                         Ammonia in San Joaquin Valley
In January - February 2013, the DISCOVER-AQ (Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality) field campaign, launched by the National Aeronautics and Space Administration (NASA) took place in the SJV. During the field campaign, aircraft measurements of PM2.5 and its precursors, including ammonia were made in agricultural and urban areas of the SJV. The measurements clearly showed that there was excessive ammonia in the SJV that can fully neutralize nitric acids and sulfuric acids, implying that the formation of ammonium nitrate was limited by NOx emissions instead of ammonia emissions.  CMAQ air quality modeling simulation of the DISCOVER-AQ period indicated that the model has decent agreement with observation for ammonia at Fresno (Figure 1) and under predicted ammonia by a few factors near areas with significant ammonia emissions such as Porterville (Figure 2).  The extent of under prediction could potentially be made worse by the fact that model prediction was based on hourly average concentrations in 4kmx4km horizontal grid cells while aircraft recorded second by second measurements. However while ammonia was under-predicted in the model, it was found that there was generally enough modeled NH3 to fully neutralize modeled nitric acid and sulfate in the model, indicating that in the model ammonium nitrate formation was also limited by NOx emissions, despite that ammonia was under predicted by the model. The chemistry of ammonium nitrate formation also indicated when ammonia is in excess, under prediction of ammonia mixing ratios means model tends to over predict the sensitivity of ammonium nitrate formation to ammonia emission reduction.

Satellite measurements of infrared radiation allow for quantification of atmospheric ammonia (NH3). The IASI instrument, which is housed aboard European Space Agency's MetOP-A satellite, passes over California each day around 10:00 am local time, providing measurements of column-integrated NH3. The IASI measurements data processed by CARB are recorded in 12 x 12 km grid cells. In order for IASI data to be directly compared against NH3 column densities predicted by CMAQ, CMAQ outputs are first re-gridded from 4 km to the same 12 km grid used for IASI data, then filtered to only select grid cells that overlap in space-time with a valid IASI pixel. Figure 3 shows the annual average of column NH3 in 2017 from IASI and CMAQ and the model is biased low for column NH3 in the SJV. This bias is most noticeable in Tulare county, where both the model and satellite show a NH3 hotspot, but the model shows about half as much NH3 as the satellite. Taking the sum of all NH3 measured over the SJV, as in Figure 4, the model comes in about 25% lower than the satellite (248 versus 330 DU), implying that the model low-bias is present valley-wide. 






Figure 1. Comparison of observed and modeled vertical distributions of ammonia at Fresno based on NASA P3B flight at 12pm and 14pm on Jan 20.


Figure 2. Comparison of observed and modeled vertical distributions of ammonia at Porterville based on NASA P3B flight at 10am and 12pm on Jan 20.


 


Figure 3. Maps of annual average NH3 from CMAQ (Model; left) IASI (Satellite; middle) and the percentage difference (DU, 1 DU = 2.69e16 molecules/cm[2]).



Figure 4. Barplot showing the sum of all grid cells in Figure 3.




Figure 5. Scatterplot of annual column NH3 in 2017 from satellite vs. CMAQ simulation

