To convert CH4 emissions to CO2 equivalence I multiply the emissions x GWP. but what units must my emissions be in, kg? they're currently: mg m-2 d-1 . How can I convert the current units to the correct ones ?
Use Global Warming Potential (GWP) as conversion factor from CH4 to CO2eq. Be careful in choosing the value assigned to GWP as it changes according to the time horizon you consider (typically the reference is 100 years). Check sources in IPCC assessment reports, the Environmental Protection Agency (EPA) or other scientific literature to find the currently established values along with their academic/governmental reference.