Large-scale perspective on the meteorological modulation of air quality over China in winter
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Date
25/01/2023Item status
Restricted AccessEmbargo end date
25/01/2024Author
Jia, Zixuan
Metadata
Abstract
Rapid economic and industrial growth in China has led to serious air pollution with
high concentrations of suspended fine particulate matter (PM2.5), in particular
during winter. On a regional scale, meteorological conditions play a major role in
modulating the accumulation, transport, removal and transformation of air
pollutants. These meteorological conditions are affected by large-scale circulation
patterns over China, dominated by the East Asian winter monsoon (EAWM).
However, previous studies of how the large-scale winter circulation modulates air
quality in China primarily focused on the North China Plain. The study of regional
differences in the dominant large-scale circulation patterns needed to project future
climate-driven PM2.5 concentration changes is far from complete. On longer inter-annual timescales, the EAWM is in turn influenced by El Niño–Southern
Oscillation (ENSO) and the ensuing Pacific-East Asia teleconnection pattern.
Better understanding of the ENSO-EAWM relationship and changes in this
relationship under global warming is needed.
Firstly, the influence of large-scale circulation on daily PM2.5 variability through its
direct effect on key regional meteorological variables over three major populated
regions of China (Beijing–Tianjin–Hebei, BTH; the Yangtze River Delta, YRD; the
Pearl River Delta, PRD) is examined, based on a new high-resolution air quality
reanalysis dataset for China for five winters from December 2013 to February 2018.
In BTH, a shallow East Asian trough curbs northerly cold and dry air from the
Siberian High, enhancing PM2.5 pollution levels. Weak near-surface southerly
winds in eastern and southern China, associated with a weakened Siberian High,
suppress horizontal dispersion, contributing to air pollution accumulation over
YRD. In PRD, weak southerly winds and precipitation deficits over southern China
are conducive to high PM2.5 concentrations. To account for these dominant large-scale circulation–PM2.5 relationships, we propose three new circulation-based
indices: a 500 hPa geopotential height-based index for BTH, a sea level pressure-based index for YRD and an 850 hPa meridional wind-based index for PRD.
These
three indices can effectively distinguish clean days from heavily polluted days in
these regions, assuming PM2.5 variability is solely due to meteorology.
Subsequently, the influence of the winter large-scale circulation on daily PM2.5
concentrations and on the sensitivity of PM2.5 to emissions over major populated
regions of China with a focus on YRD is investigated, using the United Kingdom
Earth System Model, UKESM1. Weak flow of near-surface cold, dry air from the
north and weak inflow of maritime air are conducive to air pollution over YRD for
1999–2019. These provide favourable conditions for the accumulation of local
pollution but limit the transport of air pollutants into YRD from the north for 2014–
2019. Based on the dominant large-scale circulation, we construct a new index
using the north-south pressure gradient to project PM2.5 concentrations over the
region. We show that this index can effectively distinguish different levels of
pollution over YRD and explain changes in PM2.5 sensitivity to emissions from
local and northern regions. We then project future changes in PM2.5 concentrations
using this index under the weak climate and air pollutant mitigation scenario (SSP3-
7.0). We find an increase in PM2.5 concentrations over YRD due to climate-driven
circulation changes that is expected to partially offset the effect of emission control
measures in the near-term future.
Finally, changes in the relationship between ENSO and the EAWM at various
global warming levels during the 21st century are examined based on experiments
from the Max Planck Institute Grand Ensemble (MPI-GE) that represent the upper
boundary of the range of emissions scenario (RCP 8.5). The externally forced
component of this relationship (i.e. forced by greenhouse gases and anthropogenic
aerosol emissions) strengthens under moderate warming (+1.5 ◦
C), and then
weakens for +3 ◦
C warming. These changes are characterised by variations in
strength and location of the core of El Niño-related warming and associated deep
convection anomalies over the equatorial Pacific leading to circulation anomalies
across the Asian-Pacific region. Under global warming, the ENSO–EAWM
relationship is strongly related to the background mean state of both the EAWM
and ENSO, through changesin the EAWM strength and a shift of the ENSO pattern.
Anthropogenic aerosols also play a key role in influencing the ENSO–EAWM
relationship under moderate warming (up to 1.5 ◦C).
These results demonstrate the importance of understanding the occurrence of days
with elevated PM2.5 concentrations and explaining changes in the sensitivity of
PM2.5 to emissions from local and surrounding regions from a large-scale
perspective. These findings could help project the occurrence of heavily polluted
PM2.5 days during wintertime and assess future emission control strategies for PM2.5
air quality improvement under climate change. Furthermore, the ENSO–EAWM
relationship is shown to have a substantial inter-decadal variation under global
warming. This may further improve the accuracy of future predictions of air quality
in China.