The use of trajectory cluster analysis to interpret trace gas measurementsat Mace Head, Ireland

A first step in interpreting the wide variation in trace gas concentrations measured over time at a given site is to classify the data according to th e prevailing weather conditions. In order to classify measurements made dur ing two intensive field campaigns at Mace I-lead, on the west coast of Irel and, an objective method of assigning data to different weather types has b een developed. Air-mass back trajectories calculated using winds from ECMWF analyses, arriving at the site in 1995-1997, were allocated to clusters ba sed on a statistical analysis of the latitude, longitude and pressure of th e trajectory at 12 h intervals over 5 days. The robustness of the analysis was assessed by using an ensemble of back trajectories calculated for four points around Mace Head. Separate analyses were made for each of the 3 pear s, and for four 3-month periods. The use of these clusters in classifying g round-based ozone measurements at Mace Head is described, including the nee d to exclude data which have been influenced by local perturbations to the regional flow pattern, for example, by sea breezes. Even with a limited dat a set, based on 2 months of intensive field measurements in 1996 and 1997, there are statistically significant differences in ozone concentrations in air from the different clusters. The limitations of this type of analysis f or classification and interpretation of ground-based chemistry measurements are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.