Evaluating NOx emissions in the LA basin and their implications for O3 and NOx during CalNex-2010

We evaluate NOx emissions in the Los Angeles Basin during the CalNex-2010 field campaign by analyzing O3 and NOy observations using WRF/Chem. Model simulations are conducted at 4-km spatial resolution over the basin and the results are compared against ARB surface measurements as well as CalNex surface and aircraft observations. We adjust the 2005 National Emissions Inventory (NEI'05) for the CalNex simulations with a 24% reduction in NOx and 28% reduction in CO according to the emission statistics from California ARB. WRF/Chem O3 reproduces the observed diurnal cycle and day-to-day variations in surface O3 (r2 = 0.92, n = 114, p = 0.01) across the basin. Model results underestimate daytime O3 at the CalNex-LA supersite (Caltech) by 20-40 ppbv during May 29-30 when O3 levels approached 80-100 ppbv. The underestimate is in large part because of the deeper than observed PBL heights in the model and the model's inability to simulate observed strong stratospheric intrusion that penetrated deep into the basin on May 29th (Langford et al., 2011). Titration due to excessively high NOx emissions at downtown L.A. sites leads to near-zero nighttime O3. The vertical profiles of CO below 3 km show good agreement with the observation (r2=0.64, n=576, p=0.01), but with ~90 ppb overestimation for the altitude below 400 m altitude. Model NO2 and NOy concentrations along the June 2-3 NOAA P-3 flights over the basin are biased high by 106% and 48%, respectively. Model results capture the overall variability in the LP-DOAS observed NO2 and O3 but overestimate NO2 below 500 m altitude. A 45% reduction of NOx emissions from 2005 to 2010, as implied by OMI NO2 columns (Russell et al., 2010), significantly improves model comparisons of NO2, NOy and O3 during CalNex.