ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-6949-2016Three-North Shelter Forest Program contribution to long-term increasing
trends of biogenic isoprene emissions in northern ChinaZhangXiaodongHuangTaohuangt@lzu.edu.cnZhangLeimingShenYanjieZhaoYuanGaoHongMaoXiaoxuanJiaChenhuiMaJianminjianminma@lzu.edu.cnKey Laboratory for Environmental Pollution Prediction and Control, Gansu Province College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, ChinaAir Quality Research Division, Environment Canada, Toronto, Ontario, CanadaCAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing, ChinaJianmin Ma (jianminma@lzu.edu.cn) and Tao Huang (huangt@lzu.edu.cn)7June201616116949696021November201521January20163May201618May2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/6949/2016/acp-16-6949-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/6949/2016/acp-16-6949-2016.pdf
To assess the long-term trends of isoprene emissions in northern China and
the impact of the Three-North Shelter Forest Program (TNRSF) on
these trends, a database of historical biogenic isoprene emissions from 1982
to 2010 was developed for this region using a biogenic emission model for
gases and aerosols. The total amount of the biogenic isoprene emissions
during the 3 decades was 4.4 Tg in northern China and 1.6 Tg in the TNRSF,
with annual emissions ranging from 132 000 to 176 000 t yr-1 and
from 45 000 to 70 000 t yr-1, respectively, in the two regions.
Isoprene emission fluxes have increased substantially in many areas of the
TNRSF over the last 3 decades due to the growing trees and vegetation
coverage, especially in the central north China region where the highest
emission incline reached to 58 % from 1982 to 2010. Biogenic isoprene
emissions produced from anthropogenic forests tended to surpass those
produced from natural forests, such as boreal forests in northeastern China.
The estimated isoprene emissions suggest that the TNRSF has altered the
long-term emission trend in north China from a decreasing trend during 1982
to 2010 (slope =-0.533, R2=0.05) to an increasing trend for the
same period of time (slope = 0.347, R2=0.014), providing strong
evidence for the change in the emissions of biogenic volatile organic
compounds (BVOCs) induced by the human activities on decadal or longer
timescales.
Introduction
While trees and plants can efficiently remove pollutants from the atmosphere
(Nowak et al., 2006, 2014; Myles et al., 2012; Camporn, 2013; Fenn et al.,
2013; Adon et al., 2013; Zhang et al., 2015), they also play a role in air
pollution through atmospheric chemistry. It has been widely acknowledged
that terrestrial ecosystems release large quantities of reactive biogenic
volatile organic compounds (BVOCs) into the atmosphere as a significant
product of biosynthetic activities of trees and plants (Purves et al., 2004;
Zemankova and Brechler, 2010). BVOCs play important roles in tropospheric
chemistry, carbon budget, and global climate change (Purves et al., 2004;
Nichol and Wong, 2011; Aydin et al., 2014). For example, BVOCs are
precursors of surface ozone formation in the presence of nitrogen oxide
(NOx) (Penuelas et al., 2009; Penuelas and Staudt, 2010). It has been
shown that VOC emissions from biogenic sources have far exceeded those from
anthropogenic sources (Guenther et al., 1995; Aydin et al., 2014).
Among the three dominant VOCs (isoprene, monoterpenes, oxygenated compounds)
contributing to BVOC emission fluxes, isoprene accounts for 70 % of the
total BVOC emissions globally (Guenther et al., 2006; Helmig et al., 2013;
Aydin et al., 2014) and about 50 % in China (Song et al., 2012; Li et
al., 2013). In particular, terrestrial plant foliage is thought to be the
major source of atmospheric isoprene which releases over 90 % of isoprene
from global forests (Lamb et al., 1987; Guenther et al., 2006). Extensive
investigations have been conducted over the past several decades to assess
BVOC emissions and their potential influences on tropospheric chemistry and
the carbon cycle (Lamb et al., 1987; Geron et al., 2006; Müller et al.,
2008; Chang et al., 2009; Pacifico et al., 2009; Zemankova and Brechler,
2010; Guo et al., 2013; Calfapietra et al., 2013). Efforts have been also
made to measure and simulate BVOC emissions in China (Wei et al., 2007; Chen
et al., 2009; Song et al., 2012; Li et al., 2013). A recent study by Song et
al. (2012) revealed that the annual BVOC emissions in eastern China was 11.3×106 t, of which 44.9 % was isoprene, followed by monoterpenes
at 31.5 %, and other VOCs at 23.6 %. The study also showed high
isoprene emissions in boreal forests in northeastern China, on the
Qinling–Ta-pa mountains in central China, and in southern China. Li et
al. (2013) estimated China's total BVOC emissions as 42.5 Tg in 2003, of
which 55 % was isoprene emission.
BVOC emissions are often thought to be static on decadal or longer timescales
because forest coverage from regional to global scales is assumed to
be at steady state (Sanderson et al., 2003; Purves et al., 2004). However,
there are concerns for the potential impacts of climate change and changes
in underlying vegetation coverage on isoprene emissions because leaf-level
emission intensity depends on biological and meteorological conditions
(Turner et al., 1991; Constable et al., 1999; Ashworth et al., 2010; Arneth
et al., 2008, 2011). Several modeling studies were conducted to assess the
interactions between biogenic isoprene emissions and climate change as well
as the human activities (Constable et al., 1999; Sanderson et al., 2003).
Using the USDA (the United States Department of Agriculture) Forest Service
Inventory Analysis (FIA), Purves et al. (2004) estimated decadal changes in
BVOC emissions in the eastern US between the 1980s and 1990s caused by
changes in the extent, structure, and species composition of forests. They
attributed these changes to human-induced deforestation and reforestation.
Arneth et al. (2008, 2011) compared the responses of the simulated BVOC
emissions derived using different models to climate and vegetation changes.
They found that increasing forest area could add several tens of percent to
future isoprene emissions. Climate change could also exert influences on
isoprene emissions via the changes in temperature and CO2. The latter
can benefit forest productivity and leaf growth via fertilization effect.
Steiner et al. (2002) simulated the effect of human-induced land use changes
due to urbanization and agriculture on BVOC emissions. Their results
revealed that the increasing anthropogenic emissions of VOCs subject to
urbanization overall enhanced total VOC emissions. Most of the existing
studies were carried out using climate models subject to projected climate
and land cover change scenarios.
The three northern regions shelter forest (TNRSF) program in China, also
known as “the Green Great Wall”, began in 1978 and will terminate in 2050.
Figure 1 illustrates the TNRSF regions, including 11 provinces and two
megacities, Beijing and Tianjin, as highlighted in the figure caption and
marked in the figure. The program aims to increase China's forest coverage
from 5 % in the 1970s to 15 % by 2050. By the end
of the fourth phase of the program in 2010, the
vegetation coverage over the TNRSF had already reached 12.4 % (Wang et
al., 2011; Central Government of China, 2012). The
program has achieved great successes in mitigating local ecological
environment and climate, despite the debates on the effectiveness of the
TNRSF in improving the ecological environments in northern China and negative
influences of the program on groundwater storage in arid and semi-arid
regions (Pang, 1992; Cheng and Gu, 1992; Parungo et al., 1994; Hu et al.,
2001; Zhong et al., 2001; Ding et al., 2005; Liu et al., 2008; Yan et al.,
2011; Zheng and Zhu, 2013; Fang et al., 2001; Tan et al., 2007; Zhang et al.,
2013). Recently, the TNRSF impact on air quality was also investigated (Zhang
et al., 2015), which showed that the increased vegetation coverage in the
TNRSF has increased its efficiency in removing air contaminants from the
atmosphere as supported by the increasing modeled dry deposition velocities
and fluxes of sulfur dioxide (SO2) and NOx in many places of the
region during the past 3 decades.
The Three-North Shelter Forest Program (TNRSF) in northern China
(defined also by green color in the inner figure (right lower corner of
Fig. 1) and three regions of the TNRSF. The northwest China region of the
TNRSF, defined by grey color, includes Xinjiang, Gansu, the north of Qinghai,
Ningxia, west inner Mongolia, and the north of Shaanxi, many places in this
part of the TNRSF, particularly in Gansu, Ningxia, and west inner Mongolia,
are not covered by forest but by shrubs; the central north China region,
defined by orange gold color, includes the north of the Shanxi and Hebei
provinces, Beijing, Tianjin, and central inner Mongolia; the northeast China
region, defined by brass color, includes east inner Mongolia, parts of the
Liaoning, Jilin, and Heilongjiang provinces. Red, blue, and yellow circles in
the inner figure indicate three small areas in the TNRSF, farmland, and the
boreal forest from which isoprene emission flux are extracted for comparison
(see Results and Discussions sections). Two megacities, Beijing and Tianjin
in the central north China region, are also indicated.
Given its unique status in large-scale artificial afforestation in the human
history, the TNRSF might provide significant insights into understanding
human-induced biogenic VOC emissions on a long-term scale. In the present
study, a framework combining satellite remote sensing data, a biogenic
emission model, and uncertainty analysis was first developed to estimate
BVOC emissions in northern China. Seasonal and annual biogenic isoprene
emission inventories were then developed from 1982 to 2010. Finally, the
potential influences of the development and expansion of the TNRSF on the
long-term trends of the biogenic isoprene emissions were investigated to
discern evidence of decadal or longer-term changes in BVOC emissions from
large-scale forest restorations induced by the human activities. The newly
generated historical isoprene emissions inventories over northern China will
also be useful for assessing past, current, and future air quality and
climate issues.
MethodologyBVOC emission model
The MEGAN2.1 (Model of Emissions of Gases and Aerosols from Nature
version 2.1) (Guenther et al., 2012) which is an updated version of MEGAN2.0
(Guenther et al., 2006) and MEGAN2.02 (Sakulyanontvittaya et al., 2008), was
used here to estimate BVOC emissions in northern China. This new version
includes additional compounds, emission types, and various controlling
processes. For BVOC emissions, MEGAN2.1 is primarily driven by biological and
meteorological factors, including vegetation type with which the emission
factors of BVOCs are assigned, air and leaf temperatures, light, leaf age and
leaf area index (LAI), solar radiation/photosynthetically active radiation
(PAR), wind speed, humidity, and soil moisture (Guenther et al., 2006, 2012;
Pfister et al., 2008; Arneth et al., 2011). MEGAN2.1 was set up over northern
China with a grid spacing of 0.25∘× 0.25∘
latitude/longitude to produce gridded daily and monthly emission fluxes.
Meteorological data used in the MEGAN2.1 employed the 6-hourly objectively
analyzed data from the 1∘× 1∘ latitude/longitude
NCEP (National Centers for Environmental Prediction, 2016) Final Operational
Global Analysis. These data were then interpolated
into the TNRSF grids on the spatial resolution of 0.25 × 0.25
latitude/longitude. PAR was calculated from solar radiation provided by the
big-leaf dry deposition model (Zhang et al., 2002). Twenty-two land types
were used, including an additional crop type which was not specified in the
MEGAN2.1. These land types at each model grid were identified using the
surface roughness lengths estimated from satellite remote sensing data (Zhang
et al., 2015). Guenther et al. (2012) reported the differences in MEGAN2.1
modeled annual isoprene emissions as a result of changing plant functional
type (PFT) (24 %), LAI (29 %), and meteorology (15 %) input data.
This suggests that LAI is one of crucial variables in the model.
Domain-averaged annual emission flux
(micromoles m-2 h-1) of isoprene over the TNRSF from 1982 to
2010. Red dotted line indicates linear trend of emission fluxes and shading
stands for ±1 standard deviation of emission fluxes.
LAI
LAI data with 0.25∘× 0.25∘ latitude/longitude resolution
from 1982 to 2010 were derived from the satellite remote sensing data of the
normalized difference vegetation index (NDVI) for the same period. Detailed
descriptions of the procedures generating LAI data for the TNRSF region were
presented in Zhang et al. (2015).
Uncertainty analysis
Although the BVOC emissions model was well established for different
vegetation types, there were uncertainties in the estimate of BVOC emission
fluxes. Some of these uncertainties are generated from inaccurate emission
factors, empirical algorithms, and input data used in the model (Hanna et
al., 2005; Guenther et al., 2012). Situ et al. showed that, in addition to
the emission factors, PAR and temperature also created large uncertainties in
the MEGAN model (Situ et al., 2014). A Monte Carlo technique was used to
evaluate uncertainties of modeled isoprene emissions by MEGAN2.1 (Hanna et
al., 2005; Guenther et al., 2006, 2012; Situ et al., 2014). In the
uncertainty analysis, each input parameter in MEGAN2.1 for isoprene
emissions, including LAI, leaf temperature (a function of air temperature),
PAR, emission factors, several empirical coefficients related to past leaf
temperatures, and solar zenith, was treated as a random variable with a
normal distribution. The MEGAN2.1 model for BVOC emissions was run repeatedly
100 000 times at the 95 % confidence level based on the coefficients of
variation (CV, %) of these input parameters. The Monte Carlo simulations
showed that the isoprene emissions reached approximately a normal
distribution, ranging from 0.05 to 5.29 micromole m-2 h-1 with
the variation from 97 to 211 %. Details for the uncertainty analysis are
presented in the Supplement (Table S1, Fig. S1).
ResultsIsoprene emission inventory in TNRSF
Figure 2 shows the TNRSF domain-averaged annual biogenic isoprene emissions
(micromoles m-2 h-1) aggregated from monthly values. The
magnitudes of isoprene emissions estimated in the present study agree with
China's BVOC emission inventory established previously, particularly in
the natural forests (Song et al., 2012; Li et al., 2013), as elaborated
below. A long-term increasing trend up to 2007, although with fluctuations
in certain years, was observed (Fig. 2). The emissions in the central north
region of the TNRSF exhibited the strongest increasing trend with the
highest emission increase by 58 % over the 30-year period.
Figure S2 illustrates the MEGAN2.1 simulated isoprene emission fluxes across
the TNRSF in 1982, the early stage of the TNRSF construction, and 2010, the
end of the fourth phase (2001–2010) of the program, respectively. Compared
with the emission fluxes in 1982, higher isoprene emissions in the
central north China region and lower emission fluxes in the northeast region
and eastern inner Mongolia region of the TNRSF were identified in 2010. The
differences in the biogenic isoprene emissions between 1982 and 2010 were
calculated as Edif=E2010-E1982. The spatial pattern of
Edif (Fig. 3) is consistent with the emission fluxes in 1982
and 2010, as shown in Fig. S2a and b. Positive differences of Edif were
observed in the mountainous areas of west Xinjiang, Shaanxi, eastern Gansu
provinces, and the central north China region, suggesting increasing
isoprene emissions associated with the expansion of the TNRSF in these
regions.
Differences of emission flux (E2010-E1982,
micromoles m-2 h-1) of isoprene between 1982 and 2010. The
emission fluxes in these 2 years are shown in Fig. S2a and b in the
Supplement.
(a) Gridded annual isoprene biogenic emissions
(micromoles m-2 h-1) in the year 2000 over northern China with spacing
1/4∘× 1/4∘ latitude/longitude; (b) slopes
of linear regression relationships between annual mean isoprene emission flux
(micromoles m-2 h-1) and the time sequence (or linear trend)
from 1982 to 2010 across northern China.
As mentioned previously in the Introduction, in addition to forest expansion, biogenic
isoprene emissions are also associated with climate change via changes in
mean temperature (Sanderson et al., 2003) and PAR (Guenther et al., 2006,
2012; Situ et al., 2014). Since the influence of climate change on BVOC is
beyond the scope of this article, we shall not assess detailed associations
between climate change (mean temperature) and isoprene emissions from the
TNRSF. Nevertheless, in Sect. 4, we shall briefly discuss the potential
influence of the changes in annual mean air temperature and PAR on long-term
trends of biogenic isoprene emissions in the TNRSF.
Slopes of linear regression relationships between summer mean
isoprene emission flux (micromoles m-2 h-1) and the time
sequence (or linear trend) from 1982 to 2010 across the TNRSF.
Isoprene emission trend in the TNRSF and northern China
Decadal or longer time trends in isoprene emissions over the TNRSF and
northern China can provide some insights into the impact of the large-scale
artificial afforestation on BVOC emissions – the knowledge that is needed to
address air quality, climate, and ecosystem issues. Figure 4 illustrates
modeled isoprene emission fluxes (micromoles m-2 h-1) in 2000
(Fig. 4a), after 20 years of construction of the TNRSF, and the slopes
(trends) of the linear regression relationship between isoprene emissions and
the time sequence of 1982–2010 (Fig. 4b) over northern China, respectively.
High isoprene emissions can be found in the regions extending from the
northeast Qinghai province to the Ta-pa Mountains, the boreal forest in
northeast China, central north China, and the Tianshan Mountain and Pamirs in
the Xinjiang province. The spatial pattern of the estimated emissions in
northeastern China is similar to Song et al. (2012)'s results from 2008 to
2010 (Song et al., 2012). They showed high isoprene emissions from the boreal
forest in northeastern China and Qinling–Ta-pa mountains.
The total annual isoprene emission, summed from annual emissions of the
model grids that fall within the TNRSF domain, ranged from 45 000 to 70 000 t yr-1 during 1982–2010 for the whole TNRSF (the area encircled by
the blue solid line in Fig. 4), and from 132 000 to 176 000 t yr-1
for all of northern China (Fig. 4). This is equivalent to a total emission of
1.6 and 4.4 Tg, respectively, for the two regions during the past 3
decades from 1982 to 2010. It is worth noting that, although the TNRSF
accounts for 59 % of the total area of northern China and 42 % of
mainland China (Zhang et al., 2015), it covers almost all arid and
semi-arid regions in northern China. Vegetation coverage in these regions
was still sparse after 30 years of construction of the TNRSF, and shrubs,
instead of trees, are major plant types in the northwest China region of the
TNRSF. The isoprene emissions are considerably low in these regions, as
shown by Figs. 4 and 5. In addition, as shown by Fig. 4, the region of
northern China defined in this study extends virtually to 30∘ N.
Although the isoprene emissions in the TNRSF only accounted for 37 % of
the total emissions in northern China, the relatively strong increasing
trend (Fig. 2) in the TNRSF (slope = 0.881, R2=0.335) has reversed
the negative trend (slope =-0.533, R2=0.05) of the total annual
isoprene emissions in northern China, which did not take the isoprene
emissions in the TNRSF into consideration, to the positive trend
(slope = 0.347, R2=0.014) from 1982 to 2010 in northern China, as
shown in Fig. S3.
To highlight the contribution of the TNRSF to the increasing isoprene
emissions, the trend of the gridded isoprene emissions over the TNRSF was
further investigated. As expected, the estimated monthly emission fluxes
showed dramatic seasonal variations with the largest values in summer and
the lowest values in winter, consistent with the seasonal changes in LAI
over the TNRSF (figure not shown). Figure 5 presents the gridded trends of
the summer biogenic isoprene emissions across the TNRSF from 1982 to 2010.
The summer emission fluxes exhibited a similar annual pattern to the annual
emissions (Fig. 4b) but were greater than the annual emissions, as shown by
Fig. 5. Positive trends of the emissions were observed in the mountainous
and surrounding areas of the Junggar Basins (north Xinjiang), eastern
Qinghai province in the northwest China region of the TNRSF, the
central north China region, and the western Liaoning province in the northeast
China region of the TNRSF. These provinces and locations are marked in Fig. 1. In particular, the largest positive trends can be observed in the areas
north of the two megacities – Beijing and Tianjin. These two megacities have
been targeted as key cities to be protected by the TNRSF from sandstorms
from the north. Extensive tree planting activities have been promoted to the
north of these two megacities (Central Government of China, 2012).
Annual variations of emission fluxes of isoprene averaged over three
regions of the northeast, central north, and northwest China region of the
TNRSF. The dotted straight line represents linear trend of isoprene emission
fluxes in the central north China region.
Figure 6 shows the isoprene emissions from 1982 to 2010 averaged over
northwest China, the central north China, and the northeast China regions of
the TNRSF, respectively. It can be identified again that the domain-averaged
isoprene emissions in the central north China region of the TNRSF exhibited
a clear increasing trend with the slope of 0.0004 (R2=0.35,
p=0.002), whereas statistically insignificant and relatively weak trends
of isoprene emissions were found in the northeast China (slope = 0.00003,
R2=0.032, p=0.484) and northwest China (slope = 0.00009,
R2=0.27, p=0.012) regions of the TNRSF, respectively. The increase
of isoprene emissions over the central north China region can be attributed
to continuous expansion of forest coverage. Compared with the central north
region of the TNRSF, the forests in the northeast China region are mixed
with natural forests. These natural forests already reached the steady state
before the 1980s, so they would not contribute to the increasing trend of
biogenic isoprene emissions. As shown by Fig. 4b, the isoprene emissions in
most places of northeast China show almost no trends. The northwest China
region of the TNRSF is arid and semi-arid area with low precipitation.
Shrubs, instead of trees, were planted in many places of this part of the
TNRSF regions, resulting in low biogenic isoprene emissions.
Trends of isoprene emissions were also compared between those within and
outside the TNRSF and in natural forests. Three small areas were selected
for the comparison, each consisting of four grid points, in the central north
China region of the TNRSF (marked by the red circle in the inner map of Fig. 1), a farmland outside the TNRSF (blue circle), and in the boreal forest of
northeast China (the Greater Khingan mountains, marked by a yellow circle in
Fig. 1), respectively. Trends in annually averaged isoprene emissions from
these three small areas are shown in Fig. 7. A significant increasing trend is
only seen in the area within the TNRSF. The levels of isoprene emissions in
the other two small areas were almost uniformly distributed for the last
3 decades.
Comparison with the previous emission data
No extensive and direct measurements of BVOC emissions across the TNRSF have
been ever carried out. Several field campaigns were conducted to measure
BVOC emissions in northern China but these monitoring programs were not
typically designated for the TNRSF (Klinger et al., 2002; Wang et al.,
2003). Li et al. (2013) established an emission inventory of BVOCs
(isoprene, monoterpenes, sesquiterpene and other VOCs) over China using
MEGAN2.1 model. Their results showed that annually averaged isoprene
emission fluxes ranged from 0 to 22 µg m-2 h-1 in 2003 in
northern Xinjiang, Qinghai, Gansu, and Shaanxi provinces in the northwest
China region of the TNRSF, and western inner Mongolia. The average isoprene
emission fluxes estimated in the present study for the same regions and the
same year ranged from 0.01 to 18.2 µg m-2 h-1, agreeing
reasonably well with Li et al. (2013)'s data. Li et al. (2013)'s inventory also
showed high isoprene emission flux in the central north China region,
including the north of Shanxi and Hebei provinces, Beijing, and the natural
(boreal) forest area in northeast China, ranging from 22 to 880 µg m-2 h-1. While the lower limit of their estimated flux agrees well
with our lowest emission flux of 20.4 µg m-2 h-1, the upper
limit of their emission flux was 880 µg m-2 h-1, is a factor of
4 higher than our value (122.4 µg m-2 h-1) for the same
region. Li et al. (2013) adopted more locally updated species-specific
emission factors and a vegetation classification based on a new vegetation
investigation in the late 1990s and early 2000s in China. Their calculation
also used hourly and diurnal meteorological (temperature, radiation, winds)
data. Our estimated fluxes used the emission factors specified in the
MEGAN2.1 (Guenther et al., 2012) and vegetation types classified by the
roughness lengths (Zhang et al., 2002, 2015). In addition, our model input
daily meteorological data. These different input data to the MEGAN model
likely resulted in the difference of the isoprene emission fluxes between Li
et al. (2013) and our results. Song et al. (2012) simulated BVOC emissions in
eastern China from 2008 to 2010. A portion of their model domain in eastern
China was superimposed with the central north and the northeast China
regions of the TNRSF defined in our study. The annually averaged isoprene
emission fluxes from 2008 to 2010 from Song et al. (2012)'s model simulations ranged
from 10 to 100 µg m-2 h-1 in the inner Mongolia region, and
100–1000 g m-2 h-1 in the north of the Shanxi and Hebei provinces,
Beijing, and Tianjin, which were higher than our results of 0 to 32.6 µg m-2 h-1 and 20.4 to 122.4 µg m-2 h-1,
respectively, in these two regions. Song et al. (2012) used MEGAN2.04 model with
different emission factors adjusted based on China's principal vegetation
species (Song et al., 2012). These could also lead to different biogenic
isoprene emissions.
Annual variation and trend of isoprene emission flux spatially
averaged over three small areas in and outside the TNRSF in central north
China and the natural (boreal) forest region as marked in Fig. 1. The
left-hand side y axis scales the trend of isoprene emission fluxes in the TNRSF
region and boreal forest in northeast China and the right-hand side y axis
scales the emission flux from the farmland outside the TNRSF.
Discussions
Overall the estimated biogenic isoprene emission fluxes across the TNRSF
illustrated an increasing trend from the 1980s onward (Fig. 2). The incline
trend was most significant in the central north region of the TNRSF where
most intensive afforestation has been carried out in north China (Zheng and
Zhu, 2013), in order to protect the national capital (Beijing) region from
dust and sandstorms. The increasing biogenic isoprene emissions can be
attributed to the development of the TNRSF. The forest expansion in the TNRSF
can be identified by the satellite-derived LAI, as seen in Fig. S4a and b.
The linear increasing trend of the LAI across the TNRSF is consistent with
the modeled isoprene emission fluxes. The maximum increase (58 %) of the
isoprene emissions from 1982 to 2010 in the central north region of the TNRSF
seems to agree well with the model prediction by Arneth et al. (2008, 2011)
based on projected land use changes. Their modeling results suggested that
increasing forest area could lead to several tens of percent of change in
biogenic isoprene emissions.
As shown above, the significant incline trend of the annual total isoprene
emissions in the TNRSF has affected the long-term trend of the emissions in
northern China. This implies that the increasing emission trends across the
TNRSF could alter the large-scale BVOC emissions not only in the TNRSF but
also in northern China considering that the TNRSF occupies 59 % of
northern China and 42 % of all of mainland China. Future impacts of the
TNRSF on BVOC emissions may be even stronger with continuous increases of
vegetation coverage till the end of the program in 2050.
While BVOC emissions vary on short timescales, the global BVOC emissions
are often assumed to change little on a long-term (e.g., decadal) scale
(Purves et al., 2004; Sindelarova et al., 2014) considering the steady state
of global forests. Since BVOCs can partition onto or form particles in the
atmosphere after oxidation, their emissions could affect aerosol formation,
cloud condensation nuclei, and climate (Makkonen et al., 2012; Penuelas and
Staudt, 2010). Identification of the impact of climate change on BVOC
emissions is not straightforward if regional or global forests reach a steady
state. The evidence identified in this study suggested that the human-induced
BVOC emissions via large-scale afforestation exert strong influence on
long-term BVOC emissions and should be taken into consideration in projected
climate change scenarios, at least on a regional scale, such as in northern
China. As a precursor of secondary organic aerosols and tropospheric ozone,
the significant incline of biogenic isoprene emissions also carries
significant implications to the air quality in northern China. Heavy air
pollution in the Beijing–Tianjin–Hebei area (Fig. 1) has been widely known
nationally and internationally, characterized by year-round high levels of
fine particular matter (PM2.5) and high surface ozone concentrations in
the summertime. The Chinese government has decided to extend the TNRSF as one
of the primary measures to reduce and remove air pollutants from the
Beijing–Tianjin–Hebei area (Chinese Environmental Protection Agency, 2013).
As shown in Figs. 5 and 6, the TNRSF in the central north region covering a
large part of the Beijing–Tianjin–Hebei area has already gained the most
rapid development as compared to the other two northern regions of the TNRSF
(Fig. 1), leading to marked incline of isoprene emissions. However, it is not
yet clear if and how the extension of the TNRSF could otherwise improve local
air quality. Our previous study suggested that the TNRSF played a moderate
role in removing SO2 and NOx (Zhang et al., 2015). Under the
rapidly increasing NOx emissions in the past decade due to the rapidly
increasing number of private vehicles in the Beijing–Tianjin–Hebei area, it
is necessary to assess the interactions between BVOC emissions from the TNRSF
and local air quality in this region.
In addition to its long-term trend, isoprene emissions also exhibited
short-term interannual fluctuations, as also observed in Fig. 2. Factors
causing the fluctuations or interannual changes in the emission fluxes depend
on meteorological and biological processes. Afforestation and deforestation
often took place during the course of the TNRSF construction due to favorable
or unfavorable weather and climate conditions for tree growth. For example,
10–50 % of trees planted since the late 1970s in the central north
region of the TNRSF were reported dead since 2007 (Zhang et al., 2013; Tan
and Li, 2015), causing visible decline of the forest coverage and isoprene
emissions in this region after 2007, as shown in Fig. 2. The lower isoprene
emissions in 2010 in the northeast China region and eastern inner Mongolia
region of the TNRSF as compared with those in 1982 were inconsistent with the
increasing trend of the emissions. The forest coverage in the northeast China
region did not show considerable change between 1982 and 2010. On the other
hand, lower annual temperatures (e.g., by around 1 ∘C) in 2010 were
evident over the northeast China region of the TNRSF than those in 1982, as
shown by the differences of annual surface temperatures (SATs, ∘C)
between 1982 and 2010 (Tdif=T2010-T1982, Fig. S5a),
which likely caused lower biogenic emissions in 2010 (Purves et al., 2004;
Arneth et al., 2008, 2011). Negative Tdif in the northeast China
region of the TNRSF corresponded nicely to negative Edif
(Fig. 3), indicating the strong association between SATs and isoprene
emissions. In addition, compared with the increasing trend of LAI in the
northeastern China region of the TNRSF (Fig. S4a), no statistically
significant increasing trends of the isoprene emissions are discerned in this
region. Figure S5b displays the trend of annual SATs in the northeast China
region of the TNRSF from 1982 to 2010. Overall the SATs exhibited a
decreasing trend, caused mostly by declining SATs since the late 1990s. Since
temperature plays a key role in canopy BVOC emissions (Guenther et al., 2012;
Li et al., 2013), the lack of the incline trend of the isoprene emission
fluxes in the northeast China region of the TNRSF might be attributable to
the decreasing SAT from the late 1990s.
Another environmental factor that may exert the influence on the trend of
isoprene emissions is solar radiation/PAR (Situ et al., 2014). Analogous to
the response of the BVOC emissions to temperature, increasing radiation
could also enhance the isoprene emissions, or vice versa, particularly on
a daily or monthly basis. To elucidate potential association between the
long-term trend of biogenic isoprene emissions and PAR, we estimated the
trend of the flux of PAR (Guenther et al., 1995) over the TNRSF from 1982 to
2010. Results are shown in Fig. S6. Positive trends can be observed in the
northwest China region of the TNRSF (Xinjiang, Gansu) and inner Mongolia. In
contrast to the positive trends of isoprene emissions in the central north
China region of the TNRSF, PAR in this region exhibited negative trends. Hu
et al. (2010) have calculated the long-term changes in PAR in Beijing using a
broadband global solar radiation data set. Their result revealed a
significant declining trend of PAR from the late 20th century. They
attributed the decrease of PAR to increasing aerosol emissions from large
amounts of fossil fuel combustion due to rapid economic development and
industrialization in north China, including the Beijing–Tianjin–Hebei region, in
the past several decades. The increase in anthropogenic aerosol particles
can both absorb and scatter solar radiation in the atmosphere, contributing
to the decreasing PAR. Within and proximate to north China where most heavy
industries in China are located, the central north China region is the
mostly contaminated area in the TNRSF by particulate matter and other air
pollutants. Higher aerosol loading to this region was at least partially
responsible for the decrease in the trend of PAR. This means that, while
PAR contributes significantly to daily and monthly changes as well as
spatial distribution in biogenic isoprene emissions in the TNRSF, it is
unlikely to overwhelm the long-term trend of isoprene emissions.
The comparison between the isoprene emission trends and the emissions in
2000 in northern China also carries a significant implication for the
human-induced BVOC emissions. As shown in Fig. 4b, the trend of isoprene
emissions from 1982 to 2010 over northern China showed a rather different
spatial pattern from its emissions in 2000 (Fig. 4a). No significant trends
were observed in the boreal forest in northeastern China, though a larger
amount of isoprene was emitted from the forest in this region in 2000. This
implies that this natural forest was likely under a steady state from which
the biogenic isoprene emissions were not altered on the decadal basis
(Sanderson et al., 2003; Purves et al., 2004).
Although the Qinling–Ta-pa mountains exhibited the highest emissions in 2000
(Fig. 4a), negative trends of the biogenic isoprene emissions dominated this
area, indicating the declining of the emissions over the period of 1982–2010.
This is consistent with the decreasing vegetation coverage
during this period in this region, as shown by the negative trends of the
LAI in northern China (Fig. S4). On the other hand, most
positive trends of LAI can be identified in the central north region and
along the foot of Tianshan Mountain in west China (see the areas encircled
by the solid blue line in Fig. 4). This manifests that the TNRSF exerts
strong influences on biogenic VOC emissions, particularly on their decadal
variation, though the magnitude of emissions might not be higher than that
from natural forests in northeast China (Fig. 4a). Results further imply
that the TNRSF is very likely the major source contributing to the
increasing biogenic isoprene emissions over the past 30 years and for many years
to come in northern China. Climate change has been thought also to play an
important role in the changes in the biogenic emissions of isoprene on decadal or
longer timescales because it can alter temperature and vegetation coverage
(Turner et al., 1991; Sanderson et al., 2003). It is unknown if and to what
extent the increasing vegetation coverage and temperature over the TNRSF
were induced by climate change. Evidence shows that the human-induced
afforestation contributed mostly to the increased vegetation coverage over
the TNRSF and northern China (Wang et al., 2011), as shown by Fig. S4a, and
hence to the increased biogenic isoprene emissions.
Among the three small areas within the TNRSF, in the farmland, and in the
boreal forest of northeast China (Fig. 7), the emission values increased by
nearly 5 times from 1982 to 2010 in the area within the TNRSF with the slope
of 0.0018 (R2=0.55). On the other hand, no statistically
significant increasing trends of biogenic isoprene emissions were found in
the farmland and the boreal forest, though the higher emissions were
observed in the boreal forest. More interestingly, the biogenic isoprene
emissions in the selected small area of the central north China region tend
to surpass the isoprene emissions in the boreal forest from 2004 onward.
This can be partly attributed to rapidly growing forest coverage and higher
temperatures in this region as compared to northeastern China. The large
area of foliage trees planted in this region also played a role for
relatively high and increasing isoprene emissions as compared with the
boreal forests in northeastern China where coniferous trees are major tree
species which release relatively lower isoprene to the atmosphere as
compared to broadleaf trees in the selected area in the central north China
region of the TNRSF (Guenther et al., 2012).
Conclusions
Gridded monthly and annual biogenic isoprene emissions in northern China
were modeled for the period of 1982–2010 and were then applied to assess
the long-term trends of the biogenic isoprene emissions in the TNRSF in
order to discriminate the signals of the human activities in decadal and
longer-term trends of BVOCs on large spatial scales. Significant impacts of
the TNRSF on the BVOC emissions in northern China were identified during the
past 3 decades. Annual isoprene emissions in many places of the TNRSF
region, especially in the central north China region, exhibited an inclining
trend. The maximum increase in the isoprene emission flux reached 58 %
between 1982 and 2010, indicating important roles of the human activities on
BVOC emissions. The comparison of isoprene emission fluxes among the
central north China region of the TNRSF, farmland, and the boreal forest in
northeastern China outside the TNRSF revealed that the biogenic isoprene
emissions in some areas of the central north China region of the TNRSF
produced by man-made forests have surpassed the emissions from the natural
forests. This suggests that the TNRSF was a main contributor to the decadal
or longer-term changes in BVOCs in northern China. The impact of the TNRSF
on BVOC emissions is expected to be stronger in the coming years along with
continuous development of the TNRSF program till 2050. Since BVOCs are a major
precursor of tropospheric ozone, future studies are needed to investigate
how the increased BVOCs in the TNRSF contribute to ozone formation,
especially in the case of concurrently increasing NOx emissions in
northern China.
Data availability
The meteorological data used in the MEGAN2.1 model compiled by NCEP Final
Operational Global Analysis are available at
http://rda.ucar.edu/datasets/ds083.2/ (National Centers for
Environmental Prediction, 2016).
The Supplement related to this article is available online at doi:10.5194/acp-16-6949-2016-supplement.
Acknowledgements
This work is supported by the National Natural Science Foundation of China
through grants 41371478 and 41371453.
Edited by: A. B. Guenther
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