are banned from roads and sent to wrecking yards for recycling. Consequently,
the estimated share of fuel consumption by vehicles meeting Euro IV and Euro V
standards increased from 2% in 2010 to 66% in 2017.
2.3.3 Comparison with Observations
Rapid decreases in SO 2 and NO x emissions after 2013 are confirmed by satellitebased observations (de Foy et al. 2016; Krotkov et al. 2016; van der A et al. 2017;
Koukouli et al. 2018). We estimate that NO x and SO 2 emissions in Eastern China
decreased by 21% and 59% during 2013–2017, respectively, lower than the 30% and
73% decreases in OMI-observed NO 2 and SO 2 columns for the same region and time
period (Zheng et al. 2018). Surface SO 2 concentrations decreased by 57% over
Eastern China for the period of 2013–2017, in good agreement with the estimated
emission trend. In contrast, surface NO 2 concentrations only decreased by 9% for the
same time, significantly lower than the estimated trend in NO x emissions.
Different trends between emissions and concentrations could be attributed to
many factors. First, temporal and spatial patterns of emissions and concentrations
are impacted by variations in meteorology, atmospheric transport, and chemical
reactions. Interannual variabilities can result in remarkable variations in column
and surface concentrations (Uno et al. 2007), which may partly explain the disagreement between changes in emissions and observations for a single year. Surface
observations are more sensitive to surface emissions than high-stack emissions, and
satellite-based column observations better respond to high-stack emissions due to
different transport patterns, in which both will contribute to the differences when
comparing trends. In addition, chemical partitioning of NO 2 /NO x may also contribute to discrepancies between trends in emissions and observations (Lamsal et al.
2011; Valin et al. 2011). The discrepancies between bottom-up and top-down
emission estimates indicate that uncertainties from other aspects might exist.
Second, uncertainties in observations can also contribute to the discrepancies. In
situ observations are usually thought to be more accurate. However, surface NO 2
concentrations obtained from China’s national monitoring network relied on chemiluminescence measurements, which can significantly overestimate NO 2 concentrations (Lamsal et al. 2010) and then contribute to discrepancies between emissions
and surface observations. Satellite retrievals are subject to larger uncertainties, for
instance, tropospheric SO 2 columns are quite uncertain due to difficulties in isolating
anthropogenic SO 2 signals from ozone and volcanic SO 2 (Krotkov et al. 2006). Most
uncertainties in satellite retrievals are systematic and canceled when trends are
compared. However, influences of aerosols on satellite NO 2 retrievals (Lin et al.
2015) may impact the reliability of the NO 2 column trend due to the large decrease in
aerosol concentrations over the discussed period. SO 2 columns are less sensitive to
small- and near-surface emissions (Li et al. 2017a), which may lead to an underestimation of the SO 2 budget in China for most recent years and a disagreement
between emission and SO 2 column trend when high-stack emissions (e.g., power
plants) were significantly reduced.
2 Anthropogenic Emissions of SO 2 , NO x , and NH 3 in China
25
the estimated share of fuel consumption by vehicles meeting Euro IV and Euro V
standards increased from 2% in 2010 to 66% in 2017.
2.3.3 Comparison with Observations
Rapid decreases in SO 2 and NO x emissions after 2013 are confirmed by satellitebased observations (de Foy et al. 2016; Krotkov et al. 2016; van der A et al. 2017;
Koukouli et al. 2018). We estimate that NO x and SO 2 emissions in Eastern China
decreased by 21% and 59% during 2013–2017, respectively, lower than the 30% and
73% decreases in OMI-observed NO 2 and SO 2 columns for the same region and time
period (Zheng et al. 2018). Surface SO 2 concentrations decreased by 57% over
Eastern China for the period of 2013–2017, in good agreement with the estimated
emission trend. In contrast, surface NO 2 concentrations only decreased by 9% for the
same time, significantly lower than the estimated trend in NO x emissions.
Different trends between emissions and concentrations could be attributed to
many factors. First, temporal and spatial patterns of emissions and concentrations
are impacted by variations in meteorology, atmospheric transport, and chemical
reactions. Interannual variabilities can result in remarkable variations in column
and surface concentrations (Uno et al. 2007), which may partly explain the disagreement between changes in emissions and observations for a single year. Surface
observations are more sensitive to surface emissions than high-stack emissions, and
satellite-based column observations better respond to high-stack emissions due to
different transport patterns, in which both will contribute to the differences when
comparing trends. In addition, chemical partitioning of NO 2 /NO x may also contribute to discrepancies between trends in emissions and observations (Lamsal et al.
2011; Valin et al. 2011). The discrepancies between bottom-up and top-down
emission estimates indicate that uncertainties from other aspects might exist.
Second, uncertainties in observations can also contribute to the discrepancies. In
situ observations are usually thought to be more accurate. However, surface NO 2
concentrations obtained from China’s national monitoring network relied on chemiluminescence measurements, which can significantly overestimate NO 2 concentrations (Lamsal et al. 2010) and then contribute to discrepancies between emissions
and surface observations. Satellite retrievals are subject to larger uncertainties, for
instance, tropospheric SO 2 columns are quite uncertain due to difficulties in isolating
anthropogenic SO 2 signals from ozone and volcanic SO 2 (Krotkov et al. 2006). Most
uncertainties in satellite retrievals are systematic and canceled when trends are
compared. However, influences of aerosols on satellite NO 2 retrievals (Lin et al.
2015) may impact the reliability of the NO 2 column trend due to the large decrease in
aerosol concentrations over the discussed period. SO 2 columns are less sensitive to
small- and near-surface emissions (Li et al. 2017a), which may lead to an underestimation of the SO 2 budget in China for most recent years and a disagreement
between emission and SO 2 column trend when high-stack emissions (e.g., power
plants) were significantly reduced.
2 Anthropogenic Emissions of SO 2 , NO x , and NH 3 in China
25