Riinu Ots

The UK particulate matter air pollution episode of March–April 2014: more than Saharan dust

A period of elevated surface concentrations of airborne particulate matter (PM) in the UK in spring 2014 was widely associated in the UK media with a Saharan dust plume. This might have led to over-emphasis on a natural phenomenon and consequently to a missed opportunity to inform the public and provide robust evidence for policy-makers about the observed characteristics and causes of this pollution event. In this work, the EMEP4UK regional atmospheric chemistry transport model (ACTM) was used in conjunction with speciated PM measurements to investigate the sources and long-range transport (including vertical) processes contributing to the chemical components of the elevated surface PM. It is shown that the elevated PM during this period was mainly driven by ammonium nitrate, much of which was derived from emissions outside the UK. In the early part of the episode, Saharan dust remained aloft above the UK; we show that a significant contribution of Saharan dust at surface level was restricted only to the latter part of the elevated PM period and to a relatively small geographic area in the southern part of the UK. The analyses presented in this paper illustrate the capability of advanced ACTMs, corroborated with chemically-speciated measurements, to identify the underlying causes of complex PM air pollution episodes. Specifically, the analyses highlight the substantial contribution of secondary inorganic ammonium nitrate PM, with agricultural ammonia emissions in continental Europe presenting a major driver. The findings suggest that more emphasis on reducing emissions in Europe would have marked benefits in reducing episodic PM2.5 concentrations in the UK.

Evaluation of Organic Aerosol and Its Precursors in the SILAM Model

Volatility basis-set (VBS) was implemented in the atmospheric chemistry and transport model SILAM for modelling organic aerosol (OA). We present the evaluation of the concentrations of OA and its precursors against observations available in the EBAS database. SILAM simulations with biogenic and anthropogenic emissions from different inventories and different assumptions about the chemical composition of the VOC emissions and OA volatility are analyzed. The contributions of different precursors to the total OA are evaluated.

Modelling Past and Future Changes in Secondary Inorganic Aerosol Concentrations in the UK

The FRAME (Fine Resolution Atmospheric Multi-pollutant Exchange) model is a Lagrangian atmospheric transport model with a horizontal grid resolution of 5 × 5 km2. This work uses FRAME to calculate annual average concentrations of three secondary inorganic aerosol species (NH4 +, NO3 −, SO4 2−) and gas phase SO2 over the United Kingdom for the years 2000–2010. Modelled concentrations are compared to measurements from 12 monitoring sites which are operated with the UK Eutrophying and Acidifying Air Pollutants (UKEAP) Acid Gases and Aerosols Network (AGANet). The results showed good spatial correlation between measurements of gas and aerosol concentrations and modelled values (R2 > 0.8). However FRAME underestimated decreases in sulphate and ammonium aerosol concentrations over the 11-year period and overestimated changes in SO2 concentrations relative to measurements. Ongoing work to investigate temporal changes in atmospheric oxidation rates should clarify this difference. FRAME showed a more modest reduction in nitrate concentrations (compared to ammonium and sulphate), which is in agreement with measurements.