“…These reactions constitute the first step in the process of photochemical O 3 production in the atmosphere. Total OH reactivity measurements in the urban atmosphere have been conducted in New York City, USA (Ren et al, 2003a(Ren et al, , 2003b, Mexico City, Mexico (Shirley et al, 2006), Mainz, Germany (Sinha et al, 2008), Pearl River Delta, China (Hofzumahaus et al, 2009;Lou et al, 2010), and Houston, USA (Mao et al, 2010). The values of the OH reactivity observed during these measurements vary widely, between 10 and 200 s À1 , which reflects considerable regional diversity in the urban atmosphere.…”