Representativeness and application of long-term trace gas and photolysis measurements for evaluating local air quality
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Date
12/11/2021Author
Walker, Hannah
Metadata
Abstract
Networks of long-term measurements of trace gases are critical for understanding spatio-temporal trends in air pollutants. This data is used to assess long-range and trans-boundary transport of emissions, quantify effects on public health, develop mitigation
strategies and examine the impact of implemented policy changes. As part of the European Monitoring and Evaluation Programme (EMEP), the UK operates two “super sites” which have provided a suite of co-located measurements for this purpose. These
supersites have been running for decades, and are located in rural background conditions, with the intention of being representative of the north and south of the country.
A Monitor for AeRosols and Gases in ambient Air (MARGA; Metrohm Applikon, NL)
has been included in these sites’ measurements for over a decade. However its gaseous
measurements of nitric acid (HNO3) have been demonstrated to include potential artefacts from other oxidised reactive nitrogen species (NOy), such as dinitrogen pentoxide
(N2O5). This interference has not yet been formally quantified. Other NOy measurements at either site are infrequent. Nitryl chloride (ClNO2) in particular was first measured in the UK in 2012, and has been measured only sporadically since.
Meteorological variables are similarly measured in networks to provide locally representative data, which are utilised in atmospheric chemistry and chemical transport
models. Photolysis reactions are key drivers of atmospheric chemistry, initiating many
reaction routes via the production of reactive radical species. As such, accurate estimation of photolysis rate constants (or photolysis frequencies; j-values) are imperative
for understanding subsequent reactions and predicting accurate pollutant concentrations. Photolysis rate constants are highly influenced by local meteorology (e.g. clouds, aerosols), but capturing the spatio-temporal variability of these changing conditions is
challenging, and often computationally costly. Consequently, modelled j-values are often parameterised or determined for unrepresentative local conditions, and results are
not validated beyond model conception. Some studies apply adjustment factors to these
model results to account for local conditions, but these have not yet been standardised
nor explored.
Part of this PhD research presents a systematic analysis of a measurement-driven adjustment factor (MDAF) to adjust clear-sky or cloud-free modelled j-values to capture
changes in the local meteorology. MDAFs were derived from the ratios of j-values
from both filter- and spectral radiometer measurements and clear-sky estimates from
the Tropospheric Ultraviolet and Visible radiative transfer model (TUV). MDAFs were
examined in terms of space (3 UK sites), time resolution (hourly to annual averages),
photolysis reactions (12 studied), optical inlet used (4-π sr and 2-π sr) and qualitative
impact on model chemical schemes. MDAFs derived from j(NO2) were found to be
seasonally similar around the UK, but specific to local environments at higher time resolutions, demonstrating the importance of local j-value measurements. Downwelling
(2-π) MDAFs demonstrated a slight increase with solar zenith angle (SZA), which was
amplified when measurements of upwelling j(NO2) were considered (4-π). Increased
surface albedo (snow cover) resulted in approximately 36% lower downwelling compared with 4-π MDAF, but the difference was negligible at other times. Derivations of
MDAF for the 12 different atmospheric photolysis reactions were grouped using hierarchical cluster analysis (HCA). The groupings of the photolysis reactions were found
to be driven by the extent to which a species photodissociates at longer (UVA) wave-lengths. MDAFs derived from j(NO2) measurements were deemed an applicable reference for local adjustment of the j-values for other photodissociations at wavelengths
>350 nm. For j-values of photodissociations at shorter wavelengths, adjustment using
MDAFs based on a reference of j(O1D) resulted in lower total error. The presence
of clouds had a greater influence on reducing cloud-free model results of j(NO2) (approx. 45%). Shorter wavelengths, such as those required for the photolysis rate constant
j(O1D), are scattered more readily in clear skies, and thus resulted in a lower magnitude difference (20%).
The other part of this PhD investigated atmospheric composition at the two UK supersites, by assessing the impact of the relocation of the southern EMEP supersite from
Harwell to Chilbolton Observatory, and deploying an iodide chemical ionisation mass
spectrometer (I – CIMS) to measure NOy species at the northern supersite (Auchencorth Moss). Meteorological normalisation was used on a concatenated time series of
pollutant concentrations pre- and post-relocation from Harwell to Chilbolton Observatory, to identify any resulting effects of the move on these time series. Of all the
species considered, only nitrogen oxides (NOx) and ammonia (NH3) had a step change
in concentration, both increasing. The additional contributing sources at Chilbolton
Observatory were identified. As a consequence, the long-term time series of NOx and
NH3 should be considered to be restarted following the relocation, and the new site not
strictly representative of the wider area it is intended to be. The aim of the CIMS study
at Auchencorth Moss was to measure HNO3 and N2O5 to quantify the interference in
co-located MARGA measurements, as well as to contribute the first Scottish ClNO2
measurements. The challenges of this study, and future work required is discussed.
This PhD research has demonstrated a new potential application of meteorological normalisation for air quality site relocations, which will become more pertinent in future
years where background sites will on occasion need to be relocated due to local development. Furthermore, this study has emphasised the importance of measuring local
photolysis rate constants to account for highly variable local conditions. It provides
discussion around making existing measurements standardised and accessible, so as to
make more frequent model validation or implementation of MDAF-like metrics easier,
and to improve modelled estimations of local photolysis rate constants without significantly increasing computational cost. This PhD research explores the ongoing need
to measure both atmospheric chemical components and photolysis rate constants to
understand changes in the atmosphere as pollutant emission abatement policies are implemented under real local conditions.