5A: FRAPPÉ analysis beyond the final report

The final report for NCAR’s Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) funded by NSF and the Colorado Department of Public Health and Environment (CDPHE) was submitted to the CDPHE in July 2017. FRAPPÉ took place jointly with 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)  in summer 2014 to study summertime ozone pollution in the Northern Colorado Front Range Metropolitan Area (NFRMA). In the final report we focused on characterizing the contributions of the different photochemical drivers to surface ozone in the NFRMA. We found that significant adjustments were necessary to the reported emissions from the oil and gas sector as well as the mobile emission sector and that these two groups of emissions also show to be the major contributors to locally produced ozone in the NFRMA.

In a follow-up study, we are now looking beyond the emission sectors and characterize the role different volatile organic compounds (VOCs) play in local ozone production. We apply the Integrated Reaction Rate (IRR) capability, which we introduced in the Weather Research and Forecasting model with Chemistry (WRF-Chem) Version 4 and the chemical tendencies diagnostics for an in-depth analysis of the ozone formation in various NFRMA regions. Our study focuses on a case study for 12 August 2014. Campaign observations and model results show efficient ozone production within the NFRMA on this sunny and calm day, driven by high nitrogen oxides (NOx) and high VOCs and also continued ozone production during transport into the mountains.

As shown in Figure 1 below, for both, the larger Denver metro area, which is strongly influenced by urban emissions, and for Weld County, which is strongly influenced by emissions from oil and gas operations, the dominant VOCs are identified as formaldehyde (CH2O) followed by higher alkanes (BIGALK) and acetaldehyde (CH3CHO). While BIGALK concentrations are for the most part related to direct emissions, for daytime CH2O and CH3CHO concentrations we estimate that chemical production is a largest contributor over direct emissions, with alkanes and alkenes important pre-cursor species. Isoprene from industrial and biogenic sources and propane (C3H8), emitted from oil and gas operations, rank as the fourth most important VOC in the Denver area and Weld County, respectively. This study can provide valuable policy information into the chemical fingerprint of surface ozone in the NFRMA.

WACCM PM2.5
Larger Denver Metro Area Weld County
Diurnal cycles of VOCs. Diurnal cycles of VOCs.
Figure 1: Diurnal cycle of the amount of VOCs reacting with OH in (ppb/hr). The six VOCs with the highest contribution are colored. Click to display larger images.