['< sod > it is found on earth and exists in very small concentrations and in the atmosphere at about 1 ppb. < ref name = " owenpickering1997 " > { { cite book | author = owen, lewis a. ; pickering, kevin t | title = an introduction to global environmental issues | url = https : / / books. google. com / books? id = 6nvxmjonu38c & pg = pa33 | year = 1997 | publisher = taylor & francis | isbn = 978 - 0 - 203 - 97400 - 1 | pages = 33 – } } < / ref > < ref name = " taylorsimpson1987 " > { { cite journal | last1 = taylor | first1 = j. a. | last2 = simpson | first2 = r. w. | last3 = jakeman | first3 = a. j. | title = a hybrid model for predicting the distribution of sulphur dioxide concentrations observed near elevated point sources | journal = ecological modelling | volume = 36 | issue = 3 – 4 | year = 1987 | pages = 269 – 296 | issn = 0304 - 3800 | doi = 10. 1016 / 0304 - 3800 ( 87 ) 90071 - 8 } } < / ref > < eod > < sod > it is found on earth and exists in very small concentrations and in the atmosphere at about 1 ppb. < ref name = " owenpickering1997 " > { { cite book | author = owen, lewis a. ; pickering, kevin t | title = an introduction to global environmental issues | url = https : / / books. google. com / books? id = 6nvxmjonu38c & pg = pa33 | year = 1997 | publisher = taylor & francis | isbn = 978 - 0 - 203 - 97400 - 1 | pages = 33 – } } < / ref > < ref name = " taylorsimpson1987 " > { { cite journal | last1 = taylor | first1 = j. a. | last2 = simpson | first2 = r. w. | last3 = jakeman | first3 = a. j. | title = a hybrid grid model for predicting the distribution of sulphur dioxide concentrations observed near elevated point sources | journal = ecological modelling | volume = 36 | issue = 3 – 4 | year = 1987 | pages = 269 – 296 | issn = 0304 - 3800 | doi = 10. 1016 / 0304 - 3800 ( 87 ) 90071 - 8 } } < / ref > { { clarify | reason = not clear where, when, how much variability in time or space | date = march 2016 } } < eod >']