Atmospheric oxidants (O3, NO and NO2) and particle size distributions (6–700 nm) were studied at a semi-rural background location in Denmark during 2009 to evaluate the sources of particulate matter. Aitken mode particles (30–110 nm) and accumulation mode particles (110–700 nm) anti-correlated or showed no correlation with O3. The lower Aitken mode (<30 nm), also referred to as the nucleation mode, typically peaked around noon from April to September and correlated positively with O3, which indicates that photochemistry is likely to be responsible for most of the nucleation mode particles in this period of the year. In the months with low global radiation (October–March), NO2 correlated positively with the lower Aitken mode, which typically peaked in the morning and afternoon on working days, indicating that traffic is the most important source of nucleation mode particles in that period of the year. A strong correlation was observed for NO2 and Aitken mode particles throughout the year, which indicate a common traffic source. We also observed growth of Aitken mode particles during high NOX episodes. While photochemistry and traffic were major sources to Aitken mode particles, a source apportionment analysis identified Secondary Inorganic Aerosols (SIA), as the largest source to PM10 (36% including secondary marine aerosols) in the semi-rural background troposphere in Denmark. Ammonium nitrate was a larger source than ammonium sulfate to the SIA mass fraction, where particulate nitrate exceeded that of sulfates by 2.6 times on a molar basis. Indications are that NO2, as a precursor for e.g. ammonium nitrate is probably more important than SO2, as a precursor for ammonium sulfate, in the formation of SIA. In this way, NO2 from anthropogenic emissions and following conversion to particulate nitrates thus appeared to impact semi-rural atmospheric particulate matter in both the Aitken mode and mass wise in PM10.