Seasonal and diurnal variations of near-surface atmospheric CO₂ concentration within a residential sector of the urban CO₂ dome of Phoenix, AZ, USA

Over most of an entire year (315 days), we obtained 1-min averages of near-surface (2-m height) atmospheric CO₂ concentration, temperature and wind speed in a residential area of a suburb of Phoenix, AZ. Daily minimum CO₂ concentrations, which occurred during the afternoon, were nearly invariant over the year, averaging 390.2±0.2ppm. Daily maximum CO₂ concentrations, however, which occurred at night, varied seasonally with the air temperature, exhibiting a mean peak of 490.6ppm about 2h before midnight during the coldest part of the year (December-January) and 424.3ppm just before sunrise during the warmest part of the year (July-August). Reevaluating prior assessments of the strength of the urban CO₂ dome at the center of Phoenix, our results suggest a mean cold-season maximum there of 619.3ppm, which is 67.4% greater than the rural background value. At our residential site, however, the mean cold-season maximum was only 32.6% greater than the surrounding rural mean. Averaged over the entire night, this enhancement dropped to 25.4% in the cold season and 10.9% in the warm season, while over the daylight period it averaged 10.5% and 10.1% in the cold and warm seasons, respectively. CO₂ concentrations were greater on weekdays than on weekends from 0415 to 0830 in the warm season and from 0445 to 1045 in the cold season. During peak morning traffic, the maximum weekday-weekend CO₂ differential was 35.9ppm in the cold season and 22.0ppm in the warm season.