制作新的绘图标签时在列表中使用expression

如何解决制作新的绘图标签时在列表中使用expression

我正在尝试将expression（）应用于列表的一部分，以便以ug / m3的形式为3加上上标，以便更改图上的标签：

new_labels <- c("CO" = "Carbon Monoxide (ppb)","NO" = "Nitric Oxide (ppb)","NO2" = "nitrogen dioxide (ppb)","NOX" = "nitrogen Oxides (ppb)","O3" = "Ground-Level Ozone (ppb)","SO2" = "Sulphur dioxide (ppb)","PM" = expression("Fine Particulate Matter(ug/"~m^3~")"))

但是它会自动将expression（）应用于整个列表，而不仅仅是括号内的脚本，并为我提供：

> new_labels
expression(CO = "Carbon Monoxide (ppb)",NO = "Nitric Oxide (ppb)",NO2 = "nitrogen dioxide (ppb)",NOX = "nitrogen Oxides (ppb)",O3 = "Ground-Level Ozone (ppb)",SO2 = "Sulphur dioxide (ppb)",PM = "Fine Particulate Matter(ug/" ~ m^3 ~ ")"))

我想要的输出是：

> print(as.data.frame(new_labels))
                         new_labels
CO            Carbon Monoxide (ppb)
NO               Nitric Oxide (ppb)
NO2          nitrogen dioxide (ppb)
NOX           nitrogen Oxides (ppb)
O3         Ground-Level Ozone (ppb)
SO2           Sulphur dioxide (ppb)
PM  Fine Particulate Matter(ug/m^3)

以3作为上标。