**5.7 Partial carbon dioxide pressure (PCO2)**

Generally, the PCO2 versus saturated atmospheric pressure for the lake is above equilibrium for the study period. However, several peaks (4.5 X 10−2 and 2.5 X 10−2 bar) were observed during the lake color change in November 2014 etc. with corresponding pH decreases (6.71). Similarly, these characteristics where increases in PCO2 are characterized by decrease in pH were observed in Popocapetl volcanic activities [28]. The increased PCO2 and decreased in pH characterized by lake water color change activities suggest low pressure CO2 gas influx or bubbling degassing input and its ionization to form weak carbonic acid.
