In the frame of their own research activities, the partners have developed expertise and activities in the sector of modelling the atmospheric pollution processes, at several spatial and time scales. In particular, during the last 30 years, the above bodies have collaborated in the development and fine-tune of a Lagrangian particle model of transport, diffusion and chemical transformation of the atmospheric pollutants, called "SPRAY". This code was officially born in 1987 by means of a 2-week working session at the EDF laboratories ("Direction des Etudes et Reserche", Chatou, Paris, France) attended by Eng. J. Moussafir (EDF researcher), Dr. D. Anfossi (researcher at CNR-ICGF, Turin), Dr. G. Brusasca (researcher at ENEL-DSR, Milan) and Dr. P. Zannetti (consultant of a USA company); this code version was completed and made operative at "Servizio Ambiente" of ENEL-DSR (Milan, Italy) by Dr. G. Tinarelli, in collaboration with Brusasca and Anfossi. These three researchers constituted the base core of the code development and validation during the first years; in 1990 Prof. E. Ferrero of "Università del Piemonte Orientale" joined the development group and was followed in 1994 by Dr. S. Trini Castelli of CNR-ISAC (formerly ICGF, Turin). J. Moussafir had continuously followed the developments and gave the name SPRAY to the code when he resigned from EDF to found the company ARIA Technologies SA. In 1996-97, Dr. S. Alessandrini carried out his Master Degree thesis on the code, supervised by Tinarelli and Anfossi, and collaborated to the code development by means of the funding of "Fondo di Ricerca per il Sistema Elettrico" as an employee of RSE (formerly "ERSE SpA", formerly "CESI RICERCA SpA", formerly "CESI SpA - ramo d'azienda Ricerca di Sistema-"), where he was hired in 2001. During the same year, Brusasca and Tinarelli funded ARIANET (partner of ARIA Technologies) and kept developing the code. Many  interactions with national and international research groups have provided a relevant number of publications on international journals and new researchers have participated to the development of the code: in particular, in 2010 Dr. L. Mortarini (CNR-ISAC, Turin) joined the team, in 2015 Dr. A. Amicarelli (RSE).