Impacts of Large-Scale Land-Use Change on the Uptake of Polycyclic

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Impacts of large-scale land-use change on the uptake of polycyclic aromatic hydrocarbons in the artificial Three Northern Regions Shelter Forest across northern China Tao Huang, Xiaodong Zhang, Zaili Ling, Leiming Zhang, Hong Gao, Chongguo Tian, Jiujiu Guo, Yuan Zhao, Li Wang, and Jianmin Ma Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04835 • Publication Date (Web): 05 Nov 2016 Downloaded from http://pubs.acs.org on November 7, 2016

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Environmental Science & Technology

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Impacts of large-scale land-use change on the uptake of polycyclic

2

aromatic hydrocarbons in the artificial Three Northern Regions

3

Shelter Forest across northern China

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Authors: Tao Huang†, Xiaodong Zhang†, Zaili Ling†, Leiming Zhang‡, Hong Gao†, Chongguo ∥ Tian§, Jiujiu Guo†, Yuan Zhao†, Li Wang1, Jianmin Ma*† Affiliation: † Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province; College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, P. R. China ‡ Air Quality Research Division, Environment and Climate Change Canada, Ontario M3H 5T4, Canada § Key Laboratory of Coastal Zone Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai, China ∥ CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China Corresponding author: Jianmin Ma College of Earth and Environmental Sciences, Lanzhou University, 222, Tianshui South Road, Lanzhou 730000, China Email: [email protected]

ACS Paragon Plus Environment


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TOC/Abstract Art Forest filter

PAHs

TNRSF Xinjiang Ningxia

15

PHE BaP

1.5

10 y = 0.016x + 1.008 R² = 0.861

1

5 0.5 0

0 1990

44

2

y = 0.122x + 10.14 R² = 0.845

1995

2000 Year

2005

Removal of BaP (ng m2 d -1)

Removal of PHE (ng m2 d-1)

Qinghai

2010

45 46

Abstract This study quantifies the influence of large-scale land-use change induced

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by the artificial Three-Northern Regions Shelter Forest (TNRSF) across northern

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China on the environmental cycling of organic chemicals. Atmospheric removal and

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long-term trends of two polycyclic aromatic hydrocarbon (PAH) species,

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phenanthrene (PHE) and benzo[a]pyrene (BaP), resulting from increasing vegetation

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coverage and soil organic carbon in the TNRSF over the last two decades were

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examined. Field sampling data and modeling result showed that the total atmospheric

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removal of PHE by TNRSF increased from 36.4 tons in 1990 to 76.8 tons in 2010,

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increasing at a rate of 5.6% yr-1, and BaP from 2.2 to 4.5 tons, increasing at a rate of

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5.2% yr-1. Three model scenarios were designed to distinguish the effects of

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atmospheric emissions, and with and without TNRSF on the environmental fate of

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PAHs. Approximately 1-4% of PHE and BaP emitted in northern China were

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removed by the TNRSF during 1990-2010. Model simulations revealed that the

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TNRSF enhanced atmospheric removal of PHE by 29% and BaP by 53% compared

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with the simulation without the TNRSF, manifesting marked contributions of land-use


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change by the artificial TNRSF, the largest afforestation activity in human history, to

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the atmospheric removal of organic chemicals.

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1. Introduction

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Human activities significantly impact the biogeochemical cycling of organic

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chemicals (OCs), including persistent organic pollutants (POPs) and POP-like

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chemicals such as polycyclic aromatic hydrocarbons (PAHs),1 through climate and

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land-use change. Extensive research on the interactions between climate change and

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OCs environmental cycling has been conducted in the last decade.2-6

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al.7 assessed the influence of temperature and land-use changes on the reemission of

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OCs from background and agricultural soils and showed that land-use change may

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significantly affect trends of atmospheric OCs. However, their investigation was

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based on a small area and projected land-use changes. The long-term environmental

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effects of OCs induced by large-scale land-use changes has not yet been investigated

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since land-use changes by human activities are often not readily linked with the

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biogeochemical cycling of OCs.

Komprda et

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The Three Northern Regions Shelter Forest (TNRSF) program, also known as

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the Great Green Wall, was initiated in 1978 with the aims of preventing and slowing

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soil and water erosion in northern China. The TNRSF program is the largest

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ecosystem restoration project in the world and in human history. The project extends

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from 1978 to 2050 and was designated to cover a total area of 4.1 million km2 (42.7%

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of the land area in China) (Figure S1 of Supporting Information, SI). However, many

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land areas in the TNRSF region are not presently covered by forests, especially in



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many places in northwestern China where shrubs, instead of trees, are major

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vegetation types. By the end of the fourth phase of the TNRSF program (2010), the

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forest coverage in this region had increased from 5% in 1978 to 12.4%, leading to a

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significant artificial land-use change in northern China.8-10 While the TNRSF aims to

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improve regional ecological environments,11-15 it was recently found to play a role in

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the removal of sulfur dioxide (SO2) and nitrogen oxide (NOx) through dry

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deposition.16 It was shown that the increasing vegetation coverage in the TNRSF over

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the past 30 years had increased its efficiency in removing air contaminations from the

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atmosphere. Compared with criteria air pollutants such as SO2 and NOx, the pathways

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in the uptake of organic pollutants by forests are more complex, as they involve

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air-canopy exchange, air-soil exchange subjected to the soil organic carbon (SOC)

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content, and soil-vegetation exchange.

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Since organic chemicals tend to partition from gas and liquid phases to organic

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phases,17 the spatial distribution of atmospheric OCs is often driven by the gradient of

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organic carbon content in soils. The strong association of OCs with organic carbon

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pools in soils is therefore often taken into account when assessing the environmental

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fate of OCs. Forests have been identified as a very important terrestrial storage

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compartment for OCs because soils in equilibrium with forest ecosystems have a high

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carbon density.17-24 Large surface roughness and leaf area index (LAI) of forests can

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also lead to rapid uptakes of OCs in forest canopies, there by reducing air pollutants

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levels through increasing deposition to forest leaves and underlying floors.25-29

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Matzner showed that bulk deposition of PAHs could be elevated under forest


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canopies.27 Simonich and Hites estimated that 44% of the PAHs emitted from the

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northeast United States (US) were deposited into forests.29 McLachlan and Horstmann

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linked the uptake of OCs by forests with a ‘‘forest filter effect’’, defined as the forest

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filtering airborne OCs from the atmosphere and transferring them to soil. They

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reported that annually averaged deposition of an organic chemical to forested soils

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was approximately a factor of 3-5 greater than that to bare soils.28 Abundant

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observational evidence has revealed higher concentrations of OCs in forest soils than

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nearby treeless soils.24,29,30,31 However, almost all previous studies in the associations

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between OCs environmental cycling and forests canopies were conducted in natural

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forests under a steady state32 and have no significant changes in leaf area, SOC, and

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forest biomass for a long-term period. Since increasing forest biomass and carbon

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storage across China has been largely attributed to artificial forestation,33-36 the

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significant land-use change induced by the TNRSF provide a unique opportunity to

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elucidate the impact of large scale artificial land-use change on the environmental fate

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of OCs. It is interesting to know how and to what extent the TNRSF would alter the

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cycling of OCs in the terrestrial environment. This will also contribute to our

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understanding of the interactions between human induced land-use changes and the

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biogeochemical cycle of organic pollutants.

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In this study, two PAHs congeners, phenanthrene (PHE) and benzo[a]pyrene

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(BaP), were selected to investigate the impact of the TNRSF on the uptake of OCs

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from the atmosphere. Given their POP-like properties, PAHs have been categorized as

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POPs by the European Environment Agency and the Convention on Long-Range



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Transboundary Air Pollution Protocol.1 PHE is primarily in gas phase and BaP is

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mostly particle-bound which enables us to separately elucidate the influence of the

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TNRSF on the environmental cycling of gas-phase and particle-bound OCs. Field

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monitoring evidence for the uptake of PHE and BaP and changes in SOC by the

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TNRSF was firstly provided. A modified high-resolution atmospheric transport model

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was then applied to simulate PAHs removal in the TNRSF. Our study aims to measure

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the interactions between artificial large-scale land use change and the environmental

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fate of atmospheric contaminants through (1) assessing the potential contribution and

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significance of large-scale land-use change by artificial forests to the removal of OCs

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from the atmosphere; (2) quantifying the roles of the TNRSF in the depletion of OCs

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in a man-made large-scale forest; and (3) examining the long-term trend of the

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depletion of OCs in northern China resulting from the TNRSF development in the

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past 20 years and future.

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2. Methodology

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2.1 In-situ field measurement

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A field campaign was conducted to measure the organic carbon content and soil

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concentrations of PAHs at several sites in the TNRSF to examine soil absorption of

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PAHs in the artificial forest and verify the SOC calculated by the satellite retrieved

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LAI. These sites are located in the Central-North region of the TNRSF near Beijing

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and Tianjin, the two megacities in northern China (Figures. S1, S2). Soil samples

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were collected at 4 paired sampling sites (4 inside forests and the other 4 outside


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forests) during August 7-12, 2015. The sampling sites, samples collections, laboratory

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processing and chemical analysis can be found in the Supporting Information

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(Section SI2, Figure S2, Table S1).

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2.2 Atmospheric transport modeling

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An

atmospheric

transport

model

(CanMETOP,

Canadian

Model

for

154

Environmental Transport of Organochlorine Pesticides)37 was used to simulate

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long-term (1990-2010) trends of PHE and BaP in multiple environmental

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compartments. The major features of the model are the same as the global version of

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CanMETOP38; however, the horizontal spatial resolution is increased to 1/4° latitude

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by 1/4° longitude with 14 vertical levels. The CanMETOP considers the

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three-dimensional atmospheric advectiosn, eddy diffusion, dry/wet deposition, and

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degradation in air and soil. Modifications of the model in this study include coupling

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a fugacity based Level IV multimedia fate and transport module 37 for simulating OCs

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in soils and air-soil exchange processes, updating several trans-media or mass

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exchange processes and incorporating air-forest-soil exchange processes. The model

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domain covers the TNRSF and the surrounding regions (Figure S3). The annual

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gridded emission inventories of PHE and BaP across the model domain from 1990 to

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2010 were established according to Zhang and Tao.39 (Figures S3 and S4). Although

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fine particles of PAHs can be transported globally,40 this study did not account for the

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influence of external PAH sources from other countries in their budget in northern

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China via long-range transport. Since China has been the largest PAH emission source

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worldwide,39 local emissions dominated the environmental fate of PAHs. The model



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parameterization and uncertainty analysis are provided in SI (Sections SI3-SI7). In

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the literature, model evaluations have been carried out by comparisons between the

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modeled and measured air concentrations in the model domain. Details of the model

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evaluation are presented in SI (Section SI6). Overall, the results show that the

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modeled air concentrations agree with the sampled data at r>0.8 (p1.96). The blue circle denotes the regions where 12 grid points were selected to project air-soil net exchange flux and air-canopy net exchange flux in this area until 2050. The positive values indicate an increasing trend of net exchange fluxes from 1990 to 2010, and vice versa.

434 4%

Remova l rate derived from Scenario 2 Remova l rate derived from Scenario 3 Difference of remova l rate between Scenarios 2 and 3

Removal

3%

2%

1%

0% 1990

435

1995

2000

2005

2010

Year


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(a) PHE

436 2.5%

Removal ra te derived from Scenario 2 Removal ra te derived from Scenario 3

Removal

2.0%

Difference of removal ra te between Scenarios 2 and 3

1.5% 1.0% 0.5% 0.0% 1990

437

1995

2000

2005

2010

Year

438 439 440 441 442 443 444

(b) BaP Figure 4 Comparisons between annual average uptake fraction for PHE (a) and BaP (b) from 1990 to 2010 derived from scenario 2 (annual PHE and BaP emissions from 1990 to 2010 and annual LAI and SOC for the same time period) and scenario 3 (annual PHE and BaP emissions averaged over 1990 to 2010 and fixed LAI and SOC in 1978). The solid black lines represent the differences of the annual mean uptake fraction by the TNRSF between model scenarios 2 and 3.

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3.4 Projected atmospheric removal of PAHs with and without TNRSF

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Further insight on the significance of the TNRSF in the removal of the selected

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PAHs can be gained by comparing modeled fluxes between model Scenarios 2 and 3.

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As mentioned, Scenario 3 virtually postulated no TNRSF by using the fixed LAI and

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SOC in 1978 over the TNRSF region whereas Scenario 2 took into account the annual

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changes in the SOC and LAI over the TNRSF. Both scenarios incorporated annually

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varying emissions of 1990 through 2010 over northern China. The fractions of

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atmospheric removal, defined in Section 3.2, were also estimated for the results of

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Scenario 3 and compared with those from Scenario 2 (Figure 4). The multi-year mean

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removal fractions in Scenario 2 were 2.8% for PHE and 1.5% for BaP, whereas those

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in Scenario 3 were approximately 2% for PHE and 0.7% for BaP. This suggests that

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the TNRSF has enhanced the atmospheric removal by 29% for PHE and 53% for BaP.

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In particular, due to the greater increasing trend of LAI and SOC in the Central-North



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region (Figures S15 and S16), the artificial forest in this region has enhanced the

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atmospheric removal by 43% for PHE and 72% for BaP. For BaP, the differences of

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the removal fractions with and without the TNRSF exhibited an increasing trend

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(Figure 4b, solid black line), confirming the increasing contribution of the TNRSF to

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the atmospheric removal of this most toxic PAH congener. These results also imply

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that, although the TNRSF merely removes 1%-4% of PHE and BaP from the

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atmosphere, the depletion of PAHs in the atmosphere would be much lower without

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the presence of the TNRSF.

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Since the TNRSF program will continue until 2050, the LAI and SOC across the

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TNRSF would very likely increase in coming years until its biomass reaches a steady

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state. To project potential changes in PAHs exchange fluxes in the future resulting

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from continued increases of LAI and SOC over the TNRSF, we estimated the linear

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regressions between PAHs fluxes and the SOC and LAI across the TNRSF using the

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linear models of LAI and SOC as shown in Figures S15 and S16. In this way, we

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extended the modeled air-soil net exchange flux and air-canopy net exchange fluxes

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to 2050. Figure S17 shows projected air-soil and air-canopy net exchange fluxes in a

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small area marked by the blue solid line in Figure 3 where the LAI and SOC

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exhibited statistically significant increasing trends (r>0.8, p

Impacts of Large-Scale Land-Use Change on the Uptake of Polycyclic

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