Explicit modeling of organic chemistry and secondary organic aerosol partitioning for Mexico City and its outflow plume J. Lee-Taylor1, S. Madronich1, B. Aumont2, A. Baker3,*, M. Camredon2, A. Hodzic1, G. S. Tyndall1, E. Apel1, and R. A. Zaveri4 1National Center for Atmospheric Research, Boulder, Colorado, USA 2LISA, UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Créteil, France 3University of California, Irvine, CA, USA 4Pacific Northwest National Laboratory, Richland, Washington, USA *now at: Max Planck Institute for Chemistry, Mainz, Germany
Abstract. The evolution of organic aerosols (OA) in Mexico City and its outflow is
investigated with the nearly explicit gas phase photochemistry model GECKO-A
(Generator of Explicit Chemistry and Kinetics of Organics in the
Atmosphere), wherein precursor hydrocarbons are oxidized to numerous
intermediate species for which vapor pressures are computed and used to
determine gas/particle partitioning in a chemical box model. Precursor
emissions included observed C3-10 alkanes, alkenes, and light aromatics, as
well as larger n-alkanes (up to C25) not directly observed but estimated by
scaling to particulate emissions according to their volatility. Conditions
were selected for comparison with observations made in March 2006 (MILAGRO).
The model successfully reproduces the magnitude and diurnal shape for both
primary (POA) and secondary (SOA) organic aerosols, with POA peaking in the
early morning at 15–20 μg m−3, and SOA peaking at
10–15 μg m−3 during mid-day. The majority (≥75%) of the model SOA stems
from reaction products of the large n-alkanes, used here as surrogates for
all emitted hydrocarbons of similar volatility, with the remaining SOA
originating mostly from the light aromatics. Simulated OA elemental
composition reproduces observed H/C and O/C ratios reasonably well, although
modeled ratios develop more slowly than observations suggest. SOA chemical
composition is initially dominated by δ-hydroxy ketones and nitrates
from the large alkanes, with contributions from peroxy acyl nitrates and, at
later times when NOx is lower, organic hydroperoxides. The simulated
plume-integrated OA mass continues to increase for several days downwind
despite dilution-induced particle evaporation, since oxidation chemistry
leading to SOA formation remains strong. In this model, the plume SOA burden
several days downwind exceeds that leaving the city by a factor of >3.
These results suggest significant regional radiative impacts of SOA.
Citation: Lee-Taylor, J., Madronich, S., Aumont, B., Baker, A., Camredon, M., Hodzic, A., Tyndall, G. S., Apel, E., and Zaveri, R. A.: Explicit modeling of organic chemistry and secondary organic aerosol partitioning for Mexico City and its outflow plume, Atmos. Chem. Phys., 11, 13219-13241, doi:10.5194/acp-11-13219-2011, 2011.