How to read and display data from a CO2 logger connected via RS-232?
I'm assuming that you can already read the string into a buffer so I will start at the buffer. Your code should go something like this (you will need to import the string library):

String wholeString = String(yourLoadedCharBuffer);
int startTag = wholeString.indexOf("&lt;co2&gt;");
int endTag = wholeString.indexOf("&lt;/co2&gt;");

String co2FullString = wholeString.substring(startTag+5,endTag-1);


This gives us "5.2527637e2", you add 5 to the start tag because it is 4 characters long and you want to start at the "5", and you subtract 1 from the endtag, because we don't want the first character of it.

int eStart = co2FullString.indexOf("e");
String decimalString = co2FullString.substring(0,eStart-1);
String powerString = co2FullString.substring(estart+1, co2FullString.length-1);

char decimalCharArray[] = decimalString.toCharArray();
char powerCharArray[] = powerString.toCharArray();

float decimal = atof(&amp;decimalCharArray[0]);
float power = atof(&amp;powerCharArray[0]);

float finalCO2Value = pow(decimal,power);