The depletion in atmospheric ozone layer in last century was due to :

The depletion of the stratospheric ozone layer during the last century was primarily caused by chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODS) [3][5]. CFCs are highly stable in the lower atmosphere, allowing them to drift into the stratosphere where intense ultraviolet radiation breaks them down, releasing chlorine atoms [1][2]. A single chlorine atom can destroy over 10,000 to 100,000 ozone molecules through a catalytic chain reaction [1][4]. This scientific discovery in the 1970s and the subsequent observation of the Antarctic 'ozone hole' in the 1980s led to the 1987 Montreal Protocol, which aimed to phase out the production and consumption of these harmful chemicals [3]. While other gases like nitrous oxide also contribute to depletion, CFCs were the primary culprit for the significant thinning observed in the 20th century [6].

Sources

1. [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC1817831/
2. [5] https://www.sciencedirect.com/topics/economics-econometrics-and-finance/ozone-pollution
3. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > ozone depletion > p. 13
4. [2] https://www.epa.gov/ozone-layer-protection/basic-ozone-layer-science
5. [4] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Ozonosphere > p. 276
6. [6] https://research.noaa.gov/the-montreal-protocol-banned-this-family-of-ozone-depleting-chemicals-why-are-some-still-increasing/