The Country Level Air Quality Calculator (CLAQC) is an open-source modeling tool that utilizes national sectoral emissions and weather data to forecast monthly and annual concentrations of PM2.5 and O3. CLAQC leverages the recent advancements in the CAMS system, employing CAMS global gridded emissions and CAMS reanalysis pollutant concentrations to improve the accuracy of its predictions. One of the notable strengths of CLAQC is its ability to provide country-specific and sectoral information. We have developed two methodological approaches, namely elastic net modeling and extreme gradient boosting regressor, that can effectively predict annual average concentrations for nearly all countries. Although both methods show good performance for the country's yearly average, the sectoral contributions are not robust enough for the elastic net models. The tool can simulate a vast range of policy scenarios and can be integrated into national policy assessment and optimization frameworks. Finally, we present a method selection framework for each country to optimize performance, and an online tool displaying model results.