Impact of Convection and Lightning-induced Nitrogen Oxide Emission on Atmospheric Composition: 2019 Case Study

Lightning-generated nitrogen oxide (LNO) is a major source of nitrogen oxides (NOx=NO+NO2) in the free troposphere, bearing a crucial role in tropospheric ozone chemistry. LNO significantly influences ozone formation and increases concentrations of HNO3 and PAN in the free troposphere, which is vital for establishing natural background ozone levels. The recent deployment of Geostationary Lightning Mappers (GLM) on the GOES-East and GOES-West satellites has provided a new opportunity for estimating and incorporating LNO into chemical transport models (CTMs). To leverage this capability, we have developed the Geostationary Lightning Mapper Nitrogen Oxides (GLM-NOx) Emissions Processor, designed to generate model-ready LNO emissions data. This tool utilizes a novel technique to derive LNO estimates from GLM observations, which are then applied in air quality simulations for the summer of 2019 to assess the impact on tropospheric chemical composition.

LNO emission estimates are derived from lightning optical energy observed by the GLM. Given that the GLM's detection efficiency decreases with increasing satellite view angle, observations from both the GLM-east and GLM-west were combined to improve the accuracy of LNO estimates across the contiguous United States (CONUS). This approach involves calculating the total column of LNO and then distributing it vertically within the column. The vertical distribution is informed by monthly profiles derived from the NASA Lightning Nitrogen Oxides Model (LNOM), which uses multiyear data from the North Alabama Lightning Mapping Array (LMA).

The Weather Research and Forecasting (WRF) model, in conjunction with the Community Multiscale Air Quality (CMAQ) photochemical model and the Sparse Matrix Operator Kernel Emissions (SMOKE) model, was employed for atmospheric simulations during the summer of 2019. These simulations were conducted both with and without LNO to examine its effect on atmospheric composition. During the period from June to September 2019, lightning events generated approximately 0.174 TgN of NOx across the contiguous United States (CONUS) which accounted for 11.4% of the total NOx emissions in the region during this timeframe. On average, the presence of LNO led to a 1–2% increase in tropospheric ozone concentration, equivalent to an increase of 0.3–1.5 parts per billion by volume (ppbv) in the atmospheric column. The most substantial enhancement in ozone, reaching up to 40 ppb, was observed at approximately 6 km above ground level, while the increase was minimal near the surface.

The southeastern United States experienced the most pronounced rise in ground-level ozone, with back trajectory analysis showing that LNO contributed to an ozone increase of up to 12 ppb in this region. Additionally, comparisons with lidar and ozonesonde measurements taken over Huntsville, Alabama, indicated that incorporating LNO into the model improved its performance, particularly with respect to underestimations of ozone. Our findings suggest that the current rate of LNO production used in the emissions model needs to be revised upward.

Furthermore, the backward trajectory analyses identified the scenarios contributing to the significant increases in ozone. For lightning activities occurring at night or in areas limited by volatile organic compounds (VOCs), the air mass can travel a considerable distance before encountering fast reacting VOCs necessary for substantial ozone production. In contrast, daytime lightning activity in regions where nitrogen oxides (NOx) are limited leads to significant ozone production. This was evident in the case of the outflow boundary ahead of a southwestward-moving mesoscale storm system over Houston, Texas.