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

Y. Sim Tang, Christine F. Braban, U. Dragosits, I. Simmons, D. Leaver, Netty van Dijk, J. Poskitt, Sarah Thacker, Manisha Patel, H. Carter, M. Gloria Pereira, P.O. Keenan, Alan Lawlor, Christopher Conolly, K. Vincent, Mathew R. Heal, Mark A. Sutton

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

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 for each of the other measured species. The ranges in site-annual mean concentrations (nmol m−3) in 2015 for the gases were: HNO3: 0.7–17; HCl: 2.4–21; SO2: 0.9–10, while those for aerosol were: NO3−: 6.3–53; Cl−: 22–89; SO42−: 2.2–11; Na+: 20–74; Ca2+: < lod  2.6; Mg2+: 1.3–6.8. The largest concentrations of HNO3, SO2, and aerosol NO3− and SO42− are found in south and east England and smallest in western Scotland and Northern Ireland. For HCl, highest concentrations are in the southeast and southwest, that may be attributed to dual contribution from anthropogenic (coal combustion) and marine sources (reaction of sea salt with acid gases to form HCl). The spatial distributions of Na+ and Cl− were similar, with largest concentrations at coastal sites in the south and west and at Shetland, reflecting a contribution from sea salt (NaCl) since a near 1 : 1 relationship was also observed in their concentrations. Temporally, peak concentrations in HNO3 and NO3− occurred in late spring and early summer, due to photochemical processes and transboundary pollutant transport. The spring peak in SO42− concentrations coincides with the peak in concentrations of NH3 and NH4+, and are therefore likely attributable to formation of (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, accompanied by large reductions also in the aerosol components, with concentrations of NO3− and NH4+ in molar excess over SO42−. At the same time, a positive trend in HNO3 : NO3− and NH3 : NH4+ ratios, contrasting with a negative trend in SO2 : SO42− ratio provides evidence of a change in the particulate phase from (NH4)2SO4 to NH4NO3, with indications that atmospheric lifetime of HNO3 and NH3 has increased. 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.
Original languageEnglish
Pages (from-to)16293-16324
Number of pages32
JournalAtmospheric Chemistry and Physics
Early online date25 May 2018
Publication statusPublished - 16 Nov 2018

Keywords / Materials (for Non-textual outputs)

  • Atmospheric pollution
  • monitoring network
  • acid gases
  • SO2
  • HNO3
  • HCl
  • NO3-N
  • nitrate aerosol
  • chloride aerosol


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