Observing U.S. Regional Variability in Lightning NO₂ Production Rates

Abstract

Lightning is a large and variable source of nitrogen oxides (NOx ≡ NO + NO2) to the upper troposphere. Precise estimates of lightning NOx (LNOx) production rates are needed to constrain tropospheric oxidation chemistry; however, controls over LNOx variability are poorly understood. Here, we describe an observational analysis of variability in LNO2 with lightning type by exploiting U.S. regional differences in lightning characteristics in the Southeast, South Central, and North Central United States. We use satellite NO2 measurements from the Ozone Monitoring Instrument with Berkeley High Resolution vertical column densities, a combined lightning data set derived from the Earth Networks Total Lightning Network and National Lightning Detection NetworkTM measurements, and hourly winds from the European Centre for Medium‐Range Weather Forecasts climate reanalysis data set (ERA5) over May–August 2014–2015. We find evidence that cloud‐to‐ground (CG) strokes produce a factor of 9–11 more NO2 than intracloud (IC) strokes for storms with stroke rates of at least 2,800 strokes·cell−1·hr−1. We show that regional differences in LNO2 production rates are generally consistent with regional patterns CG and IC stroke frequency and stroke current density. A comparison of stroke‐based and flash‐based CG/IC LNO2 estimates suggests that CG LNO2 is potentially underestimated when derived with flash data due to the operational definition of CG lightning. We find that differences in peak current explain a large portion of CG/IC LNO2 variability, but that other factors must also be important, including minimum stroke rate. Because IC and CG strokes produce NOx in distinct areas of the atmosphere, we test the sensitivity of our results against the atmospheric NO2 vertical distribution assumed in the a priori profiles; we show that the relative CG to IC LNO2 was generally insensitive to the assumed NO2 vertical distribution.

Estimates of lightning NO<sub><i>x</i></sub> production based on high-resolution OMI NO<sub>2</sub> retrievals over the continental US

“…Although this sensitivity anal-ysis is false in some regions, it allows the calculation of an upper limit on the NO 2 due to LNO and LNO 2 profiles. As discussed in Laughner and Cohen (2017), the scattering weights are uniform under cloudy conditions and the sensitivity of NO 2 is nearly constant with different pressure levels because of the high albedo. However, the relative distribution of LNO 2 within the UT should be taken carefully into consideration.…”

“…3.1 and 3.4. The bimodal profile modified from the standard Ott et al 2010profile (Laughner and Cohen, 2017) is employed as the vertical distribution of lightning NO (LNO) in WRF-Chem, while outputs of LNO and LNO 2 profiles are defined as the difference of vertical profiles between simulations with and without lightning.…”

“…The first part is the NO 2 above clouds (pixels with CRF > 0.9) and the second part is the NO 2 detected from cloud-free areas. A threshold of 3×10 15 molecules cm −2 , the typical urban NO 2 concentration, was applied to mask the contaminated grid cells (Beirle et al, 2010;Laughner and Cohen, 2017). The main difference between Lapierre et al (2020) and Pickering et al (2016) is the air mass factor for lightning (AMF LNO x ) implemented in the basic algorithm.…”

“…The Weather Research and Forecasting (WRF) model coupled with chemistry (WRF-Chem) has been employed to evaluate the convective transport and chemistry in many studies (Barth et al, 2012;Wong et al, 2013;Fried et al, 2016;. Meanwhile, Laughner and Cohen (2017) showed that the OMI AMF is increased by ∼ 35 % for summertime when LNO 2 simulated by WRF-Chem is included in the a priori profiles to match aircraft observations. The simulation agrees with observed NO 2 profiles and the bias of AMF related to these observations is reduced to < ±4 % for OMI viewing geometries.…”

Estimates of Lightning NO&lt;sub&gt;x&lt;/sub&gt; Production based on High Resolution OMI NO&lt;sub&gt;2&lt;/sub&gt; Retrievals over the Continental US

“…On another hand, Lapierre et al (2019) constrained LNO 2 to 1.1 ± 0.6 mol NO 2 /stroke for intracloud (IC) strokes and 10.0 ± 4.9 mol NO 2 /stroke for cloud-to-ground (CG) strokes over the continental US (CONUS). LNO 2 per stroke was scaled to 54.4 mol NO x /flash by strokes per flash and the ratio of NO to NO 2 in the UT.…”