dry gaseous deposition.

The effect of dry and wet deposition of condensable vapors on secondary organic aerosols concentrations over the continental US

Abstract. The effect of dry and wet deposition of semi-volatile organic compounds (SVOCs) in the gas phase on the concentrations of secondary organic aerosol (SOA) is reassessed using recently derived water solubility information. The water solubility of SVOCs was implemented as a function of their volatility distribution within the WRF-Chem regional chemistry transport model, and simulations were carried out over the continental United States for the year 2010. Results show that including dry and wet removal of gas-phase SVOCs reduces annual average surface concentrations of anthropogenic and biogenic SOA by 48 and 63% respectively over the continental US. Dry deposition of gas-phase SVOCs is found to be more effective than wet deposition in reducing SOA concentrations (−40 vs. −8% for anthropogenics, and −52 vs. −11% for biogenics). Reductions for biogenic SOA are found to be higher due to the higher water solubility of biogenic SVOCs. The majority of the total mass of SVOC + SOA is actually deposited via the gas phase (61% for anthropogenics and 76% for biogenics). Results are sensitive to assumptions made in the dry deposition scheme, but gas-phase deposition of SVOCs remains crucial even under conservative estimates. Considering reactivity of gas-phase SVOCs in the dry deposition scheme was found to be negligible. Further sensitivity studies where we reduce the volatility of organic matter show that consideration of gas-phase SVOC removal still reduces average SOA concentrations by 31% on average. We consider this a lower bound for the effect of gas-phase SVOC removal on SOA concentrations. A saturation effect is observed for Henry's law constants above 108 M atm−1, suggesting an upper bound of reductions in surface level SOA concentrations by 60% through removal of gas-phase SVOCs. Other models that do not consider dry and wet removal of gas-phase SVOCs would hence overestimate SOA concentrations by roughly 50%. Assumptions about the water solubility of SVOCs made in some current modeling systems (H* = H* (CH3COOH); H* = 105 M atm−1; H* = H* (HNO3)) still lead to an overestimation of 35%/25%/10% compared to our best estimate.

Impact of dry deposition of semi-volatile organic compounds on secondary organic aerosols

Dry deposition of semi-volatile organic compounds (SVOC) is not currently treated in most chemical transport models of air quality and this omission has been identified as a possible major source of uncertainty. The effect of dry deposition of SVOC on the concentration of secondary organic aerosols (SOA) is investigated in summertime with the chemical transport model CHIMERE that simulates SOA concentrations by means of molecular SOA surrogate species. Omitting dry deposition could overestimate SOA concentrations by as much as 50%. This overestimation is larger during nighttime due to higher relative humidity.

Regional differences in gas–particle partitioning and deposition of semivolatile organic compounds on a global scale

Variability in gas–particle partitioning of semivolatile organic compounds (SOCs) and related atmospheric processes (particle-associated deposition, rain washout and degradation) are investigated on a global scale. Two different sorption approaches (one using the octanol-air partition coefficient, K OA, and one based on poly-parameter linear free energy relationships, ppLFER) and two different atmospheric box models (unit-world and highly spatially and temporally resolved) are applied. In the unit-world model, the overall deposition and atmospheric fate of SOCs calculated with the K OA-based sorption approach are similar to the ones calculated with the ppLFER approach. Rain washout dominates the atmospheric removal of polar chemicals in the unit-world model while non-polar chemicals are removed mainly through degradation or particle-associated deposition. In contrast, big differences and a high sensitivity to the selected sorption approach are found in the spatially and temporally resolved model. The highly resolved geographic variability cannot be represented using the K OA-based approach if aerosol components other than OM are of importance for sorption. In particular, aerosols in dry regions (desert) and regions with low OM aerosols (arctic, some oceanic regions) are more appropriately described by the ppLFER approach. With the ppLFER approach, good agreement between modeled deposition fluxes and measurement data are found for higher chlorinated PCBs and TCDD/Fs. In general, we recommend the ppLFER approach for highly resolved environmental fate models.

Particulate Dry Deposition and Overall Deposition Velocities of Polycyclic Aromatic Hydrocarbons

Previous studies have shown that the dry deposition of semivolatile organic compounds to the Great Lakes can account for a significant fraction of their total inputs. However, there is no generally accepted method to directly measure dry deposition. In this study the particulate dry deposition of polycyclic aromatic hydrocarbons (PAHs) was measured using smooth surrogate surfaces during the winter of 1996–1997 in Chicago. Concurrently, ambient air samples were collected. Average particulate Σ13-PAH fluxes and ambient concentrations were 120±28μg/m2d and 30±16ng/m3, respectively. The measured particulate dry deposition fluxes were similar to those measured in other urban areas. Overall dry deposition velocities of PAHs calculated using the dry deposition fluxes and ambient concentrations averaged 4.5±3.1cm/s. This value is higher than values typically used to estimate PAH particulate deposition, however, it agrees well with values determined using similar techniques. The overall dry deposition velocity for individual PAHs generally decreased with increasing molecular weight. This finding is consistent with the previous experimental studies that have shown that a greater fraction of the higher molecular weight PAHs are associated with fine particles relative to the lower molecular weight compounds.

Atmospheric deposition of semivolatile organic compounds to two forest canopies

The deposition of polycyclic aromatic hydrocarbons, hexachlorobenzene and polychlorinated biphenyls, dibenzo- p-dioxins and dibenzofurans to a coniferous and a deciduous forest canopy was measured by simultaneously sampling bulk deposition below the canopies and in an adjacent clearing for one year. In addition, ambient air was sampled continuously, with separate analysis of the gaseous and particle-bound phases. The deposition of almost all compounds was higher under the forest canopies than in the clearing. The excess deposition to the forest sites was attributable to equilibrium partitioning between the atmosphere and the canopy vegetation, kinetically limited gaseous deposition, or particle-bound deposition. Which of these deposition processes dominated for a given compound was shown to depend on the octanol–air partition coefficient of the chemical and its gas/particle partitioning. Deposition velocities–to our knowledge the first for SOCs to forests–were calculated by dividing the excess deposition by the air concentrations. The gaseous deposition velocities were 0.78 cm s -1 to the coniferous canopy (annual weighted average) and 3.6 cm s -1 to the deciduous canopy (6 month summer average). These values are high compared to deposition velocities to forest canopies that have been measured for inorganic gases, reflecting the fact that lipophilic organic chemicals are taken up by the leaf/needle cuticle and not just via the stomata. The dry particle bound deposition velocities for particle diffusion and impaction were 0.05 and 0.73 cm s -1 for the coniferous and deciduous canopies, respectively. These values are considerably lower than the gaseous deposition velocities, underlining the importance of gaseous deposition for the accumulation of semivolatile organic compounds in forest ecosystems.

Dry Deposition of Semivolatile Organic Compounds to Lake Michigan

The Lake Michigan Urban AirToxics Study(LMUATS) was carried out from July 8 to August 9, 1991, in the southern Lake Michigan basin. The investigation was the first aimed at assessing the impact of the Chicago/Gary urban plume on the deposition of hazardous air pollutants (HAPs) to Lake Michigan. The concentrations of a large number of semivolatile organic compounds (SOCs) were measured at four locations (Kankakee, IL ; Chicago, IL ; over Lake Michigan onboard the R/V Laurentian ; and South Haven, Ml). These sites were chosen as they are often linked by air mass transport with air masses reaching South Haven, after passing through Chicago, often having an over-water path to the downwind site. The University of Michigan R/V Laurentianwas also deployed off-shore of Chicago to measure the concentrations of HAPs as they were transported out over the water. A hybrid-receptor deposition (HRD) modeling approach, which utilized measured atmospheric concentrations ata a receptor site together with observed meteorological data in a lagrangian dispersion modeling framework, was used to estimate both gas exchange across the air-water interface and particle dry deposition of SOCs to Lake Michigan. The atmospheric concentrations of selected pesticides, PCBs, and PAHs measured during the LMUATS were used as input to the HRD model. The comparison between the measured and calculated ambient concentrations gives reasonable results with measured/calculated concentration ratios in the range of 0.3-1.9 for pesticides, 0.9-2.6 for PCBs, and 0.5-3.8 for PAHs. A comparison of deposition fluxes obtained in this study with those found in literature reveals large differences for some compounds, especially for those that are primarily in the particle phase. This finding suggests that assuming constant values of critical parameters controlling the transfer processes of gaseous and particulate SOCs from the atmosphere to the water surface may result in estimates with large uncertainties.

Deposition of semivolatile organic compounds to spruce needles

The deposition of atmospheric tetrachlorobenzene, pentachlorobenzene, hexachlorobenzene,α-HCH,gg-HCH, DDT, DDE and the PCB congeners 52, 101, 138, 153 and 180 to spruce needles (Picea abies) was estimated for a period of 9 months. Accumulation in spruce as a result of dry gaseous deposition, particle-bound deposition and wet deposition was calculated on the basis of the corresponding deposition rates and the compounds' concentrations in the different atmospheric compartments. The comparison of the calculated values with the concentrations of the compounds measured in 9-month-old spruce needles showed that for many compounds each deposition pathway could explain a large pan of the concentrations found in the needles.

Deposition of semivolatile organic compounds to spruce needles

The accumulation of atmospheric HCB, Lindane, DDT, DDE and the PCB congeners 52, 101, 138, 153 and 180 in spruce needles (Picea abies) was investigated at outdoor locations and in greenhouses supplied with ambient air. The air supply of the greenhouses was modified to dinstinguish between gaseous and particle-associated deposition of the compounds. Accumulation of the compounds occurred in all spruce except those grown in the greenhouse where the gaseous concentration of the compounds was reduced. Spruce grown in the greenhouse supplied with particle-free ambient air behaved similarity to those grown outdoors. Protecting the spruce located outdoors from rain did not affect the accumulation. The results show that under spring and summer conditions at a typical central European rural environment dry gaseous deposition is the dominant pathway of these compounds to needles.

Deposition of semivolatile organic compounds to spruce needles

The deposition of atmospheric tetrachlorobenzene, pentachlorobenzene, hexachlorobenzene, α-HCH, γ-HCH, DDT, DDE and the PCB congeners 52, 101, 138, 153 and 180 to spruce needles (Picea abies) was estimated for a period of 9 months. Accumulation in spruce as a result of dry gaseous deposition, particle bound deposition and wet deposition was calculated on the basis of the corresponding deposition rates and the compounds’ concentrations in the different atmospheric compartments. The comparison of the calculated values with the concentrations of the compounds measured in 9-month-old spruce needles showed that for many compounds each deposition pathway could explain a large part of the concentrations found in the needles.

The measurement of atmospheric concentrations and deposition of semi-volatile organic compounds

A physical description of semi-volatile organic compounds (SVOCs) is given, both in terms of their characteristic nature in the atmosphere and the processes which control their deposition. The currently adopted methods used to measure the atmospheric concentrations of SVOCs have been summarised, along with their shortcomings. Techniques and associated problems have also been reviewed for measurements of atmospheric deposition. A summary has been given of the requirements necessary for a full assessment of atmospheric SVOCs and their deposition.