Modeling Organic Aerosols during MILAGRO: Application of the CHIMERE Model and Importance of Biogenic Secondary Organic Aerosols
The meso-scale chemistry-transport model CHIMERE is used to assess our understanding of major sources and formation processes leading to a fairly large amount of organic aerosols [OA, including primary OA (POA) and secondary OA (SOA)] observed in Mexico City during the MILAGRO field project (March 2006). Chemical analyses of submicron aerosols from aerosol mass spectrometers (AMS) indicate that organic particles found in the Mexico City basin have a large fraction of oxygenated organic species (OOA), which have strong correspondence with SOA, and that their production actively continues downwind of the city. The SOA formation is modeled here by the first-generation oxidation of anthropogenic (i.e., aromatics, alkanes) and biogenic (i.e., monoterpenes and isoprene) precursors and their partitioning into both organic and aqueous phases. The near-surface model evaluation shows that predicted OA correlates reasonably well with measurements during the campaign, however it remains a factor of 2 lower than the measured total OA. Fairly good agreement is found between predicted and observed POA within the city suggesting that anthropogenic and biomass burning emissions are reasonably captured. Consistent with previous studies in Mexico City, large discrepancies are encountered for SOA species, with a factor of 5-10 model underestimate. When only anthropogenic SOA precursors were considered, the model was able to reproduce within a factor of two the sharp increase in SOA concentrations during the late morning at both urban and near-urban locations. However, predicted SOA concentrations were unrealistically low when photochemistry was not active, especially overnight. These nighttime discrepancies were not significantly reduced when greatly enhanced partitioning to the aerosol phase was assumed. Model sensitivity results suggest that observed nighttime SOA concentrations are strongly influenced by the regional background (~2µg/m3) from biogenic origin, which is transported from the coastal regions into the Mexico City basin. The relative contribution of biogenic SOA to monthly mean SOA levels is estimated to be more than 30% within the city and up to 65-90% at the regional scale (even in the immediate vicinity of the city), which is consistent with measurements of modern carbon during low biomass burning periods. The anthropogenic emissions of isoprene and its nighttime oxidation by NO3 were also found to enhance the SOA mean concentrations within the city by an additional 15%. Our results confirm the large underestimation of the SOA production by traditional models in polluted regions (estimated to 10-20 Tons within the Mexico City metropolitan area during the daytime), and emphasize for the first time the role of biogenic precursors in this region, indicating that they cannot be neglected in modeling studies.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 973983
- Report Number(s):
- PNNL-SA-66066; KP1701000; TRN: US201007%%166
- Journal Information:
- Atmospheric Chemistry and Physics, 9(18):6949-6981, Vol. 9, Issue 18
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
AEROSOLS
ALKANES
AROMATICS
BIOMASS
CARBON
COASTAL REGIONS
EVALUATION
ISOPRENE
MASS SPECTROMETERS
MEXICO
ORIGIN
OXIDATION
PHOTOCHEMISTRY
PRODUCTION
SENSITIVITY
SIMULATION
URBAN AREAS
secondary organic aerosol
MILAGRO field campaign
Mexico City
air quality modeling
isoprene chemistry
AMS data