In situ measurements of potential SOA formation and OA aging during GoAmazon2014/15 & Direct measurements of gas-particle partitioning and mass accommodation coefficients in environmental chambers

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Description

Secondary organic aerosols (SOA) are an important contributor to climate forcing, and many of their fundamental processes remain poorly understood. During GoAmazon2014/15, the potential of air masses to form SOA and the aging of ambient OA were investigated by exposing ambient air to OH or O3 using an oxidation flow reactor (OFR). Oxidation of ambient air led to variable SOA formation (~1–10 ug/m3). Typically, more SOA was produced during nighttime versus daytime, suggesting higher SOA-precursor gases at night. More SOA was formed in the dry than wet season. O3 oxidation led to several times less SOA formation than OH oxidation. SOA formed from OH was up to an order of magnitude larger than can be explained from aerosol yields of measured primary VOCs. This, along with previous measurements at other locations, suggests that most SOA was formed from typically unmeasured S/IVOC gases (e.g., VOC oxidation products). SOA yields in the OFR were measured by standard addition of VOCs to ambient air, and found to be similar to published chamber yields. Preliminary PMF factor analysis shows production of secondary oxidized OA. Factors that are primary emissions or require reactive uptake (e.g., HOA, BBOA, IEPOX-SOA) were not formed, but were depleted at high ages (at different rates). Relationships between SOA formation and different anthropogenic and biogenic influences are investigated. Most aerosol models use gas-particle equilibrium partitioning theory as default treatment of gas-aerosol transfer, despite questions about its validity and applicability. We have conducted fundamental SOA formation experiments in a Teflon environmental chamber using a novel fast-product-burst method. A simple chemical system rapidly produces low-volatility gas products that are competitively taken up by liquid OA seed and chamber walls. Clear differences in the kinetics versus OA seed amounts allow us to separate and quantify the influence of wall and particle uptake. We reproduce gas- and aerosol-phase observations using a kinetic box model, from which we also quantify the aerosol mass accommodation coefficient (α). We measure an average α of 0.6, in agreement with some previous work, suggesting values approaching unity for low-volatility species. α appears to decrease as volatility increases. Using the model and derived parameters, we propose a modeling framework for correcting SOA mass yields and a wall-loss SOA yield correction factor, Φ, that varies ~1-4 for typical experiments.