Weekly and Seasonal Trends in the Diurnal Variation of CO2 Mixing Ratio in Pasadena, CA

Diurnal variations in CO₂ mixing ratio ([CO₂]) in urban areas reflect changing proportions of biogenic and anthropogenic sources and changes in meteorological conditions (e.g., London, England: Rigby et al., 2008, Atm. Env. 42, 8943-8953). We have monitored [CO₂] in Pasadena, CA almost continuously since 2001 using an infrared gas analyzer. In a typical day there is a low [CO₂] plateau at about ~10 AM-4 PM (all times given as Pacific Standard Time) and a high [CO₂] plateau at ~9 PM-3 AM, as observed previously for both CO (e.g., Riverside, CA: Gentner et al., 2009, Env. Sci. Tech. 43, 4247-4252) and CO₂ (e.g., Vancouver, British Columbia, Canada: Reid and Steyn, 1997, Atm. Env. 31, 3101-3114; Phoenix, AZ: Idso et al., 2002, Atm. Env. 36, 1655-1660; Salt Lake City, UT: Pataki et al., 2007, Oecolog. 152, 307-322; London, England: Rigby et al., 2008). The midday low and nighttime high in [CO₂] are probably due to draw-down by photosynthesis during daylight hours and respiration at night, accompanied by diurnal changes in the mixed-layer depth resulting from formation and destruction of a nocturnal temperature inversion layer (e.g., Reid and Steyn, 1997). The amplitude of the diurnal variation in Pasadena ranges from ~20 ppm in June to ~70 ppm in December. We typically observe a maximum in [CO₂] at ~5-9 AM on weekday mornings. This peak is smaller on weekends, generally being smallest on Sundays. This morning [CO₂] peak coincides with increased traffic on surface streets in Los Angeles due to weekday morning rush hour (Chinkin et al., 2003, J. Air Waste Mgmt. Assoc. 53, 829-843) it has also been observed by Reid and Steyn (1997) and Idso, et al. (2002 ) in Vancouver, BC, Canada, and Phoenix, AZ, respectively. There is no corresponding peak that can be associated with afternoon rush hour, perhaps because the time period of the afternoon commute is ill-defined in Pasadena and/or increased emissions from this time of day contribute to the evening increase in [CO₂] along with decreases in photosynthesis and development of a temperature inversion layer, which decreases the depth of the mixed layer (Idso et al., 2002). Several seasonal patterns are observed in the monthly average diurnal trends for each day of the week. During summer months, there can be a slight midday increase in [CO₂] that is not observed during winter months. This increase could be associated with a midday minimum in photosynthesis due to water stress in Pasadena’s semi-arid environment (Deng et al., 2000, Photosynthetica 38, 187-192). The absolute minimum [CO₂] during summer days is later (4-6 PM) than it is during the winter (11 AM-2 PM). In addition, the morning peak shifts to later in the day when sunrise comes later. Finally, we observe that the morning rush hour peak is more pronounced in winter months (~30-40 ppm mid-week) than during the summer (~0-5 ppm), perhaps because of seasonal differences in diurnal changes in mixed-layer depth.