There is PKGBUILD in AUR. The simplest way to install is using yaourt:
yaourt -S co2mon-git
co2mon packages can be installed from official repo:
dnf install co2mon
# macOS
brew install cmake pkg-config hidapi
# Ubuntu
apt-get install cmake g++ pkg-config libhidapi-dev
mkdir build
cd build
cmake ..
make
./co2mond/co2mond