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Satellite remote sensing of air quality in the Energy Golden Triangle, Northwest China Yanjie Shen, Xiaodong Zhang, Jeffrey Robert Brook, Tao Huang, Yuan Zhao, Hong Gao, and Jianmin Ma Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.6b00182 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016
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Satellite Remote Sensing Of Air Quality In The Energy Golden Triangle In Northwest China
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Yanjie Shen1, Xiaodong Zhang1, Jeffrey R. Brook2, Tao Huang1, Yuan Zhao1，Hong Gao1*, Jianmin Ma1,3*
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Affiliation:
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College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China
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Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province,
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Air Quality Research Division, Atmospheric Science and Technology Directorate, Environment Canada, 4905 Dufferin St., Toronto, Ontario, Canada M3H 5T4
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Tel: +86 15293166921, fax: +86-931-8911843, email:
[email protected] ；
[email protected] 16
Word account: 2632; Figures: 3
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Abstract
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This article presents the first assessment of air quality in the Energy Golden Triangle (EGT) in northwest China. Using the planetary boundary layer (PBL) column density (PCD) of SO2 and the tropospheric column density (TCD) of NO2 retrieved from the Ozone Monitoring Instrument (OMI), we show that the column densities of both SO2 and NO2 exhibit an increasing trend from 2005 to 2014 in the Ningdong energy and
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CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China
*Corresponding author: Jianmin Ma, Hong Gao
chemical industrial base (NECIB) within the EGT，in contrast to the rapid and widespread decrease of SO2 emissions in northern China. This is largely attributed to the rapid development of the energy industry in this region. It is expected that SO2 and NO2 emitted from the EGT would increasingly contribute to the total emissions of these two air pollutants in northern China.
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1. Introduction
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Fossil fuel industries are major sources of air pollutants,1 Due to rapid
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industrialization and urbanization over the past decades, the increasing demands for
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energy supply in China have imposed serious adverse effects on air quality，
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particularly in northern China where most of the heavy industry is located. Under the
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national strategy for energy development and safety during the 21st century, oil,
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natural gas, coal mining industries，and thermal power generators have been relocated
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towards energy-abundant northwest China,2,3 Consequently, the Energy Golden
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Triangle (EGT), which is located in this region (Figure S1 of Supporting Information,
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SI), is now the largest new energy and chemical industry base in China providing 25%
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of the country’s energy production,4 The total fossil fuel resources in the EGT are
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equivalent to 2 trillion tons of standard coal and account for approximately 40% of the 2
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country's total. In particular, the Ningdong energy and chemical industrial base
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(NECIB) in the Ningxia Hui Autonomous Region,5 a sub-region in the EGT (Figure
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S1), is a nationally prioritized region for the promotion of the development of
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large-scale coal mining bases, thermal power generation, and the chemical industry.
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To guarantee national energy security and to promote the regional economy，the EGT
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energy program has been accelerating since 2010 reaching the national goals for coal
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production of 1.45×109 tons, oil output of 5.4×107 tons, and natural gas production of
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5.5×1010 m3 in 2015.4 ,6 Given the low annual precipitation ranging from 100 mm to
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400 mm and sparse vegetation coverage,7,8 the poor ecological environments in the
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EGT region are uniquely susceptible to atmospheric pollution. It is hypothesized that
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the increased release of pollutants to the air, water and land that is associated with
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rapid development in the EGT will have a considerable impact on the local
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environment. These effects can also be expected to extend to the downstream regions
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in eastern China via atmospheric transport and river flows. In particular, the EGT
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region is located in the upper reaches of the Yellow River, the second largest river in
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China, which passes through the EGT and has been the main source of water supply
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not only to local industries, but also to a vast region of eastern and central China.
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Knowledge of the magnitude and extent of the environmental contamination and
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impact associated with EGT development is very limited. No detailed air quality
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monitoring data are available and the limited local government reports and air quality
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assessments indicate that air quality in the EGT has been improving over the past
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decade. These assessments state that levels of most criteria air pollutants were well 3
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below the national standard.5,9 Here we attempt to independently assess the air quality
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in the NECIB and EGT using satellite remote sensing data to quantify the influence of
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the rapid development in the EGT on the local environment since 2005. Through the
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use of satellite-based observations, it is possible to obtain valuable information on
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emissions, spatial-temporal patterns, and annual trends of many air contaminants10-12.
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In particular, air quality remote sensing with relatively high spatial resolution has
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been improved rapidly in the last several years. Notably, the Ozone Monitoring
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Instrument (OMI) on the NASA EOS Aura platform provides daily global coverage in
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combination with small ground pixel sizes (nominally 13 × 24 km2 at nadir,
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minimum 13 × 12 km2 at nadir),12-15 and the Infrared Atmospheric Sounding
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Interferometer.15,16 This article presents the satellite retrieved column densities of
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sulfur dioxide (SO2) and nitrogen dioxide (NO2) as well as their trends from 2005 to
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2014 over the EGT and NECIB with a high spatial resolution, aiming to fill critical
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knowledge gaps and understand air quality in the EGT given the shifting energy base
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in northern China. Awareness of the implications of these large scale national policies
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are important for assessing and minimizing impacts.
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2. Materials and Methods
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The OMI-derived PCD ( DU) of SO2 and TCD (1015 molec cm-2) of NO2 with a fine
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spatial and temporal resolution were collected from the NASA EOS and KNMI
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(Koninklijk Nederlands Meteorologisch Instituut), respectively.15,18 The operational
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OMI PBL data are processed using the highly sensitive band residual difference (BRD)
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algorithm to facilitate the acquisition of information on near-surface SO2 emission16. 4
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The NO2 vertical column densities are obtained using a differential optical absorption
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spectroscopy (DOAS) algorithm that converts NO2 slant column densities to vertical
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columns.19,20 The SO2 PCD and NO2 TCD have been validated against their respective
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monitored air concentrations in northern China.14,20 In the present study, the
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OMI-retrieved column densities of SO2 and NO2 have been compared with the
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available daily and monthly ambient air concentration data of these two chemicals at
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official air quality monitoring stations and in the China National Environmental
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Monitoring Center (http://106.37.208.233:20035/) near the central NECIB region.
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Details of the comparisons are presented in the SI (text and Figures S2 and S3).
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Two emission inventories for SO2 and NOx (nitrogen dioxide, NOx=NO+NO2
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where NO is nitric oxide) are available in China. The first one covers East Asia with
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gridded emission data before 2006.21 The other provides gridded SO2 and NOx
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emission data from four emission sectors (industry, residential, power generation, and
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transportation) for every two years from 2008 to 2012.22 Figures S4 and S5 display
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the SO2 and NOx annual emissions from different emission sectors in 2008, 2010, and
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2012,22 averaged over the NECIB and over a selected region of northern China (see
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below for description of this region).
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3. Results and Discussion
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3.1 OMI-retrieved SO2 and NO2 in EGT and NECIB
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Figure 1 shows the spatial distribution of SO2 PCD and NO2 TCD over the EGT and
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four bordering provinces (inner map), averaged over 2005-2014 at a 0.25°×0.25°
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latitude/longitude resolution. Higher SO2 PCDs (Figure 1a) and NO2 TCDs (Figure 5
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1b) can clearly be observed in Ningdong (Yinchuan), Erdos, and the north of Yulin
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(also seeing Figure S1). The higher 2005-2014 mean SO2 PCD and NO2 TCD extend
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from the central NECIB region to Shizuishan in Ningxia and Wuhai in Inner
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Mongolia. Wuhai and Shizuishan are traditional coal mining areas in northwest China.
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The energy industries in these two regions have rapidly expanded since the early
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2000s to provide raw coal to the Ningdong chemical and thermal power industries.
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Due to SO2 emissions from coal mining industry, the daily ambient air
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concentrations of SO2 in Wuhai were considerably higher than in Yinchuan (Figure
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S6a). Conversely there were relatively higher levels of NO2 over Yinchuan compared
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to Wuhai (Figure S6b), which was likely due to the larger number of motor vehicles
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in Yinchuan (population 2.1 million) compared with that in Wuhai (population 0.56
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million).
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Since the coal mining and power generation industries in the NECIB sub-area
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of the EGT have been experiencing the fastest expansion in recent several years, we
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compared the monthly mean PCD of SO2, averaged over the NECIB with northern
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China (Figure 2a, the NECIB and northern China are marked in Figure S1), where
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air pollution levels, especially fine particulate matter (PM2.5), have received
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significant international attention during the past three decades. Here we include in
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northern China two mega cities (Beijing, Tianjin) and three provinces ( Liaoning,
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Shangdong, and Hebei), which make up one of China's most populated and
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industrialized regions. This region will be referred to as the northern China region
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hereafter. Result from 2005 to 2014 shown in Figure 2a indicate that the SO2 PCDs in 6
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the northern China region were much higher than those in the NECIB before 2011.
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However, since 2011, the monthly SO2 PCDs over the NECIB have been similar to
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and higher than the northern China region in magnitude. This is attributed to
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decreasing SO2 emissions in the northern China region and increasing emissions in
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the NECIB. The strong decline of SO2 emission from power generation after 2006 in
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the northern China region (Figure S4c) was a result of regulations to reduce SO2
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emission in this region,21,22 including application of flue-gas desulfurization, and the
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relocation of oil and coal mining industries as well as thermal power generations to
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northwest China.23,24
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The 2005-2014 mean SO2 PCDs and NO2 TCDs are generally consistent with the
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mean SO2 and NOx emissions over 2008, 2010, and 201222 in the same region, as
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shown in Figure 2b, c and Figure S7. The annual SO2 emissions from different
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emission sectors averaged over the NECIB and the northern China region are
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illustrated in Figure S4. Although the emissions from the residential and
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transportation sectors in the northern China region were considerably higher than
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those averaged over the NECIB in the later 2000s because of much higher population
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density in the northern China region (Figure S4b and d), the emissions from these
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two sectors accounted for less than 10% of the total SO2 emission. In contrast to the
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continuously declining SO2 emission in the northern China region, for the major
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emission sectors of industry and power generation (Figures 2a, b and Figure S4a, c),
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the SO2 emissions averaged over the NECIB increased after 2006 and became much
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higher than the domain-averaged emissions from power plants over the northern 7
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China region, in line with the SO2 PCD changes, as illustrated in Figure 2a and b. As
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a result, the domain-averaged total SO2 emission over the NECIB has exceeded the
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mean emission averaged over the northern China region in 2012 (Figure 2b and S4e).
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Similarly, the domain-averaged NOx emissions from power generation in the NECIB
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increased rapidly and was 2.2 times higher than the emissions averaged over the
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northern China region in 2012 (Figure 2c and S5). Although the mean NOx emissions
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in the NECIB from the other three emission sectors were still lower than those
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averaged over the northern China region, the total NOx emission from all emissions
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sectors in the NECIB increased more rapidly than those averaged over the northern
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China region due to markedly increasing NOx emission from power generation in the
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NECIB (Figure S5e).
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Figure S8a and b further compare the annual emissions of SO2 and NOx from
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power generation21 with annually averaged column densities of SO2 and NO2
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retrieved from OMI over the EGT in 2008, 2010, and 2012, showing agreement
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between the emissions and column densities of SO2 and NO2. Knowing that the
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thermal power industry contributed primarily to the NECIB and EGT development6,
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the association between SO2 and NOx emission patterns and the column densities of
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these two criteria air pollutants confirmed that the coal mining and thermal power
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industry in this region likely played a significant role worsening local air quality.
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3.2 Trends of OMI-retrieved column densities of SO2 and NO2
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Table S1 presents the non-parametric Mann-Kendall statistical test (Z)25 and Sen’s
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estimated slopes (Q)26 for the monthly mean SO2 PCD and NO2 TCD in the NECIB, 8
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EGT, and the northern China region. The statistically significant declining trend of
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SO2 PCD in the northern China region is indicated by the negative Z value (-6.698,
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p
