Distinct bimodal size distribution in number concentration and light absorption of sub-500 nm brown carbon particles

Brown carbon (BrC) aerosols impact climate and air quality through light absorption, but their size-resolved characteristics remain unclear. This study employs a novel positive matrix factorization (PMF) approach constrained by light absorption and marker fragment to derive the size distribution of the BrC number concentration and light absorption at high time and size resolutions. Our results show distinct bimodal patterns in the BrC number concentration for the sub-500 nm particles at Mace Head, the west coast of Ireland, with peaks at ~20 nm and 107 nm, attributable to new particle formation (nucleation mode) and subsequent growth processes to the accumulation mode, respectively. Light absorption also exhibited a bimodal distribution, with peaks at 137 nm and increasing values to 484 nm. This difference highlights that the larger particles (e.g. around 484 nm), though fewer in number (~3.6% of the total BrC particles), contribute significantly (~70%) to light absorption due to high mass concentration. The direct solar absorption of BrC relative to black carbon ranges from 1.7% to 4.8%, with a slight increase for particles larger than 100 nm, emphasizing the importance of larger particles in BrC radiative effects. These results offer insights into the size-resolved properties of sub-500 nm BrC, enhancing our understanding of BrC properties and potentially reducing uncertainties in aerosol-radiation interactions.