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

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

Aqueous-phase nitration of phenol by N2O5 and ClNO2

Nitrophenols are present in the atmospheric gas phase and in cloud and rainwater. Their formation via aqueous-phase reactions of phenol with the nitronium ion, NO2+, arising from N2O5 and ClNO2 partitioning into the aqueous phase, has been proposed but not verified experimentally. Here we demonstrate for the first time that gaseous N2O5 and ClNO2 partitioning into dilute aqueous solutions of phenol yields 2- and 4-nitrophenol (and 4-nitrosophenol), but no dinitrophenol isomers. The rate of nitration does not vary significantly between 5 and 20 degrees C, presumably because of opposing temperature dependences in Henry’s law partitioning and reaction rate coefficients. The rate coefficient for reaction of NO2+ with phenol could not be directly quantified but is evidently large enough for this reaction to compete effectively with the reaction between NO2+ and water and to provide a feasible route to nitrophenol production in the atmosphere

Experimentally determined Henry's Law coefficients of phenol, 2-methylphenol and 2-nitrophenol in the temperature range 281-302 K

The Henry’s Law coefficient is a key physical parameter in the partitioning, and hence environmental fate, of a chemical species between air and water. Despite the acknowledged polluting potential of phenol, 2-methylphenol (o-cresol) and 2-nitrophenol, there is extremely poor agreement in the literature of their Henry’s law coefficients and, in particular, no apparent systematic measurement of the variation with temperature. Here a temperature controlled column-stripping method was employed to determine the Henry's Law coefficients for these compounds over the temperature range 281-302 K. Coefficients were derived from regression fits to the observed rates of loss from the liquid phase as a function of column depth in order explicitly to take account of potential non-attainment of equilibrium between liquid and gas phases. Temperature dependent expressions summarising the Henry's Law coefficient of phenol, o-cresol and 2-nitrophenol over the stated temperature range are ln H (/ M atm-1) = 5850/T - 11.6, ln H (/ M atm-1) = 6680/T - 15.4 and ln H (/ M atm-1) = 6270/T - 16.6, respectively (to within 15% of all measured values in this work). A thorough comparison with previous literature-published values has been undertaken

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 interactive effects of N addition with P and K availability on N status of Sphagnum

Little information exists concerning the long-term interactive effect of nitrogen (N) addition with phosphorus (P) and potassium (K) on Sphagnum N status. This study was conducted as part of a long-term N manipulation on Whim bog in south Scotland to evaluate the long-term alleviation effects of phosphorus (P) and potassium (K) on N saturation of Sphagnum (S. capillifolium). On this ombrotrophic peatland, where ambient deposition was 8 kg N ha−1 yr−1, 56 kg N ha−1 yr−1 of either ammonium (NH4+, Nred) or nitrate (NO3−, Nox) with and without P and K, were added over 11 years. Nutrient concentrations of Sphagnum stem and capitulum, and pore water quality of the Sphagnum layer were assessed. The N-saturated Sphagnum caused by long-term (11 years) and high doses (56 kg N ha−1 yr−1) of reduced N was not completely ameliorated by P and K addition; N concentrations in Sphagnum capitula for Nred 56 PK were comparable with those for Nred 56, although N concentrations in Sphagnum stems for Nred 56 PK were lower than those for Nred 56. While dissolved inorganic nitrogen (DIN) concentrations in pore water for Nred 56 PK were not different from Nred 56, they were lower for Nox 56 PK than for Nox 56 whose stage of N saturation had not advanced compared to Nred 56. These results indicate that increasing P and K availability has only a limited amelioration effect on the N assimilation of Sphagnum at an advanced stage of N saturation. This study concluded that over the long-term P and K additions will not offset the N saturation of Sphagnum

Sphagnum can ‘filter’ N deposition, but effects on the plant and pore water depend on the N form

The ability of Sphagnum moss to efficiently intercept atmospheric nitrogen (N) has been assumed to be vulnerable to increased N deposition. However, the proposed critical load (20 kg N ha− 1 yr− 1) to exceed the capacity of the Sphagnum N filter has not been confirmed. A long-term (11 years) and realistic N manipulation on Whim bog was used to study the N filter function of Sphagnum (Sphagnum capillifolium) in response to increased wet N deposition. On this ombrotrophic peatland where ambient deposition was 8 kg N ha− 1 yr− 1, an additional 8, 24, and 56 kg N ha− 1 yr− 1 of either ammonium (NH4+) or nitrate (NO3−) has been applied for 11 years. Nutrient status of Sphagnum and pore water quality from the Sphagnum layer were assessed. The N filter function of Sphagnum was still active up to 32 kg N ha− 1 yr− 1 even after 11 years. N saturation of Sphagnum and subsequent increases in dissolved inorganic N (DIN) concentration in pore water occurred only for 56 kg N ha− 1 yr− 1 of NH4+ addition. These results indicate that the Sphagnum N filter is more resilient to wet N deposition than previously inferred. However, functionality will be more compromised when NH4+ dominates wet deposition for high inputs (56 kg N ha− 1 yr− 1). The N filter function in response to NO3− uptake increased the concentration of dissolved organic N (DON) and associated organic anions in pore water. NH4+ uptake increased the concentration of base cations and hydrogen ions in pore water though ion exchange. The resilience of the Sphagnum N filter can explain the reported small magnitude of species change in the Whim bog ecosystem exposed to wet N deposition. However, changes in the leaching substances, arising from the assimilation of NO3− and NH4+, may lead to species change

The production and degradation of trichloroacetic acid in soil: results from in situ soil column experiments

Previous work has indicated that the soil is important to understanding biogeochemical fluxes of trichloroacetic acid (TCA) in the rural environment, in forests in particular. Here, the hydrological and TCA fluxes through 22 in situ soil columns in a forest and moorland-covered catchment and an agricultural grassland field in Scotland were monitored every two weeks for several months either as controls or in TCA manipulation (artificial dosing) experiments. This was supplemented by laboratory experiments with radioactively-labelled TCA and with irradiated (sterilised) soil columns. Control in situ forest soil columns showed evidence of net export (i.e. in situ production) of TCA, consistent with a net soil TCA production inferred from forest-scale mass balance estimations. At the same time, there was also clear evidence of substantial in situ degradation within the soil (~70% on average) of applied TCA. The laboratory experiments showed that both the formation and degradation processes operate on time scales of up to a few days and appeared related more with biological rather than abiotic processes. Soil TCA activity was greater in more organic-rich soils, particularly within forests, and there was strong correlation between TCA and soil biomass carbon content. Overall it appears that TCA soil processes exemplify the substantial natural biogeochemical cycling of chlorine within soils, independent of any anthropogenic chlorine flux