Measurement report: Characterization of severe spring haze episodes and influences of long-range transport in the Seoul metropolitan area in March 2019
Authors: Kim Hwajin , Zhang Qi , Sun Yele 
Published: October 2020

Abstract. Severe haze episodes have occurred frequently in the Seoul
metropolitan area (SMA) and throughout East Asian countries, especially
during winter and early spring. Although notable progress has been attained
in understanding these issues, the causes of severe haze formation have not
yet been fully investigated. SMA haze is especially difficult to understand,
because the area is impacted by both local emissions from anthropogenic and
biogenic activities and emissions transported from upwind sources. Here, we
investigated the emission sources and formation processes of particulate
matter (PM) during three haze episodes measured in early spring of 2019,
from 22 February to 2 April, using a high-resolution aerosol mass
spectrometer (HR-AMS). Overall, the average concentration of nonrefractory submicron aerosol
(NR-PM1) + BC (black carbon) was 35.1 µg m−3, which was composed of
38 % organics, 12 % SO4, 30 % NO3, 13 % NH4, and 5 %
BC. The organics had an average oxygen-to-carbon ratio (O∕C) of 0.52 and an
average organic mass to organic carbon ratio (OM∕OC) of 1.86. Seven distinct
sources of organic aerosols (OAs) were identified via positive matrix
factorization (PMF) analysis of the HR-AMS data: vehicle-emitted
hydrocarbon-like OA (HOA), cooking OA (COA), solid-fuel-burning emitted OA
(SFOA), and four different types of oxidized secondary OA with varying oxidation
degrees and temporal trends. Of the 40 d of the measurement period, 23 were identified as haze days
(daily average: >35 µg m−3), during which three
severe haze episodes were recorded. In particular, PM1 concentration
exceeded 100 µg m−3 during the first episode when an alert was
issued, and strict emission controls were implemented in the SMA. Our results
showed that nitrate dominated during the three haze episodes and accounted
for 39 %–43 % of the PM1 concentration on average (vs. 21 %–24 % during
the low-loading period), for which there were indications of regional-transport influences. Two regional-transport-influenced oxidized organic aerosols (OOAs), i.e., less oxidized OOA2 (LO-OOA2) and more oxidized OOA2 (MO-OOA2), contributed
substantially to the total PM1 during the haze period (12 %–14 % vs.
7 % during the low-loading period), as well. In contrast, HOA and COA only
contributed little (4 %–8 % vs. 4 %–6 % during the low-loading period) to
the PM1 concentration during the haze days, indicating that local
emissions were likely not the main reason for the severe haze issues. Hence,
from simultaneous downwind (SMA) and upwind (Beijing) measurements using
HR-AMS and ACSM (aerosol chemical speciation monitor) over the same period,
the temporal variations in PM1 and each chemical species showed peak
values on the order of Beijing (upwind) to the SMA for approximately 2 d. Furthermore, lead (Pb) derived from HR-AMS measurements was observed
to increase significantly during the haze period and showed good
correlations with MO-OOA2 and LO-OOA2, which is consistent with regional sources.
A multiple linear regression model indicated that the transported regionally
processed air masses contributed significantly to Pb in the SMA (31 %),
especially during the haze period, although local burning was also
important by contributing 38 %. The above results suggest that regional
transport of polluted air masses might have played an important role in the
formation of the haze episodes in the SMA during early spring.