Global Co Distributions Surface CO

Background Atmosphere, CO varies both temporally and spatially. Figure I presents a smoothed representation of the surface distribution of CO in the background marine boundary layer (MBLI as a function of latitude and time. The surface illustrates that CO mixing ratios in both hemispheres exhibit seasonal variation, and

Figure 1 Smooth surface representing she distribution of CO in the marine boundary layer. The surface was created from 38 time series determined from sampling locations in the NOAA CMUL Cooperative Air Sampling Program. CO mi\ing ratios were combined in 5 latitude bands, longitudinal differences were averaged, and the combined time series were smoothed in both time and space."4

Figure 1 Smooth surface representing she distribution of CO in the marine boundary layer. The surface was created from 38 time series determined from sampling locations in the NOAA CMUL Cooperative Air Sampling Program. CO mi\ing ratios were combined in 5 latitude bands, longitudinal differences were averaged, and the combined time series were smoothed in both time and space."4

although there are considerable interannual variations, repeatable patterns occur from year to year. Most notable is the seasonal cycle and the interhemispheric gradient. Greatest CO mole fractions in the MBL [200 to 225nmol CO/mol air (ppb)] are found in the high latitudes of the Northern Hemisphere during late winter/early spring. The high Northern Hemisphere also exhibits the greatest seasonal amplitude (120 to 140 ppb). The imbalance of sources in winter (mostly anthropogenic pollution from the midlatitudes) and sink (when OH levels are lowest) leads to an accumulation of CO in the high Northern Latitudes. Lowest CO mixing ratios in the boundary layer (40 to 50 ppb) are found during the southern summer, where low concentrations are further depressed by reaction with OH. The interhemispheric gradient also exhibits a strong seasonality. The largest difference between the high northern and high southern hemispheres (~ 150 ppb) occurs in February/March and the minimum difference (10 to 20 ppb) occurs in September/October.17'18'24

Polluted Atmosphere. CO levels in urban locations and areas of regional-scale pollution are greater than those found in the background atmosphere, with CO mixing ratios in urban areas often reaching ppm level, orders of magnitude greater than those found in the background troposphere. CO is defined as a criteria species for urban pollution. The lifetime of CO is on the order of several months, and emissions can be transported far from the original source region.25 Even in areas far distant from CO sources, wide-scale, diffuse pollution may enhance CO levels (up to twice background levels). Air parcels downwind of areas where combustion occurs can also show elevated levels of 03.26'27 The enhanced 03 often reflects its photochemical production [Eqs. (2)-(4)], which is favored in environments having both high CO and NOt.28

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