On December 31, 2019, the Chinese authorities reported to the World Health Organization (WHO) the outbreak of a new strain of coronavirus that causes a serious disease in the city of Wuhan, China. This outbreak was classified as SARS-CoV2 and is the cause of the COVID-19 disease. On March 11, 2020, the WHO declares it a Pandemic and today it is considered the greatest challenge in global health that humanity has faced since World War II and it is estimated that between 40 and 60% of the population worldwide will catch the virus. This has caused enormous challenges in countries around the world in social, economic, environmental and obviously health issues. These challenges are mainly due to the effects of the established quarantines in almost all capitals and major cities around the world, from Asia, Europe to America. However, these lockdown which began worldwide from January 23, have had a significant impact on the environment and on the air quality of cities as recently reported by NASA (National Aeronautics and Space Administration) and ESA (European Space Agency), with reductions according to them of up to 30% in some of the epicenters such as the case of Wuhan. Knowing that air pollution causes approximately 29% of lung cancer deaths, 43% of COPD deaths, and 25% of ischemic heart disease deaths, it is important to know the effects of quarantines in cities regarding air quality to take measures that favor populations and urban ecosystems when the emergency ends. Therefore, this paper describes the behavior of PM 2.5 emissions particulate matter from the 50 most polluted capital cities in the world according to the WHO, measured before-after the start of the quarantine. Likewise, the impact at the local and global level of this emissions behavior, which averaged 12% of PM 2.5 decrease in these cities.

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Currently, developed countries have been implementing sustainable development policies specifically in Sustainable Consumption and Production (SCP). Therefore, companies consider the products and services life cycle, where Green Procurement (GP) represents a key tool for sustainability in the Supply Chain (SC). Moreover, the future on green policies in countries like Colombia has a long way to go on issues such as energy production, consumption and processes within the supply chain. In the GP area, there are three relevant issues in sustainable development policies in Colombia: 1) issues linked to barriers in the implementation such as tariffs, taxes, incentives and promotion; 2) motivation and drivers for the implementation for both public and private sector and 3) performance impacts related to the models used in order to implement in companies. The latter one is the main interest of this paper. Regarding this work, it identifies the mathematical models most widely used in SC processes, especially the issues associated with purchase processes, selection and hiring of suppliers in large and medium-sized enterprises; public or private. Moreover, the paper analyzes the models regarding their advantages and disadvantages on which some of these can be used in GP processes due its high impact. Finally, a possible mathematical model used in GP processes based on petri nets (PN) for to their characteristics and application in complex systems.