Characteristics of submicron aerosol number size distribution and new particle formation events measured in Seoul, Korea, during 2004–2012

The diurnal and seasonal characteristics of the submicron aerosol size distribution measured in Seoul for a long term (2004–2012) were analyzed. The average diurnal variation of aerosol number size distribution showed a single modal distribution but the modal diameter varied around 35 nm and the total aerosol concentration varied diurnally with the maximum during the rush hour. To identify the dominant patterns of the diurnal variation of aerosol number size distribution, the cyclostationary empirical orthogonal function (CSEOF) method was employed. The first CSEOF mode revealed the dominant diurnal pattern that represented an urban life pattern. The principal component (PC) values of the first CSEOF mode showed clear seasonal variation. The PC values were the largest in winter months due to winter monsoon that brought polluted air masses from China, and larger traffic emission amount during the winter season. New particle formation (NPF) events were found to occur on 12.3% of the total measurement days in Seoul when classified with a subjective method. NPF frequency varied seasonally, with the maximum in spring. The second CSEOF mode was found to represent NPF events. A way to identify NPF event days by the CSEOF method was proposed and generally performed well but with some exceptions. The favorable conditions for NPF events in Seoul were found to be low RH, low condensation sink, and low cloud cover, which is consistent with some previous studies. Lastly, the relative contribution of NPF to ultrafine aerosol in Seoul was estimated to be ~32%.