Evidence of change in UK atmospheric composition as a result of Icelandic volcanic emissions in 2014

Abstract relates to a poster presentation at the European Geosciences Union General Assembly 2015

Evaluation of local measurement-driven adjustments of modelled cloud-free atmospheric photolysis rate coefficients

Photolysis rate constants (j-values) play a crucial role in atmospheric chemistry modelling, but capturing the variability in local conditions needed for their accurate simulation is computationally challenging. One approach is to adjust modelled clear-sky estimates using ratios of measured-to-modelled j-values of a reference photolysis, typically j(NO2) or j(O1D). However, application of such adjustments to other photolysis reactions introduces uncertainty. Using spectral radiometer data from the UK, this study examines how hourly measurement driven adjustment factors (MDAF) across a set of 12 photolysis reactions group together using cluster analysis, and evaluates the uncertainties in using j(NO2) and j(O1D)-derived MDAF values to adjust modelled j-values of other photolysis reactions. The NO2-MDAF reference is suitable for adjusting photolysis reactions that absorb at λ > 360 nm (HONO, methylglyoxal, ClNO2, ClONO2 → Cl), which are largely independent of solar zenith angle and total ozone column (<31% error). In particular, NO2-MDAF is a good reference for j(HONO) and j(ClNO2). The O1D-MDAF performed better at adjusting modelled j-values for species that predominantly photodissociate at λ < 350 nm, such as HNO3, H2O2, CH3CHO, HCHO → H, HCHO → H2 and ClONO2 → ClO (errors ≤ 30%). However, j(O1D) radiometers require more data processing to account for local conditions. The maximum error determined using NO2-MDAF was within a factor of two (91% for j(H2O2)), which may still be acceptable in some instances. It is important that MDAFs are used to improve accuracy and uncertainty in simulated j-values caused by variation in local conditions

Process-based modelling of NH3 exchange with grazed grasslands

In this study the GAG model, a process-based ammonia (NH3) emission model for urine patches, was extended and applied for the field scale. The new model (GAG_field) was tested over two modelling periods, for which micrometeorological NH3 flux data were available. Acknowledging uncertainties in the measurements, the model was able to simulate the main features of the observed fluxes. The temporal evolution of the simulated NH3 exchange flux was found to be dominated by NH3 emission from the urine patches, offset by simultaneous NH3 deposition to areas of the field not affected by urine. The simulations show how NH3 fluxes over a grazed field in a given day can be affected by urine patches deposited several days earlier, linked to the interaction of volatilization processes with soil pH dynamics. Sensitivity analysis showed that GAG_field was more sensitive to soil buffering capacity (β), field capacity (θfc) and permanent wilting point (θpwp) than the patch-scale model. The reason for these different sensitivities is dual. Firstly, the difference originates from the different scales. Secondly, the difference can be explained by the different initial soil pH and physical properties, which determine the maximum volume of urine that can be stored in the NH3 source layer. It was found that in the case of urine patches with a higher initial soil pH and higher initial soil water content, the sensitivity of NH3 exchange to β was stronger. Also, in the case of a higher initial soil water content, NH3 exchange was more sensitive to the changes in θfc and θpwp. The sensitivity analysis showed that the nitrogen content of urine (cN) is associated with high uncertainty in the simulated fluxes. However, model experiments based on cN values randomized from an estimated statistical distribution indicated that this uncertainty is considerably smaller in practice

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

Reactive nitrogen fluxes and gas-aerosol interactions above a semi-natural forest in the Po Valley, Italy

The Po Valley, Italy, is known to be a nitrogen hotspot through the co-emissions of nitrogen oxides (NOx) and ammonia (NH3) due to intensive agriculture and industry within the region. Due to the regions poor air quality there have been a number of studies to understand the atmospheric composition and the tropospheric chemistry. Studies on the deposition of reactive N to the local ecosystems are however limited due to the complexities of measuring species such as NH3. The following study presented took place above an oak-hornbeam forest “Bosco Fontana” near Mantova, situated in the Po Valley, Italy with the aim to determine the importance of individual N species to the dry deposition budget and understand the impact of the chemical interactions and changes in the gas-aerosol partioning. Water soluble gases (NH3, HONO and HNO3) and their counter-part aerosol species (NH+ 4 and NO− 3 ) were measured using an online wet chemistry instrument called the GRadient of Aerosols and Gases Online Registration (GRAEGOR, ECN, NL). The fluxes were calculated using a modified gradient method, with concentration measurements at 2 heights. In addition, NH+ 4 and NO− 3 species were also measured by eddy covariance using an aerosol mass spectrometer (AMS, Aerodyne Inc.). Eddy Covariance was also used to measure NO fluxes. Nitric acid (HNO3) as expected had the fastest deposition rate (Vd) of 18.80 mm s−1 of all the N species measured. The study however did demonstrate that the deposition of NH+ 4 and NO− 3 was greatly enhanced during the day due to the evaporation during deposition close to the surface of the canopy, which resulted in the Vd of HNO3 to be reduced. Overall, the largest deposition flux over the forest was from NH3, with an average of -253.42 ng m−2 s−1, which accounted for 75% of the total N deposition budget during the period presented. The aerosols (NH+ 4 and NO−3) combined accounted for 19% and HNO3 contributed just 5% to the total N deposition budget. Taking this budget,measured over 2 weeks, an inferred annual budget of 75 Kg N ha−1 yr−1, which is greater than previously measured at the same site using a throughfall method for N deposition

Deriving a speciated atmospheric nitrogen budget at Auchencorth Moss, a background site in south east Scotland

Abstract relates to a poster presentation at the European Geosciences Union General Assembly 2015

Changing supersites: Assessing the impact of the southern UK EMEP supersite relocation on measured atmospheric composition

In January 2016 the United Kingdom's southern European Monitoring and Evaluation Programme (EMEP) level-2 air pollution monitoring 'supersite' was relocated from Harwell, Oxfordshire to Chilbolton Observatory, Hampshire. As no co-location study was undertaken, this work retrospectively investigates whether the supersite relocation has led to discontinuities in the time series of concentrations of commonly studied gaseous pollutants (NOx, NH3, SO2 and O3) and particulate matter (PM2.5 and PM10). Two years of measurements pre- and post-relocation (2014–15 and 2016–17 respectively) were analysed in conjunction with meteorological variables and local emission data. The deweather package was applied to the concatenated time series to minimise the influence of meteorology. Similar average concentrations of PM2.5, PM10, SO2 and O3 were observed, but there were substantial differences in that of NOx and NH3 (increase by factors of ~1.6 and ~3, respectively). The considerably higher NH3 concentrations at Chilbolton are attributed to the close proximity of mixed farmland, in particular to a strong south-westerly source contributing to ~50% of the annual average. NOx and PM concentrations in easterly winds arriving at Chilbolton are ~2.7 and ~1.5 times larger than at Harwell, from sources including the M3 motorway and Greater London. Westerly concentrations of NOx remain similar, therefore despite a higher frequency of westerly wind, annual mean concentrations are larger. Lower concentrations of PM arriving from the west result in similar annual averages. The secondary inorganic and black carbon components of PM were broadly similar between the sites. The differences in average NOx and NH3 at Chilbolton must be taken into account when considering long-term regional trends based on the southern UK supersite data

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

Using a co-created transdisciplinary approach to explore the complexity of air pollution in informal settlements

We present novel co-created transdisciplinary research that uses arts and humanities methods to explore air pollution in an informal settlement (Mukuru) in Nairobi, Kenya. Air pollution is a well-documented major human health issue, but despite many air pollution reduction interventions designed to improve health, these are frequently ineffective. Often this is because they fail to account for local knowledge, cultural practices and priorities of the intended recipients. Designing solutions therefore requires in-depth exploration of relevant issues with stakeholders. Researchers worked collaboratively with local residents to develop a range of methods to explore understandings of air pollution including interviews, storytelling, participatory mapping and theatre. Together, we uncovered contrasting definitions of air pollution, differing perceptions of who was responsible for enacting solutions, and overall a view that air pollution cannot be seen in isolation from the other issues faced by settlement residents. The methods used also allowed us to communicate about the topic with a wide audience. While we acknowledge that this research approach is more time consuming than traditional approaches, we urge other researchers wishing to address multifactorial problems, such as air pollution to use a mixture of qualitative, participatory and creative methods to engage with a wide range of stakeholders to elicit new and unexpected understandings that may not otherwise emerge.Additional co-authors: Charlotte Waelde, Anna Walnycki, Megan Wainwright, Jana Wendler, and Mike Wilso