A study of large-scale episodes of new particle formation observed at a rural site in southern England which is a receptor site for air masses with varying degrees of pollution is presented. Large-scale episodes of new particle formation are infrequent at Harwell (26 episodes for 1999-2001; data capture of 55%), and it is shown that the most favorable situation for new particle formation in southern England is also infrequent. All episodes but three occurred when clean cool arctic or polar maritime air masses arrived in the UK bringing days of high solar radiation and thin cloud cover. The incursion of these cool air masses may be associated with passage of a cold front and leads to stronger winds and probably strong turbulent mixing; all favor new particle production. More exceptionally, nucleation occurred in warm polluted continental or tropical maritime air masses which shows that different conditions are also possible but much less probable. Higher concentrations of SO2 and higher relative humidity favor higher particle number in agreement with nucleation processes involving H2SO4 and H2O. Taken together, these parameters are insufficient to predict when a burst of particle formation due to nucleation will occur at Harwell, and other important variables must be influential.