What are the Most Important Precursors of Urban Secondary Organic Aerosol (SOA)?   - What are the Most Important Precursors of Urban Secondary Organic Aerosol (SOA)?

Secondary organic aerosol (SOA), formed from the oxidation of gas-phase organic compounds, is the largest component of fine particles (PM2.5). Three major new groups of urban precursors have been identified: semivolatile and intermediate volatility species (S/I/VOCs), volatile chemical products (VCPs), and biogenic VOCs emitted within the urban area. The relative importance of each source remains controversial.

We measured SOA formation potential (SOA-FP), i.e. the species present in the gas-phase, but in units of how much SOA they can form, in Los Angeles during Summer 2022. We separately quantified SOA-FP from VOCs vs. S/IVOCs, to our knowledge the first measurements with this separation. ~31% of the total SOA-FP is due S/IVOCs. Results are compared to two box models: both performed well for SOA but poorly for SOA-FP, and both estimated a much higher fraction from S/IVOCs vs. the observations. This suggests important gaps in urban SOA models. Our results do not support monoterpenes being a dominant contributor to SOA-FP in LA, contrary to two major recent studies. 

SOA-FP was also measured indoors in a real house in Colorado. To our knowledge these are the first measurements of SO-FP in a real, lived-in house. Indoor SOA-FP was an order-of-magnitude higher than outdoors. Activities such as rapid ventilation, cooking or cleaning greatly perturbed indoor SOA-FP for a few hours, but the house quickly returned to background SOA-FP values. This confirms that surface reservoirs are very large and are able to replenish and sustain this high SOA potential. Contrary to most of the indoor literature, precursors other than monoterpenes dominated indoor SOA.