The time of the day is also a very important factor in the amount of photochemical smog present
Photo Courtesy of:  WanLoon and Duffy. "Evnivronmental Chemistry, Global Perspective"
-  early in the morning ( ~ 6 AM), traffic increases the emissions of both nitrogen oxides and VOC's as people drive to work;                                                                                                                                                 
-  later in the morning ( ~ 8-10 AM), traffic dies down and the NOx and volatile organic compounds begin to form NO2, thus increasing its concentration; 
                                                                                                                                 
-  as the sunlight becomes more intense later in the day( ~ 12 AM-4 PM), NO2 is broken down and its by-products form increasing concentrations of ozone; 
                                                                                           
-  at the same time ( ~ 12 AM-4 PM), some of the nitrogen dioxide can react with the VOC to produce toxic chemicals such as PAN;                                                                                                                                                       
-  as the sun goes down ( ~ after 4 PM), the production of smog ceases because many of the reactions are photochemical and need sunlight .
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