Nocturnal Chemistry in the Urban Boundary Layer of Los Angeles

The State of California continues to face air quality challenges in its major urban areas. Ozone and particulate matter (PM) levels in the South Coast Air Basin exceed the California and Federal standard regularly, and the ever growing population makes the development of further strategies to improve air quality a priority. To support the State’s air pollution mitigation goals this project focused on the investigation of nocturnal chemistry in the Los Angeles basin. While often ignored, nocturnal chemistry is important, as the processes occurring at night set the stage for air quality conditions on the next day. To support other research efforts to better understand the processes underlying air pollution the University of California Los Angeles (UCLA) organized the CalNex Los Angeles supersite, which was located on the campus of the California Institute of Technology. The supersite, which hosted more than 40 research groups, yielded a highly comprehensive data set of gaseous and particulate pollutants and meteorological parameters from May 15, 2010 to June 16, 2010. As part of Calnex-LA, UCLA’s LP-DOAS instrument continuously measured vertical concentration profiles of O3, NO2, HCHO, SO2, HONO and NO3 in four altitude intervals: 3378 m, 78-121 m, 121-255 m, and 255-556 m above the ground. This unique dataset was interpreted using a 1-dimensional chemistry and transport model to yield the following main conclusions: • Nocturnal atmospheric composition and chemistry in Los Angeles is highly altitude dependent. • The high levels of the dominant nocturnal radical species NO3, in particular above 200 m altitude, showed that active nocturnal radical chemistry is occurring. • Uptake of N2O5 on particles was found to be responsible for an altitude-averaged nocturnal loss of nitrogen oxides of 0.7 to 1.3 ppb/h. which is comparable to the daytime NOx loss. • N2O5 chemistry led to the efficient formation of ClNO2, an important Cl atom precursor in the morning, which was observed by the collaborators from the University of Calgary. • Nitrous acid, HONO, an important hydroxyl-radical precursor, showed elevated mixing ratios in the lowest 100 m of the atmosphere. Surface conversion of NO2 to HONO on the ground was identified as the most likely source. • Elevated daytime HONO mixing ratios show the presence of an active daytime HONO source. A photo-enhanced conversion of NO2 on the ground was identified as the most likely HONO source. • The photolysis of morning and daytime HONO was identified as a major contributor to the daytime primary radical budget, with a potentially large influence on ozone levels. The results show that the nocturnal chemistry of nitrogen species strongly influences air quality in Los Angeles. Nighttime and daytime nitrous acid chemistry, which is currently poorly represented in urban air quality models, is an important contributor to the hydroxyl radical budget, thus impacting ozone chemistry. The data acquired in the project as well as the scientific conclusions from this project can be used to validate and improve models of the Los Angeles airshed and will thus lead to more accurate description of ozone and particle formation.