Analysis of Submicron Aerosol Number Size Distribution in Seoul Using the Cyclostationary Empirical Orthogonal Functions

Aerosols are known to play an important role in climate change directly by scattering or absorbing solar radiation and indirectly by acting as cloud condensation nuclei (CCN) and thus altering cloud radiative properties. Whether aerosols can act as CCN or not depends on the size and chemical composition of aerosol particles. Aerosol number size distribution varies with time and especially when a new particle formation (NPF) event occurs, we can witness the growth of the particles. Since this is a major mechanism that can produce CCN active particles, understanding NPF mechanism is crucially important.

Here we analyze the Scanning Mobility Particle Sizer (SMPS) data measured during 2004-2012 at the Yonsei University campus (37.6^oN, 127.0^oE) located near the center of Seoul, a highly urban capital city of South Korea. The mobility diameter range was 10-450 nm and the measurement was almost continuous except when the instrument was deployed for field measurements or not operational due to repair.

For cyclostationary processes such as the diurnal variation of the aerosol size distribution, CycloStationary Empirical Orthogonal Function (CSEOF) method is useful for identifying dominant patterns and therefore this method is employed here. The first and second CSEOF modes suggest the seasonal variation of aerosol number size distribution and NPF events, respectively. There are clear seasonal and diurnal variations of aerosol number concentration and size distribution and they are found to be due to seasonal monsoon and traffic emission.

Overall the relative frequency of NPF events is the highest in spring and the average for the whole year is 12.3%. Based on the Principal Component (PC) time series of the second CSEOF mode, we classify NPF and non-NPF days. When NPF events occur, relative humidity and condensation sink are generally lower than the average values. This may suggest that high relative humidity and condensation sink suppress NPF in Seoul. More will be discussed at the conference.