Exploring the vertical profile of atmospheric organic aerosol: comparing 17 aircraft field campaigns with a global model

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<sup>−3</sup> 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 (<i>R</i><sup>2</sup>=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<sup>−1</sup> 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<sup>−1</sup> 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<sup>−1</sup> to 25 kJ mol<sup>−1</sup>). These results illustrate that models may require both additional sources and additional sinks to capture the observed concentrations of organic aerosol

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

“…Monthly mean OA concentrations are archived from a separate v9-02 GEOS-Chem simulation including primary emissions from combustion and secondary production from biogenic and anthropogenic hydrocarbons (Pye et al, 2010). An evaluation of modeled OA against aircraft observations globally is presented in Heald et al (2011), which showed no systematic bias in remote environments but an underestimate of median concentrations in polluted regions. Aqueous-phase concentrations of Hg 0 , O 3 , and HOCl are determined from the gas-phase partial pressures and Henry's law coefficients (Table 1).…”

Impacts of bromine and iodine chemistry on tropospheric OH and HO<sub>2</sub>: comparing observations with box and global model perspectives

“…The model framework includes gas-aerosol partitioning of semi-volatile organic compounds (Liao et al, 2007;Henze et al, 2007Henze et al, , 2009Fu et al, 2008;Heald et al, 2011;Wang et al, 2011), and heterogeneous chemistry (Jacob, 2000). Coupling between gas-phase chemistry and sulfateammonium-nitrate aerosol is described by Park et al (2004) and Pye et al (2009).…”

“…The uptake coefficient for N 2 O 5 uses the parameterisation by Evans and Jacob (2005), while that for HO 2 uses the parameterisation of Thornton et al (2008). A full description of the organic aerosol chemistry in the model is given by Heald et al (2011).…”

Diurnal Variability in Secondary Organic Aerosol Formation over the Indo-Gangetic Plain during Winter Using Online Measurement of Water-Soluble Organic Carbon

“…Given the above mentioned complexity, the primary and secondary fractions of WSOC in atmospheric aerosols are still poorly constrained. Poor understanding on SOA formation on temporal and spatial scale leads to measurement-modeling mismatch (Heald et al, 2011), and among major causes of large uncertainty in the assessment of aerosol effects on air quality and climate.…”