Investigation of organic carbon profiles and sources of coarse PM in Los Angeles

Source apportionment analyses are essential tools to determine sources of ambient coarse particles (2.5 <dₚ < 10 μm) and to disentangle their association and contribution from other pollutants, particularly PM₂.₅ (<2.5 μm). A semi-continuous sampling campaign was conducted using two virtual impactors/concentrators to enhance coarse particulate matter concentrations coupled with an online thermal-optical EC/OC monitor to quantify coarse PM–bound organic carbon volatility fractions (OC₁-OC₄) in central Los Angeles during the winter, spring, and summer of 2021. The total OC and its volatility fraction concentrations, meteorological parameters (i.e., wind speeds and relative humidity), vehicle miles traveled (VMT), and gaseous source tracers (i.e., O₃ and NO₂) were used as inputs to positive matrix factorization (PMF) model. A 3-factor solution identified vehicular emissions (accounting for 46% in the cold phase and 26% in the warm phase of total coarse OC concentrations), secondary organic carbon (27% and 37%), and re-suspended dust (27% and 37%) as the primary organic carbon sources of coarse PM. The re-suspended dust factor showed a higher contribution of more volatile organic carbons (i.e., OC₁ up to 77%) due to their re-distribution on dust particles, whereas the SOA factor was the dominant contributor to less volatile organic aerosols (i.e., OC₄ up to 54%), which are the product of reactions at high relative humidity (RH). Our findings revealed that the total OC concentrations in the coarse size range were comparable with those of previous studies in the area, underscoring the challenges in curtailing coarse PM-bound OC sources and the necessity of developing effective emission control regulations on coarse PM. The results from the current study provide insights into the seasonal and temporal variation of total OC and its volatility fractions in Los Angeles.