Effects of Ground Level Ozone on Vegetation

All the experiments conducted in this project use ozone exposures that either are already currently occurring or are expected to occur over UK uplands this century, if predictions of increasing „background‟ ozone exposure are correct. Uplands are particularly vulnerable to such increases, because they are windy and wet – conditions which favour ozone uptake by vegetation. Consequently, the observations on ozone effects on upland plant species are likely to occur in the foreseeable future, if they are not already happening. The novel field ozone exposure system at Keenley Fell (Northumberland) has been operating since spring 2007. Seasonal average increases in ozone exposure at 10 m from the release point were very small in 2007, increasing to around 5 ppb above ambient in 2008 and 2009. The effects of the ozone treatments on this conservation-managed grassland should be seen in the context of this very modest increase in annual mean exposure. Average deposition velocity for ozone at Keenley Fell was between 1 mm/s (night) and 5 mm/s (mid-day), varying with weather conditions. These flux data will assist with the modelling of ozone deposition to upland grasslands. Limited flux measurements for CO2 showed uptake during the day and release at night, as expected, and will contribute to modelling interactions between ozone and carbon fluxes under these conditions. The ozone treatments at Keenley Fell had no effect on total above-ground production in any of the three years. However, there was a cumulative decrease in forb biomass, with an associated change in forb species composition, over the three years. This was accompanied by an increase in grass biomass. These effects of ozone are antagonistic to the objectives of the conservation management at this site. The biomass and flowering of a small number of individual forb species were significantly reduced by ozone exposure at Keenley Fell. Importantly, these species included the hemi-parasite Rhinanthus minor which is frequently used to enhance species diversity in this type of conservation management. Hence, the observed effects of ozone on species composition may partly have been caused through its adverse effects on Rhinanthus . Data compiled from several experiments show adverse effects of ozone on semi-natural plant species at concentrations as low as 30 ppb, with 12 of the species studied showing effects at concentrations below 50 ppbv. Effects on roots were greater than on shoots, with potentially significant implications for overwintering, drought tolerance and carbon sequestration

Measurement of dry deposition to bulk precipitation collectors using a novel flushing sampler

Bulk precipitation samplers, which are continuously open, also sample gases and particles deposited on the funnel surface. Wet-only samplers, which open only during precipitation, avoid this problem, but can be bulky (leading to disruption of air flow and droplet collection) and need electrical power. We describe here a simple battery-powered modification to a standard bulk sampler that allows the separate measurement of deposition to the funnel surface and wet deposition by washing the funnel surface when precipitation is detected. Comparison of this design with a standard bulk sampler over 3 months at a site in eastern Scotland showed that dry deposition to the funnel surface contributed around 20% of sulphate, 20-30% of nitrate and 20-40% of ammonium ions. There was also a significant loss of ammonium and nitrate in the modified sampler, presumably in the tubing, even though a biocide had been added to the sample bottles. This observation has implications for bulk samplers of similar design, with a sample bottle at ground level. Deposition of sea salts and calcium was greater to the flushing collectors than to the bulk collectors, implying that regular cleaning of funnel surfaces with 10% methanol solution subtly alters the capture efficiency for larger particles

UK (CEH) and ACTRIS

Details of CEH involvement through EMEP supersites at Auchencorth and Harwell in the ACTRIS Programm

Systematic biases in measurement of urban nitrogen dioxide using passive diffusion samplers

Measurement of nitrogen dioxide using passive diffusion tube over 22 months in Cambridge, U.K. are analysed as a function of sampler exposure time, and compared with NO2 concentrations obtained from a co-located chemiluminescence analyser. The average ratios of passive sampler to analyser NO2 at a city centre site (mean NO2 concentration 22 ppb) are 1.27 (n = 22), 1.16 (n = 34) and 1.11 (n = 7) for exposures of 1, 2 and 4-weeks, respectively. Modelling the generation of extra NO2 arising from chemical reaction between co-diffusing NO and O3 in the tube gave a ratio (modelled/measured) of 1.31 for 1-week exposures. Such overestimation is greatest when NO2 constitutes, on average, about half of total NOx (= NO + NO2) at the monitoring locality. Although 4- week exposures gave concentrations which were not significantly different from analyser NO2, there was no correlation between the datasets. At both the city-centre site and another semi-rural site (mean NO2 concentration 11 ppb) the average of the aggregate of four consecutive 1-week sampler exposures or of two consecutive 2-week sampler exposures was systematically greater than for a single 4-week exposure. The results indicate two independent and opposing systematic biases in measurement of NO2 by passive diffusion sampler: an exposure-time independent chemical overestimation with magnitude determined by local relative concentrations of NO and O3 to NO2, and an exposuretime dependent reduction in sampling efficiency. The impact of these and other potential sources of systematic bias on the application of passive diffusion tubes for assessing ambient concentrations of NO2 in short (1-week) or long (4-week) exposures are discussed in detail

VOC chemical climate and O3 variation: impact of emissions on regional O3 increment

Understanding the role of individual volatile organic compounds (VOCs) in the formation of surface ozone is important for the effective targeting of ozone mitigation strategies. The UK operates two European Monitoring and Evaluation Programme (EMEP) monitoring ‘supersites’ where concurrent measurement of 27 VOCs, NOx and ozone allows the relationships between these precursors and ozone to be explored. This work presents the relative contribution of measured VOCs on ozone formation at the ‘supersites’, including spatial variation across the UK, and temporal changes between 1999 and 2012. The study was undertaken using the impact-centred chemical climatology framework (Malley et al 2014) VOC concentrations are made up from both regional and local emissions. Regional components of ozone concentrations are distinguished from hemispheric background ozone and measured ozone concentrations which show depletion due to the local NOx environment. Increased VOC photochemical cycling is observed during periods of regional ozone formation, and the contribution of individual VOCs to this total measured VOC cycling is discussed. The drivers of this photochemical depletion, such as meteorology and emissions are evaluated. Back trajectories are coupled with gridded VOC emission maps to estimate the exposure of trajectories to VOC emissions for the four days prior to their arrival at the receptor site. These emissions are disaggregated into 11 broad source sectors, and their contribution is evaluated. Finally the implications of the level of source disaggregation available are discussed in terms of its limitations on VOC emissions speciation to estimate the exposure of receptor sites to individual VOCs.. Using the SNAP sector and the NFR code sector data, it is demonstrated that a greater level of source sector disaggregation would be beneficial for atmospheric model studies and policy determination

Atmospheric Concentrations and Deposition of Trichloroacetic Acid in Scotland: Results from a 2-Year Sampling Campaign

The first long-term concurrent measurements of trichloroacetic acid (TCA) in rainwater, in cloudwater, and in air (both gas and particle phase) are reported. Measurements were made weekly between June 1998 and April 2000 at a rural forested upland site in SE Scotland. Rainwater TCA concentration did not differ significantly between two elevations (602 and 275 m asl), with precipitation-weighted mean values of 0.77 and 0.70 mu g L-1, respectively ( n> 75). The precipitation-weighted mean concentration of TCA in cloudwater at the highest elevation was 0.92 mu g L-1, yielding an average cloudwater enrichment factor of 1.2, considerably lower than for other inorganic ions measured. Rainwater and cloudwater TCA concentrations did not vary systematically with season. Since wet precipitation depth also did not vary systematically with season, the wet deposition fluxes of TCA were likewise invariant (annual fluxes at the highest elevation of 880 and 130 mu g m-2, respectively, for rain and cloud interception to spruce forest). Weekly integrated concentrations of TCA in air (gas and particle) were very low (median 25 pgm-3, range<LOD- 110 pg m-3). The estimated upper limit for annual dry deposition of TCA at this site was 20 mu g m-2, assuming a deposition velocity of 2 cm s-1. Concentrations of TCA in air correlated reasonably strongly with concentrations in rainwater, with a partition ratio approximately equal to the Henry’s law coefficient. On average, only about 23% of TCA measured in Edinburgh air was associated with the particle phase. These measurements are consistent with the observed high scavenging ratio of TCA (ratio of concentration in air to concentration in rainwater). Overall, these data confirm that the atmosphere is an important source of TCA to the environment and that precipitation is the dominant transfer mechanism. In line with previous work, the atmospheric deposition flux is greater than expected from the current understanding of atmospheric production of TCA from anthropogenic precursors. It is suggested that aqueous-phase processes could lead to greater atmospheric conversion of chlorinated solvent precursors to TCA than is currently accepted

Evaluation of the pathways of tropospheric nitrophenol formation from benzene and phenol using a multiphase model

Phenols are a major class of volatile organic compounds (VOC) whose reaction within, and partitioning between, the gas and liquid phases affects their lifetime within the atmosphere, the local oxidising capacity, and the extent of production of nitrophenols, which are toxic chemicals. In this work, a zero-dimension box model was constructed to quantify the relative importance of different nitration pathways, and partitioning into the liquid phase, of mono-aromatic compounds in order to help elucidate the formation pathways of 2- and 4-nitrophenol in the troposphere. The liquid phase contributed significantly to the production of nitrophenols for liquid water content (Lc) values exceeding 3x10-9, and for a range of assumed liquid droplet diameter, even though the resultant equilibrium partitioning to the liquid phase was much lower. For example, in a "typical" model scenario, with Lc=3x10-7, 58% of nitrophenol production occurred in the liquid phase but only 2% of nitrophenol remained there, i.e. a significant proportion of nitrophenol observed in the gas phase may actually be produced via the liquid phase. The importance of the liquid phase was enhanced at lower temperatures, by a factor ~1.5-2 at 278K c.f. 298K. The model showed that nitrophenol production was particularly sensitive to the values of the rate coefficients for the liquid phase reactions between phenol and OH or NO3 reactions, but insensitive to the rate coefficient for the reaction between benzene and OH, thus identifying where further experimental data are require

P and K additions enhance canopy N retention and accelerate the associated leaching

This study evaluated the interactive effects of combined phosphorus (P) and potassium (K) additions on canopy nitrogen (N) retention (CNR) and subsequent canopy leaching at a long-term N manipulation site on Whim bog in south Scotland. Ambient deposition is 8 kg N ha-1 year-1 and an additional 8, 24, and 56 kg N ha-1 year-1 of either ammonium (NH4+) or nitrate (NO3-) with or without P and K has been applied over 11 years. Throughfall N deposition below Calluna vulgaris and foliar N and P concentrations were assessed. Results showed that 60% for low dose and 53% for high dose of NO3- contrasting with 80% for low dose and 38% for high dose of NH4+ onto Calluna was retained by Calluna canopy. The CNR was enhanced by P and K addition in which 84% of NO3 - and 83% of NH4+ for high dose were retained. CNR for NO3- increased the canopy leaching of dissolved organic N (DON) and associated organic anions. NH4+ retention increased canopy leaching of magnesium and calcium through ion exchange. Even over 11-years N exposure without P and K, foliage 29 N:P ratio of Calluna did not increase, suggesting that N exposure did not lead to N saturation of Calluna at Whim bog. Our study concluded that increases in P and K availability enhance CNR of Calluna, but accelerate the associated canopy leaching of DON and base cations, depending on foliar N status

Long-Term Exposure of Sitka Spruce Seedlings to Trichloroacetic Acid

Trichloroacetic acid (TCA) has been implicated as an airborne pollutant responsible for adverse effects on forest health. There is considerable debate as to whether TCA observed in trees and forest soils is derived from atmospheric deposition or from in situ production. This experiment reports the results from treating 4-year-old Sitka spruce ( Picea sitchensis(Bong.) Carr) plants in a greenhouse over a growing season with TCA supplied either to the soil or to the foliage at concentrations of 10 and 100 ng mL-1. Similar uptake of TCA by needles was observed for both modes of treatment, with significant accumulation of TCA (300 ng g-1 dry wt) at the higher concentration. Larger concentrations in stem tissue were seen for the foliar-applied TCA (280 ng g-1) than for the soil-applied TCA (70 ng g-1), suggesting that direct stem uptake may be important. Six months after treatments stopped, TCA concentrations in the needles of plants exposed to 100 ng mL-1 TCA were still enhanced, showing that biological degradation of TCA in needles was slow over the winter. By contrast, no significant enhancement of TCA in soil could be detected in the directly treated soils even during the experiment. The protein content of needles treated with the higher concentration of TCA by either route was significantly smaller than for the controls, but there was no effect of TCA on the conjugation of 1-chloro-2,4-dinitrobenzene in roots nor on the conjugation of 1,2-dichloro-4-nitrobenzene in needles