Since the closure of UK airspace on Thursday staff in our operations centre at King’s have been analysing pollution measurements for possible ground level impacts of the volcanic plume. Elevated concentrations of SO2 and PM10 have been detected over the weekend. These are not thought to be linked to the volcanic eruption as discussed below. Yesterday over 16,000 notifications were sent to users of the LondonAir i-phone application. We will continue to analyse pollution concentrations for possible impacts of the Icelandic volcano. Our next planned update will report changes to air pollution concentrations around airports following the closure of UK air space.

Initially our analysis focused on looking for co-incident elevations of SO2 and PM10 reflecting the likely composition in the volcanic plume. A series of SO2 peaks have been measured in the areas covered by our networks. These have affected sites in east London at Greenwich and Bexley on Friday and sites in south Essex, at Castle Point and Thurrock on Saturday and Sunday. Elevated concentrations of NOX have been associated with the SO2 peaks (indicating a combustion source) and each of these events has shown an easterly progression with local wind direction. This is consistent with the normal pattern of plume grounding from industrial sources in the east Thames area.

On Friday afternoon a veering of wind direction from northeast towards southeast was linked to a sharp increase in PM10 concentrations at certain background sites. This increase of around 20 ug m-3 appears to be mainly coarse PM (i.e. PM10- PM2.5) particles, with coarse PM comprising around 50% of background concentrations of total PM10 during Friday afternoon. Although this is a change in the nature of PM concentrations that prevailed earlier in the week, similar patterns of concentration changes have been measured before, for instance during a secondary PM10 episode in May 2009.

Since Friday afternoon concentrations of volatile PM have been increasing steadily across south east England, consistent with increasing concentrations of secondary particulate. In addition, widespread ‘moderate’ O3 was measured throughout south east England over the weekend with the greatest concentrations being measured on Sunday afternoon in Sussex, east Surrey and west Kent where concentrations of over 120 ug m-3 were attained.

On Sunday afternoon, elevated concentrations of PM10 were measured at sites to the south of London, across Sussex, east Surrey and west Kent. PM10 concentrations of over 80 ug m-3 were measured at roadside sites in Horsham, Chichester and Sevenoaks and at background sites in Sevenoaks, Eastbourne, Reigate and Banstead and Mole Valley. During Sunday afternoon PM10 concentrations at these sites exceeded those in London.

Key to understanding these pollution events is to use back trajectory analysis to look at where air has travelled over before it reaches us. Although the air present over the Sussex, east Surrey and west Kent on Sunday afternoon was over Iceland four days previously, this air also passed over parts of Germany, Belgium, the Netherlands and north France during the preceding 36 hours, as shown in the illustration. By comparison, the air present over London during Sunday afternoon had spent less time over mainland Europe. Coupled with concentrations of O3 and volatile PM it is most likely that the elevated PM across Sussex, east Surrey and west Kent on Sunday was due to secondary PM from pollutants emitted over the near continent rather than volcanic ash.

In summary, although we have no direct evidence of a volcanic ash contributing to our PM concentrations over the weekend, the possibility of a minor contribution cannot be ruled out. Detection of PM from this source would require mineralogical analysis of collected PM samples. Such analysis is not routinely undertaken in the UK networks.

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Item date 19/04/2010