<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 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>