This thesis presents mobile air pollution measurements as a novel way to measure and assess particulate air pollution in urban, suburban and rural areas. Increased particulate air pollution shows a significant potential for adverse health effects in the human airways. High levels of particulates are mainly found in urban and urban-influenced regions and are to a large extent attributable to increased traffic density in the last decades. Worldwide, federal agencies regulate particle emissions and ambient concentrations by means of aerosol mass concentration standards (aerosol: suspension of liquid or solid compounds in a gas, usually air). Commonly regulated parameters are PM10 and PM2.5 (particle mass concentration of particles with aerodynamic diameters smaller than 10 µm and 2.5 µm, respectively), which usually are monitored by a network of stationary measurements. Diesel engines contribute a major fraction to road traffic particulate emissions and typically produce major amounts of particles that are more than one order of magnitude smaller than 2.5 µm in diameter. On occasion of the project “Diesel Aerosol Sampling Methodology”, performed at the University of Minnesota (USA), real-world emissions from diesel engines were measured in mobile truck-chase experiments while driving on the road. The mobile emission laboratory (MEL) contained a variety of instruments for the physical characterization of aerosol particles in the 0.003 – 2.5 µm size range and was ideal for the measurement of highly dynamic vehicle exhaust plumes. Selected diesel trucks were followed and their exhaust was captured and analyzed a few seconds after its emission from the stack. Measured diesel particle number size distributions were typically characterized by a nanoparticle mode (diameter d<50 nm, consisting of hydrocarbons and a small percentage of sulphuric acid and water, in most cases evaporating clearly below 300 oC), a diesel exhaust accumulation mode (80 nm <d<140 nm, “diesel soot”, i.e. agglomerates of elemental and organic carbon, coated with other condensable species like semi-volatile hydrocarbons, nitrates and sulfates) and a coarse mode (d>1 µm, particulate matter reentrained from the engine cylinders into the exhaust stream). The nanoparticle mode typically contained 1-20 % of the particle mass and more than 90 % of the particle number. The truck-chase experiments found that under ambient conditions large number concentrations of nanoparticles are formed, especially at cold ambient temperatures and at high average traffic speeds. They are rapidly formed by nucleation during the dilution and cooling of freshly emitted gaseous exhaust in the atmosphere. Hydrocarbons originating from engine lube oil and sulphuric acid from sulfur containing fuels are main gaseous precursors of primary ultrafine particles. Their lifetime is rather short (< 10 minutes in urban areas, corresponding to an area of influence of 100 m – 1 km). Both specific chase experiments and more general measurements in urban traffic showed that nanoparticles are not only produced by diesel vehicles, but also by mixed on-road traffic. Due to experimental conditions suppressing nanoparticle formation, the nanoparticle mode has hardly been observed in chassis dynamometer (test bench) measurements in the past. Therefore, chase experiments are an important complement to laboratory studies. Taking into account that they require highly time-resolved real-time instrumentation, combined condensation particle counter (CPC), photoelectric aerosol sensor (PAS) and diffusion charging sensor (DC) measurements have been shown to be a novel method for the determination of the relative fraction of nanoparticles in ambient air and the coating degree of combustion accumulation mode particles with condensable species. These three robust instruments have time resolutions lower than 10 seconds and are easy to use in field experiments. At the Paul Scherrer Institute (PSI) another mobile pollutant measurement laboratory was constructed for highly time-resolved on-road ambient concentration measurements of a large set of gas phase, aerosol, geographical and meteorological parameters. During the project YOGAM (Year of Gas Phase and Aerosol Measurements) the spatial and temporal variation of fine (d<2.5 µm) and ultrafine (d< 100 nm) particles in urban, suburban and rural areas in the Zürich (Switzerland) area was assessed in 2001/2002 by means of on-road measurements along a specified route on 6-10 days per season. Particle background number concentrations (d>3 nm) were 35000 ± 30000 cm-3 (mean ± standard deviation) for urban regions and 15000 ± 12000 cm-3 for rural areas. Governed by meteorology (namely wind speed, wind direction, degree of vertical mixing and precipitation), the day-to-day variation was significantly larger than the spatial variation, but within one day a consistent regional pollution pattern was found. Freeway-influenced areas showed the highest background number concentrations (>80000 cm-3), and peak concentrations from single vehicles easily reached 400000 cm-3. Agreeing with the results from the U.S. study, ambient particle number size distributions were mainly characterized by a combustion exhaust accumulation mode at d=80-140 nm with number concentrations depending on the distance to anthropogenic activity, and an ultrafine mode showing rather variable number concentrations. In urban areas, the ultrafine mode was dominated by combustion nanoparticles (d<50 nm) with an increased formation potential at low temperatures. These anthropogenic ultrafine particles are also defined as primary ultrafines. Contrastingly, levels of ultrafine particles were much lower in rural areas. Sporadic episodes with increased ultrafine particle levels during warm and sunny spring days suggested the presence of secondary ultrafine particles, i.e. products from gas-to-particle conversion after photochemical reactions in aged urban air plumes, or products from biogenic gaseous precursors. Concluding, the YOGAM mobile measurements have been shown to be suitable for long-term pollutant assessments, to obtain valuable information on spatial variability. Despite the fact that respective equipment currently lacks an official accreditation for legal use, both project described in this thesis support that future regulation of emissions and ambient aerosol concentrations will be required to not longer be solely based on mass measurements, but also on particle number or surface area concentration.