Acid gases and aerosol measurements in the UK (1999–2015): regional distributions and trends

Abstract

Abstract. The UK Acid Gases and Aerosol Monitoring Network (AGANet) was established in 1999 (12 sites, increased to 30 sites from 2006), to provide long-term national monitoring of acid gases (HNO3, SO2, HCl) and aerosol components (NO3−, SO42−, Cl−, Na+, Ca2+, Mg2+). An extension of a low-cost denuder-filter pack system (DELTA) that is used to measure NH3 and NH4+ in the UK National Ammonia Monitoring Network (NAMN) provides additional monthly speciated measurements for the AGANet. A comparison of the monthly DELTA measurement with averaged daily results from an annular denuder system showed close agreement, while the sum of HNO3 and NO3− and the sum of NH3 and NH4+ from the DELTA are also consistent with previous filter pack determination of total inorganic nitrogen and total inorganic ammonium, respectively. With the exception of SO2 and SO42−, the AGANet provides, for the first time, the UK concentration fields and seasonal cycles for each of the other measured species. The largest concentrations of HNO3, SO2, and aerosol NO3− and SO42− are found in southern and eastern England and smallest in western Scotland and Northern Ireland, whereas HCl are highest in south-eastern, south-western, and central England, that may be attributed to dual contribution from anthropogenic (coal combustion) and marine sources (reaction of sea salt with acid gases to form HCl). Na+ and Cl− are spatially correlated, with largest concentrations at coastal sites, reflecting a contribution from sea salt. Temporally, peak concentrations in HNO3 occurred in late winter and early spring attributed to photochemical processes. NO3− and SO42− have a spring maxima that coincides with the peak in concentrations of NH3 and NH4+, and are therefore likely attributable to formation of NH4NO3 and (NH4)2SO4 from reaction with higher concentrations of NH3 in spring. By contrast, peak concentrations of SO2, Na+, and Cl− during winter are consistent with combustion sources for SO2 and marine sources in winter for sea salt aerosol. Key pollutant events were captured by the AGANet. In 2003, a spring episode with elevated concentrations of HNO3 and NO3− was driven by meteorology and transboundary transport of NH4NO3 from Europe. A second, but smaller episode occurred in September 2014, with elevated concentrations of SO2, HNO3, SO42−, NO3−, and NH4+ that was shown to be from the Icelandic Holuhraun volcanic eruptions. Since 1999, AGANet has shown substantial decrease in SO2 concentrations relative to HNO3 and NH3, consistent with estimated decline in UK emissions. At the same time, large reductions and changes in the aerosol components provide evidence of a shift in the particulate phase from (NH4)2SO4 to NH4NO3. The potential for NH4NO3 to release NH3 and HNO3 in warm weather, together with the surfeit of NH3 also means that a larger fraction of the reduced and oxidized N is remaining in the gas phase as NH3 and HNO3 as indicated by the increasing trend in ratios of NH3 : NH4+ and HNO3 : NO3− over the 16-year period. Due to different removal rates of the component species by wet and dry deposition, this change is expected to affect spatial patterns of pollutant deposition with consequences for sensitive habitats with exceedance of critical loads of acidity and eutrophication. The changes are also relevant for human health effects assessment, particularly in urban areas as NH4NO3 constitutes a significant fraction of fine particulate matter ( < 2.5 µm) that are linked to increased mortality from respiratory and cardiopulmonary diseases.

Pan-European rural atmospheric monitoring network shows dominance of NH<sub>3</sub> gas and NH<sub>4</sub>NO<sub>3</sub> aerosol in inorganic pollution load

“…The DELTA ® method used in the NEU Level 1 network is based on the system developed for the UK Acid Gas and Aerosol monitoring network (AGANet, Tang et al, 2018b). Full details of the DELTA ® method and air concentration calculations in the NEU network are provided by Tang et al (2009Tang et al ( , 2018b. The method uses a small 6 V air pump to deliver low air sampling 25 rates of between 0.2 to 0.4 L min -1 , a high sensitivity gas meter to record the typically monthly volume of air collected and a DELTA ® denuder-filter pack sample train to collect separately the gas and aerosol phase components.…”

“…The NitroEurope (NEU) Level 1 network was operated between November 2006 and December 2010 to deliver the core measurements of reactive nitrogen gases (NH3, HNO3) and aerosols (NH4 + , NO3 -) for the project (Figure 1). A low-volume denuder-filter pack method, the 'DEnuder for Long-Term Atmospheric sampling' system (DELTA ® , Sutton et al, 2001a;Tang et al, 2009Tang et al, , 2018b with time-integrated monthly sampling was used, which made implementation at a large number of sites 20 possible. Other acid gases (SO2, HCl) and aerosols (SO4 2-, Cl -, Na + , Ca 2+ , Mg 2+ ) were also collected at the same time and measured by the DELTA ® method.…”

“…DELTA ® measurements were co-located with all NEU Level 3 sites with advanced flux measurements (Skiba et al, 2009), and with the network of main CarboEurope-IP CO2 flux monitoring sites (www.carboeurope.eu) (Flechard et al, 2011(Flechard et al, , 2020. Two of the UK sites in the NEU DELTA ® network are existing UK NAMN (Tang et al, 2018a) and AGANet sites (Tang et al, 2018b). These are Auchencorth Moss (UK-Amo) and Bush EBu) located in Southern Scotland.…”

“…There are also networks with a focus on specific N components, for example, the national NH3 monitoring networks in the Netherlands (LML, van Zanten et al, 2017) and in the UK (National Ammonia Monitoring Network, NAMN;Tang et al, 2018a), or compliance monitoring across Europe in the case of SO2 and NOx. The UK is unique in having an extensive set of speciated gas and aerosol monitoring data from the Acid Gas and Aerosol Network (AGANet), with measurements from 1999 to the present (Tang et al, 2018b). 30 In this context, there is an ongoing need for cost-effective, easy-to-operate, time-integrated atmospheric measurement for the respective gas and aerosol phases at sufficient spatial scales.…”

Alkaline air: changing perspectives on nitrogen and air pollution in an ammonia-rich world

“…+ concentrations have decreased significantly, consistent with declining SO 2 and HNO 3 (figure 2b). This has led to less formation of ammonium sulfate and ammonium nitrate, which will have also helped maintain gaseous NH 3 levels [38,39]. As a consequence, acid rain is now a thing of the past for UK conditions.…”