An air quality and boundary layer dynamics analysis of the Los Angeles basin area during the Southwest Urban NO_<i>x</i> and VOCs Experiment (SUNVEx)

Abstract

The NOAA Chemical Sciences Laboratory (CSL) conducted the Southwest Urban NOx and VOCs Experiment (SUNVEx) to study emissions and the role of boundary layer (BL) dynamics and sea-breeze (SB) transitions in the evolution of coastal air quality. The study presented utilizes remote sensing and in situ observations in Pasadena, California. Separate analyses are conducted on the synoptic conditions during ozone (O3) exceedance (>70 ppb) and non-exceedance (<70 ppb) days, as well as the fine-structure variability of in situ chemistry measurements during BL growth and SB transitions. Diurnal analyses spanning August 2021 revealed a markedly different wind direction during evenings preceding O3 exceedance (northerly) versus non-exceedance (easterly) days. Increased O3 occurred simultaneously with warmer and drier conditions, a reduction in winds, and an increase in volatile organic compounds (VOCs) and fine particulate matter (PM2.5). While the average BL height was lower and surface pressure was higher, the day-to-day variability of these quantities led to an overall weak statistical relationship. Investigations focused on the fine-structure variability of in situ chemistry measurements superimposed on background trends were conducted using a novel multivariate spectral coherence mapping (MSCM) technique that combined the spectral structure of two or more independent measurements through a wavelet analysis as reported by maximum-normalized scaleograms. A case study was chosen to illustrate the MSCM technique, where the dominant peaks in scaleograms were identified and compared to BL height during the growth phase. The temporal widths of peaks (τmax) derived from VOC and nitrogen oxide (NOx) scaleograms, as well as scaleograms combining VOCs, NOx, and variations in BL height, indicated a broadening with respect to time as the BL increased in depth. A separate section focused on comparisons between τmax and BL height during August 2021 revealed uncorrelated or weakly correlated scatter, except in the case of VOCs when really large τmax and relatively deep BL heights were ignored. Instances of large τmax and relatively deep BL heights occurred near sunrise and as onshore flow entered Pasadena, respectively. Wind transitions likely influenced both the dynamical evolution of the BL and tracer advection and thus offer additional challenges when separating factors contributing to the fine structure. Other insights gained from this work include observations of descending wind jets from the San Gabriel Mountains that were not resolved by the High-Resolution Rapid Refresh (HRRR) model and the derivation of intrinsic properties of oscillations observed in NOx and O3 during the interaction between an SB and enhanced winds above the BL that flowed in opposition to the SB.