Introduction
Nitrogen dioxide (NO2), sulfur dioxide (SO2), and formaldehyde
(HCHO) are important atmospheric trace gases that play a major role
in atmospheric chemical processes. NO2 participates in the
formation of ozone (O3) and reacts with hydroxyl radicals (OH), the
strongest oxidizing agents in the atmosphere, to produce aerosols and acid
rain, which are harmful to both buildings and human health (Seinfeld and
Pandis, 2006; Lelieveld and Dentener, 2000; Lelieveld et al., 2002).
NO2 may also have important impacts on the greenhouse effect
(Solomon et al., 1999). In addition to natural sources,
high-temperature combustion processes, e.g., fossil fuel burning and accidental
and intentional biomass burning, are estimated to contribute major
emissions of nitrogen oxides (NOx=NO2+NO)
(Lee et al., 1997). SO2 contributes to the formation
of sulfate aerosols and acid rain, both of which have negative effects on
the climate and human health and lead to building acid corrosion
(Hutchinson and Whitby, 1977; Pope and Dockery, 2006; Longo et al.,
2010). The dominant anthropogenic emissions of SO2 are the burning of
fossil fuels, smelters, and oil refineries, whereas the discharge of active
volcanoes is the major natural source. HCHO is the predominant product of
the oxidation of many volatile organic compounds (VOCs) by OH radicals and is
abundant throughout the atmosphere. Therefore, elevated HCHO levels can be
related to the emission of reactive non-methane volatile organic compounds
(NMVOCs) originating from biogenic, pyrogenic, or anthropogenic sources
(Fu et al., 2007; Millet et al., 2008; Stavrakou et al., 2009a, b).
The differential optical absorption spectroscopy (DOAS) technique is widely
used to identify and quantify different kinds of atmospheric trace
gases. The DOAS principle makes use of the fact that narrow trace gas
absorption structures can be separated from broad band absorption and
atmospheric scattering (Platt and Stutz, 2008). The
multi-axis differential optical absorption spectroscopy (MAX-DOAS)
instrument is designed to observe scattered sunlight under different viewing
angles closed to the horizontal and the zenith directions, which can provide
high sensitivity to tropospheric aerosols and trace gases (Hönninger et
al., 2004). In the past decades, the MAX-DOAS method has been successfully
used for observations of many atmospheric trace gases, such as NO2, SO2, HCHO, HONO, and others, on different platforms. The most common
application is ground-based measurement (e.g., Irie et al., 2011; Pinardi
et al., 2013; Wang et al., 2014; Chan et al., 2015; Xing et al., 2017).
Meanwhile, mobile platform observations have been developed rapidly, e.g.,
car-based observations (Johansson et al., 2008; Shaiganfar et al., 2011, 2017;
Wang et al., 2012), aircraft observations
(Baidar et al., 2013; Dix et al., 2016), and ship-based
observations (Sinreich et al., 2010; Takashima et al., 2012; Peters et
al., 2012; Schreier et al., 2015; Hong et al., 2018).
Usually, the trace gas concentrations are very low in remote marine
environments, considering there are no emission sources except the ship
traffic and some other natural sources. Previous ship-based MAX-DOAS studies
reported that NO2 vertical column densities (VCDs) were basically low
(<0.50×1015 molec cm-2) due to the absence of
obvious NOx emission sources nearby, and the NO2 concentrations
in the
marine boundary layer extracted from profile retrieval are <30 pptv
in the open and clean tropical sea area of the South China and Sulu seas
(Schreier et al., 2015). Time series of SO2
magnitudes were found to be consistent with tropospheric NO2 in
this area
and occasionally increased if the measurements were taken in a busy shipping
lane. Over the western Pacific and Indian oceans, the background value of
NO2 concentration was less than ∼0.2 ppbv over the remote ocean
(Takashima et al., 2012). Peters et al. (2012) found that HCHO VCDs over the
remote ocean exhibit a diurnal pattern with maximum values of 4×1015 molec cm-2 at noontime over the western Pacific Ocean, and
corresponding retrieved peak concentrations were up to 1.1 ppbv at higher
altitudes around 400 m. However, these pollutant concentrations increased
to a high value when the measurements were taken close to the shore, busy
ports, or vessels (Takashima et al., 2012; Peters et al., 2012; Schreier
et al., 2015). So far, the air quality of the marine boundary layer along the Chinese coast has rarely been reported on.
In this study, we used ship-based MAX-DOAS measurements to report the
column densities and temporal–spatial distributions of NO2, SO2,
and HCHO in marine environments over the East China Sea (ECS) area in
June 2017. During this campaign, the cruise ship mainly navigated the sea
area surrounding the Yangtze River Delta (YRD) region, which is the confluence of
the coastal shipping routes and inland water transportation on the Yangtze
River. It is the busiest waterway of the ECS area and also one of the
three key Ship Emission Control Zones (ECZs) of China. The YRD coastal port
cluster is composed of more than 15 ports, of which Shanghai and
Ningbo–Zhoushan have served as the two largest container ports in the world
since 2013. With the flourishing shipping industry, the throughput of YRD
ports continuously strikes new highs and causes considerable ship
emissions of SO2, NOx, and PM2.5, which has significant
impacts on local and regional air pollution in both offshore and inland
areas of the YRD region (Fan et al., 2016; Zhang et al., 2017). Due to rapid urbanization and industrialization, as well as an expanding
population, the continental YRD region also suffers from ecological
degradation and environmental problems at the same time, e.g., atmospheric
fine-particle and O3 pollution (Chen et al., 2017; Song et al., 2017).
In this paper, both VCDs and vertical distributions of NO2,
SO2, and HCHO from ship-based MAX-DOAS measurements, as well as the
ozone profiles from onboard lidar, have been reported for the ECS
area covering the Yangtze River estuary and surrounding YRD region waters for the first time.
The spatial distributions of these gaseous pollutants by ship-based
measurements were compared to satellite observations and further used to
discuss air pollutant transport between continental and sea areas.
Moreover, ozone formation over the sea was investigated. These observed
data sets are vital for better understanding of the air quality in the
marine boundary layer along the coastline of China and helpful for
regulating air pollution in coastal areas.
Methodology
The measurement cruise
The ship-based measurement campaign was implemented over the offshore marine
area of the ECS covering the Yangtze River estuary and YRD region coast in
summer from 2 to 29 June 2017 (Fig. 1a). Before departure,
measurement instruments were installed and debugged at Gongqing port (point
A in Fig. 1b, 31.33∘ N, 121.55∘ E) of Shanghai on 1 June 2017. As indicated in Fig. 1b, the ship set sail on 2 June 2017 from
Gongqing port by way of Yangshan port (B, 30.62∘ N,
122.09∘ E) and the Daishan islands (C, 30.25∘ N,
122.16∘ E) and Hangzhou Bay. After encircling the Zhoushan Islands
(29.99∘ N, 122.20∘ E), the ship moved forward to the Shengsi
Islands (D, 30.71∘ N, 122.45∘ E) and Huaniao Island (E,
30.85∘ N, 122.68∘ E). Then, the ship was headed to
Lianxing port in Jiangsu Province (F, 31.72∘ N, 121.87∘ E) and passed around Huaniao Island before sailing back to Gongqing port on
29 June 2017. As shown in Fig. 1 the ship cruise routes not only covered the
busy waters of the YRD region but also passed though some clean marine areas 100 km away from the continental coast.
Location (a) and cruise routes (b) of the
ship-based campaign from 2 to 29 June 2017 (DOY 153 to 181).
Ship-based MAX-DOAS measurements
Instrument setup
An integrated and fully automated MAX-DOAS instrument was fixed on a 1.5 m high tripod top on the stern deck of the ship. This compact instrument
consists of an ultraviolet spectrometer (AvaSpec-ULS2048L-USB2) covering the
spectral range of 300–460 nm with a spectral resolution of 0.6 nm, a
one-dimensional charge-coupled device detector (Sony ILX511, 2048 individual pixels), and a
stepper motor driving the telescope to collect scattered sunlight from
different elevation angles (angle between the horizontal and the viewing
directions, α). In addition, the controlling electronic
devices, connecting fiber, and other necessary devices are mounted inside
too. To avoid the impact of emission plumes from the ship itself, the
azimuthal angle of the telescope unit was kept at 130∘ relative
to the heading direction of the observation ship. The telescope scanned
in the sequence of elevation angles of 3,
5, 7, 10, 15, 30, and 90∘. The
duration of an individual spectrum measurement was about 30 s and each
scanning sequence took about 4 min. The daily measurements were
automatically controlled by a built-in computer combined with a spectral
collection software when the solar zenith angle (SZA) was less than
75∘. Moreover, a high-precision Global Position System (GPS) data
receiver was configured to record the real-time coordinate positions and the
track of the ship cruise.
Typical DOAS spectral fittings for (a) NO2,
(b) O4, (c) SO2, and (d) HCHO.
The spectrum was collected at an elevation of 15∘ at 10:13 LT on
7 June 2017. The black curves show the reference absorption cross section
scaled to the measured atmospheric spectrum (red curves) by DOAS fitting.
The spatial distributions of the trace gas VCDs of
(a) NO2, (b) SO2, and (c) HCHO
along the cruise route of the ship-based campaign in June 2017.
Spectral analysis
Based on the DOAS principle, the measured scattered sun-light spectra are
analyzed using the QDOAS spectral fitting software suite developed by
BIRA-IASB (http://uv-vis.aeronomie.be/software/QDOAS/, last access: 23 October 2018). The
detailed configuration of the spectral fitting is listed in Table 1. The
fitting wavelength intervals of NO2, O4, SO2, and HCHO are
338–370, 338–370, 305–317.5, and 336.5–359 nm, respectively. Trace
gas absorption cross sections of NO2 at 220 and 298 K
(Vandaele et al., 1998), SO2 at 298 K (Vandaele et al., 2009), HCHO at 297 K (Meller and Moortgat,
2000), O3 at 223 and 243 K (Serdyuchenko et al., 2014), O4
at 293 K (Thalman and Volkamer, 2013), BrO at 223 K (Fleischmann
and Hartmann, 2004), the Ring spectrum, a Fraunhofer reference spectrum, and
a low-order polynomial are included in the DOAS fitting. The wavelength
calibration was performed using a high-resolution solar spectrum
(Chance and Kurucz, 2010). The dark current spectrum and electronic offset
spectrum were used to correct measured spectra before the spectra
analysis.
DOAS spectral fitting of NO2, O4, SO2,
and HCHO.
Parameter
Data source
Trace gases
NO2 & O4
SO2
HCHO
Wavelength range
338–370 nm
305–317.5 nm
336.5–359 nm
O4
Thalman and Volkamer (2013), 293 K
√
×
√
NO2
Vandaele et al. (1998), 220 K, 298 K, I0 correction (1017 molec cm-2)
√
√(only 298 K)
√(only 298 K)
SO2
Vandaele et al. (2009), 298 K
×
√
×
HCHO
Meller and Moortgat (2000), 297 K
√
√
√
O3
Serdyuchenko et al. (2014), 223 K, 243 K, I0 correction (1020 molec cm-2)
√
√
√
BrO
Fleischmann et al. (2004), 223 K
√
√
√
Ring
Ring spectra calculated with QDOAS according to Chance and Spurr (1997)
√
√
√
Polynomial degree
5
5
5
Wavelength calibration
Based on a high-resolution solar reference spectrum (SAO 2010 solar spectra)
The variations in trace gas tropospheric VCDs with longitude:
(a) NO2, (b) SO2, and (c) HCHO.
The spectral analysis yields the measured slant column densities (SCDs), the
integrated trace gas concentration along the light path through the
atmosphere. For MAX-DOAS spectral analysis, the measured spectrum at
90∘ was selected as the Fraunhofer reference spectrum for the
DOAS fitting of the measured spectra at other elevation angles in each scan
sequence. Thus the generated results are the difference of the SCDs between
the measured spectrum and that of the Fraunhofer reference spectrum, usually
referred to as differential slant column densities (DSCDs). Figure 2 shows a
typical DOAS spectral fitting of the measured spectrum collected at an
elevation of 15∘ at 10:13 local time (LT) on 7 June 2017. The
retrieved DSCDs of NO2, O4, SO2, and HCHO are 2.28×1016, 1.90×1043, 1.84×1016, and 4.46×1016 molec cm-2, respectively. All these fittings
displayed the evident absorption structures of the trace gases and fairly
low residuals, which demonstrates the good performance of the spectral
fitting. In this study, a threshold of residual <2.5×10-3 is used to filter the unsatisfied fitting results of NO2,
O4, and HCHO. Afterwards, the qualified DSCD results remain
99.37 %, 99.37 %, and 99.79 %, respectively. Considering the weak
scattered sunlight signals and low signal-to-noise ratio around 300 nm,
where the SO2 has strongly structured absorption, the threshold of
residual for SO2 is set to 5.0×10-3, and 70.05 % of
the fitting results match this criterion.
Retrieval of the trace gas VCDs and profiles
To obtain the tropospheric VCD of trace gas, the
DSCDs have to be converted using tropospheric differential air mass factors
(DAMFs) by Eq. (1) (Wagner et al., 2010):
VCDtrop=DSCDsDAMFs=DSCDsαAMFα-AMF90∘.
The air mass factor (AMF) calculation is solved using the
geometric approximation method (Hönninger et al., 2004; Wagner et
al., 2010), which is simple and convenient and is simultaneously also
validated by radiative transfer simulations (Solomon et al.,
1987; Shaiganfar et al., 2011).
AMFα=1/sin(α)
Using Eqs. (1) and (2), the tropospheric DAMF was estimated to be 2.86
and 1 for elevation angles of 15 and 30∘,
respectively. Previous ground-based MAX-DOAS studies show that the most
appropriate choice for the elevation angle would probably be 30∘
for the geometric approximation approach (Halla et
al., 2011; Brinksma et al., 2008). Nevertheless, an elevation of 15∘
also works well for the conversion of DSCDs into VCDs in a ship-based MAX-DOAS
campaign since the last scattering point is generally above the trace gas
layer for an elevation angle of 15∘ in the lower boundary layer over the
sea (Schreier et al., 2015). In addition, due to the longer
light path through the boundary layer, the observations at a 15∘
elevation angle are more sensitive compared to at a 30∘ elevation angle. Consequently,
the VCDs of NO2, SO2, and HCHO were attained from DSCDs
of a
15∘ elevation angle by the geometric approximation method in this
study.
Daily 24 h backward trajectories of air masses at 300 m in
altitude for (a) 2 to 5 June, (b) 6 to 10 June,
(c) 11 to 22 June, and (d) 23 to 29 June 2017.
Time series (a) and correlation analysis (b) of
the tropospheric NO2 VCDs measured by ship-based MAX-DOAS and the OMI
satellite during this campaign.
Time series (a) and correlation analysis (b) of
the tropospheric SO2 VCDs measured by ship-based MAX-DOAS and the OMPS
satellite during this campaign.
To obtain the vertical distribution of trace gases, we used the HEIPRO
algorithm (HEIdelberg PROfile, developed by IUP Heidelberg) for MAX-DOAS
profile retrieval (Frieß et al., 2006, 2011, 2016). The HEIPRO retrieval algorithm is based on the
optimal estimation method (OEM; Rodgers, 2000) and coupled with the
radiative transfer model SCIATRAN (Rozanov et al., 2005) as the
forward model. Because the existence of aerosol has strong impacts on the
scattered light path in the atmosphere, the retrieval algorithm takes into
account the aerosol profile retrieval first and then adopts the retrieved
aerosol scenario to profile the trace gases. In this study, an a priori exponential
decay profile with a scale height of 1.0 km is used as the initial
profile for both the aerosol and trace gas retrievals. The total aerosol
optical depth and trace gas VCD of the a priori profile are 0.2 and 7.27×1015 molec cm-2, respectively. The uncertainty of the a priori
aerosol and trace gas profiles is set to 100 % and correlation
length is set to 0.5 km. In the radiative transfer model, the parameters of
single-scattering albedo, asymmetry parameter, and ground albedo are assumed
to be 0.92, 0.68, and 0.06 for the marine environment. The retrieved profile
of aerosol extinction and trace gas concentration has the resolution of a
fixed grid of 200 m from the sea surface to 3 km in altitude. The criteria that the
relative error of profile retrieval is larger than 50 % and degree of freedom
of signal is smaller than 1.0 are used to filter the profile results.
Afterwards, about 1.1 %, 23.4 %, and 7.2 % of all measurements were
discarded for NO2 SO2, and HCHO profile retrievals, respectively.
OMI and OMPS satellite data
The Ozone Monitoring Instrument (OMI) was launched on July 2004 onboard the
NASA Aura satellite (Levelt et al., 2006). It is an imaging
spectrometer covering the wavelength range from 270 to 500 nm, which
receives the light signal of scattered light in the Earth's atmosphere and
reflected by the Earth's surface. OMI aims to monitor ozone, NO2, and
other minor trace gas distributions with high spatial resolution (about 13×24km2) and global daily coverage. OMI is operated on a
sun-synchronous orbit, and the overpass time is about 13:45 LT. However, OMI
has suffered from a so-called “row anomaly” and lost several cross-track
position data (Boersma et al., 2011). In this study, we use
USTC OMI tropospheric NO2 products (Liu et al., 2016; Su et al.,
2017). To generate the USTC OMI products, the NO2 SCDs are retrieved
from the OMI Level 1B VIS global radiances data (OML1BRVG) based on the DOAS
method. To convert into NO2 VCDs more accurately, AMFs are calculated
with the input of the localized NO2 and atmospheric temperature and
pressure profiles derived from WRF-Chem chemistry transport model
simulations. The WRF-Chem simulations have used the National Centers for Environmental Prediction (NCEP)
Final Operational Global Analysis (FNL) meteorological data.
Time series (a) and correlation analysis (b) of
the tropospheric HCHO VCDs measured by ship-based MAX-DOAS and the OMPS satellite
during this campaign.
The monthly averaged spatial distributions of the trace gas VCDs
of (a) NO2, (b) SO2, and
(c) HCHO of the OMI and OMPS satellite observations in June 2017.
The Ozone Mapping and Profiler Suite (OMPS) instrument was launched on 28 October 2011 onboard the Suomi National Polar-orbiting Partnership
(Suomi NPP) satellite. The OMPS nadir mapper (OMPS NM) is one of three
sensors of the OMPS suite of instruments. It contains a UV spectrometer
covering
the wavelength range between 300 and 380 nm with a full width at half maximum
(FWHM) of 1 nm. It has a high spatial resolution of 50×50km2 and high time resolution of daily global coverage (Dittman et
al., 2002; Seftor et al., 2014; González Abad et al., 2016). Its Equator
crossing time in the ascending node is 13:30 LT. In this study, the OMPS
satellite observation data were used to retrieve the USTC OMPS tropospheric
SO2 and HCHO products. Similar to the USTC OMI tropospheric NO2
VCDs, the SO2 and HCHO VCDs are produced with a two-step approach
too; i.e., first the SCDs of SO2 and HCHO are retrieved from the measured
scattered sunlight spectra and are then converted to the SO2 and HCHO VCDs by
applying the calculated AMFs based on the WRF-Chem chemistry transport model
simulation results.
Ozone lidar
During this campaign, an O3 lidar, co-located with the
MAX-DOAS instrument, was also onboard and was developed by the Anhui Institute of Optics and
Fine Mechanics (AIOFM) using differential absorption lidar (DIAL)
technology. The laser pulse of the lidar is at 316 nm, usually with the
energy of about 90 mJ and a repetition frequency of 10 Hz. The laser beam is
emitted with a divergence of 0.3 milliradian (mrad) and the receiving
telescope has a field of view (FOV) of 0.5 mrad, resulting in an overlap
height of approximately 300 m. The O3 profiles in the lower troposphere
were obtained using DIAL retrieval algorithms. The lidar observation has a
high vertical resolution of 7.5 m and a temporal resolution of about 12 min.
In order to improve the signal-to-noise ratio, the retrieved vertical
distribution O3 concentrations were averaged on a 100 m grid.
Additionally, the O3 concentration profiles with relative errors above
20 % were removed from the further discussion.
Comparison of OMI and ship-based measurements of NO2 VCDs on
(a) 2, (b) 7, (c) 16, and (d) 27 June.
The ship-based measurements were plotted overlapping the base map of OMI
products, and the wind field is indicated with black arrows.
Results and discussion
Trace gas tropospheric VCDs
Based on the spectral analysis and the geometric AMF approach, we obtained
the VCDs of different trace gases along the ship cruise combined with the
GPS-received geo-position data. Figure 3 shows the spatial distributions of
NO2, SO2, and HCHO VCDs along the route over the ECS area. The
missing data are due to power failure and instrumental malfunction
during the campaign as well as measurements taken under bad weather
conditions (e.g., heavy rain). During the campaign, the NO2 VCDs
varied from 1.00×1015 to 5.52×1016 molec cm-2 with a mean value
of 6.50×1015 molec cm-2. As shown in Fig. 3a, high NO2 VCDs,
almost three times the average of the whole cruise, were observed at
the ship lanes of the south channel of the Yangtze River estuary and along
the route
to Lianxing port (located in Qidong of Jiangsu Province), as well as the
busy port of Ningbo–Zhoushan. The SO2 VCDs ranged from 1.00×1015 to 1.77×1016 molec cm-2 with an average of 4.28×1015 molec cm-2.
Figure 3b shows the elevated SO2 value (i.e., >8.63×1015 molec cm-2, about the 95th percentile value,
∼2.02 times mean value) are appeared in the same places as
NO2, such as the ship lanes close to the Gongqing port, Ningbo–Zhoushan
port. For HCHO, the averaged VCD is 7.39×1015 molec cm-2 in the range of 1.02×1015 to
3.16×1016 molec cm-2. As in Fig. 3c, the enhanced HCHO
columns were found in the section of the cruise in Hangzhou Bay, which
is different than NO2 and SO2 spatial distribution. Moreover, high
HCHO VCDs >1.0×1016 molec cm-2
were also appeared over the same hot spots as NO2 and SO2.
Three typical observation periods in characteristic observation
areas.
Vertical profiles of NO2, SO2, and HCHO
concentrations during the three typical observation periods:
(a) measurements taken at Hangzhou Bay and the Zhoushan Islands from 7 to
10 June, (b) measurements carried out at a relatively clean area from 16 to
19 June, and (c) measurements implemented in the area of the
Yangtze River estuary on 26 to 29 June.
The coastal waters of the YRD region, including Jiangsu, Shanghai, and Zhejiang,
are the busiest sea area of the ECS, and the continental YRD region is also one
of the most developed industrial city clusters of China or even the world.
Therefore, previous studies found that the air quality in coastal sea and
inland areas was affected by ship-emitted pollutants under cruising and
maneuvering conditions together with continental anthropogenic
pollutants (e.g., Zhao et al., 2013; Fu et al., 2014). In order to
investigate impacts of ship emissions, we obtained the dependence of
NO2, SO2, and HCHO VCDs on longitude in Fig. 4. It can be found
that most of the peaks of trace gases occurred at the geo-locations of
busy ports and ship lanes, whereas lower values are observed at remote
oceanic areas (Fan et al., 2016). The spatial distribution of
NO2, SO2, and HCHO over sea areas is mainly dominated by the local
emission sources of ships, ports, and even coastal factories.
In addition, the spatial distribution of trace gases is also influenced
significantly by meteorological conditions, especially wind speed and
wind direction. Here, we calculated 24 h backward trajectories of air masses 300 m in
altitude by applying the HYSPLIT (Hybrid Single-Particle
Lagrangian Integrated Trajectory) model, which was developed by the National
Oceanic and Atmospheric Administration Air Resource Laboratory (NOAA ARL)
(http://ready.arl.noaa.gov/HYSPLIT.php, last access: 23 October 2018) (Stein et
al., 2016). Global Data Assimilation System (GDAS) meteorological data
with a spatial resolution of 1∘×1∘ and 24
vertical levels were used in the trajectory simulation process. Figure 5
displays the daily 24 h backward trajectory results for four periods,
which illustrate the origin of the air masses arriving at the endpoint
(indicated by black triangle) at 04:00 UTC (12:00 LT).
In Fig. 5a and c, the air masses originated from a clean sea area from 2 to 5 and 11 to 22 June 2017. This suggests that the
observed air pollutants were less impacted by airflow patterns and
were instead mainly from local emission sources. For example, a high
concentration of pollutants was reported on 2 and 3 June, during which the
measurements were implemented on busy ship lanes in the south channel of the
Yangtze River estuary. In contrast, the trace gas VCDs were much lower
during most days from 11 to 22 June, when the measurements were taken over the clean sea area. Figure 5b and d show the air masses
coming from inland areas from 6 to 10 and 23 to 29 June, respectively. As
shown in Fig. 3, high values of NO2, SO2, and HCHO VCDs, the
corresponding 95th percentiles of which are 1.81×1016, 1.05×1016, and 1.31×1016 molec cm-2 were found during the ship cruise from the Shengsi Islands
to Lianxing port and back to Huaniao Island on 26 and 27 June 2017. The air mass
originated from the coastal industrial zone on 26 June and the city center of
Shanghai on 27 June. These pollution episodes can mainly be attributed
to pollutants transported from inland cities and coastal areas combined with
ship emissions in nearby waters.
Comparison with OMI and OMPS satellite products
In order to compare the ship-based MAX-DOAS and satellite data, we have to
make them comparable for temporal and spatial coverage. The ship-based
MAX-DOAS-measured VCDs are averaged for 13:00 to 14:00 LT according to the OMI
and OMPS instrument overpass times of about 13:45 and 13:30 LT. The
satellite products are averaged within a 10 km radius of the center position
of the ship cruise between 13:00 and 14:00 LT considering the cruising speed
at
around 8–15 km h-1. Moreover, satellite data with larger error
(relative error >100 %) and cloud impacts (cloud fraction
>0.5) were excluded from the intercomparison. Hence, there
remain 14 days of observation for NO2, SO2, and HCHO VCD
comparison.
Figure 6a shows the time series of the NO2 VCD intercomparison
between ship-based MAX-DOAS measurements and OMI satellite observations.
These two data sets agree well with each other and have a high correlation
coefficient (R) of 0.83 in Fig. 6b. However, OMI satellite observations
were higher than the ship-based MAX-DOAS results on some days, which is
different from the comparisons over continental areas where the satellite
observations are usually much smaller than ground-based data (Liu et al.,
2016). The larger discrepancies on 8, 15, and 20 June were observed in the
remote ocean area, implying possible larger uncertainties of the VCD
retrieval in such a clear marine environment.
Time series of the vertical profiles of the O3
concentrations measured by the ozone lidar in June 2017.
For the intercomparison with ship-based MAX-DOAS measurements, the space-based products of
SO2 and HCHO VCDs were retrieved from the OMPS satellite. The time series
of the SO2 VCDs measured by ship-based MAX-DOAS and retrieved from OMPS
satellite observations are displayed in Fig. 7a. These space-based and shipborne data exhibited similar temporal trends during the campaign, showing a
correlation coefficient (R) of 0.76 in Fig. 7b. Figure 8a presents the
time series of the HCHO VCDs measured by ship-based MAX-DOAS together with
the satellite data retrieved from OMPS observations, which also show good
agreement, with a correlation coefficient (R) of 0.69 in Fig. 8b. In addition,
we also found that the trace gas VCDs of NO2, SO2 and HCHO from
space-based observation by the OMI and OMPS satellites are higher than ship-based
MAX-DOAS measurements in a marine environment.
To characterize the spatial distribution of tropospheric NO2,
SO2,
and HCHO VCDs, the monthly averaged tropospheric products of OMI satellite
NO2, and OMPS SO2 and HCHO in June 2017 are shown in Fig. 9.
The satellite data were error (relative error >100 %) and cloud
(could fraction >0.50) filtered and gridded at a high spatial
resolution of 0.05∘×0.05∘. Due to the
different emission sources and formation mechanisms, these three trace gases
show distinct features of spatial distributions. In Fig. 9a, the hot
spots of NO2 distribution were centered at the coast of the
Yangtze River at Shanghai and Jiangsu Province, the Ningbo–Zhoushan port,
the and Shengsi Islands. For the spatial distributions of SO2 in Fig. 9b, Qidong in Jiangsu province, the northwest part of Shanghai, and
Hangzhou Bay, and even over some sea areas
where there are dense waterways and ship lanes, relatively high values are expressed. In addition, the main hot spots
are
located at Shanghai for HCHO spatial distributions in
Fig. 9c. In summary, all three trace gases have high values in some
polluted continental areas, e.g., the Shanghai city center and northwest area,
while hot spots over sea areas are mainly consistent with the heavy
vessel and port emission areas.
Since daily satellite observations can provide a detailed regional view
of the spatial distribution of gaseous pollutants, the daily
distribution of NO2 VCDs was further compared between OMI data and
ship-based results. Figure 10 presents the comparison for 2, 7, 16, and
27 June; the trajectories of ship-based measurements are shown in
color, indicating cruise routes with the white lines, and the position
of ship-based measurements at 13:00 to 14:00 LT are marked by black
points. It can be observed that the color-coded ship-based measurements
generally achieve a good agreement with the spatial distribution of
satellite data, except for somewhere on 27 June.
To reveal some typical pollution transport process, the wind files (black
arrow) were plotted overlapping each other. Combining with the wind information,
the air masses came from clean sea areas on 2 and 16 June but originated
from polluted inland areas on 7 and 27 June. Therefore, the observed
NO2 VCDs over inland and sea areas were substantially lower on 2 and 16 June compared to measurements on 7 and 27 June. Under oceanic wind
conditions, hot spots of NO2 VCDs are mainly located in the inland
areas and sea areas with high shipping emission intensities, which can be
attributed to the impacts of local emissions. When the wind blew from the continent
on 7 and 27 June, the NO2 pollution
spread from inland to the downwind water areas close to the coast and even
to the sea areas far from the coast. This suggests significant
influence of the pollutants transported from inland on the air quality over
seawaters.
Tropospheric NO2, SO2, and HCHO profiles
In order to obtain vertical distribution of trace gases, we followed the
method described in Sect. 2.2.3 to retrieve the vertical profiles of NO2,
SO2, and HCHO. Daily profile results are available and three typical
observation periods are presented for different characteristic areas, as
indicated in Fig. 11. The measurements during cycle 1 from 7 to 10 June were
located at Hangzhou Bay and the Zhoushan Islands. In cycle 2 from 16 to 19 June, the ship cruise was in the clean waters far away from the coastline.
In cycle 3 from 26 to 29 June, measurement was carried out through clean to polluted areas over the waters of
Shanghai, Qidong, and areas around the Yangtze River estuary.
The daily averaged vertical profiles of O3 concentrations
and HCHO/NO2 ratios at different altitudes between 09:00 and
15:00 LT
on 7 to 10 June 2017.
Figure 12a–c show the diurnal variations in the vertical profiles of
NO2, SO2, and HCHO concentrations during these three cycles. It is
obvious that the trace gas concentrations of NO2, SO2, and HCHO
in cycle 2 were lower than those of the others, which can also be confirmed
by the spatial distribution of trace gas VCDs in Fig. 3. By extracting the
lowest 500 m grids of the retrieved profiles, the observed concentrations of
NO2, SO2, and HCHO were <3, <3, and
<2 ppbv in the clean marine boundary layer. This can be explained by
the fact that the measurements of cycle 2 were performed in a relatively
remote sea area, which is far away from the YRD continental region and less
impacted by inland emission sources and often receives clean air masses from the remote ocean (in Fig. 5c). However, higher levels of the trace gases were found during
cycles
1 and 3. For example, the concentrations of NO2, SO2, and HCHO in
the marine boundary layer all increased to high values on 9 and 29 June.
To track the cruise on 9 June, the ship passed through the channel
between the Ningbo and Zhoushan Islands, where the Ningbo–Zhoushan port is located, the
world's largest port by container throughput. The Ningbo–Zhoushan
port has been reported to account for about one-third of the national
port-level emissions in China (Fu et al., 2017). The hourly
variation in pollutant emissions reached a peak during
09:00–14:00 LT and varied with vessel type (Yin et al., 2017).
The backward trajectory on 9 June shows that the air mass originated from
the coastal area. It is inferred that this pollution episode was mainly
attributed to the ship emissions from coastal and oceangoing vessels, as
well as cargo handling equipment in the port areas. On 29 June,
observations were performed along the Yangtze River upstream until
121.15∘ E and then back to the Gongqing port of Shanghai. This
waterway is the only channel going upstream of the Yangtze River and is
consequently dense with inland ships. Moreover, Taicang port and some
industrial zones were distributed along the coastline areas. Therefore,
industrial factories and ship emissions, as well as transports from
inland cities, contributed together to these elevated pollutant levels.
In addition, the vertical distributions of NO2, SO2, and HCHO
in the
marine boundary layer have unique features. The high NO2 concentrations
were observed close to the sea surface and decreased with height. For the layer below 500 m, the lowest and highest NO2
concentrations were found to be <3 ppbv in cycle 2 and >10 ppbv during cycles 1 and 3. Almost all the measured NO2
concentrations in the marine boundary layer during this campaign are larger
than the background value over the western Pacific and Indian oceans
(<0.2 ppbv) (Takashima et al., 2012) and over the South China and
Sulu seas (<30 pptv) (Peters et al., 2012). Due to the
sulfur-containing marine fuels, ship emissions are the primary source of
SO2 over the seas. Therefore, intermittent enhanced SO2
signals were detected during the whole cruise as shown in Fig. 12, even for
the relatively clean area in cycle 2, where the SO2 concentrations
exceeded 3 ppbv at times. It implies that the frequently observed SO2
pulses are the emissions from the kinds of vessels in the vicinity or even
from the cruise ship itself.
As distinguished from NO2 and SO2 vertical profiles,
the highest HCHO concentrations are located at elevated altitudes (about
500 m) during cycles 1 and 3 since there are no HCHO sources from
the sea surface. A similar phenomenon was also reported in a study over the
remote western Pacific Ocean, where the highest concentrations of HCHO
occurred at altitudes of 400 m (Peters et al., 2012). Furthermore,
extremely high HCHO concentrations of >5 ppbv appeared as
the ship cruised along coastal and busy port areas in cycles 1 and 3, while
the low concentration of about 1.2 ppbv was measured in cycle 2. However, the
observed lowest levels of HCHO in the marine boundary layer of the ECS area were almost
equal to the highest value (∼1.1 ppbv) measured in the remote
western Pacific Ocean (Peters et al., 2012). The behavior of NO2,
SO2, and HCHO concentrations highlighted the obvious shipping emissions
along the ship lanes and close the busy ports and further significant
impacts on the regional air quality over the ECS areas.
Ozone formation
Figure 13 presents the onboard DIAL-observed vertical distributions of
ozone concentrations from 300 m up to 2 km above sea level (km a.s.l.) during the
campaign. Except the absence on 13 and 28 June due to power failure,
there were 26 days of measurement results. It is found that the ozone
concentrations in the marine environment showed a characteristic vertical
structure, the O3 concentrations increased with altitude from
300 m to 1.0 km a.s.l., and high values >100 ppbv were mostly
distributed at altitudes higher than 1 km a.s.l. However, the high ozone
concentrations were detected from 300 m and spread to 1.4 km on 7 and 8 June. For the diurnal patterns, the O3 concentrations usually began to
increase in the morning with the enhancing solar radiation and accumulated to
arrive at the daily peak in the afternoon, then declining with the decrease
in
sun illumination. Similar diurnal variations were also reported in
previous studies over continental areas, e.g., measurements in nearby
Hangzhou, Zhejiang Province (Su et al., 2017). The study indicated the daytime
intense photochemical processes in the marine boundary layer.
In order to investigate the formation and consumption processes of ozone, we
integrated the vertical profiles of O3 by lidar, NO2, and HCHO
profiles from MAX-DOAS together. We averaged the daily profiles of O3
and the ratio of HCHO/NO2 during the intense photochemical periods between
09:00 and 16:00 LT. Figure 14 shows the vertically resolved comparisons
between O3 and HCHO/NO2 ratio profiles on 7 to 10 June.
Referring
to Fig. 12, the NO2 concentrations were higher on 9 and 10 but lower on
7 and 8, while the HCHO concentrations remained at high levels during all of
cycle 1 from 7 to 10 June. Accordingly, the ratios of HCHO to NO2 on 7
and 8 June were higher than those on 9 and 10 June. Meanwhile, the O3
concentrations ranged at different altitudes from 70 to 100 ppbv in the
marine boundary layer on 7 and 8 June, however, they were below 60 ppbv on 9 and 10 June. It can be inferred that the high-O3 episodes on 7 and 8 June were
controlled by the NOx regime of ozone formation because the O3
concentration dropped significantly with the increase in NOx
concentration and simultaneous decrease in HCHO/NO2 ratio. As shown in
Fig. 5b, air masses on 7 and 8 June originated from northwest inland areas;
however, they originated from southwest coastal areas on 9 and 10 June. Air masses
transportation from different regions may also have contributed to this ozone
pollution episode. Furthermore, it can be concluded that the high O3
concentrations exceeding 60 ppbv can be expected with ratios of HCHO to
NO2 larger than 1.5 and vice versa during this case.
Summary and conclusions
In this paper, ship-based MAX-DOAS and ozone lidar measurements were
performed in the YRD region over the ECS area from 2 to 29 June 2017. During
this campaign, the measured VCDs of NO2, SO2, and HCHO were first
reported to be 6.50×1015, 4.28×1015, and 7.39×1015 molec cm-2, respectively, for the ECS area. In order to
provide validation of space-based observation over marine areas, the ship-based
measured tropospheric NO2, SO2, and HCHO VCDs were compared with satellite products. NO2, SO2, and HCHO showed good
agreement between MAX-DOAS results and satellite products, with a
correlation coefficient R of 0.83, 0.76, and 0.69, respectively. Furthermore,
the spatial distribution of trace gases along the ship cruise demonstrated
that the enhanced pollution of trace gases is usually related to the
emissions from vessels in nearby waterways and busy ports. In general,
the levels of trace gases decreased with distance from the coastline,
whereas the exceptional case of high values observed on 26 and 27 June at a
relatively remote sea area is mainly owed to the transport process from
continental areas with the favor of meteorological conditions.
The daily vertical profiles of NO2, SO2, and HCHO were obtained by
the retrieval from MAX-DOAS measurements using the HEIPRO algorithm. The trace
gas concentrations in the bottom of the marine boundary layer are <3,
<3, and <2 ppbv for NO2, SO2, and HCHO,
respectively, over the relatively clean offshore areas far away from the YRD
region. However, we also frequently found elevated SO2 concentration
during the cruise, which is attributed to the nearby ship emissions.
Combining
the ratio of HCHO/NO2 profiles from ship-based MAX-DOAS with O3
vertical profiles from the ozone lidar, typical O3 formation was
identified to be related to the increase in NO2 concentration and
relatively lower HCHO/NO2 ratios. This study highlighted the strong
impacts of shipping emissions on the air quality in the marine boundary layer of
ECS areas, which need to be regulated urgently in the coming future,
especially for the YRD region where the world's two biggest ports are located.