The influence of hydrogen addition on carbonaceous aerosols produced by an ethylene flame

Abstract. Combusting hydrogen alongside carbon-based fuels has been proposed to reduce CO₂ emissions and combat climate change. However, combustion-generated aerosol particles can also cause a significant radiative forcing on climate. Since addition of novel fuels alters the combustion process, it also influences particle formation inside the fame and consequently the properties of the emitted aerosols. To investigate this, combustion-generated particles from various ethylene/hydrogen mixtures are sampled in the post-flame regime. The size distribution and light absorption properties of the particles are measured using a scanning mobility particle sizer (SMPS) and a multi-wavelength aethalometer. In addition, the particles are sampled on quartz-fiber filters and the mass concentrations of organic, elemental and total carbon (OC, EC, and TC) are measured using a thermo-optical OC-EC analyzer. The geometric mean diameter of the emitted particles decreased from 300 nm down to 150 nm upon increasing the hydrogen mole fraction in the fuel from 0 % to 50 %, while the EC/TC fraction decreased from 70 % to 35 %. The light absorption of methanol-dissolved OC were measured using UV-vis analysis, showing no dependence on flame parameters or fuel composition, and no significant light absorption at wavelengths larger than 500 nm. For combustion-generated particles, the mass absorption cross section σ of the total carbonaceous aerosol (i.e. the absorption coefficient normalized to TC mass concentration) is reported as a function of EC/TC ratio at wavelengths of 370, 590 and 880 nm. At a wavelength of 880 nm, σ is slightly higher than expected of an external mixture of OC and EC, indicating some absorption enhancement due to OC coating. At wavelengths of 590 and 370 nm, σ is much higher than that expected for a mixture of colorless OC and EC and this enhancement is attributed to light absorbing non-refractory species, also called brown carbon (BrC). The absorption Ångström exponent (370–660 nm) increased from 1.3 up to 3.8 with increasing hydrogen mole fraction in the fuel, especially at lower flame temperatures, indicating an increasing contribution of BrC to the light absorption of the emitted particles. It is concluded that BrC is a precursor to EC during particle formation, in line with the existing literature, and that it matures less efficiently into EC in the hydrogen containing flame.