There seems some confusion in what the CO2 background level in atmosphere is different to the CO2 levels near ground. The atmospheric CO2 background concentration had been speculated by C. Keeling since 1955 and had been measured by him since 1958 at the Mauna Loa observatory ( Hawaii, ~4 km altitude). It represents the CO2 levels in higher troposphere and near sea surface (MBL= marine boundary layer) as measured in world wide network by NOAA listed in WDCGG.
Near ground the CO2 levels are highly influenced by local sources, therefore their concentrations show large variations especially over the continents. Over sea surface the water absorption provide a small SEAS (seasonal variation).
The vertical CO2 profiles are the key to estimate background levels from near ground measurements. These are characterized by large seasonal fluctuation (SEAS) near ground on continents in non well-mixed environments and small variations in the higher troposphere or over sea surface (MBL) in well-mixed environments. Because all CO2 sources are assumed to come from lithosphere there is a physical connection from the ground to the higher layers. Fig. 1 shows the most important globally CO2 sources and sinks in the lithosphere- atmosphere boundary layer. Anthropogenic sources and others below 1% of total emissions according to IPCC [IPCC 2007] had been omitted. Within the atmosphere there exists a CO2 gradient with a somewhat lower concentration and better mixing in the higher troposphere.
Fig. 3 CO2 sources and sinks in the boundary layer of the lithosphere-troposphere.
1: ocean degassing/absorption, 2: photosynthesis, 3: respiration, 4: submerse geological degassing; 5: limestone weathering, 6: surface coal oxidation, 7: volcanic degassing and subduction degassing, 8: precipitation absorption, 9: soil respiration. CO2 flux < 1% of total emissions (IPCC) omitted.
The main globally effective controllers for CO2 flux in the lithosphere/atmosphere system are the oceans (1) and the biomass (2, 3, 9). The phytoplankton in the surface layer of the oceans act as controlling agent for ocean bound CO2.
The amount of geological surface flux of CO2 from continent is greatly underestimated according to Mörner and Etiope 2002. Limestone weathering, surface coal oxidation and non-volcanic degassing are not quantified in detail in the IPCCs carbon cycle. Also the submerse fluxes in the oceans has not been quantified. [IPCC 2007]. Local sources and sinks control local mixing ratios.
Let´s take a look at typical continental station far from human influence, Harvard Forest (USA), a NOAA GlobalViewCO2 station part of the global WDCGG CO2 network. Near ground the atmospheric parameters are measured on a tower at different hights, NOAA has measured from altitudes of 500 m to 8 km by aeroplane at that location.
From the NOAA Globalview-CO2 sampling locations [NOAA 2009] I have chosen the vertical CO2 gradient from the Harvard Forest site as an example for a typical continental location with vegetation at a typical latitude (lat 42,547N, lon -72,17E).
Fig. 4 Vertical profile of CO2 ( deviations from 0) at Harvard Forest (USA), lat 42,54N, lon -72,17E, measured by aeroplane at different altitudes of 500, 1500, 2500, 3500, 4500, 5500, 6500 and 7500 m.(data from NOAA Globalview-CO2 2009)
Fig. 4 shows the larger SEAS fluctuation near ground (500 m, 21,5975 ppm) and the smaller variation of the background levels at higher altitudes (7500 m: 7,138 ppm). The SEAS average is nearly identical for 500 m: 0, 0,099225 and 7500 m: -0,00551667( 0,1047 ppm difference).
Figure 5 shows the measured SEAS near ground (29 m) at Harvard Forest station USA (Ameriflux), a typical continental station with strong vegetational influence. Please note large seasonal variations in the order of 100 ppm up to 500 ppm maximum. Because the annual average of the SEAS near ground is very close to the background level in the higher troposphere we can easiliy calculate using simple nonlinear regression methods the CO2 MBL level according to NOAA within about 1% accuracy. MBL average 1991-2007 (NOAA): 367,56 ppm; CO2 -wind-speed-background- approximation (CWBA) for 1991-2007 results in 372 ppm. Error: +1,19 % (fig. 4)