The 2018 figure is the average of seasonally-adjusted figures for June and July, 2018 (which is a good projection for the annual average). Previous years are annual averages.
Direct measurements of CO2 at Mauna Loa Observatory, in Hawaii began in March, 1958. This is the annual average data we’ve graphed:
For monthly data see:
1850-1958 data is from http://data.giss.nasa.gov/modelforce/ghgases/Fig1A.ext.txt (ice cores).
1800-1850 CO2 data is from http://www1.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/law/law_co2.txt (Law Dome ice cores, 75 year smoothed).