) data(airquality) contains measurements of ozone concentration in New York from May to September 1983, together with other relevant variables.

(a) Plot the ozone concentration over time. Look at how different plotting types ("l", "h") affect the appear- ance. The EPA standard for ozone until recently was 120ppb, 140ppb was moderate nonattainment and 160ppb serious nonattainment. Indicate these with horizontal lines in appropriate colors. Use the text() function to annotate the severe ozone days with their dates.

(b) Ozone is produced by chemical reactions in the air that require sunlight. Draw a scatterplot of ozone and solar radiation.

(c) Perhaps wind or temperature are responsible for the shape of the plot. Use the coplot function to draw scatterplots of ozone and solar radiation for different levels of wind speed, of temperature, and of both at once. Do the same thing with the trellis command xyplot.