Atmospheric Brown Clouds: the ABCs of climate change and long range pollution transport

Atmospheric Brown Clouds (ABCs) are composed of submicrometer aerosols, including black carbon and other constituents (e.g., sulfate, organics). Globally, ABCs are a major agent of climate change and long range pollution transport.  Single particle mass spectra indicate ABC particles in size ranges that efficiently scatter (e.g., 0.2-1.0 μm) are comprised predominantly of internal mixtures that include organic carbon. The vertical profile of organic carbon is not well simulated in atmospheric models and this contributes substantially to uncertainty in climate projections because radiative scattering is altitude dependent. Changes in emissions, SOA partitioning parameters, (among other efforts) do not improve model-predicted vertical profiles, but inclusion of aqueous phase organic chemistry (e.g., cloud processing of VOCs) does. Organic “brown” carbon, often associated with humic-like substances (HULIS) is ubiquitous in the atmosphere.  Sources are thought to include multi- or mixed-phase atmospheric processes (e.g., aqueous phase chemistry in cloud droplets). It has been demonstrated in recent laboratory experiments that products of aqueous phase oxidation of water soluble atmospheric gases such as, glyoxal, methylglyoxal and phenols include HULIS and other light-absorbing products.  In this work I explore how inclusion of aqueous phase chemistry in clouds in 3-dimensional atmospheric models changes the predicted vertical profile of particulate carbon.  Inclusion of multiphase atmospheric chemistry improves closure with measurements of short-lived climate forcers (SLCFs), particularly aloft.