Methylglyoxal-Urea Brown Carbon Aerosol: A Loss Process for Urea

Recent studies reveal that urea (CO(NH₂)₂) is often a significant component of tropospheric reactive nitrogen in both the gas- and the condensed-phases; however, little is known about urea sources and sinks. Although it is generally assumed that deposition is the major sink, aqueous reactions in aerosols and clouds may be possible but have yet to be considered. Here, we report a study of the aqueous reactions of urea with methylglyoxal as a function of pH using optical property measurements as a proxy for reaction. UV-vis absorption spectroscopy is used to monitor bulk-phase browning while cavity ringdown and photoacoustic spectroscopies are used to measure the aerosol optical properties. We observe the reaction of urea with methylglyoxal produces brown carbon at low and high pH, with little absorption at mid-pH ranges. The urea brown carbon products absorb well into the visible range, providing a greater overlap with the solar emission spectrum than previously studied brown carbon systems including that of methylglyoxal and ammonium. These experiments suggest that urea could be chemically transformed in aqueous aerosol with the products contributing to aerosol absorption.