Meteorological measurements at Auchencorth Moss from 1995 to 2016

The Auchencorth Moss atmospheric observatory has being measuring meteorological parameters since 1995. The site was originally set‐up to measure the deposition of sulphur dioxide at a site that represented the vegetation and climate typical of NW Europe, in relatively clean background air. It is one of the longest running flux monitoring sites in the region, over semi‐natural vegetation, providing infrastructure and support for many measurement campaigns and continuous monitoring of air pollutants and greenhouse gases. The meteorological sensors that are used, data processing and quality reviewing procedures are described for a set of core measurements up to 2016. These core measurements are essential for the interpretation of the other atmospheric variables

The Importance of Capturing Local Measurement-Driven Adjustment of Modelled <i>j</i>(NO₂)

Accurate photolysis rate constants are essential for simulation of local air quality but their values can vary substantially with changes in local meteorological and surface conditions. This study demonstrates the use of local radiometer measurements for capturing via hourly measurement-driven adjustment factors (MDAF) the temporal resolution needed to adjust clear-sky or cloud-free model estimates of j(NO2). Measurements simultaneously at two sites in the UK (Auchencorth Moss and Manchester) showed that TUV (v5.3) model estimates of j(NO2)↓ in cloud-free conditions (used as an example of modelled j-values) were, on average, approximately 45% larger than measured j(NO2)↓, which would lead to substantial model bias in the absence of local adjustment. At Auchencorth Moss, MDAF values based on 4π and 2π radiometer inlets generally agreed very well with each other (<6% average difference). However, under conditions of particularly high surface albedo (such as snow cover), increased upwelling local diffuse radiation yielded an MDAF derived using total radiation (sum of ↓ and ↑ components) ~40% larger than the MDAF derived using only ↓ radiation. The study has demonstrated: (1) the magnitude of potential impact of local conditions—principally cloud cover, but also changes in surface albedo—on assumed j-values; (2) that whilst annual mean MDAF values are similar at Auchencorth Moss and Manchester, there is no contemporaneous correlation between them at hourly resolution; hence MDAF values derived at one site cannot readily be applied at another site. These data illustrate the need to routinely deploy long-term radiometer measurements alongside compositional measurements to support atmospheric chemistry modelling

Evaluation of isoprene light response curves for bryophyte-dominated ecosystems and implications for atmospheric composition

Isoprene is emitted from numerous plant species in response to light and temperature and parameterisations of these relationships, based on observations from a few vascular plant species, have been shown to be broadly applicable to many different vegetation types. Here, we investigate their performance when applied to an ecosystem dominated by bryophytes. Over a six-week period, emissions of isoprene were measured above a Scottish peat bog. The light response derived on the basis of both canopy-scale flux and whole-plant enclosure measurements, deviated from the classical response, showing no sign of saturation within the observed range. We attribute this response to the canopy architecture of moss hummocks, which may attenuate light differently compared to a grass canopy. Both existing big-leaf and canopy-level emission algorithms, developed for vascular plants but commonly used for moorland vegetation, failed to replicate the observed fluxes, overestimating at low light intensities (<1000 μmol m−2 s−1 photosynthetically active radiation) and underestimating during daytime clear sky conditions. The light response was optimised for bryophyte-dominated ecosystems using measured fluxes and incorporated into the EMEP4UK chemical transport model and applied exclusively to moorland. The revised parameterisation resulted in a small reduction in the average annual isoprene emissions in the northern latitudes (5%), but peak isoprene emissions and concentrations increased by up to a factor of two. Yet, no significant change in average or maximum surface ozone concentrations was observed, reflecting that the northern latitudes are in a chemical regime that is strongly NOx limited, in part due to the spatial segregation with the urban sources of NOx. We conclude that, the anticipated increase in isoprene emissions from the northern latitudes in response to climate change is unlikely to contribute towards ozone-related air quality issues, as long as NOx pollution does not increase. However, the non-saturating light response may be equally applicable to non-vascular plants elsewhere, including in the tropics

Representativeness and application of long-term trace gas and photolysis measurements for evaluating local air quality

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

Impacts of the 2014-2015 Holuhraun eruption on the UK atmosphere

Volcanic emissions, specifically from Iceland, pose a pan-European risk and are on the UK National Risk Register due to potential impacts on aviation, public health, agriculture, the environment and the economy, both from effusive and explosive activity. During the 2014-2015 fissure eruption at Holuhraun in Iceland, the UK atmosphere was significantly perturbed. This study focuses one major incursion in September 2014, affecting the surface concentrations of both aerosols and gases across the UK, with sites in Scotland experiencing the highest sulfur dioxide (SO2) concentrations. The perturbation event observed was confirmed to originate from the fissure eruption using satellite data from GOME2B and the chemical transport model, EMEP4UK, which was used to establish the spatial distribution of the plume over the UK during the event of interest. At the two UK European Monitoring and Evaluation Program (EMEP) supersite observatories (Auchencorth Moss, SE Scotland and Harwell, SE England) significant alterations in sulfate (SO42-) content of PM10 and PM2.5 during this event, concurrently with evidence of an increase in ultrafine aerosol, most likely due to nucleation and growth of aerosol within the plume, were observed. At Auchencorth Moss, higher hydrochloric acid (HCl) concentrations during the September event (max = 1.21 µg m-3, c.f annual average 0.12 µg m-3 35 in 2013), were assessed to be due to acid displacement of chloride (Cl-) from sea salt (NaCl) to form HCl gas rather than due to primary emissions of HCl from Holuhraun. The gas and aerosol partioning at Auchencorth moss of inorganic species by thermodynamic modelling, confirmed the observed partioning of HCl. Using the data from the chemical thermodynamic model, ISORROPIA-II, there is evidence that the background aerosol, which is typically basic at this site, became acidic with an estimated pH of 3.8 during the peak of the event. Volcano plume episodes were periodically observed by the majority of the UK air quality monitoring networks during the first 4 months of the eruption (August – December 2014), at both hourly and monthly resolution. In the low resolution networks, which provide monthly SO2 averages, concentrations were found to be significantly elevated at remote “clean” sites in NE Scotland and SW England, with record high SO2 concentrations for some sites in September 2014. For sites which are regularly influenced by anthropogenic emissions, taking into account the underlying trends, the eruption led to statistically unremarkable SO2 concentrations (return probabilities >0.1, ~10 months). However for a few sites, SO2 concentrations were clearly much higher than has been previously observed (return probability 3000 months). The Holuhraun Icelandic eruption has resulted in a unique study providing direct evidence of atmospheric chemistry perturbation of both gases and aerosols in the UK background atmosphere. The measurements can be used to both challenge and verify existing atmospheric chemistry of volcano plumes, especially those originating from effusive eruptions, which have been under-explored, due to limited observations available in the literature. If all European data sets were collated this would allow improved model verification and risk assessments for future volcanic eruptions of this type

Neural Network Analysis to Evaluate Ozone Damage to Vegetation Under Different Climatic Conditions

Tropospheric ozone (O-3) is probably the air pollutant most damaging to vegetation. Understanding how plants respond to O(3)pollution under different climate conditions is of central importance for predicting the interactions between climate change, ozone impact and vegetation. This work analyses the effect of O(3)fluxes on net ecosystem productivity (NEP), measured directly at the ecosystem level with the eddy covariance (EC) technique. The relationship was explored with artificial neural networks (ANNs), which were used to model NEP using environmental and phenological variables as inputs in addition to stomatal O(3)uptake in Spring and Summer, when O(3)pollution is expected to be highest. A sensitivity analysis allowed us to isolate the effect of O-3, visualize the shape of the O-3-NEP functional relationship and explore how climatic variables affect NEP response to O-3. This approach has been applied to eleven ecosystems covering a range of climatic areas. The analysis highlighted that O(3)effects over NEP are highly non-linear and site-specific. A significant but small NEP reduction was found during Spring in a Scottish shrubland (-0.67%), in two Italian forests (up to -1.37%) and during Summer in a Californian orange orchard (-1.25%). Although the overall seasonal effect of O(3)on NEP was not found to be negative for the other sites, with episodic O(3)detrimental effect still identified. These episodes were correlated with meteorological variables showing that O(3)damage depends on weather conditions. By identifying O(3)damage under field conditions and the environmental factors influencing to that damage, this work provides an insight into O(3)pollution, climate and weather conditions.Peer reviewe

Assessing the bias of molybdenum catalytic conversion in the measurement of NO2 in rural air quality networks

The measurement method of NO2 with continuous analysers is specified for EU Ambient Air Quality Directive compliance reporting, which provides a consistent methodology and concurrent NO measurements (85/203/EEC-NO2). While the established method of measurement of NO2, following conversion of NO2 to NO using a molybdenum-conversion process, has known interference uncertainties (due to conversion of other oxidised nitrogen (NOy) chemicals, the consistency and traceability of compliance measurement is important. This study compared three continuous NO2 analyser instruments: a Thermo-NOx molybdenum convertor chemiluminescence analyser (Model 42C, ThermoFisher Scientific Inc., MA, USA), a photolytic chemiluminescence analyser (T200UP, Teledyne Technologies Inc., San Diego, USA) and a Cavity Attenuated Phase Shift (CAPS) analyser (T500U, Teledyne Technologies Inc., CA, USA). The instruments were run for over a year at the Auchencorth Moss long-term peatland monitoring site (Southeast Scotland) which is a low NOx atmosphere away from sources. NOy and NHx chemicals were also measured concurrently. This study concludes that there is a strong artefact in molybdenum catalyst chemiluminescent instruments as a result of unselective catalysis of airborne NOy compounds that causes an overestimate of NO2 measured in the atmosphere. The observed artefact in concentration measurements is likely to be observed at the entire UK scale as almost the entirety of the rural air network relies on molybdenum catalyst instruments. We therefore recommend that molybdenum catalyst instruments should be phased out and replaced in air quality monitoring networks with molecule specific (spectroscopy) instrumentation (equivalent in cost, such as those described in this study) that do not suffer from the same measurement artefacts

Trends in atmospheric evaporative demand in Great Britain using high-resolution meteorological data

Observations of climate are often available on very different spatial scales from observations of the natural environments and resources that are affected by climate change. In order to help bridge the gap between these scales using modelling, a new dataset of daily meteorological variables was created at 1 km resolution over Great Britain for the years 1961–2012, by interpolating coarser resolution climate data and including the effects of local topography. These variables were used to calculate atmospheric evaporative demand (AED) at the same spatial and temporal resolution. Two functions that represent AED were chosen: one is a standard form of potential evapotranspiration (PET) and the other is a derived PET measure used by hydrologists that includes the effect of water intercepted by the canopy (PETI). Temporal trends in these functions were calculated, with PET found to be increasing in all regions, and at an overall rate of 0.021 ± 0.021 mm day−1 decade−1 in Great Britain. PETI was found to be increasing at a rate of 0.019 ± 0.020 mm day−1 decade−1 in Great Britain, but this was not statistically significant. However, there was a trend in PETI in England of 0.023 ± 0.023 mm day−1 decade−1. The trends were found to vary by season, with spring PET increasing by 0.043 ± 0.019 mm day−1 decade−1 (0.038 ± 0.018 mm day−1 decade−1 when the interception correction is included) in Great Britain, while there is no statistically significant trend in other seasons. The trends were attributed analytically to trends in the climate variables; the overall positive trend was predominantly driven by rising air temperature, although rising specific humidity had a negative effect on the trend. Recasting the analysis in terms of relative humidity revealed that the overall effect is that falling relative humidity causes the PET to rise. Increasing downward short- and longwave radiation made an overall positive contribution to the PET trend, while decreasing wind speed made a negative contribution to the trend in PET. The trend in spring PET was particularly strong due to a strong decrease in relative humidity and increase in downward shortwave radiation in the spring