Aircraft Pilot Observation (APOB) Measurements of Summertime Ozone in the Residual Layer during 2008 – 2013 in the San Joaquin Valley (SJV), CA.

Abstract:

The California Air Resources Board (CARB) has been operating the Aircraft Pilot Observation (APOB) campaign since 2008 to measure ozone in the upper air, mostly in the residual layer, with an objective to understand the impact of residual layer ozone on daily surface ozone concentrations in the San Joaquin Valley (SJV) where both federal and state standards are often violated. A contracted flight operates daily between 5 – 6 AM over Fresno during the summer months from June – September, and makes continuous measurements of ozone throughout the residual layer up to ~3000 m above ground. The objectives of the current study are to 1) examine the correlation between the residual layer ozone and the surface level 1-hr and/or 8-hr maximum ozone concentrations by analyzing the 2008 – 2013 APOB dataset, and 2) evaluate an air quality model’s ability to capture vertical ozone profiles, especially in the residual layer, by comparing the modeling results with the measurements. Preliminary analysis shows that the summertime annual median ozone in the residual layer (between 150 – 3000 m above ground) has decreased by approximately 36% over the 2008 – 2013 period. The time series of daily 1-hr and 8-hr maximum ozone concentrations at a downtown Fresno monitoring site, and daily maximum ozone concentrations in the residual layer show similar patterns, although ozone in the residual layer shows stronger decreasing trends compared to the surface measurements. This, perhaps, indicates that the ozone in the residual layer over Fresno contains contributions from surrounding areas during the daytime. In addition, the ozone trend in the residual layer follows a similar decreasing trend as the reduction of basin-wide emissions of ozone precursors such as NOx and VOCs, which have decreased by 31% and 8%, respectively over the 2005 – 2010 period. The US EPA’s CAMQ model will be utilized to model ozone concentrations during the summer months of 2012, and the vertical distributions will be compared with those obtained from the APOB measurements to determine how well the model predicts the measured profiles. Potential improvements to the modeling will be tested if found necessary.

Keywords: residual layer, ozone, San Joaquin Valley, CMAQ