Modeling organic aerosols during MILAGRO: importance of biogenic secondary organic aerosols

Abstract

Abstract. 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 contain 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 one-step oxidation of anthropogenic (i.e. aromatics, alkanes), biogenic (i.e. monoterpenes and isoprene), and biomass-burning SOA precursors and their partitioning into both organic and aqueous phases. Conservative assumptions are made for uncertain parameters to maximize the amount of SOA produced by the model. 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, with a factor of 2–10 model underestimate. When only anthropogenic SOA precursors were considered, the model was able to reproduce within a factor of two the sharp increase in OOA concentrations during the late morning at both urban and near-urban locations but the discrepancy increases rapidly later in the day, consistent with previous results, and is especially obvious when the column-integrated SOA mass is considered instead of the surface concentration. The increase in the missing SOA mass in the afternoon coincides with the sharp drop in POA suggesting a tendency of the model to excessively evaporate the freshly formed SOA. Predicted SOA concentrations in our base case were extremely low when photochemistry was not active, especially overnight, as the SOA formed in the previous day was mostly quickly advected away from the basin. These nighttime discrepancies were not significantly reduced when greatly enhanced partitioning to the aerosol phase was assumed. Model sensitivity results suggest that observed nighttime OOA concentrations are strongly influenced by a regional background SOA (~1.5 μg/m3) of biogenic origin which is transported from the coastal mountain ranges into the Mexico City basin. The presence of biogenic SOA in Mexico City was confirmed by SOA tracer-derived estimates that have reported 1.14 (±0.22) μg/m3 of biogenic SOA at T0, and 1.35 (±0.24) μg/m3 at T1, which are of the same order as the model. Consistent with other recent studies, we find that biogenic SOA does not appear to be underestimated significantly by traditional models, in strong contrast to what is observed for anthropogenic pollution. The relative contribution of biogenic SOA to predicted monthly mean SOA levels (traditional approach) is estimated to be more than 30% within the city and up to 65% at the regional scale which may help explain the significant amount of modern carbon in the aerosols inside the city 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 as 10–20 tons within the Mexico City metropolitan area during the daily peak), and emphasize for the first time the role of biogenic precursors in this region, indicating that they cannot be neglected in urban modeling studies.

An overview of the MILAGRO 2006 Campaign: Mexico City emissions and their transport and transformation

“…During the high BB periods both datasets reported increases of modern carbon of about 15% of the total OC , which is consistent with the estimates from several other apportionment techniques as discussed above. A substantial fraction of non-fossil organic carbon (≥37%, ) is present during very low wildfire periods, which suggests the importance of urban sources of modern carbon such as food cooking or biofuel use (Hildemann et al, 1994;Mugica et al, 2009;Christian et al, 2010) and/or regional sources such as biogenic SOA (e.g., Stone et al, 2010a;Hodzic et al, 2009) and others. Hodzic et al (2010b) present the first comparison of modern carbon measurements with the predictions of a 3-D model, and find that the observed concentrations are similar to the measurements of Aiken et al (2010) but lower than those of Marley et al (2009a).…”

“…The surface meteorological network, boundary layer measurements, and radar wind profiler measurements have also been used to examine the performance of meteorological predictions made by coupled meteorological-chemistryparticulate models, such as WRF-Chem Tie et al, 2009;Zhang et al, 2009a;Hodzic et al, 2009), to understand how forecast errors in meteorology affect predictions of trace gases and particulate matter. In general, the synoptic-scale circulations are simulated reasonably well by mesoscale models, although the details (i.e., timing and strength) of local and regional winds affected by terrain variations around Mexico City are more difficult to reproduce by models.…”

Diurnal variations of polar organic tracers in summer forest aerosols: A case study of a Quercus and Picea mixed forest in Hokkaido, Japan

“…Recently, Jaoui et al (2010) reported two new nitrogen-containing compounds (the structures are not identified) from isoprene oxidation under acidic conditions, which could be potentially served as tracers for isoprene SOA formation. In addition, highly time-resolved characterization of organic aerosol by using aerosol mass spectroscopy (AMS) (e.g., Hodzic et al, 2009) has been often reported in urban and rural sites as well as a continuous measurement of water-soluble organic carbon (WSOC) coupled to a particle-into-liquid sampler (PILS) (Miyazaki et al, 2006). These studies provide insights into the sources and processes that influence SOA production and their spatial and seasonal distributions.…”

The composition and variability of atmospheric aerosol over Southeast Asia during 2008

“…Over the study region, our calculations show that SOA contributes approximately 23.5 % to total OA. Our model SOA contribution is significantly lower than those reported for other remote locations (Jimenez et al, 2009) and this model issues will be explored in sensitivity calculations. There is an approximate balance between sources and sinks of BC and of OA.…”

“…By increasing the gas-particle partitioning coefficients of all SOA by a factor [%] of 100, we account for subsequent generations of oxidations that further lower their volatility. A similar factor was necessary to match model and observed organic aerosol over the Amazon basin (Hodzic et al, 2009). Table 8 shows that for this calculation we see the largest increases over tropical ecosystems, as expected, where the emissions of BVOCs and resulting production of SOA is largest.…”