Emissions and concentrations of CO 2 , CO , CH 4 , and H 2 S in exhaust gases from Welch Ranch vents 7 - 12 were measured twice during the campaign , once on 13 May 2009 and again on 15 May 2009 . Results from Wilcoxon signed - rank tests for paired data indicate that median CO 2 emissions were significantly lower ( p b 0.05 ; 51 % lower , on average ) on the second sampling day relative to the first , but that emissions for CO and H 2 S did not significantly differ ( p > 0.05 ) between days . These findings indicate that temporal variations in concentrations and fluxes can be large , even over small , daily timescales .