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

Y. Sim Tang, Christine F. Braban, Ulrike Dragosits, Ivan Simmons, David Leaver, Netty van Dijk, Janet Poskitt, Sarah Thacker, Manisha Patel, Heather Carter, M. Glória Pereira, Patrick O. Keenan (+5 others)
2018 Atmospheric Chemistry and Physics Discussions  
<p><strong>Abstract.</strong> 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 (HNO<sub>3</sub>, SO<sub>2</sub>, HCl) and aerosol components (NO<sub>3</sub><sup>&amp;minus;</sup>, SO<sub>4</sub><sup>2&amp;minus;</sup>, Cl<sup>&amp;minus;</sup>, Na<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>). An extension of a low-cost denuder-filter pack system (DELTA) that is
more » ... em (DELTA) that is used to measure NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> 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 HNO<sub>3</sub> and NO<sub>3</sub><sup>&amp;minus;</sup> and the sum of NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup> from the DELTA are also consistent with previous filter pack determination of total inorganic nitrogen and total inorganic ammonium, respectively. With the exception of SO<sub>2</sub> and SO<sub>4</sub><sup>2&amp;minus;</sup>, 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<span class="thinspace"></span>m<sup>&amp;minus;3</sup>) in 2015 for the gases were: HNO<sub>3</sub>: 0.7&amp;ndash;17; HCl: 2.4&amp;ndash;21; SO<sub>2</sub>: 0.9&amp;ndash;10, while those for aerosol were: NO<sub>3</sub><sup>&amp;minus;</sup>: 6.3&amp;ndash;53; Cl<sup>&amp;minus;</sup>: 22&amp;ndash;89; SO<sub>4</sub><sup>2&amp;minus;</sup>: 2.2&amp;ndash;11; Na<sup>+</sup>: 20&amp;ndash;74; Ca<sup>2+</sup>: <<span class="thinspace"></span>lod<span class="thinspace"></span><span class="thinspace"></span>2.6; Mg<sup>2+</sup>: 1.3&amp;ndash;6.8. The largest concentrations of HNO<sub>3</sub>, SO<sub>2</sub>, and aerosol NO<sub>3</sub><sup>&amp;minus;</sup> and SO<sub>4</sub><sup>2&amp;minus;</sup> 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<sup>+</sup> and Cl<sup>&amp;minus;</sup> 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<span class="thinspace"></span>:<span class="thinspace"></span>1 relationship was also observed in their concentrations. Temporally, peak concentrations in HNO<sub>3</sub> and NO<sub>3</sub><sup>&amp;minus;</sup> occurred in late spring and early summer, due to photochemical processes and transboundary pollutant transport. The spring peak in SO<sub>4</sub><sup>2&amp;minus;</sup> concentrations coincides with the peak in concentrations of NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup>, and are therefore likely attributable to formation of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> from reaction with higher concentrations of NH<sub>3</sub> in spring. By contrast, peak concentrations of SO<sub>2</sub>, Na<sup>+</sup> and Cl<sup>&amp;minus;</sup> during winter are consistent with combustion sources for SO<sub>2</sub> 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 HNO<sub>3</sub> and NO<sub>3</sub><sup>&amp;minus;</sup> was driven by meteorology and transboundary transport of NH<sub>4</sub>NO<sub>3</sub> from Europe. A second, but smaller episode occurred in September 2014, with elevated concentrations of SO<sub>2</sub>, HNO<sub>3</sub>, SO<sub>4</sub><sup>2&amp;minus;</sup>, NO<sub>3</sub><sup>&amp;minus;</sup> and NH<sub>4</sub><sup>+</sup> that was shown to be from the Icelandic Holuhraun volcanic eruptions. Since 1999, AGANet has shown substantial decrease in SO<sub>2</sub> concentrations relative to HNO<sub>3</sub> and NH<sub>3</sub>, accompanied by large reductions also in the aerosol components, with concentrations of NO<sub>3</sub><sup>&amp;minus;</sup> and NH<sub>4</sub><sup>+</sup> in molar excess over SO<sub>4</sub><sup>2&amp;minus;</sup>. At the same time, a positive trend in HNO<sub>3</sub><span class="thinspace"></span>:<span class="thinspace"></span>NO<sub>3</sub><sup>&amp;minus;</sup> and NH<sub>3</sub><span class="thinspace"></span>:<span class="thinspace"></span>NH<sub>4</sub><sup>+</sup> ratios, contrasting with a negative trend in SO<sub>2</sub><span class="thinspace"></span>:<span class="thinspace"></span>SO<sub>4</sub><sup>2&amp;minus;</sup> ratio provides evidence of a change in the particulate phase from (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> to NH<sub>4</sub>NO<sub>3</sub>, with indications that atmospheric lifetime of HNO<sub>3</sub> and NH<sub>3</sub> 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 NH<sub>4</sub>NO<sub>3</sub> constitutes a significant fraction of fine particulate matter (<<span class="thinspace"></span>2.5<span class="thinspace"></span>&amp;mu;m) that are linked to increased mortality from respiratory and cardiopulmonary diseases.</p>
doi:10.5194/acp-2018-489 fatcat:ztxx2stxw5eu3cjmfgahiimmqu