1Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA
2School of Earth, Atmospheric and Environmental Sciences, University of Manchester, UK
3Department of Chemistry and Biochemistry, and CIRES, University of Colorado, Boulder, CO, USA
4School of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, GA, USA
5ESRL Chemical Sciences Division, NOAA, Boulder, CO, USA
6University of California - San Diego, La Jolla, CA, USA
7Department of Atmospheric and Ocean Sciences and Laboratory for Climate and Ocean-Atmosphere Studies, Peking University, Beijing, China
8National Centre for Atmospheric Science, University of Manchester, UK
*now at: Tofwerk AG, Thun, Switzerland
**now at: US EPA, Washington, DC, USA
***now at: US National Academy of Sciences, Washington, DC, USA
Received: 29 Aug 2011 – Discussion started: 12 Sep 2011
Abstract. The global organic aerosol (OA) budget is highly uncertain and past studies suggest that models substantially underestimate observed concentrations. Few of these studies have examined the vertical distribution of OA. Furthermore, many model-measurement comparisons have been performed with different models for single field campaigns. We synthesize organic aerosol measurements from 17 aircraft campaigns from 2001–2009 and use these observations to consistently evaluate a GEOS-Chem model simulation. Remote, polluted and fire-influenced conditions are all represented in this extensive dataset. Mean observed OA concentrations range from 0.2–8.2 μg sm−3 and make up 15 to 70% of non-refractory aerosol. The standard GEOS-Chem simulation reproduces the observed vertical profile, although observations are underestimated in 13 of the 17 field campaigns (the median observed to simulated ratio ranges from 0.4 to 4.2), with the largest model bias in anthropogenic regions. However, the model is best able to capture the observed variability in these anthropogenically-influenced regions (R2=0.18−0.57), but has little skill in remote or fire-influenced regions. The model bias increases as a function of relative humidity for 11 of the campaigns, possibly indicative of missing aqueous phase SOA production. However, model simulations of aqueous phase SOA suggest a pronounced signature in the mid-troposphere (2–6 km) which is not supported in the observations examined here. Spracklen et al. (2011) suggest adding ~100 Tg yr−1 source of anthropogenically-controlled SOA to close the measurement-model gap, which we add as anthropogenic SOA. This eliminates the model underestimate near source, but leads to overestimates aloft in a few regions and in remote regions, suggesting either additional sinks of OA or higher volatility aerosol at colder temperatures. Sensitivity simulations indicate that fragmentation of organics upon either heterogeneous or gas-phase oxidation could be an important (missing) sink of OA in models, reducing the global SOA burden by 15% and 47% respectively. The best agreement with observations is obtained when the simulated anthropogenically-controlled SOA is increased to ~100 Tg yr−1 accompanied by either a gas-phase fragmentation process or a reduction in the temperature dependence of the organic aerosol partitioning (by decreasing the enthalpy of vaporization from 42 kJ mol−1 to 25 kJ mol−1). These results illustrate that models may require both additional sources and additional sinks to capture the observed concentrations of organic aerosol.
Revised: 24 Nov 2011 – Accepted: 06 Dec 2011 – Published: 15 Dec 2011
Heald, C. L., Coe, H., Jimenez, J. L., Weber, R. J., Bahreini, R., Middlebrook, A. M., Russell, L. M., Jolleys, M., Fu, T.-M., Allan, J. D., Bower, K. N., Capes, G., Crosier, J., Morgan, W. T., Robinson, N. H., Williams, P. I., Cubison, M. J., DeCarlo, P. F., and Dunlea, E. J.: Exploring the vertical profile of atmospheric organic aerosol: comparing 17 aircraft field campaigns with a global model, Atmos. Chem. Phys., 11, 12673-12696, doi:10.5194/acp-11-12673-2011, 2011.