It is increasingly accepted that although exposure to elevated concentrations of PM(10) is associated with an increased risk of mortality and morbidity, the relationship may not be causal. Rather, there is evidence that number concentrations may be a more appropriate metric than mass concentrations in evaluating health risk. Number concentrations are not routinely monitored and spatial and temporal patterns are poorly quantified. CO and NO, are co-pollutants with their major urban source in common with fine particles, i.e. road vehicle emissions; are routinely monitored in many cities and are also related to ill health. Datasets of particle number concentration measurements from approximately month-long field campaigns in Manchester, Edinburgh and Birmingham (UK) are compared with simultaneous concentrations of CO and NO, from nearby fixed monitors. It was found that it might be possible to reliably predict particle number concentrations (diameters > 100 nm) on an hourly basis in Manchester city centre from knowledge of NO, or CO concentrations alone. The influences of meteorology, spatial variability in emissions and lack of co-location upon the correlations are investigated using cluster analysis. The cluster analysis revealed that these relationships may vary between cities and are dependent upon monitor location but in ways that can be ascribed. For two out of three sites there existed a linear relationship between average cluster aerosol and gas concentrations. This indicates that although airmass aging disrupts the short-term linear relationship, the relationship in the average survives. An emission ratio of particles (approx. 100-500 nm diameter) to NO(x) of approximately 50 cm(-3) ppb(-1) was estimated in Manchester and Birmingham. Particle mass spectrometry measurements indicated that organic compounds dominated these particles and an emission rate of 0.58 ton km(-2) a(-1) of organic particulate matter from road transport has been estimated for the Greater Manchester conurbation. (c) 2005 Elsevier Ltd. All rights reserved. ; Times Cited: 11