Figure 1 (a) Map showing the distribution of average surface
SO2 mass concentrations in July in eastern China from
OMI satellite reproduced dataset . (b) Measurement overview. Map showing
the surface measurement sites (black circles with white numbers) and
flight route (black dotted lines). The numbers from 1 to 9 correspond to
the measurement sites in Heze, Jining, Liaocheng, Jinan, Zibo, Binzhou
and 3 sites in Beijing. The spatial distributions of sea level pressure
and 10-m wind field at 16:00 LT on July 28, 2018 are also shown
corresponding to the areas in the black box marked in (a). The surface
position of the warm front is marked with the symbol of a red line with
semicircles. The meteorology data were downloaded from the ERA5
reanalysis website.
We conducted aircraft measurements of trace gases and aerosols from July
to August 2018 in Jilin province of NE China. A Y-12 aircraft of the
Jilin Weather Modification Office was used for the study. This
twin-engine turboprop aircraft was similar to a Twin Otter. The aircraft
campaign was conducted out of Baicheng city (122.84°E, 45.63°N), which
is located 300 km from Changchun (the capital of Jilin, with a
population of 7 million). The airport is located in the north (generally
upwind direction) of the city. The sampling inlet was mounted at the
bottom of the airframe with a forward-facing inlet connected to the
measurement instruments. O3 and SO2 were
measured with Thermo instruments (TEI 49i and 43i), and the aerosol
scattering coefficients were measured with a commercial integrating
nephelometer (TSI3563), which detects the light scattering coefficient
(Bsp) at 450 nm, 500 nm, and 700 nm. All trace gas instruments were
calibrated at the ground base and different altitudes on several
selected flights. An Aerodyne time-of-flight Aerosol Chemical Speciation
Monitor (TOF-ACSM) (Fröhlich et al., 2013; Sun et al., 2020) was
deployed and fixed in the aircraft cabin for the chemical
characterization of non-refractory PM2.5(NR-PM2.5). The detailed descriptions and data analysis
of the instrument are in Text S1.
For multiple-surface measurements, the inorganic ions of
PM2.5 were measured with a Monitor for Aerosols and
Gases in ambient Air (MARGA, designed and manufactured by Applikon
Analytical B.V., the Netherlands) (ten Brink et al., 2007) at 9 ground
supersites in the NCP with three in the megacity Beijing and the other
six in the Shandong Province, located from South to North in Heze,
Jining, Liaocheng, Jinan, Zibo, and Binzhou, separately (Fig. 1b).
2.2 Designed aircraft experiments under the influence of mid-latitude
cyclone
Previous studies have indicated the important role of mid-latitude
cyclone in transporting pollutants from the NCP to NE China (Eckhardt et
al., 2004; Stohl, 2001; Li et al., 2012; Dickerson et al., 2007; Oshima
et al., 2013; Ding et al., 2009; Wu et al., 2018). In this study, we
specially designed an aircraft experiment on July 30, 2018 under the
influence of a cyclone. As shown in Fig. 1b, a deep mid-latitude cyclone
was located over Mongolia (centered at 107°E, 45°N). The cyclone was
characterized by a low-pressure system on the ground with a warm front
extending from the center to the western edge of inner Mongolia. Wind
vectors clearly show that warm and humid air from the NCP moved
northwardly until encountered cold air mass from the north Mongolia and
accordingly formed the convergence belt. Previous studies have
identified the warm conveyor belt (WCB) from the distribution of 500 hPa
specific humidity due to moisture advection associated with the lifting
effect (Kiley and Fuelberg, 2006; Ding et al., 2009). The 500 hPa
specific humidity field (Fig. S1) consistently shows a moist band over
the front, revealing the strong vertical venting in this region. The
distribution of specific humidity and wind filed indicate that a
WCB-like circulation extended from the warm sector of the frontal system
to NE China, where we conducted our aircraft study.
In addition to this specially designed experiment, we have also
conducted 4 aircraft experiments respectively on August 2, August 3,
August 8 and August 9. All the experiments were carried out during the
daytime.
2.3 Positive matrix factorization (PMF)
Positive matrix factorization (PMF) (Paatero and Tapper, 1994) with the
PMF2.exe algorithm was performed on the TOF-ACSM organics mass spectra
to investigate various organic aerosol (OA) sources and processes. PMF
analysis was performed with an Igor Pro-based PMF Evaluation Tool
(Ulbrich et al., 2009), and the results were evaluated following the
procedures detailed in Ulbrich et al. (2009) and Zhang et al. (2011).
The total OA of all aircraft experiments was resolved into a
hydrocarbon-like OA (HOA) factor and an oxygenated OA (OOA) factor. The
detailed PMF results are described in Text S2.
2.4 The WRF-Chem model
The WRF-Chem model is an online-coupled chemical transport model that
considers multiple physical and chemical processes, including emission
and deposition of pollutants, advection, diffusion, gaseous chemical
transformation, and aerosol chemistry (Grell et al., 2011). In this
study, the WRF model (version 3.9.1) was employed to simulate weather
conditions using the 1° x 1°NCEP (National Centers for Environmental
Prediction) FNL Operational Global Analysis data (ds083.2). Meanwhile,
the NCEP ADP Global Upper Air Observational Weather Data (ds351.0) was
assimilated to improve the model’s meteorological reproduction.
The simulation is conducted from 1 July 2018 to 20 August 2018. The
detailed configurations of the model are listed in Table S1. In this
study, the model domain was centered at 35° N and 110° E with a 20 km ×
20 km spatial resolution to cover the eastern China and its surrounding
areas. On vertical distribution, 30 vertical layers are set from the
surface to the top pressure of 50 hPa, and 10 of which are set below 1
km to better resolve the processes within boundary layer. Natural
emissions such as biogenic, sea salt and dust are included online into
the model runs for this study. For biogenic emissions, online biogenic
emissions are calculated through MEGAN (Model of Emissions of Gases and
Aerosols from Nature) module. It estimates the net emission rates of
isoprene, monoterpene and other biogenic VOCs from terrestrial
ecosystems into the above-canopy atmosphere. In addition to natural
emissions, the anthropogenic emissions of CO, NOx,
SO2, NH3, BC, OC, PM2.5.
PM10 and VOCs are set based on the MEIC database
(Multi-resolution Emission Inventory for China) (Huang et al., 2018).
To investigate the contributions of each individual physical and
chemical process to variations of atmospheric nitrate and sulfate
concentrations, we performed a diagnostic analysis in WRF-Chem modeling
(Wang et al., 2020). The detailed diagnostic analysis and model
evaluation are described in Text S3.
2.5 Lagrangian dispersion modeling
Lagrangian particulate dispersion modeling (LPDM) was used to study the
transport pathways and trace the potential sources of air masses during
the campaign using HYSPLIT (Stein et al., 2015), following the method
developed by Ding et al. (2013). Briefly, for each hour during the study
period, the model was run several days backwardly with 3,000 particles
released every hour at a specific height over the site. The model
calculated the particle position with mean wind and turbulent transport
after being released from the receptor point. The residence time of
particles at specific height columns was used to identify the footprint
retroplume. The spatiotemporal distributions of these particles were
used to identify the potential source regions and their relative
contributions to the air masses at the measurement sites.
3. Results and discussions
3.1 Overall aircraft observation results
Fig. 2 shows the overall results of these 5 aircraft experiments
conducted in the NEC. The overall NR-PM2.5 was dominated
by secondary species, including sulfate, ammonium, nitrate and OOA since
most of the flight time is outside the boundary layer, where the
influence of surface primary emission is supposed to be small. Pollution
episodes were captured by our flight on July 30 and August 3 with
relatively higher mass concentrations of NR-PM2.5.
Sulfate, a secondary inorganic specie usually formed over a regional
scale (Sun et al., 2011) contributed a significant fraction of
NR-PM2.5 on July 30 (43%) and August 3 (54%). The
sulfur oxidation ratios (defined as SOR =
nSO42-/
(nSO42- + nSO2), where
n represents molar concentration) were substantially higher on these two
days, indicative of overall aged secondary aerosols. HOA, which
represents primary organic aerosols (POA) usually associated with
freshly traffic emissions (Zhang et al., 2011), contributed notably to
NR-PM2.5 mass loadings during clean situations and its
contribution were comparable on August 2 (13%), August 8 (15%) and
August 9 (17%). While extremely lower contributions of HOA and
relatively higher SOA to POA ratios were observed on pollution days,
indicating the possible influence of regional transport rather than
local sources. fm/z is defined as the fraction of the
signal at the given m/z of the organic mass spectra derived from ACSM
observation results. The organic aerosols of these two pollution cases
(July 30 and Aug 3) both had higher values of f44, thus
were more oxidized and aged, which was caused by extensive oxidation
processes. These sulfate-dominated pollution episodes in the NEC are
likely to be caused by regional transport due to extremely aged
secondary aerosols while relatively lower values of f44and SOR together with notable fraction of HOA during clean situations
indicate the importance of local sources.
Among these aircraft experiments, flight on July 30 was designed to
investigate the role that mid-latitude cyclone plays in transporting air
pollution and the aerosol chemistry alongside. Here we present the
detailed analysis and in-depth understanding of this regional transport
air pollution.