Here is one method using the dplyr package.
library(dplyr)
#>
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#>
#> filter, lag
#> The following objects are masked from 'package:base':
#>
#> intersect, setdiff, setequal, union
DF <- data.frame(prop25 = c(1,2,3,NA, 5), pm25 = c(11,NA, 22, NA, NA))
DF
#> prop25 pm25
#> 1 1 11
#> 2 2 NA
#> 3 3 22
#> 4 NA NA
#> 5 5 NA
DF <- DF %>% mutate(pm25 = ifelse(is.na(pm25), prop25, pm25))
DF
#> prop25 pm25
#> 1 1 11
#> 2 2 2
#> 3 3 22
#> 4 NA NA
#> 5 5 5
Created on 2021-02-09 by the reprex package (v0.3.0)