Recent updates to the atmospheric aerosol modelling of the ECMWF IFS in support to CAMS

2021 ◽  
Author(s):  
Samuel Remy ◽  
Zak Kipling ◽  
Vincent Huijnen ◽  
Johannes Flemming ◽  
Swen Metzger ◽  
...  
Keyword(s):  
Organic Matter ◽  
Dry Deposition ◽  
Wet Deposition ◽  
Deposition Velocity ◽  
Surface Concentration ◽  
Atmospheric Composition ◽  
Small Scale ◽  
Skill Scores ◽  
Simulated Surface ◽  
The Impact

<p>The Integrated Forecasting System (IFS) of ECMWF is used within the Copernicus Atmosphere Monitoring Service (CAMS) to provide global analyses and forecasts of atmospheric composition, including aerosols as well as reactive trace gases and greenhouse gases.</p><p>The aerosol model of the IFS, IFS-AER, is a simple sectional-bulk scheme that forecasts seven species:  dust, sea-salt, black carbon, organic matter, sulfate, and  since July 2019, nitrate and ammonium.  The main developments that have been recently carried out, tested and are now contemplated for implementation in the next operational version (known as cycle 48r1) are presented here.</p><p>The dry deposition velocities are computed as a function of roughness length, particle size and surface friction velocity, while wet deposition depends mainly on the precipitation fluxes. The parameterizations of both dry and wet deposition have been upgraded with more recent schemes, which have been shown to improve the simulated deposition fluxes for several aerosol species. The impact of this upgrade on the skill scores of simulated aerosol optical depth (AOD) and surface particulate matter concentrations against a range of observations is very positive.</p><p>The simulated surface concentration of nitrate and ammonium are frequently strongly overestimated over Europe and the  United States in the current version of the IFS. Nitrate, ammonium, and their precursors nitric acid and ammonia, were evaluated against a range of ground and remote data and it was found that the recently-implemented gas-particle partitioning scheme is too efficient in producing nitrate and ammonium particles.</p><p>A series of small-scale changes, such as adjusting nitrate dry deposition velocity, direct particulate sulphate emission, and limiting nitrate/ammonium production by the concentration of mineral cations, have been implemented and shown to be effective in improving the simulated surface concentration of  nitrate and ammonium.</p><p>The representation of secondary organic aerosol (SOA) in the IFS has been overhauled with the introduction of a new SOA species, distinct from primary organic matter, with anthropogenic and biogenic components. The implementation of this new species leads to a significant improvement of the simulated surface concentration of organic carbon. An evaluation of simulated SOA concentrations at the surface against climatological values derived from observations using Positive Matrix Factorisation (PMF) techniques also shows a reasonable agreement.</p>

scholarly journals Impacts of aerosol-cloud interactions on past and future changes in tropospheric composition

2009 ◽  
Vol 9 (12) ◽  
pp. 4115-4129 ◽  
Author(s):  
N. Unger ◽  
S. Menon ◽  
D. M. Koch ◽  
D. T. Shindell
Keyword(s):  
Air Quality ◽  
Wet Deposition ◽  
Wide Spectrum ◽  
Surface Ozone ◽  
Tropospheric Chemistry ◽  
Atmospheric Composition ◽  
Sulfate Production ◽  
Aerosol Cloud ◽  
Aerosol Cloud Interactions ◽  
The Impact

Abstract. The development of effective emissions control policies that are beneficial to both climate and air quality requires a detailed understanding of all the feedbacks in the atmospheric composition and climate system. We perform sensitivity studies with a global atmospheric composition-climate model to assess the impact of aerosols on tropospheric chemistry through their modification on clouds, aerosol-cloud interactions (ACI). The model includes coupling between both tropospheric gas-phase and aerosol chemistry and aerosols and liquid-phase clouds. We investigate past impacts from preindustrial (PI) to present day (PD) and future impacts from PD to 2050 (for the moderate IPCC A1B scenario) that embrace a wide spectrum of precursor emission changes and consequential ACI. The aerosol indirect effect (AIE) is estimated to be −2.0 Wm−2 for PD-PI and −0.6 Wm−2 for 2050-PD, at the high end of current estimates. Inclusion of ACI substantially impacts changes in global mean methane lifetime across both time periods, enhancing the past and future increases by 10% and 30%, respectively. In regions where pollution emissions increase, inclusion of ACI leads to 20% enhancements in in-cloud sulfate production and ~10% enhancements in sulfate wet deposition that is displaced away from the immediate source regions. The enhanced in-cloud sulfate formation leads to larger increases in surface sulfate across polluted regions (~10–30%). Nitric acid wet deposition is dampened by 15–20% across the industrialized regions due to ACI allowing additional re-release of reactive nitrogen that contributes to 1–2 ppbv increases in surface ozone in outflow regions. Our model findings indicate that ACI must be considered in studies of methane trends and projections of future changes to particulate matter air quality.

scholarly journals Evaluating and Improving the Impact of the Atmospheric Stability and Orography on Surface Winds in the WRF Model

2016 ◽  
Vol 144 (7) ◽  
pp. 2685-2693 ◽  
Author(s):  
Raquel Lorente-Plazas ◽  
Pedro A. Jiménez ◽  
Jimy Dudhia ◽  
Juan P. Montávez
Keyword(s):  
Atmospheric Stability ◽  
Wrf Model ◽  
Grid Cell ◽  
Momentum Equation ◽  
Small Scale ◽  
Surface Winds ◽  
Boundary Layer Height ◽  
Stable Conditions ◽  
Simulated Surface ◽  
The Impact

Abstract This study assesses the impact of the atmospheric stability on the turbulent orographic form drag (TOFD) generated by unresolved small-scale orography (SSO) focusing on surface winds. With this aim, several experiments are conducted with the Weather Research and Forecasting (WRF) Model and they are evaluated over a large number of stations (318 at 2-m height) in the Iberian Peninsula with a year of data. In WRF, Jiménez and Dudhia resolved the SSO by including a factor in the momentum equation, which is a function of the orographic variability inside a grid cell. It is found that this scheme can improve the simulated surface winds, especially at night, but it can underestimate the winds during daytime. This suggests that TOFD can be dependent on the PBL’s stability. To inspect and overcome this limitation, the stability conditions are included in the SSO parameterization to maintain the intensity of the drag during stable conditions while attenuating it during unstable conditions. The numerical experiments demonstrate that the inclusion of stability effects on the SSO drag parameterization improves the simulated surface winds at diurnal, monthly, and annual scales by reducing the systematic daytime underestimation of the original scheme. The correction is especially beneficial when both the convective velocity and the boundary layer height are used to characterize the unstable conditions.

scholarly journals Influence of local air pollution on the deposition of peroxyacteyl nitrate to a nutrient-poor natural grassland ecosystem

2014 ◽  
Vol 14 (14) ◽  
pp. 20383-20416
Author(s):  
A. Moravek ◽  
P. Stella ◽  
T. Foken ◽  
I. Trebs
Keyword(s):  
Dry Deposition ◽  
Deposition Velocity ◽  
Bowen Ratio ◽  
Grassland Ecosystem ◽  
Autumn Period ◽  
Mixing Ratios ◽  
Underlying Mechanisms ◽  
Threaten Species ◽  
The Impact ◽  
Natural Grassland

Abstract. Dry deposition of peroxyacetyl nitrate (PAN) is known to have a phytotoxic impact on plants under photochemical smog conditions, but it may also lead to higher productivity and threaten species richness of vulnerable ecosystems in remote regions. However, underlying mechanisms or controlling factors for PAN deposition are not well understood and studies on dry deposition of PAN are limited. In this study, we investigate the impact of PAN deposition on a nutrient-poor natural grassland ecosystem situated at the edge of an urban and industrialized region in Germany. PAN mixing ratios were measured within a 3.5 months summer to early autumn period. In addition, PAN fluxes were determined with the modified Bowen ratio technique for a selected period. The evaluation of both stomatal and non-stomatal deposition pathways was used to model PAN deposition over the entire summer-autumn period. We found that air masses at the site were influenced by two contrasting pollution regimes, which lead to median diurnal PAN mixing ratios ranging between 50 and 300 ppt during unpolluted and between 200 and 600 ppt during polluted episodes. The measured PAN fluxes showed a clear diurnal cycle with maximal deposition fluxes of ~ −0.1 nmol m−2 s−1 (corresponding to a deposition velocity of 0.3 cm s−1) during daytime and a significant non-stomatal contribution was found. The ratio of PAN to ozone deposition velocities was found to be ~0.1, which is much larger than assumed by current deposition models. The modelled PAN flux over the entire period revealed that PAN deposition over an entire day was 333 μg m−2 d−1 under unpolluted and 518 μg m−2 d−1 under polluted episodes. Besides, thermochemical decomposition PAN deposition accounted for 32% under unpolluted episodes and 22% under polluted episodes of the total atmospheric PAN loss. However, the impact of PAN deposition as a nitrogen source to the nutrient-poor grassland was estimated to be only minor, under both unpolluted and polluted episodes.

scholarly journals Impact of future land-cover changes on HNO<sub>3</sub> and O<sub>3</sub> surface dry deposition

2015 ◽  
Vol 15 (23) ◽  
pp. 13555-13568 ◽  
Author(s):  
T. Verbeke ◽  
J. Lathière ◽  
S. Szopa ◽  
N. de Noblet-Ducoudré
Keyword(s):  
Land Cover ◽  
Land Cover Change ◽  
Dry Deposition ◽  
Deposition Velocity ◽  
Land Cover Changes ◽  
Vegetation Distribution ◽  
Deposition Velocities ◽  
The Future ◽  
Rcp 8.5 ◽  
The Impact

Abstract. Dry deposition is a key component of surface–atmosphere exchange of compounds, acting as a sink for several chemical species. Meteorological factors, chemical properties of the trace gas considered and land surface properties are strong drivers of dry deposition efficiency and variability. Under both climatic and anthropogenic pressure, the vegetation distribution over the Earth has been changing a lot over the past centuries and could be significantly altered in the future. In this study, we perform a modeling investigation of the potential impact of land-cover changes between the present day (2006) and the future (2050) on dry deposition velocities at the surface, with special interest for ozone (O3) and nitric acid (HNO3), two compounds which are characterized by very different physicochemical properties. The 3-D chemistry-transport model LMDz-INCA is used, considering changes in vegetation distribution based on the three future projections, RCPs 2.6, 4.5 and 8.5, and present-day (2007) meteorology. The 2050 RCP 8.5 vegetation distribution leads to a rise of up to 7 % (+0.02 cm s−1) in the surface deposition velocity calculated for ozone (Vd,O3) and a decrease of −0.06 cm s−1 in the surface deposition velocity calculated for nitric acid (Vd,HNO3) relative to the present-day values in tropical Africa and up to +18 and −15 %, respectively, in Australia. When taking into account the RCP 4.5 scenario, which shows dramatic land-cover change in Eurasia, Vd,HNO3 increases by up to 20 % (annual-mean value) and reduces Vd,O3 by the same magnitude in this region. When analyzing the impact of surface dry deposition change on atmospheric chemical composition, our model calculates that the effect is lower than 1 ppb on annual-mean surface ozone concentration for both the RCP 8.5 and RCP 2.6 scenarios. The impact on HNO3 surface concentrations is more disparate between the two scenarios regarding the spatial repartition of effects. In the case of the RCP 4.5 scenario, a significant increase of the surface O3 concentration reaching locally by up to 5 ppb (+5 %) is calculated on average during the June–August period. This scenario also induces an increase of HNO3 deposited flux exceeding locally 10 % for monthly values. Comparing the impact of land-cover change to the impact of climate change, considering a 0.93 °C increase of global temperature, on dry deposition velocities, we estimate that the strongest increase over lands occurs in the Northern Hemisphere during winter, especially in Eurasia, by +50 % (+0.07 cm s−1) for Vd,O3 and +100 % (+0.9 cm s−1) for Vd,HNO3. However, different regions are affected by both changes, with climate change impact on deposition characterized by a latitudinal gradient, while the land-cover change impact is much more heterogeneous depending on vegetation distribution modification described in the future RCP scenarios. The impact of long-term land-cover changes on dry deposition is shown to be significant and to differ strongly from one scenario to another. It should therefore be considered in biosphere–atmospheric chemistry interaction studies in order to have a fully consistent picture.

scholarly journals Mercury distribution in surface soil of China is potentially driven by precipitation, vegetation cover and organic matter

2020 ◽  
Author(s):  
Zhao-Yang Zhang ◽  
Gang Li ◽  
Lei Yang ◽  
Guo-Xin Sun
Keyword(s):  
Organic Matter ◽  
Soil Organic Matter ◽  
Solar Radiation ◽  
Dry Deposition ◽  
Vegetation Cover ◽  
Wet Deposition ◽  
Surface Soil ◽  
Wet And Dry Deposition ◽  
Continental Scale ◽  
Low Emission

Abstract Background Mercury (Hg) distribution in surface soil in China is quite uneven with relatively high concentrations in southeastern China and low concentrations in northwestern China. The reason for this is inconclusive so far, especially on the continental scale. In the present study we used the multiple linear regression model to evaluate the relative importance of these different factors and elucidate the contribution on soil Hg of major factors, such as dry and wet precipitations, vegetation cover, soil organic matter and solar radiation. Results Wet and dry deposition associated with precipitation and vegetation cover, and emissions influenced by soil organic matter (SOM), are key factors controlling Hg distribution in surface soil. In southeast China, high wet deposition associated with south Asia monsoon and dry deposition, enhanced by vegetation canopies, together with low emission caused by high vegetated surface and solar radiation, are responsible for high Hg in soil (> 0.08 mg/kg). In northeast China, medium wet Hg deposition and high dry deposition via throughfall and litterfall, low emission due to weak solar radiation and high SOM are responsible for high Hg accumulation in soil. In northwest China, low wet deposition, together with high emission by low vegetation cover (bare soil), SOM and strong solar radiation contributed to low Hg in surface soil (< 0.03 mg/kg). Conclusions We suggest that wet deposition derived from Asian monsoon, dry deposition linked to vegetated surfaces and Hg emission associated with vegetation cover, SOM and solar radiation play key roles in Hg balance in other terrestrial environments worldwide, especially in those regions with significantly high wet and dry deposition and high vegetation cover.

scholarly journals Understanding the impact of recent advances in isoprene photooxidation on simulations of regional air quality

2012 ◽  
Vol 12 (10) ◽  
pp. 27173-27218 ◽  
Author(s):  
Y. Xie ◽  
F. Paulot ◽  
W. P. L. Carter ◽  
C. G. Nolte ◽  
D. J. Luecken ◽  
...  
Keyword(s):  
Air Quality ◽  
Dry Deposition ◽  
Deposition Velocity ◽  
Chemical Mechanism ◽  
Recent Advances ◽  
Dry Deposition Velocity ◽  
Isoprene Nitrates ◽  
Isoprene Oxidation ◽  
Oxidation Chemistry ◽  
The Impact

Abstract. The CMAQ model in combination with observations for INTEX-NA/ICARTT 2004 are used to evaluate recent advances in isoprene oxidation chemistry and provide constraints on isoprene nitrate yields, isoprene nitrate lifetimes, and NOx recycling rates. We incorporate recent advances in isoprene oxidation chemistry into the SAPRC-07 chemical mechanism within the US EPA Community Multiscale Air Quality (CMAQ) model. The results show improved model performance for a range of species compared against aircraft observations from the INTEX-NA/ICARTT 2004 field campaign. We further investigate the key processes in isoprene nitrate chemistry and evaluate the impact of uncertainties in the isoprene nitrate yield, NOx (NOx = NO + NO2) recycling efficiency, dry deposition velocity, and RO2 + HO2 reaction rates. We focus our examination in the Southeastern United States, which is impacted by both abundant isoprene emissions and high levels of anthropogenic pollutants. We find that NOx concentrations increase by 4–9% as a result of reduced removal by isoprene nitrate chemistry. O3 increases by 2 ppbv as a result of changes in NOx. OH concentrations increase by 30%, which can be primarily attributed to greater HOx production. We find that the model can capture observed total alkyl and multifunctional nitrates (∑ANs) and their relationship with O3, by assuming either an isoprene nitrate yield of 6% and daytime lifetime of 6 h or a yield of 12% and lifetime of 4 h. Uncertainties in the isoprene nitrates can impact ozone production by 10% and OH concentrations by 6%. The uncertainties in NOx recycling efficiency appear to have larger effects than uncertainties in isoprene nitrate yield and dry deposition velocity. Further progress depends on improved understanding of isoprene oxidation pathways, the rate of NOx recycling from isoprene nitrates, and the fate of the secondary, tertiary, and further oxidation products of isoprene.

scholarly journals Influence of local air pollution on the deposition of peroxyacetyl nitrate to a nutrient-poor natural grassland ecosystem

2015 ◽  
Vol 15 (2) ◽  
pp. 899-911 ◽  
Author(s):  
A. Moravek ◽  
P. Stella ◽  
T. Foken ◽  
I. Trebs
Keyword(s):  
Dry Deposition ◽  
Deposition Velocity ◽  
Bowen Ratio ◽  
Grassland Ecosystem ◽  
Peroxyacetyl Nitrate ◽  
Autumn Period ◽  
Mixing Ratios ◽  
Underlying Mechanisms ◽  
The Impact ◽  
Natural Grassland

Abstract. Dry deposition of peroxyacetyl nitrate (PAN) is known to have a phytotoxic impact on plants under photochemical smog conditions, but it may also lead to higher productivity and threaten species richness of vulnerable ecosystems in remote regions. However, underlying mechanisms or controlling factors for PAN deposition are not well understood and studies on dry deposition of PAN are limited. In this study, we investigate the impact of PAN deposition on a nutrient-poor natural grassland ecosystem situated at the edge of an urban and industrialized region in Germany. PAN mixing ratios were measured within a 3.5 months summer to early autumn period. In addition, PAN fluxes were determined with the modified Bowen ratio technique for a selected period. The evaluation of both stomatal and non-stomatal deposition pathways was used to model PAN deposition over the entire summer–autumn period. We found that air masses at the site were influenced by two contrasting pollution regimes, which led to median diurnal PAN mixing ratios ranging between 50 and 300 ppt during unpolluted and between 200 and 600 ppt during polluted episodes. The measured PAN fluxes showed a clear diurnal cycle with maximal deposition fluxes of ~−0.1 nmol m−2 s−1 (corresponding to a deposition velocity of 0.3 cm s−1) during daytime and a significant non-stomatal contribution was found. The ratio of PAN to ozone deposition velocities was found to be ~0.1, which is much larger than assumed by current deposition models. The modelled PAN flux over the entire period revealed that PAN deposition over an entire day was 333 μg m−2 d−1 under unpolluted and 518 μg m−2 d−1 under polluted episodes. Additionally, thermochemical decomposition PAN deposition accounted for 32% under unpolluted episodes and 22% under polluted episodes of the total atmospheric PAN loss. However, the impact of PAN deposition as a nitrogen source to the nutrient-poor grassland was estimated to be only minor, under both unpolluted and polluted episodes.

Effects of type of flow, plants and addition of organic carbon in the removal of zinc and chromium in small-scale model wetlands

2007 ◽  
Vol 56 (3) ◽  
pp. 199-205 ◽  
Author(s):  
D. Paredes ◽  
M.E. Vélez ◽  
P. Kuschk ◽  
R.A. Mueller
Keyword(s):  
Organic Matter ◽  
Removal Rate ◽  
Typha Latifolia ◽  
Surface Flow ◽  
Synthetic Wastewater ◽  
Scale Model ◽  
Small Scale ◽  
Significant Difference ◽  
The Impact ◽  
Small Scale Model

Constructed wetlands are used for the treatment of wastewater containing metals. In order to clarify the role of plants, flow and the impact of organic matter, an investigation of three factors, each at two different levels, was carried out in small-scale model wetlands. The evaluated factors and levels were: type of flow (subsurface and surface); presence of plants (planted with Typha latifolia and unplanted) and addition of organic matter (with and without). Eight different experimental units were run for a year. The units were fed with synthetic wastewater containing chromium (VI) (1.5 mg L−1), zinc (1.5 mg L−1), macro, micronutrients and organic matter (to those units in which this factor was being investigated). Subsurface flow wetlands showed a significantly higher rate of chromium removal in comparison with surface flow systems (97 and 60 mg m−2 d−1, respectively). Planted systems removed significantly more chromium compared to unplanted systems (85 and 76 mg m−2 d−1, respectively), and the addition of organic matter increased the removal rate in a comparison with the units without it (88 and 69 mg m−2 d−1, respectively). Similar results were found for zinc; however, the addition of organic matter made no significant difference to zinc removal.

scholarly journals Rapid deposition of oxidized biogenic compounds to a temperate forest

2015 ◽  
Vol 112 (5) ◽  
pp. E392-E401 ◽  
Author(s):  
Tran B. Nguyen ◽  
John D. Crounse ◽  
Alex P. Teng ◽  
Jason M. St. Clair ◽  
Fabien Paulot ◽  
...  
Keyword(s):  
Dry Deposition ◽  
Transport Model ◽  
Large Fraction ◽  
Surface Concentration ◽  
Water Soluble ◽  
Organic Nitrates ◽  
Chemical Transport Model ◽  
Molecular Diffusivity ◽  
Deposition Velocities ◽  
Simulated Surface

We report fluxes and dry deposition velocities for 16 atmospheric compounds above a southeastern United States forest, including: hydrogen peroxide (H2O2), nitric acid (HNO3), hydrogen cyanide (HCN), hydroxymethyl hydroperoxide, peroxyacetic acid, organic hydroxy nitrates, and other multifunctional species derived from the oxidation of isoprene and monoterpenes. The data suggest that dry deposition is the dominant daytime sink for small, saturated oxygenates. Greater than 6 wt %C emitted as isoprene by the forest was returned by dry deposition of its oxidized products. Peroxides account for a large fraction of the oxidant flux, possibly eclipsing ozone in more pristine regions. The measured organic nitrates comprise a sizable portion (15%) of the oxidized nitrogen input into the canopy, with HNO3 making up the balance. We observe that water-soluble compounds (e.g., strong acids and hydroperoxides) deposit with low surface resistance whereas compounds with moderate solubility (e.g., organic nitrates and hydroxycarbonyls) or poor solubility (e.g., HCN) exhibited reduced uptake at the surface of plants. To first order, the relative deposition velocities of water-soluble compounds are constrained by their molecular diffusivity. From resistance modeling, we infer a substantial emission flux of formic acid at the canopy level (∼1 nmol m−2⋅s−1). GEOS−Chem, a widely used atmospheric chemical transport model, currently underestimates dry deposition for most molecules studied in this work. Reconciling GEOS−Chem deposition velocities with observations resulted in up to a 45% decrease in the simulated surface concentration of trace gases.

scholarly journals Mercury distribution in the surface soil of China is potentially driven by precipitation, vegetation cover and organic matter

2020 ◽  
Author(s):  
Zhao-Yang Zhang ◽  
Gang Li ◽  
Lei Yang ◽  
Xin-Jun Wang ◽  
Guo-Xin Sun
Keyword(s):  
Organic Matter ◽  
Solar Radiation ◽  
Dry Deposition ◽  
Vegetation Cover ◽  
Wet Deposition ◽  
Surface Soil ◽  
Southeastern China ◽  
Low Levels ◽  
Soil Hg ◽  
Hg Accumulation

Abstract Background: Understanding the mechanism of Hg accumulation in soil, which is a net Hg sink, at a national scale is important to protecting the environment and improving food safety. The mercury (Hg) distribution in surface soil in China is quite uneven, with relatively high concentrations in southeastern China and low concentrations in northwestern China. The reason for this distribution is inconclusive, especially at the continental scale. In this study, the relative contributions of the key impact factors, including dry and wet deposition, soil organic matter (SOM) and solar radiation to soil Hg, were evaluated.Results: Wet and dry deposition associated with precipitation and vegetation cover and emissions influenced by SOM are key factors controlling Hg distribution in surface soil. In southeastern China, high levels of wet deposition associated with the South Asia monsoon and dry deposition, enhanced by vegetation canopies, together with low levels of emissions caused by highly vegetated surfaces and solar radiation, are responsible for the high Hg levels in soil (>0.08 mg/kg). In northeastern China, moderate levels of wet Hg deposition, high levels of dry deposition via throughfall and litterfall, low emissions due to weak solar radiation and high levels of SOM are responsible for high Hg accumulation in soil. In northwestern China, low levels of wet deposition, together with high emissions levels, low vegetation cover (bare soil) and SOM and strong solar radiation, contributed to the low Hg level in the surface soil (<0.03 mg/kg).Conclusions: We suggest that wet deposition derived from the Asian monsoon, dry deposition linked to vegetated surfaces and Hg emissions associated with vegetation cover, SOM and solar radiation play key roles in the soil Hg level in China. In other terrestrial environments worldwide, especially in regions with significantly high levels of wet deposition and high amounts of vegetation cover and soil SOM, high Hg concentrations may exist in surface soil.

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