Carbon dioxide in the urban atmosphere: Dependencies and trends

Carbon dioxide data from three urban sites in New Jersey covering periods of up to 9 years have been analyzed by a variety of statistical techniques. Hourly average concentration maxima as high as 585 ppm are seen, as are minima as low as 285 ppm. The diurnal patterns appear to be influenced strongly by local mixing and turbulence conditions and seldom show the 'rush hour' peaks often found in urban carbon monoxide data. Seasonal analysis demonstrates that maxima in monthly medians of daily mean CO2 concentration occur in winter and that minima occur in summer. Such a pattern suggests a strong contribution from statinary combustion sources to urban CO2 levels, at least in the winter months. The winter-summer difference in the monthly median of daily mean CO2 is of the order of 30 ppm, a much larger difference than is found in the rural setting. The long-term trend of the baseline CO2 concentration in these urban data is not monotonic but shows a time-integrated increase of approx.0.6 ppm/yr. consistent with the range and pattern of concentration gradients observed at global background sites