这样可以吗？
library(rvest)
url='http://www.tianqi.com/air/'
d1=url %>% html_session() %>% html_nodes("li .td-1st") %>% html_text()
d2=url %>% html_session() %>% html_nodes(".td-2nd a[href*='air']") %>% html_text()
qu=url %>% html_session() %>% html_nodes(".td-4rd") %>% html_text()
d3=qu[c(seq(1,length(qu),2))]
d4=url %>% html_session() %>% html_nodes(".td-4rd em") %>% html_text()
data=data.frame(排名=d1,城市=d2,空气质量指数=d3,空气质量=d4,stringsAsFactors=FALSE)
for(i in 1:ncol(data)) Encoding(data[,i])="UTF-8"
write.table(data,file='api.txt',row.names=T,quote=F)
