Gradients of column CO₂ across North America from the NOAA Global Greenhouse Gas Reference Network

Abstract. This study analyzes seasonal and spatial patterns of column carbon dioxide (CO₂) over North America, calculated from aircraft and tall tower measurements from the NOAA Global Greenhouse Gas Reference Network from 2004 to 2014. Consistent with expectations, gradients between the eight regions studied are larger below 2 km than above 5 km. The 11-year mean CO₂ dry mole fraction (XCO₂) in the column below  ∼  330 hPa ( ∼  8 km above sea level) from NOAA's CO₂ data assimilation model, CarbonTracker (CT2015), demonstrates good agreement with those calculated from calibrated measurements on aircraft and towers. Total column XCO₂ was attained by combining modeled CO₂ above 330 hPa from CT2015 with the measurements. We find large spatial gradients of total column XCO₂ from June to August, with north and northeast regions having  ∼  3 ppm stronger summer drawdown (peak-to-valley amplitude in seasonal cycle) than the south and southwest regions. The long-term averaged spatial gradients of total column XCO₂ across North America show a smooth pattern that mainly reflects the large-scale circulation. We have conducted a CarbonTracker experiment to investigate the impact of Eurasian long-range transport. The result suggests that the large summertime Eurasian boreal flux contributes about half of the north–south column XCO₂ gradient across North America. Our results confirm that continental-scale total column XCO₂ gradients simulated by CarbonTracker are realistic and can be used to evaluate the credibility of some spatial patterns from satellite retrievals, such as the long-term average of growing-season spatial patterns from satellite retrievals reported for Europe which show a larger spatial difference ( ∼  6 ppm) and scattered hot spots.