Modeling organic aerosols in a megacity: potential contribution of semi-volatile and intermediate volatility primary organic compounds to secondary organic aerosol formation

Abstract

Abstract. It has been established that observed local and regional levels of secondary organic aerosols (SOA) in polluted areas cannot be explained by the oxidation and partitioning of anthropogenic and biogenic VOC precursors, at least using current mechanisms and parameterizations. In this study, the 3-D regional air quality model CHIMERE is applied to estimate the potential contribution to SOA formation of recently identified semi-volatile and intermediate volatility organic precursors (S/IVOC) in and around Mexico City for the MILAGRO field experiment during March 2006. The model has been updated to include explicitly the volatility distribution of primary organic aerosols (POA), their gas-particle partitioning and the gas-phase oxidation of the vapors. discrepancies with the total OA measurements. Model improvements in OA predictions are associated with the bettercaptured SOA magnitude and diurnal variability. The predicted production from anthropogenic and biomass burning S/IVOC represents 40-60% of the total measured SOA at the surface during the day and is somewhat larger than that from commonly measured aromatic VOCs, especially at the T1 site at the edge of the city. The SOA production from the continued multi-generation S/IVOC oxidation products continues actively downwind. Similar to aircraft observations, the predicted OA/ CO ratio for the ROB case increases from 20-30 µg sm −3 ppm −1 up to 60-70 µg sm −3 ppm −1 between a fresh and 1-day aged air mass, while the GRI case produces a 30% higher OA growth than observed. The predicted average O/C ratio of total OA for the ROB case is 0.16 at T0, substantially below observed value of 0.5. A much better agreement for O/C ratios and temporal variability (R 2 =0.63) is achieved with the updated GRI treatment. Both treatments show a deficiency in regard to POA ageing with a tendency to over-evaporate POA upon dilution of the urban plume suggesting that atmospheric HOA may be less volatile than assumed in these parameterizations. This study highlights the important potential role of S/IVOC chemistry in the SOA budget in this region, and highlights the need for further improvements in available parameterizations. The agreement observed in this study is not sufficient evidence to conclude that S/IVOC are the major missing SOA source in megacity environments. The model is still very underconstrained, andPublished by Copernicus Publications on behalf of the European Geosciences Union.

Climate-forced air-quality modeling at the urban scale: sensitivity to model resolution, emissions and meteorology

“…In annual basis the bias is also small while a larger underestimation is predicted for the summertime season (bias = 2.8 µg m −3 ). The latter can be due to underestimation of summertime emission fluxes (resuspension emissions are not considered in our simulations) and underestimation of secondary organic aerosols formation (Hodzic et al, 2010;Markakis et al, 2014;Solazzo et al, 2012). The overestimation in wind and precipitation also contributes to the observed PM underestimation.…”

Three-dimensional factorization of size-resolved organic aerosol mass spectra from Mexico City

“…This event is similar to a case during a 2003 campaign in Mexico City which has been studied in detail . Although most days in the Mexico City basin have a larger effect of background concentrations and advection, and the dynamic chemical and physical changes that are no doubt occurring in the aerosol (e.g., Hodzic et al, 2010) are less clear when combined with the meteorological effects, these case-study days from 2003 and 2006 are characterized by low initial background concentrations, and lower wind speed and boundary layer, which allow observation of the evolution of the emissions from the city. After midnight on 24 March 2006, the total organic concentration decreased to ∼3 µg m −3 , the lowest concentration measured during the campaign.…”

Sources and Characteristics of Summertime Organic Aerosol in the Colorado Front Range: Perspective from Measurements and WRF-Chem Modeling

“…Similar to other aerosol particles, SOA particles deteriorate air quality and visibility and impact the climate directly through absorption and scattering of radiation and indirectly through interactions with clouds (Monks et al, 2009). Despite recent advances in the measurement and modeling aspects of SOA and their precursors (e.g., Donahue et al, 2006;Ervens and Volkamer 15 2010;Hodzic et al, 2010a;de Gouw et al, 2011;Hodzic and Jimenez 2011;Shrivastava et al, 2011;Ahmadov et al, 2012;Isaacman et al, 2012;Yatavelli et al, 2012;Ehn et al, 2014;Ensberg et al, 2014;Fast et al, 2014;Lopez-Hilfiker et al, 2014;Williams et al, 2014), the full extent of SOA sources, formation processes, and therefore their impact on air quality, human health, and climate are not fully understood.…”

“…8 (Hodzic et al, 2010b;May et al, 2013), to explore the effect of POA volatility, simulations were repeated assuming non-volatile POA. In these runs and regardless of O&G emission treatments, the mean and median POA values increased by a factor of 5, bringing the predicted POA values within a factor of 2 of the PMF-based HOA concentrations.…”