require(ggplot2)
year=2013
path="C:\\dir\\"
df=read.csv(file = paste0(path, "SAM_global_",toString(year),".csv"))
colnames(df)
for (row in 2:nrow(df)){
  p1 = df[row-1,"Pa"]
  p2 = df[row,"Pa"]
  df[row, "dPa"] = p1-p2
}

library(MASS)
fit <- lm(FCH4MF0~Ta+RH+Pa+Ts_7cm+Ustar+Rain+Ts_2cm+Ts_8cm,data=df)

step <- stepAIC(fit, direction="both")
step$anova
step$coefficients
plot(fit)

require(olsrr)
model <- lm(FCH4MF0~Ta+RH+dPa+Ts_7cm+Ustar+Rain+Ts_2cm+Ts_8cm,data=df)
ols_step_both_aic(model, details = TRUE )

fm = formula(log(FCH4MF0)~Ta+RH+dPa+Ts_7cm+Ustar+Rain+Ts_2cm+Ts_8cm)
model <- lm(fm,data=df)
ols_step_both_aic(model, details = TRUE )