Temporal patterns in near-surface CO2 concentrations in the Phoenix urban CO2 dome over contrasting vegetation types

Recent measurements have revealed a CO₂ dome over the Phoenix metropolitan area. In order to quantify temporal patterns in near-surface CO₂ concentrations in this dome and the magnitude of CO₂ enhancement, we monitored CO₂ concentrations within and at the edge of the Phoenix metropolitan area. Additionally, we examined the relationship between CO₂ concentrations and microclimatic factors, as well as plant canopy photosynthesis and respiration. Levels of CO₂, photosynthetically active radiation (PAR), wind speed and air temperatures were measured at a 2-m height every 5 minutes for several months over native desert vegetation and grass turf sites within and at the edge of the metro area. CO₂ concentrations were consistently higher within the metro area, with daily means averaging 8% higher at sites within the metro area. The average daily minimum, mean and maximum CO₂ concentrations were 378, 408 and 478 ppm, respectively, at sites in the metro area, compared to 373, 378 and 392 ppm at sites at the edge of the metro area. CO₂ concentrations at all sites were higher and more variable at night than during the day, and elevated CO₂ levels in the metro area were most apparent at night. For example, nighttime CO₂ concentrations averaged 40 ppm higher in the metro area than at its edge (419 vs 379 ppm). In contrast, daytime CO₂ concentrations averaged only 17 ppm higher in the metro area than at its edge (395 vs 378 ppm). At all sites, CO₂ concentrations were usually low at high wind speeds or PAR levels. At all sites, CO₂ concentrations declined as PAR increased in the early morning; this early morning decline in CO₂ appeared to involve both photosynthetic uptake of CO₂ by vegetation as well as solar-induced convective mixing, since it was well developed even on cloudy days when photosynthetic uptake by vegetation was limited by low levels of PAR. The daily range in CO₂ concentrations was higher over grass turf sites than adjacent native desert vegetation. Fluxes of CO₂ (photosynthesis and respiration) from the plant/soil system were larger from the former sites, suggesting that vegetation type can have an impact on near-surface CO₂ concentrations.