Having fully adopted the Daily Air Quality Index (DAQI) when it came into force on 1st January 2012, we have now examined our reporting performance during two recent particulate (PM) episodes. We examined the impact of reporting PM using the new fixed midnight to midnight day required by the new DAQI, as opposed to the running 24 hour mean used by the previous index and still widely reported by other networks. The new DAQI also contains "triggers" based on hourly values which warn of predicted exposure. This short-term predictive element is a key component of the additional public information provided by the new DAQI.

The first episode examined was a PM2.5 dominated one where pollution peaked on 31st January 2012. Trigger performance was examined at all 76 PM10 and PM2.5 analysers in our database from across the southeast and London. The daily banding was correctly predicted in advance at 71 sites with the remaining 5 being PM2.5 analysers where index level 6 (moderate) was predicted but index level 7 (high) was attained by the end of the day. This under estimation for PM2.5 in some locations was attributed to the atypical temporal distribution of PM2.5 during this episode.

The original work by King’s for the Committee on the Medical Effects of Air Pollutants (COMEAP) to calculate the triggers included AURN data from across the country in the 270,000 site days used to calculate the PM10 triggers. We were therefore also keen to examine performance of triggers outside the southeast and London.

We examined trigger performance using data from the AURN PM10 at Headingley in Leeds. This site was chosen because pollution peaked at ‘high’ and hence enabled analysis of transition from ‘low’ through ‘moderate’ to ‘high’ and back again.

As in the southeast, the episode started building on 30th January, peaked on the 31st and then cleared late on 1st February. On all days examined (30th January to 2nd February), DAQI defined triggers correctly predicted the day’s pollution levels. Further, as COMEAP intended, triggers responded rapidly to rises in ambient levels and enabled prediction of daily levels as early as 0800h.

The redundant system of reporting running 24 hour means would have introduced a lag in response and would not have reported the ‘high’ peak until the last hour of the 31st (the ambient levels peaked and the trigger activated 9 hours earlier) but then would have continued to report ‘high’, when DAQI levels were ‘moderate’, right through into the afternoon of the following day. This was repeated into the 2nd February when ‘moderate’ would have been reported for 8 hours despite the episode clearing in the afternoon on the previous day.

COMEAP in their review, which led to the changes adopted by Defra for the DAQI, specifically considered this lag issue of running 24 hour reporting (section 3.3) and recommended the change to fixed 24 hour day reporting combined with triggers.

The second episode examined was primarily driven by local emissions on 6th February 2012. During this PM10 dominated episode, alerts triggered on LondonAir, and on our other websites and apps, correctly predicted the day’s banding at all 21 analysers that measured ‘moderate’ pollution.

Having looked at the predictive performance of the DAQI ‘triggers’ since the start of the year it is apparent that the erratic nature of local sources at industrial sites, including near waste transfer stations, is at times causing an over prediction of daily levels. This issue with erratic sources was identified during the original work by King’s for COMEAP, and the industrial sites were excluded from the original 270,000 site days used to calculate the PM10 triggers. Work is ongoing to identify an enhanced reporting method to reflect the site specific PM10 at these sites.

Item date 12/03/2012 16:00:00