ggplot axis labels with superscript and subscript

super-and-sub-script-labels.R

AntroSO42<-read.csv("antroSO42-.csv", header = TRUE)
Bp <- AntroSO42[ ,(2:4), ]

library(tidyverse)
Bp %>% 
  gather(value, variable, -Class) %>% 
  ggplot(aes(Class, 
             variable)) +
  geom_boxplot() +
  facet_wrap( ~ value) +
  ylab(expression(Anthropogenic~SO[4]^{"2-"}~(ngm^-3))) + 
  theme_bw(base_size = 16)