ARL Updates NWS Forecast Model to Resolve High Wintertime Ozone Issue

April 2018

The atmospheric chemistry group within NOAA’s Air Resources Laboratory (ARL) delivered new model packages and emission data to the National Weather Service (NWS) that, among other upgrades, resolved a long-term ozone forecasting issue: high wintertime ozone bias over oil/gas fields.

The operational forecasting system that provides real-time prediction of ozone and aerosols to our nation is the NWS’s National Air Quality Forecast Capability, or NAQFC. For several years, forecasters have been concerned by the NAQFC model’s prediction of high surface ozone concentration over the Northern Great Plains to a degree that warranted mitigation actions. However, the high forecast concentrations were not verified by ground monitors. This issue was first reported to the NWS by a local forecaster in Denver who observed repeated false alarms over northwestern Colorado in 2015. ARL studied the characteristics of the phenomenon and concluded that it was likely to be an emission problem. Scientists at both the Environmental Protection Agency and at NOAA’s Earth Science Research Laboratory (ESRL) concurred.

ARL scientists, together with colleagues from ESRL’s Chemical Sciences Division, worked to conduct an in-depth forensic analysis; ultimately tracing the problem’s source to an unrealistically high level of formaldehyde emissions from oil and gas extraction operations. Although NAQFC correctly modeled the total emission mass of petroleum-related air pollutants, it allocated them too much within the highly reactive categories. To compensate, ARL’s latest update to the NAQFC model provides a new chemical speciation profile (a detailed breakdown of the type and amount of individual chemical components) which allocates more of the air pollutants generated by the petroleum extraction process into chemicals that are less photo-chemically reactive. As expected, modulating photolytic reactivity effectively lowered the local production of tropospheric ozone. Testing with an NWS testbed system showed encouraging results towards a much lower false alarm rate.

In addition to the wintertime ozone update, which will go into NWS operations on May 1, 2018, ARL has recommended several system improvements to be implemented by NWS. These updates include a temporal adjustment of oil and gas emissions based on recent petroleum extraction data and new marine isoprene emissions using NOAA satellite observations of ocean colors.

Screen shots from the emissions model before and after upgrade
A Code Orange bad air quality forecast due to an erroneous ozone plume north of Denver on a winter day. Credit: NOAA
Gas flares from a stack
Gas flares from a stack. Credit: Scott Sandberg, NOAA