Water soluble aerosols and gases at a UK background site – Part 1: Controls of PM_2.5 and PM₁₀ aerosol composition

Abstract. There is limited availability of long-term, high temporal resolution, chemically speciated aerosol measurements which can provide further insight into the health and environmental impacts of particulate matter. The Monitor for AeRosols and Gases (MARGA, Applikon B.V., NL) allows for the characterisation of the inorganic components of PM₁₀ and PM_2.5 (NH₄⁺, NO₃^-, SO₄^2-, Cl^-, Na⁺, K⁺, Ca²⁺, Mg²⁺) and inorganic reactive gases (NH₃, SO₂, HCl, HONO and HNO₃) at hourly resolution. The following study presents 6.5 years (June 2006 to December 2012) of quasi-continuous observations of PM_2.5 and PM₁₀ using the MARGA at the UK EMEP supersite, Auchencorth Moss, SE Scotland. Auchencorth Moss was found to be representative of a remote European site with average total water soluble inorganic mass of PM_2.5 of 3.82 μg m⁻³. Anthropogenically derived secondary inorganic aerosols (sum of NH₄⁺, NO₃^- and nss-SO₄²⁻) were the dominating species (63 %) of PM_2.5. In terms of equivalent concentrations, NH₄⁺ provided the single largest contribution to PM_2.5 fraction in all seasons. Sea salt was the main component (73 %) of the PM_coarse fraction (PM₁₀-PM_2.5), though NO₃^- was also found to make a relatively large contribution to the measured mass (17 %) providing evidence of considerable processing of sea salt in the coarse mode. There was on occasions evidence of aerosol from combustion events being transported to the site in 2012 as high K⁺ concentrations (deviating from the known ratio in sea salt) coincided with increases in black carbon at the site. Pollution events in PM₁₀ (defined as concentrations > 12 μg m⁻³) were on average dominated by NH₄⁺ and NO₃^-, where smaller loadings at the site tended to be dominated by sea salt. As with other western European sites, the charge balance of the inorganic components resolved were biased towards cations, suggesting the aerosol was basic or more likely that organic acids contributed to the charge balance. This study demonstrates the UK background atmospheric composition is primarily driven by meteorology with sea salt dominating air masses from the Atlantic Ocean and the Arctic, whereas secondary inorganic aerosols tended to dominate air masses from continental Europe.