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NO$_{\rm x}$ formation in lean premixed and highly-strained pure hydrogen-airflamelets is investigated numerically. Lean conditions are established at anequivalence ratio of 0.7. Detailed-chemistry, one-dimensional simulations areperformed on a reactants-to-products counter-flow configuration with an appliedstrain rate ranging from $a=100 \, {\rm s}^{-1}$ to $a=10000 \, {\rm s}^{-1}$and the \texttt{GRI3.0} mechanism. Following a similar setup, two-dimensionaldirect numerical simulations are also conducted for representative strain ratesof $2000 \, {\rm s}^{-1}$ and $5000 \, {\rm s}^{-1}$. Both solutions show adecreasing NO$_{\rm x}$ trend as the applied strain rate is increased. Thisdecreasing emission outcome is highlighted for the first time in this study forlean pure-hydrogen flamelets. A deep analysis of the 2D solution underlinesthat there is no production of NO$_{\rm x}$ in the second dimension, thusproving that the emission trend is not a result of a setup preconditioning, butis instead a direct physical effect of stretch on the flame. Furthermore, adetailed analysis of the NO$_{\rm x}$ formation pathways at $a=2000 \, {\rms}^{-1}$ and $a=5000 \, {\rm s}^{-1}$ is performed. Thermal NO$_{\rm x}$ andNNH pathways are shown to both contribute significantly to the total NO$_{\rmx}$ production. While the NNH route contribution is roughly constant atdifferent strain rates, a significant decrease is observed along the thermalNO$_{\rm x}$ route. Overall, results show that lean and highly-strainedhydrogen flames experience a significant decrease of NO$_{\rm x}$. Thisproperty is discussed and analysed in the paper.

NOx emissions trends in hydrogen lean premixed flamelets at high strain rate