# A Geostationary Instrument Simulator for Aerosol Observing System Simulation Experiments
## 2 Methodology and Data
### GEOS-5 Nature Run Aerosol Simulation
#### 2.2.1 Aerosol Sources and Sinks  
The following sources of aerosols were included in the model: (1) wind-speed dependent emissions of dust and sea-salt (for details see [21]), (2) anthropogenic emissions of sulfate and carbonaceous aerosol from fossil fuel combustion, biomass burning, and biofuel consumption (described below), and (3) biogenic emissions of organic carbon (for details see [21]). Oxidation of di-methyl sulfide (DMS) and SO\({}_{2}\), which had anthropogenic and volcanic sources, was an additional chemical production source for sulfate (SO\({}_{4}\)). The aerosol sinks were dry deposition, wet removal, and convective scavenging. Aerosol hygroscopic growth was considered in computations of particle fall velocity and deposition velocity. Sulfate emissions from fossil fuel combustion were taken from the Emissions Database for Global Atmospheric Research v4.2 (EDGAR; [22]), which are provided annually at 0.1\({}^{\circ}\) resolution. For organic and black carbon fossil fuel and biofuel aerosol emissions, which are not included in EDGAR v4.2, a hybrid database was developed using 0.1\({}^{\circ}\) resolution EDGAR-HTAP emissions [23] and 1\({}^{\circ}\) resolution emissions from the Aerosol Comparisons between Observations and Models (AeroCom)[24] Phase II project. EDGAR-HTAP emission totals for 2005 over 1\({}^{\circ}\) areas were adjusted to match yearly AeroCom 1\({}^{\circ}\) emissions estimates for 2005-2007. The relative spatial distribution of 0.1\({}^{\circ}\) resolution EDGAR-HTAP emissions within in each 1\({}^{\circ}\) area was retained. Thus, the resulting emissions database draws global total emissions and inter-annual variability from AeroCom, while using EDGAR-HTAP to achieve a high spatial resolution. SO\({}_{2}\) and SO\({}_{4}\) emissions from ships were derived in the same manner from EDGAR-HTAP and AeroCom emissions. Non-shipping emissions of SO\({}_{2}\) were taken directly from EDGAR v4.1 estimates for 2005 because of errors in AeroCom emissions. DMS emissions from marine algae were based on the monthly varying climatology described in [25]. Biomass burning emissions of OC, BC, and SO\({}_{2}\) were obtained from the Quick Fire Emissions Dataset (QFED) version 2.4-r6 [26]. QFED is based on the fire radiative power (top-down) approach and includes the cloud correction method used in the Global Fire Assimilation System (GFAS) [27]. However, it also employs a more sophisticated treatment of emissions from non-observed land areas. Fire locations and fire radiative power were obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) Level 2 fire products and the MODIS geolocation products. Daily mean fire emissions were created from the MODIS Level 2 fire products re-gridded to 0.1\({}^{\circ}\)\(\times\) 0.1\({}^{\circ}\) horizontal resolution. In the model, a diurnal cycle in fire emissions that is more prominent in the tropics and gradually weakens at higher latitudes in the North hemisphere's temperate zone was imposed on the daily mean emission values. The major chemical production and loss pathways of SO\({}_{2}\), SO\({}_{4}\), and CO were accounted for by the GOCART tropospheric chemical mechanism, which includes gas-phase and aqueous-phase reactions. The chemical mechanism was driven by prescribed monthly mean concentrations of several key trace gases and radicals, namely methane (CH\({}_{4}\)), hydroxyl radical (OH), nitrate radical (NO\({}_{3}\)) and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)). The prescribed fields were generated from a simulation of the NASA Global Modeling Initiative (GMI) model driven by assimilated meteorological data from MERRA (Modern-Era Retrospective Analysis for Research and Applications) [28; 29; 30]. A diurnal variation of OH concentration was computed by scaling the monthly mean fields by the cosine of the solar zenith angle. A diurnal variation of NO\({}_{3}\) was imposed by assuming zero concentration in daylight and the monthly mean concentration at night.