An Air Quality and Boundary Layer Dynamics Analysis of the Los Angeles Basin Area During the Southwest Urban NOx and VOCs Experiment (SUNVEx)

Much of the global populace resides in coastal cities that are densely inhabited over small areas. As a result, a substantial quantity of anthropogenic emissions occurs in and around urban centers. The transport and chemical transformation between urban emissions is driven by the multi-scaled meteorology associated with the prevailing background synoptic conditions and the mesoscale wind patterns that develop in response to topographically driven flows, land-sea gradients in temperature, and the heterogeneous BL dynamics extending from an urban environment into a rural environment. Changes in the micrometeorology associated with BL turbulence acts to change the interaction volume by mixing different chemical compounds, and thus leads to the generation of chemical compounds that are the product of these interactions. In situations where a seabreeze develops, for instance, can lead to substantial gradients in both meteorological variables as well as pollutants. Not surprisingly, how this complex interaction evolves continues to be an active area of research.

In 2021, the NOAA Chemical Sciences Laboratory (CSL) led the Southwest Urban NO_x and VOCs Experiment (SUNVEx) to investigate the evolution of emissions over a two-month period along with profile measurements of the 3D winds from two Doppler lidar (DL) instruments. The in-situ chemistry packages available were located at Pasadena, California for much of August, measuring chemical species such as O₃, NO_x and NO_y, CO, CO₂, and VOCs. A similar set of in-situ instruments were fitted to a mobile Chem-van, which drove in tandem with a mobile DL aboard a pick-up truck that was motion stabilized to ensure wind measurement accuracy. In addition to measurements of the wind profile were derived BL heights and variance estimated from the residual of the wind fit, the latter of which is sometimes used as a proxy for estimating the vertical structure of turbulence.

The results that will be presented covers a nearly month-long period spanning August of 2021. An examination of the diurnal structure of ozone at the surface reveals periods of exceedance during the daytime (i.e., surface O₃>70ppb) that usually occurred in succession. Further investigation of surface ozone highlights key important findings such as the diurnal cycle of NO_x and O₃, the relative importance of NO_x to VOCs on the role of ozone, and the role that meteorology has on the evolution of ozone during a given day, specifically as it relates to surface temperature, relative humidity, and the vertical structure of the wind and turbulence. To complement the stationary measurements, we conduct spatial comparisons between stationary and mobile systems to elucidate differences between the dynamics observed from the two DLs and the differences in the chemical make-up observed from the separate in-situ chemistry packages. We also highlight examples of how changing BL conditions lead to responses in chemical concentrations that ultimately manifest as differences in interactions between chemical species.