Airborne remote sensor observations of pollution structure and transport during CALNEX 2010

The 2010 CALNEX study aimed to investigate important issues at the nexus of air quality and climate change, with a focus on emissions, chemical transformation and climate processes, and the role of transport and meteorology.  During CALNEX detailed observations of chemical, aerosol, meteorological, and radiation parameters from a variety of mobile platforms and field sites were gathered as part of an intense spring-summer observational period in Southern and Central California. The observations are currently being analyzed to improve understanding and modeling of issues such as transformation and transport of primary emissions and interaction of aerosol particles with atmospheric radiation.

As part of the CALNEX experiment, NOAA and the University of Colorado deployed an ozone/aerosol differential absorption lidar (DIAL), wind/aerosol Doppler lidar, and airborne multi-axis differential optical absorption spectrometer (AMAX-DOAS) on a NOAA Twin Otter aircraft to study the distribution and transport of boundary layer pollutants in the Sacramento Valley and the Los Angeles Basin.  The instrument package provided three-dimensional observations of wind speed and direction , aerosol backscatter at infrared and ultraviolet wavelengths, and ozone concentration, as well as column averages (with some vertical information potentially retrievable) of nitrogen dioxide, glyoxal, and other trace gases.  The Twin Otter flew 26 missions during a two-month period from mid-May to mid-July, observing several interesting events associated with long-range pollution transport, stratospheric intrusions, horizontal layering, intra- and inter-basin transport, and vertical venting along mountain ridge-tops.

Because the DIAL and Doppler lidars simultaneously measured profiles of ozone, aerosol backscatter, and wind speed and direction below the aircraft, combining data from the two measurements provides an estimate of the horizontal flux of pollutants as a function of height below the aircraft.  The capability to measure profiles was often important in the Los Angeles basin, where phenomena such as sea breeze and mountain –valley circulations can result in formation of thin layers of dirty or clean air.  The airborne measurements also provided information on the vertical mixing of pollutants along the floor and sides of the LA basin and the slopes of the Sierras.

Although mid-May through mid-July was characterized by extended periods of low pollution levels and cool temperatures, two significant multi-day pollution events occurred during the period.  During high pollution events the Twin Otter typically flew two missions- a morning mission at lower altitudes to characterize emissions with the AMAX-DOAS as the primary instrument, and an afternoon mission at altitudes above the ridges to observe three dimensional structure of ozone and aerosol after photochemistry and transport processes had impacted local pollution levels.  Significant horizontal and vertical variability was frequently observed across the basin during these periods, as indicated in Figure 1, which shows a Google Earth image of ozone measurements within the Los Angeles basin and into the Mojave Desert on July 2.  High ozone concentrations are observed at the east end of the basin; both clean and dirty layers are observed aloft at different locations within the basin.

The summer 2010 Twin Otter deployment also included a two-week period in Sacramento in conjunction with the Department of Energy-sponsored Carbonaceous Aerosols and Radiative Effects Study (CARES).  During the CARES deployment the Twin Otter provided information on the distribution of UV and IR lidar backscatter as well as wind and trace species information upwind and downwind of Sacramento and into the Sierra Nevada foothills. 

Twin Otter observations from both the Los Angeles and Sacramento studies are being compared with chemical model outputs to assess and improve model estimation of chemical and aerosol transport and representation of important physical and chemical processes.

Figure 1:  Cross section of ozone concentration in LA basin measured by the Twin Otter DIAL instrument on July 2, 2010, showing high pollution at the east end of the basin extending into the desert and clear layers aloft over the western portion.