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

• Main Authors: Y. S. Tang, C. F. Braban, U. Dragosits, I. Simmons, D. Leaver, N. van Dijk, J. Poskitt, S. Thacker, M. Patel, H. Carter, M. G. Pereira, P. O. Keenan, A. Lawlor, C. Conolly, K. Vincent, M. R. Heal, M. A. Sutton
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-11-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/16293/2018/acp-18-16293-2018.pdf

<p>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 ( &lt; 2.5&thinsp;µm) that are linked to increased mortality from respiratory and cardiopulmonary diseases.</p>