Fine particulate matter (PM
2.5) remains a major air pollutant of significant public health
concerns in urban areas. Tracking the long-term temporal variation of PM
2.5 is essential and
valuable for evaluation of existing emissions control policies. Yet, past investigations merely
focused on the bulk PM
2.5 simply using annual averaged regression, leaving gaps and details of
relationship among the PM
2.5 components, source emissions and the policies.
The objective of this thesis is to understand the PM
2.5 source contributions over Hong
Kong and the cities in the Pearl River Delta (PRD) region through comprehensive chemical
composition data including suitable organic tracers measured over the past decade (2008-2017)
and across multiple sites. The long-term temporal variation was insp...[
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Fine particulate matter (PM
2.5) remains a major air pollutant of significant public health
concerns in urban areas. Tracking the long-term temporal variation of PM
2.5 is essential and
valuable for evaluation of existing emissions control policies. Yet, past investigations merely
focused on the bulk PM
2.5 simply using annual averaged regression, leaving gaps and details of
relationship among the PM
2.5 components, source emissions and the policies.
The objective of this thesis is to understand the PM
2.5 source contributions over Hong
Kong and the cities in the Pearl River Delta (PRD) region through comprehensive chemical
composition data including suitable organic tracers measured over the past decade (2008-2017)
and across multiple sites. The long-term temporal variation was inspected for simple annual
variations as well as more sophisticated season-specific trends. The spatial differences were
also examined to understand the source origins. This thesis work has revealed several
noteworthy findings as summarized below:
(1) Source apportionment of one-year PM
2.5 data at six different locations in Hong Kong
was conducted using positive matrix factorization (PMF) receptor modelling. The input data
matrix included sugar alcohols and unresolved complex mixture (UCM) in addition to major
constituents and source indictive elements. The contrast results between PMF with and without
organic tracers revealed the problems of mixed biomass burning and industrial/coal combustion
emissions and the vanish of biogenic source in the absent of its relevant organic markers
(arabitol and mannitol). The overestimated vehicular emissions retrieved from PMF only using
EC as source indicator species can be avoided only when adding hopanes as well as the UCM into the PMF input matrix. This work highlights the advantage and importance to include
organic source tracers to achieve more refined and accurate source apportionment.
(2) Monthly averaged PM
2.5 chemical composition in the decade of 2008-2017 at urban
Tsuen Wan (TW) in Hong Kong was analyzed by the Seasonal and Trend decomposition by
LOESS method coupled with the generalized least square and autoregessive moving average
model (STL-GLS-ARMA). The trend analysis confirmed the continuing declining of PM
2.5 and
that revealed that the decline was in a rate of -1.5 μgm
-3 per year, in which the sulfate ion and
organic matter (OM) contributed the most in the reduction. EC and hopanes which closely
related to vehicle emissions control measures implemented by the Hong Kong local government
was found to have dropped by at least 60%. While the changes in species from other regional
sources, such as Al and Si from dust, K
+ and levoglucosan from biomass burning, were
relatively lower (<50%) than the locally controlled species. The disproportional reduction
between secondary sulfate/nitrate and their respective precursor gases was noted and
attributable to the nonlinear formation mechanism. In addition, the long-time series reveals that
2011, due to strong La Niña events, is an anomaly year in that most PM
2.5 components were
elevated above the adjacent years.
(3) The long-term trend analysis at urban Guangzhou (GZ) and at suburban Nansha (NS),
sites in the PRD region, was also conducted by the STL-GLS-ARMA method. Significant
decline of PM
2.5 was also recorded at GZ and NS for -3.0 and -1.9 μgm
-3yr
-1, respectively.
Sulfate and OM were the main contributors to the reduction of PM
2.5 at those sites as well.
Different from the species in Hong Kong, which all showed a decreasing trend, Cu at NS
notably increased by 70%. Using mass increment ratio between episodic and normal events as
an indicator, we found nitrate was the species attributed to the highly polluted days at GZ and
NS whereas dust materials (e.g., Al, Si) contributed most to TW, implying the different spatial
variance of PM
2.5 over the sites. This result was further supported by the evidence that the
concentration of species at GZ and NS were typically higher than those at TW even for those
originated from regional sources.
(4) Sensitivity tests using moving PMF were conducted to identify the most suitable
parameters for resolving reliable profiles more sensitive to any changes upon the series of
control measures over the past decade. The STL-GLS-ARMA trend analysis on the most
appropriate PMF results derived using a time window size of 2-year and interval of 3 months
were then conducted for the ten-year PM
2.5 data at TW. Among all the declining source factors,
regional mixed industrial sources and local vehicular emission play the dominating roles in the reduction. While shift of source origin of regional sources from southeast to surrounding areas
was found in the concentration weighted trajectories analysis.
(5) Moving PMF was also applied to the ten-year data at GZ and NS. The main
contributors to the reduction of bulk PM
2.5 over the decade were secondary nitrate and sulfate
at GZ and NS, respectively. The next significant sources contributing to the decline of PM
2.5
were secondary sulfate and residual oil for GZ versus secondary nitrate, industrial and vehicular
emissions for NS. As for the spatial and temporal variation over the three sites, the regional
source generally distributed and varied in similar pattern among them. Obvious reduction of
residual oil combustion at GZ verse the steady variation of the same source at NS and TW were
observed. It was later confirmed from the local/non-local estimates that the residual oil
combustion at GZ were mostly locally emitted. Same transition of regional source origin was
also obtained for GZ and NS. More notably, biomass burning contributed to similar amounts
regardless of source origins, implying wide presence of biomass burning activities over the
Southern China. Lastly, the episodic events over the PRD region were found mainly associated
with the regional sources including secondary inorganic aerosol, mixed industrial sources, and
biomass burning.
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