Sources and production of organic aerosol in Mexico City: Insights from the combination of a chemical transport model (PMCAMx-2008) and measurements during MILAGRO

Abstract

Urban areas are large sources of organic aerosols and their precursors. Nevertheless, the contributions of primary (POA) and secondary organic aerosol (SOA) to the observed particulate matter levels have been difficult to quantify. In this study the three-dimensional chemical transport model PMCAMx-2008 is used to investigate the temporal and geographic variability of organic aerosol in the Mexico City Metropolitan Area (MCMA) during the MILAGRO campaign that took place in the spring of 2006. The organic module of PMCAMx-2008 includes the recently developed volatility basis-set framework in which both primary and secondary organic components are assumed to be semivolatile and photochemically reactive and are distributed in logarithmically spaced volatility bins. The MCMA emission inventory is modified and the POA emissions are distributed by volatility based on dilution experiments. The model predictions are compared with observations from four different types of sites, an urban (T0), a suburban (T1), a rural (T2), and an elevated site in Pico de Tres Padres (PTP). The performance of the model in reproducing organic mass concentrations in these sites is encouraging. The average predicted PM1 organic aerosol (OA) concentration in T0, T1, and T2 is 18 μgm-3, 11.7 μgm -3, and 10.5 μgm-3 respectively, while the corresponding measured values are 17.2 μgm.3, 11 μgm.3, and 9 μgm.3. The average predicted locally-emitted primary OA concentrations, 4.4 μgm.3 at T0, 1.2 μgm.3 at T1 and 1.7 μgm-3 at PTP, are in reasonably good agreement with the corresponding PMF analysis estimates based on the Aerosol Mass Spectrometer (AMS) observations of 4.5, 1.3, and 2.9 μgm.3 respectively. The model reproduces reasonably well the average oxygenated OA (OOA) levels in T0 (7.5 μgm-3 predicted versus 7.5 μgm.3 measured), in T1 (6.3 μgm-3 predicted versus 4.6 μgm -3 measured) and in PTP (6.6 μgm-3 predicted versus 5.9 μgm.3 measured). The rest of the OA mass (6.1 μgm-3 and 4.2 μgm-3 in T0 and T1 respectively) is assumed to originate from biomass burning activities and is introduced to the model as part of the boundary conditions. Inside Mexico City (at T0), the locally-produced OA is predicted to be on average 60% locally-emitted primary (POA), 6% semi-volatile (S-SOA) and intermediate volatile (I-SOA) organic aerosol, and 34% traditional SOA from the oxidation of VOCs (V-SOA). The average contributions of the OA components to the locally-produced OA for the entire modelling domain are predicted to be 32%POA, 10%SSOA and I-SOA, and 58% V-SOA. The long range transport from biomass burning activities and other sources in Mexico is predicted to contribute on average almost as much as the local sources during the MILAGRO period. © 2011 Author(s).