etl_air_quality/
│
├── etl/
│   ├── __init__.py
│   ├── fetch.py          # API request logic
│   ├── transform.py      # Normalization & data cleaning
│   ├── load.py           # PostgreSQL and CSV writing
│   └── spark_load.py     # PySpark integration
│
├── tests/
│   └── test_transform.py # Unit tests for normalization
│
├── main.py               # Entry point
├── requirements.txt
├── config.py             # DB configs
└── README.md