If HK's poor air quality is caused by locally-produced pollution why does the air quality in HK significantly improve during Mainland Chinese National Holidays

If HK's poor air quality is caused by locally-produced pollution why does the air quality in HK significantly improve during Mainland Chinese National Holidays (when factories in Southern China shut down but HK's so called pollution-generators like public transport, shipping and power stations remain operating as per normal)? I agree that roadside pollution levels especially in Causeway Bay and Mongkok are most likely caused by local transport, but the clear-air and high visibility during Golden weeks and CNY suggest significant contributions from north of the border.