Subscript in plot title in r

Question

I would like to add the title, " Emissions from PM2.5 in the United States from 1999 to 2008" which uses the base plotting function in r. In that, I would like 2.5 to be a subscript to PM. I don't have any problem doing that if PM2.5 happens to be at the end of the string:

barplot(height = total.emissions$Emissions, names.arg=total.emissions$year,  
        xlab="Year", ylab= " Amount of emitted in tonsPM"2.5 ,   
        main = "Emissions from in the United States from 1999 to 2008PM"[2.5] )

But I can't do the same if it is in the middle of the string. If I split it into 2 parts as follows:

barplot(height = total.emissions$Emissions, names.arg=total.emissions$year,  
        xlab="Year", ylab= " Amount of PM_[2.5] emitted in tons",   
        main = expression("Emissions from PM"[2.5] "in the United States from 1999 to 2008"))

I get an error saying unexpected symbol because of the square brackets.

Answer 1

尝试使用paste函数进行expression （有关详细信息，请参阅?plotmath ），例如：

plot(0, main = expression(paste("Emissions from ", PM[2.5], " in the United States from 1999 to 2008")))