Air quality observations are real measurements of the concentration of pollutants in the air obtained from a monitoring instrument. Depending on how far from the instrument's emission sources these measurements are, they display the air pollution over a more or less extended area. For instance, instruments located very close to a trafficked street in the city are called “urban traffic stations” and provide information on the very local air pollution, which may be strongly affected by vehicle exhausts. Conversely, instruments located further away from direct pollution emission sources, for example, in large urban public parks, are called “urban background stations” and provide information on the air pollution affecting the surrounding neighbourhood. 

Official air quality monitoring stations have high installation and maintenance costs; hence, only a limited number of them focus on a few harmful chemical species that can be deployed in cities. This means that air quality observations in urban areas are quite dispersed.

Nevertheless, to provide more thorough information on air pollution, scientists rely on air quality modelling systems that mathematically simulate how pollutants disperse in the air. Such modelling systems can be used to generate air quality forecasts.

Air quality forecasts are predictions of the future concentrations of air pollutants expected over the coming days in a specific region. Forecasts are obtained by running air quality modelling systems that use estimations of expected pollutant emissions and meteorological forecasts. In the CALIOPE forecasting system, air quality forecasts are typically calculated hourly for large geographical areas using a spatial resolution of one to a few kilometres. This is why, to be computed, air quality forecasts require huge computational resources.

To produce air quality forecasts, the CALIOPE modelling system uses and combines different models to estimate anthropogenic and biogenic emissions, as well as atmospheric and weather conditions:

1. WRFv3.5.1
   The Weather Research and Forecasting Model (WRF) is an open-source numerical weather prediction model that simulates meteorological conditions, such as wind, temperature, and vertical mixing. The software is the default meteorological driver of CALIOPE and is supported and maintained by the National Center for Atmospheric Research (USA).
2. CMAQv5.0.2
   The Community Multiscale Air Quality Modeling System (CMAQ) is an offline and open-source atmospheric chemistry transport model used to predict the diffusion of chemical species influencing weather and atmospheric processes. CMAQ is supported and distributed by the Community Modeling and Analysis System (CMAS) Center.
3. HERMESv3
   The High-Elective Resolution Modelling Emission System (HERMES) is an open-source and multi-scale atmospheric emission modelling framework that computes anthropogenic gaseous and aerosol emissions for use in atmospheric chemistry models. The model is supported and maintained by the BSC-CNS. The Model of Emissions of Gases and Aerosols from Nature (MEGAN), which is supported and maintained by the University of California, Irvine (UCI), is implemented in the HERMES code for the estimation of the biogenic emissions from terrestrial ecosystems. 
4. MONARCHv2.0 

The Multiscale Online Nonhydrostatic AtmospheRe CHemistry model (MONARCH) is an advanced chemistry and aerosol modelling system representing the atmospheric chemistry cycle, including emission, transport, and deposition along with aerosol–radiation interactions. The dust component simulates and predicts the dust cycle in the atmosphere with the aim of estimating desert dust's contribution to aerosol diffusion. The BSC-CNS supports and maintains the model and provides operational dust forecasts for the World Meteorological Organization Barcelona Dust Regional Center. Since September 2023, MONARCHv2.0 dust forecasts have replaced the previous BSC-DREAM system to represent the dust component in the CALIOPE system.

Note:
For its regional calculations, CALIOPE is initialised using global meteorological data from the Global Forecasting System (GFS) model of the National Centre for Environmental Prediction (NCEP), while boundary conditions for pollutants and aerosols come from the CAMS Global atmospheric composition forecast, operated by the European Centre for Medium-Range Weather Forecasts (ECMWF).

To assess the quality of the results, CALIOPE forecasts are compared with the observed air quality measures obtained by monitoring instruments in the region of interest. For this comparison, the system uses observed data from the European Environment Information and Observation Network (EIONET), which is managed by the European Environment Agency (EEA).

CALIOPE forecasts’ numerical datasets are not available to download from the website. The provision of numerical data or any other type of query that involves adapting our products to the specific needs of users can be accommodated under collaborations in the framework of working contracts or scientific projects. Should you need further information, please contact us through the contact form.

1. Images from CALIOPE may be used solely for research, educational, and other not-for-profit purposes. If you link to any content of the CALIOPE website on external web pages, we kindly ask you to inform us through the contact form.
2. The CALIOPE team cannot guarantee that the data are correct in all circumstances. We neither accept any liability for any error or omission in the data or any loss or damage arising from its use.
3. Articles, papers, or written scientific works of any form based in whole or in part on images or other products supplied by CALIOPE must contain an acknowledgement. Below there is a citation as an example which should be adapted accordingly:

"Images/air quality forecasts are provided by the CALIOPE air quality forecasting system from the Barcelona Supercomputing Center-Centro Nacional de Supercomputación (BSC-CNS).”

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Technical information about the CALIOPE modelling system can be found in the publications list.