In 2015, the annual mean concentration of fine particulate matter (PM2.5\text{PM}_{2.5}PM2.5) in the atmosphere of a major industrial city was 84.3 μg/m384.3\ \mu\text{g/m}^384.3 μg/m3. By 2023, following the implementation of strict clean air regulations, this concentration had decreased to 32.8 μg/m332.8\ \mu\text{g/m}^332.8 μg/m3.
Calculate the percentage decrease in PM2.5\text{PM}_{2.5}PM2.5 concentration between 2015 and 2023.
Give your answer to two decimal places.
You must show your working.
