Assessing urban air quality through measurements and modelling and its implications for human exposure assessment
Abstract
Outdoor air pollution is a major contributor to adverse health effects of citizens, in
particular those living in urban environments. Air quality monitoring networks are
set up to measure air quality in different environments in compliance with national
and European legislation. Generally, only a few fixed monitoring sites are located
within a city and thus cannot represent air pollutant concentrations in urban areas
accurately enough to allow for a detailed human exposure assessment. Other
approaches to derive detailed urban air pollutant concentration estimates exist, such
as dispersion models and land-use regression (LUR) models. Low-cost portable air
quality monitors are also emerging, which have the potential to add value to existing
monitoring networks by providing measurements at greater spatial resolution and
also to provide individual-level exposure assessment. The aim of this thesis is to
demonstrate how measurements and modelling in combination allow detailed
investigations of the variability of air pollutants in space and time in urban area, and
in turn improve on the current exposure assessment methods.
Three types of low-cost portable monitors measuring NO2, O3 (Aeroqual monitors)
and PM2.5 (microPEM monitor) were evaluated against their respective reference
instruments. The Aeroqual O3 monitor showed very good correlation (r2 > 0.9) with
the respective reference instruments, but biases in the slope and intercept coefficients
indicated that calibration of Aeroqual O3 monitor was needed. The Aeroqual NO2
monitor was subject to cross-sensitivity from O3, which, as demonstrated, can be
effectively corrected by making O3 and NO2 measurements in tandem. Correlation
between the microPEM monitor and its reference instrument was poor (r2 < 0.1)
when PM2.5 concentrations were low (< 10 μg m-3), but significantly improved (r2 >
0.69) during periods with elevated PM2.5 concentrations. Relative humidity was not
found to affect the raw results of PM2.5 measurements in a consistent manner. All
three types of monitors cannot be used as equivalent or indicative methods instead of
reference methods in studies that require quantification of absolute pollutant
concentrations. However, the generally good correlations with reference instruments
reassure their application in studies of relative trends of air pollution.
Concentrations of PM2.5, ultrafine particles (UFP) and black carbon (BC) were
quantified using portable monitors through a combination of mobile and static
measurements in the city of Edinburgh, UK. The spatial variability of UFP and BC
was large, of similar magnitude and about 3 times higher than the spatial variability
of PM2.5. Elevated concentrations of UFP and BC were observed along streets with
high traffic volumes whereas PM2.5 showed less variation between streets and a
footpath without road traffic. Both BC and UFP significantly correlated with traffic
counts, while no significant correlation between PM2.5 and traffic counts was
observed. The relationships between UFP, NO2 and inorganic components of PM2.5
were further investigated through long-term measurements at roadside, urban
background and rural sites. UFP moderately correlated with NOx (NO2 + NO) and
showed varying relationships with NOx depending on the particle size distribution.
Principal component analysis and air-mass back trajectory analysis revealed that
PM2.5 concentrations were dominated by long-range transport of secondary inorganic
aerosols, whereas UFP were mainly related to varying local emissions and
meteorological conditions. These findings imply the need for different policies for
managing human exposure to these different particle components: control of much
BC and UFP appears to be manageable at local scale by restricting traffic emissions;
however, abatement of PM2.5 requires a more strategic approach, in cooperation with
other regions and countries on emissions control to curb long-range transport of
PM2.5 precursors.
A dispersion model (ADMS-Urban) was used to simulate high resolution NO2 and
O3 concentrations in Edinburgh. The effects of different emission and meteorological
input datasets on the resulting modelled NO2 concentrations were investigated. The
modelled NO2 and O3 concentrations using the optimal model setup were validated
against reference instrument and diffusion tube measurements. Temporal variability
of NO2 was predicted well at locations that were not heavily influenced by local
effects, such as road junctions and bus stops. Temporal variability of O3 was
predicted better than for NO2. Long-term spatial variability of NO2 was found to
correlate well with diffusion tube measurements, while modelled spatial variability
of O3 in ADMS-Urban compared poorly with diffusion tube measurements.
However, it was found that the O3 diffusion tube measurements may be subject to
some unidentified biases affecting their accuracy.
Land-use regression (LUR) models are widely used to estimate exposure to air
pollution in urban areas. An appropriately sized and designed monitoring network is
an important component for the development of a robust LUR model. Concentrations
of NO2 were simulated by ADMS-Urban at ‘virtual’ monitoring sites in 54 different
network designs of varying numbers and types of site, using a 25 km2 area including
much of the Edinburgh city area. Separate LUR models were developed for each
network. These LUR models were then used to estimate ambient NO2 concentrations
at all residential addresses, which were evaluated against the ADMS-Urban modelled
concentration at these addresses. The improvement in predictive capability of the
LUR models was insignificant above ~30 monitoring sites, although more sites
tended to yield more precise LUR models. Monitoring networks containing sites
located within highly populated areas better estimated NO2 concentrations across all
residential locations. LUR models constructed from networks containing more
roadside sites better characterised the high end of residential NO2 concentrations but
had increased errors when considering the whole range of concentrations. No
particular composition of monitoring network resulted in good estimation
simultaneously across all residential NO2 concentration and of the highest NO2 levels
implying a lack of spatial contrast in LUR-modelled pollution surface compared with
the dispersion model.
Finally, the results from the measurement and modelling studies presented in thesis
are synthesised in the context of current exposure assessment studies. Low-cost air-quality
monitors currently do not possess and are unlikely in the near future to
provide the robustness and accuracy to replace the existing routine monitoring
network. Development of the low-cost air-quality should be aiming at upgrading
them as the indicative method as defined in the data quality objective in the EU
directive. The monitoring sites used to build LUR models should capture well the
population distribution in the study area as opposed to capturing the greatest
pollution contrast. The traditional methods of evaluating LUR models are also
ineffective in characterising the models’ capability at estimating pollutant
concentration at residential address. Given that the dispersion models are also subject
to the availability and uncertainties in the input data, future air quality model
development should endeavour to incorporate both dispersion and land-use
regression models, where the uncertainty in the input data can be reduced by using
LUR models built on actual measurements, and the limitation in the statistical
modelling can be replaced by adopting the deterministic approach used in the
dispersion model.