Improved Atmospheric Composition Forecasting In GEOS-CF Version 2

The GMAO recently released a major update of its Composition Forecast system (GEOS-CF, version 2), substantially improving its global atmospheric composition forecasting capabilities, and providing users and with better predictions of key atmospheric constituents. Here we give an overview of the capabilities and performance of the new version.

The GEOS Composition Forecasting (GEOS-CF; Keller et al., 2021; Knowland et al., 2022) system provides daily estimates and 5-day forecasts of global atmospheric composition at 25-km horizontal resolution, extending from Earth's surface to approximately 80 km in altitude. The system embeds the GEOS-Chem model within GMAO’s GEOS model, a capability built on the long-standing collaboration between GMAO and the GEOS-Chem community.

GEOS-CF data support a variety of research and applications, including NASA satellite missions, aircraft campaigns and instrument teams, local and regional air quality forecasters, and environmental health practitioners (Malings et al., 2026). Consistent with NASA's open science principles, the system uses open-source software and provides open access to data products through multiple channels, including direct file access, interactive visualization, an Application Programming Interface (API), and the Google Earth Engine; see GEOS-CF Data Access for details. GEOS-CF also contributes to NASA’s Earth Science to Action mission to address societal challenges by supporting air quality monitoring and management in many areas of the world.

System Updates

GEOS-CF v2 includes improved atmospheric physics and chemistry, more recent emissions data, direct assimilation of satellite ozone data, and the addition of carbon dioxide (CO2) and methane (CH4) forecasts. These changes are summarized in Table 1 and described in the updated GEOS-CF File Specifications. The File Specifications also lists changes in the file naming convections and archived fields between the two versions. The CO2 and CH4 output are under evaluation prior to public release.

Evaluation of Tropospheric Composition

The updates introduced in GEOS-CF v2 have led to substantial improvements in the simulated atmospheric composition. We highlight key results, focusing on tropospheric ozone and PM_2.5, two species of prime importance for tropospheric composition and air quality.

Figure 1: Tropospheric ozone distribution from ozonesondes and GEOS-CF. The left panels show annual mean ozone columns between the surface and 300 hPa from the observations and GEOS-CF v2. The values are shown in Dobson Units (DU; 1 DU ≡ 2.69×10¹⁶ molecules cm^-2). The right panels show the ozone profiles for the northern extratropics and from the observations and from GEOS-CF v2, GEOS-CF v1, and a GEOS-CF v2 run without ozone data assimilation. The ozonesonde observations are from the SHADOZ, WOUDC, and NOAA networks.

Figure 2 compares the distribution of surface ozone over the US from GEOS-CF and EPA air quality observations. At each site, it shows the annual 95th percentile of the maximum daily 8-hour average (MDA8), a metric closely tied to ozone air quality impacts and policies. Peak MDA8 ozone values typically occur in summer afternoons, during sunny, stagnant conditions that favor photochemical production of ozone. GEOS-CF v2 reproduces the observed spatial patterns, with higher ozone values on the east coast, the Midwest, and California, but the values are uniformly biased high by about 10 ppbv, likely reflecting uncertainties in boundary layer mixing and dry deposition in the model. Still, GEOS-CF v2 shows a marked improvement in ozone compared to GEOS-CF v1, as seen in the probability distribution of MDA8 ozone values. In GEOS-CF v1, MDA8 ozone values exceeded 80 ppbv at 25% of the sites, but such values are hardly present in the observations and GEOS-CF v2. The improvement in GEOS-CF v2 surface ozone is largely due to the update to anthropogenic emissions of nitrogen oxides (NO_x) and volatile organic compounds (VOC).



Figure 3: Daily mean PM_2.5 concentrations over the US in 2023. The maps show the observed values from the EPA Air Quality Service (AQS) monitors, and those from GEOS-CF v2. The right panel shows the monthly variation in average PM_2.5 for all sites from the observations, GEOS-CF v2, and GEOS-CF v1.