Explicit modeling of organic chemistry and secondary organic aerosol partitioning for Mexico City and its outflow plume

Abstract

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 Correspondence to: J. Lee-Taylor (julial@ucar.edu) 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.

Secondary organic aerosol formation from semi‐ and intermediate‐volatility organic compounds and glyoxal: Relevance of O/C as a tracer for aqueous multiphase chemistry

“…S/IVOCs are oxidized by OH, O 3 , and NO 3 ; peroxy radical intermediates react with NO, NO 2 , NO 3 , HO 2 , or RO 2 , and alkoxy radicals react with O 2 and undergo isomerization and decomposition reactions [Aumont et al, 2005;Camredon et al, 2007]. As in Lee-Taylor et al [2011], the mechanism presented here uses n-alkanes to represent all S/IVOC precursors.…”

“…This framework for SOA formation now includes semivolatile and intermediate volatility organic compounds (S/IVOCs) [Robinson et al, 2007;Grieshop et al, 2009]. Inclusion of S/IVOCs significantly improved predictions of SOA mass in polluted areas [Dzepina et al, 2009[Dzepina et al, , 2011Hodzic et al, 2010;Lee-Taylor et al, 2011], but reveals shortcomings in predicting chemical properties such as the atomic oxygen-tocarbon ratio (O/C ratio) on short aging time scales [Dzepina et al, 2009].…”

Modeling SOA formation from the oxidation of intermediate volatility <i>n</i>-alkanes

“…With this approximation, the computational limits are reached for C 12 hydrocarbons (Aumont et al, 2008). To further decrease the number of species, a lumping protocol has been implemented in GECKO-A (Aumont et al, 2008;Valorso et al, 2011;Lee-Taylor et al, 2011). This lumping protocol is not used in the present study so that the oxidation process described is fully explicit.…”

“…Furthermore, recent numerical simulations have shown that IVOC are likely a substantial source of SOA in the plume of megacities (e.g. Dzepina et al, 2011;Tsimpidi et al, 2010;Hodzic et al, 2010;Li et al, 2011;Lee-Taylor et al, 2011) and at continental scales (Jathar et al, 2011).…”

Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation

“…The missing SOAs can be partly explained by intermediate volatility organic compounds (IVOCs) that may be emitted initially as primary organic aerosol but evaporate upon dilution and are subsequently oxidized in the gas phase, yielding substantial SOA mass (7,8). Recent modeling studies have demonstrated that IVOCs are expected to be a major precursor of SOAs in megacity outflows (9). Long-chain alkanes, mainly emitted from incomplete fuel combustion (10), constitute a substantial fraction of IVOCs.…”