Multi-satellite sensor study on precipitation-induced emission pulses of NO_x from soils in semi-arid ecosystems

Abstract. We present a top-down approach to infer and quantify rain-induced emission pulses of NO_x ( ≡  NO + NO₂), stemming from biotic emissions of NO from soils, from satellite-borne measurements of NO₂. This is achieved by synchronizing time series at single grid pixels according to the first day of rain after a dry spell of prescribed duration. The full track of the temporal evolution several weeks before and after a rain pulse is retained with daily resolution. These are needed for a sophisticated background correction, which accounts for seasonal variations in the time series and allows for improved quantification of rain-induced soil emissions. The method is applied globally and provides constraints on pulsed soil emissions of NO_x in regions where the NO_x budget is seasonally dominated by soil emissions.

We find strong peaks of enhanced NO₂ vertical column densities (VCDs) induced by the first intense precipitation after prolonged droughts in many semi-arid regions of the world, in particular in the Sahel. Detailed investigations show that the rain-induced NO₂ pulse detected by the OMI (Ozone Monitoring Instrument), GOME-2 and SCIAMACHY satellite instruments could not be explained by other sources, such as biomass burning or lightning, or by retrieval artefacts (e.g. due to clouds).

For the Sahel region, absolute enhancements of the NO₂ VCDs on the first day of rain based on OMI measurements 2007–2010 are on average 4 × 10¹⁴  molec cm⁻² and exceed 1 × 10¹⁵  molec cm⁻² for individual grid cells. Assuming a NO_x lifetime of 4 h, this corresponds to soil NO_x emissions in the range of 6 up to 65 ng N m⁻² s⁻¹, which is in good agreement with literature values. Apart from the clear first-day peak, NO₂ VCDs are moderately enhanced (2 × 10¹⁴  molec cm⁻²) compared to the background over the following 2 weeks, suggesting potential further emissions during that period of about 3.3 ng N m⁻² s⁻¹. The pulsed emissions contribute about 21–44 % to total soil NO_x emissions over the Sahel.