A new data set of annually averaged CO2 background levels directly measured from 1826 to 1960 is presented. It is based on a selection process of about 100,000 single samplesfrom more than 200,000 available near ground on land and sea, mainly in the northern hemisphere. Analysing the data, methods, sampling stations, meteorological conditions and air masses it is possible to reconstruct the past yearly CO2 background levels.
New methods to estimate annual marien boundary levels from near ground data from the historical data are presented. This allows the reconstruction within an estimated error range of ±2.5 % and a methodical error range since 1870 of ±3 %. A definite fluctuation of levels can be seen around 1860 and especially around 1940 showing levels of more than 380 ppm, almost like today. A slow rise of atmospheric CO2 difference in averages of CO2-levels in the 19th and 20th century of 2.6 % is within error range of 3 % of methods.