5A: Field Campaigns: FRAPPÉ and DISCOVER-AQ

Two major field campaigns – NCAR’s Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ, funded by NSF and the Colorado Department of Public Health and Environment, CDPHE) and the 4th deployment of the National Aeronautics and Space Administration (NASA) Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) – were conducted jointly in summer 2014 to study summertime ozone pollution in the Northern Colorado Front Range Metropolitan Area (NFRMA). To date, 27 peer-reviewed papers have been published with more in preparation.

In July 2017, a  report, summarizing findings and recommendations for Front Range Air Quality issues, has been submitted to CDPHE. The report will aid the State in the process of developing effective ozone reduction strategies. The report focused on characterizing the contributions of the different photochemical drivers to surface ozone in the NFRMA and integrated results from analysis of the campaign data, the Sparse Matrix Operating Kernel (SMOKE) modeling system, the Weather Research and Forecasting (WRF)/Community Multiscale Air Quality (CMAQ) model, the chemical boxmodel BOXMOX (an extension to the Kinetic PreProcessor (KPP)) and the NASA LaRC Steady State Model.

We found that significant adjustments were necessary to the reported emissions from the oil and gas sector as well as the mobile emission sector. These two groups of emissions also show to be the major contributors to locally produced ozone in the NFRMA.  The oil and gas sector influence more often the northern part of the NFRMA while mobile emissions dominate more in the southern metro area, on average. Figure 1a shows the average contribution of each of these two emission sectors to MDA8 ozone production for the time frame of the FRAPPÉ observations. We estimate from these model runs that the average total contributions are 30-40% each of the locally produced ozone. Figure 1a also demonstrates a geographical separation of the influence of the two emission sectors, with oil and gas dominating the northern Front Range roughly north of 40N, and mobile emissions dominating the southern part of the metro area. Figure 1b shows the steady state ozone production rates of the average of all canister samples taken over Weld County (left) and over the western Denver suburbs (right), comparing the total ozone production rate with the rate computed when dominant emissions sectors are zeroed out. It is clear that Weld County is strongly dominated by oil and gas emissions, while southwestern suburban Denver has influence from both, mobile and industrial sources.

Large, concentrated NOx emitters such as power generating stations, Denver International Airport, and NOx emissions from some large industrial plants cause a temporary reduction in ozone close to these sources but contribute to ozone production farther downwind, affecting ozone in the western suburbs of the NFRMA and the Foothills during high ozone production days (not shown). Prevailing daytime transport to the west during summer exposes the Foothills and Front Range mountains, including Rocky Mountain National Park and, occasionally, the adjacent western valleys to high ozone originating from the NFRMA, as demonstrated by the extent of ozone production to the west in Figure 1a).

Measures resulting in lowering the emissions from oil and gas as well as from mobile sources could result in substantial reductions of NFRMA ozone with as much as 10 ppb reductions of the daily maximum 8-hour ozone on high ozone days when either sector is zeroed out.

We have also identified a number of point sources which, at the time of measurement, emitted alarming amounts of highly reactive and toxic VOC. Some of these sources were not part of the emission inventory.

Figure 1: (a) CMAQ average contribution of each of NFRMA oil and gas emissions (left) and mobile emissions (right) to MDA8 ozone production
Figure 1: (a) CMAQ average contribution of each of NFRMA oil and gas emissions (left) and mobile emissions (right) to MDA8 ozone production for the time frame of the FRAPPÉ observations. (b) Steady state ozone production rates of the average of all C-130 canister samples taken over Weld County (left) and over the western Denver suburbs (right), comparing the total ozone production rate with the rate computed when dominant emissions sectors (oil and gas(OG), mobile(mobile) and industrial(ind)) are zeroed out.