Add R scipts and plot images.

Plot1.R >
Have total emissions from $PM_{2.5}$ decreased in the United States from 1999 to 2008? Using the **base** plotting system, make a plot showing the total $PM_{2.5}$ emission from all sources for each of the years 1999, 2002, 2005, and 2008.

Plot2.R >
Have total emissions from $PM_{2.5}$ decreased in the **Baltimore City**, Maryland (``fips == "24510"``) from 1999 to 2008? Use the base plotting system to make a plot answering this question.

Plot3.R >
Of the four types of sources indicated by the ``type`` (point, nonpoint, onroad, nonroad) variable, which of these four sources have seen decreases in emissions from 1999-2008 for **Baltimore City**? Which have seen increases in emissions from 1999-2008? Use the **ggplot2** plotting system to make a plot answer this question.

Plot4.R >
Across the United States, how have emissions from coal combustion-related sources changed from 1999-2008?

Plot5.R >
How have emissions from motor vehicle sources changed from 1999-2008 in **Baltimore City**?

Plot6.R >
Compare emissions from motor vehicle sources in Baltimore City with emissions from motor vehicle sources in **Los Angeles County**, California (``fips == "06037"``). Which city has seen greater changes over time in motor vehicle emissions?