The lifetime of CO (approx. 3 months) is long enough that it will be susceptible to atmospheric transports. However, its lifetime is also sufficiently short that it is not completely mixed. This is reflected in the vertical and horizontal distribution on various length scales. Measurements made by Sachse et al. (1987) using an aircraft-borne tunable diode laser operating at 4.7µm and taken at 6 km (Figure 2.5) indicate factors of 2 change in CO on a horizontal scale of order 20 km. Presumably these changes exhibit the very pronounced effects of vertical transport induced by a mountain barrier. They also found large decreases in CO mixing ratios associated with the influx of stratospheric air in a tropopause folding event (Figure 2.6).
On larger scales the measurements of Dickerson et al. (1987) (Figure 2.7) indicate enhanced CO mixing ratios at higher altitudes clearly associated with rapid convection by storm systems over continental boundary layers with enriched CO.
On a global scale the data of both Seiler and Fishman (1981) and Reichle et al. (1986) mentioned above also bear the signature of extensive transport effects and indicate at mid- tropospheric levels sources associated with the biologically active equatorial regions not immediately evident in the surface measurements. CO measurements over Saudi Arabia and northern India (Doherty et al., 1986) reflect the direct influence of transport on CO mixing ratios. In particular, low values were observed following the onset of the monsoon and the influx of CO-poor southern hemisphere air in the monsoon southwesterlies. Seasonal effects have been noted by Condon et al. (1987).