Precipitation Network and Rural Nitrogen Dioxide Network UKEAP- NO2 Net and Precip-Net

Tender description

Precip Net: Precip-Net sites collect rainwater using standard bulk rainwater collectors on a fortnightly basis to measure the bulk deposition of both acidifying and eutrophying chemical species at 48 sites. This includes calcium, chloride, potassium, magnesium, sodium, phosphate, nitrate, ammonium, sulphate, non-marine sulphate, acidity, conductivity, pH and total rainfall levels. At the two UK EMEP Supersites, daily wet only collectors are operated and samples analysed for the same ions as the bulk samplers with the addition of fluoride. With the long term time series from this network, it is possible to do trend analysis and modelling of chemical deposition via rainfall to UK ecosystems. Precip-Net is measured via bottle collectors mounted on a pole. The samples are collected fortnightly by the Local Site Operators (LSOs), returned, processed and sent for analysis. The bottle collector has a funnel with a polypropylene mesh filter at the base to prevent coarse material entering the collection chamber. Bird deterrents are also located on the top of the funnels to try to minimise contamination from bird droppings. At Auchencorth Moss and Chilbolton (The two EMEP sites) daily wet only collections are also made. This is via a wet only sampler and is used towards meeting the UK’s EMEP commitments. Ion analysis of the daily samples is undertaken, including: Na, NH4-N, K, Mg, Ca, Cl, NO3-N, SO4-S, PO4-P. In the daily samples fluoride (F-) is measured additionally. NO2 Net NO2-Net sites are a subset of the Precip-Net sites and are used to provide a rural background for the UK modelling of NO2 via the Defra Modelling Ambient Air Quality contract for annual compliance mapping at 26 sites. This supplements information delivered on a separate EA network, the Automatic Urban and Rural Network (AURN). Samples are taken using diffusion tubes. Three diffusion tubes with stainless steel grids mounted in low density polyethylene caps are used to collect the samples. The stainless steel grids are impregnated with an absorbent chemical (triethanolamine and acetone) which captures the NO2 as the gas diffuses through the tube. The tubes are left in situ for 4 weeks then analysed. Following a two-year intercomparison study during 2022 and 2023 comparing the performance of tubes with and without wind protection caps, the use of the tubes without the wind protection caps were discontinued and only those tubes with wind protection caps are now employed in the network. These tubes, provided by Gradko, are like those used in the UK Urban NO2 Network (UUNN Network). The wind protection caps help maintain the diffusion path length and hence reduces the positive sampling bias.