Modeling secondary organic aerosol formation from volatile chemical products

Volatile chemical products (VCPs) are commonly used consumer and industrial items that are an important source of anthropogenic emissions. Organic compounds from VCPs evaporate on atmospherically relevant timescales and include many species that are secondary organic aerosol (SOA) precursors. However, the chemistry leading to SOA, particularly that of intermediate-volatility organic compounds (IVOCs), has not been fully represented in regional-scale models such as the Community Multiscale Air Quality (CMAQ) model, which tend to underpredict SOA concentrations in urban areas. Here we develop a model to represent SOA formation from VCP emissions. The model incorporates a new VCP emissions inventory and employs three new classes of emissions: siloxanes, oxygenated IVOCs, and nonoxygenated IVOCs. VCPs are estimated to produce 1.67 μg m−3 of noontime SOA, doubling the current model predictions and reducing the SOA mass concentration bias from −75 % to −58 % when compared to observations in Los Angeles in 2010. While oxygenated and nonoxygenated intermediate-volatility VCP species are emitted in similar quantities, SOA formation is dominated by the nonoxygenated IVOCs. Formaldehyde and SOA show similar relationships to temperature and bias signatures, indicating common sources and/or chemistry. This work suggests that VCPs contribute up to half of anthropogenic SOA in Los Angeles and models must better represent SOA precursors from VCPs to predict the urban enhancement of SOA.

Species assigned to low-volatility, inactive species (i.e. NVOL, NROG, or IVOC) whose ozone and SOA formation are not otherwise captured in existing gas-phase surrogates are assigned to the new IVOC surrogates (SOAOXY, IVOCP3/4/5/6/5ARO/6ARO) or an existing long alkane surrogate (SOAALK) according to their 165 structure, O:C, C*, and SOA yield. ¶ All species assigned to SOAOXY, IVOCP3/4/5/6/5ARO/6ARO, and SOAALK are also mapped to existing alkane surrogates based on kOH to participate in ozone chemistry. Siloxanes are not mapped to alkane surrogates because of their low kOH. Since these alkane species do not participate in the SOA module and the VCP surrogates do not affect ozone chemistry, this double-mapping does not double-count chemistry due to the lack of overlap between these parts of the gas-and aerosol-phase mechanisms. ¶ Some compounds were manually reassigned depending on their  Skamarock et al., 2008). Gas-phase chemistry is represented using SAPRC07TIC (Pye et al., 2013;Xie et al., 2013) with the addition of the VCP chemical mechanism summarized in Table 1. Aerosol-phase chemistry is simulated using an extended version of the AERO7 mechanism, depicted in Figure 1, including all AERO7 reactions plus those of the new VCP mechanism (boxed in red) and mobile IVOCs (boxed in red in the lower left) that participate in the multigenerational aging shown in the orange boxes (Lu 240 et al., 2020). This diagram also includes a representation of the aqueous-phase cloud chemistry and removal used in the Asymmetric Convection Model (ACM) version 2 module (Binkowski & Roselle, 2003), which has been updated to include wet deposition properties for the new aerosol surrogates (Table 1)   All smaller black boxes depict species undergoing gas-phase oxidation from VOCs to semivolatile or nonvolatile SOA species.
Orange font depicts the VBS model for S/IVOCs. Red font depicts particle-phase accretion reactions while purple font depicts particle-phase hydrolysis reactions. Green font represents heterogeneous processes. Blue font shows cloud-processed aerosol and 260 yellow font shows aerosol water associated with the organic phase. Gray boxes are nonvolatile primary organic aerosol (POA) species. Double-sided arrows represent reversible processes and one-sided arrows represent irreversible processes. Dashed lines represent processes that are dependent on relative humidity. The diagram includes the AERO7 mechanism plus the three VCPforming pathways specific to this work (thick boxes in red). See AE7I Species Table (

Simulation cases
Three simulations were evaluated against the observations collected during the CalNex campaign. A "zero VCP" case removes all VCP emissions. The "CMAQv5.3.2" case is a standard CMAQ simulation with base emissions (i.e. VCP emissions from the NEI) and base chemistry (i.e. no new VCP chemistry). Finally, the "CMAQv5.3.2+VCP" case adds both 270 the VCP chemistry described above (i.e. SAPRC07TIC_AE7I_VCP) and replaces all NEI VCP emissions with VCPyderived VCP emissions. Comparisons between the "zero VCP" case and the "CMAQv5.3.2+VCP" case illustrate the complete impact of VCPy emissions on modeled SOA. In contrast, comparisons between the "CMAQv5.3.2" case and the  3.5% of the total emissions are assigned to siloxanes, 7.8% to oxygenated IVOCs, 11.8% to nonoxygenated IVOCs, and 20.4% to traditional SOA precursors, such as VOC alkanes, toluene, and other aromatics. The volatility and SOA yields of 300 species in each category are summarized in Fig. S1. Figure 2 indicates that in traditional model processing, precursors to SOA are systematically discarded from chemistry calculations. As described in Section 2.1, low-volatility emissions (i.e. NROG, NVOL, and IVOC) do not participate in SOA or radical chemistry in traditional SAPRC07TIC_AE7I which is a key issue in representing SOA mass. The inner ring of 305 Figure 2 depicts the fraction of each category that was originally assigned to inactive species (NROG, NVOL, and IVOC; hatched) versus other existing surrogates (solid). 2.6 x 10 7 kg year -1 (30.7%) of the total VCP emissions were originally assigned to these surrogates and did not participate in any atmospheric chemistry processes. Using the new speciation and mechanism, 1.8 x 10 7 kg year -1 (21.2% of total VCP emissions) were reassigned to surrogates that form SOA in the model Deleted: Fig. 2 (hatched inner ring: red, blue, orange, and purple). The remaining 8.0 x 10 6 kg year -1 (9.4% of total VCP emissions; inner ring hatched green) is comprised of species with SOA yields of zero and were not reassigned to SOA-forming surrogates.

Figure 2. Percentage of the VCP emissions assigned to each category of CMAQ surrogates using the SAPRC07TIC_AE7I_VCP
mobile sources contributed an additional 1.1 µg m -3 at noon (Lu et al., 2020). Therefore this updated CMAQ model predicted a total IVOC-derived SOA concentration of 2.35 µg m -3 , equivalent to 35% of the total observed above-background PM1 SOA concentration (6.6 µg m -3 ). Previous work stated that 40-85% of above-background oxidation rate. The increase in formaldehyde between simulation cases, therefore, cannot be largely attributed to the addition inventory includes near-zero emissions of formaldehyde, but formaldehyde is emitted from wooden furniture and emission rates increase with temperature (Y. Wang et al., 2021). This may account for some of the temperature-dependence of 635 formaldehyde bias, but likely not the entirety since the VCP emissions inventory has been evaluated with select ambient VOC measurements with low error (Seltzer et al., 2021). One possible explanation of the temperature-dependence of both the SOA and formaldehyde biases is missing sources of emissions and resulting chemistry. Previous work has shown that formaldehyde formation is particularly sensitive to the emissions/chemistry of alkenes (e.g. isoprene) and, to a lesser extent, alkanes and aromatics (Luecken et al., 2018), so these precursors likely indicate missing emissions as a source of error in 640 our model. While the radical chemistry of these hydrocarbon precursors are included in the model, additional missing chemistry may be causing some of the error. Chemical processes that have not been included in the mechanism include autooxidation (Crounse et al., 2013) -which forms low-volatility SOA -and formaldehyde potentially formed from the fragmentation of S/IVOC precursors to SOA. The inclusion of these missing emissions and/or chemistry would further impact oxidant levels, which we have shown to be an important source of modeled SOA and formaldehyde. As stated above, 645 the behavior of POA and CO bias suggest that errors in combustion emissions and PBL height cannot fully describe the temperature-dependence of SOA bias, and POA and CO are better indicators of mobile and industrial sources. Formaldehyde may instead serve as a better indicator of SOA production in urban areas where VCPs are important atmospheric constituents. While many factors may contribute to the temperature-dependence of SOA and formaldehyde bias, future work must investigate the importance of these factors and tracking the response of formaldehyde to these changes alongside SOA 650 could provide insight.

Conclusions and future work
We have shown that VCPs are a major source of SOA in urban atmospheres by introducing updated emissions and VCP-655 relevant chemistry into CMAQ that better represents SOA precursors emitted from these sources. This includes three new categories of emissions: siloxanes, oxygenated IVOCs, and nonoxygenated IVOCs. VCP emissions from the VCPy framework (Seltzer et al., 2021) were used to parameterize the new chemistry, and the mapping of VCP emitted species to model surrogates was reviewed and updated based on species structure, volatility, and estimated SOA yield.

660
The new model chemistry and emissions inventory doubles the predicted SOA concentrations above background levels, increasing the average daily maximum PM1 SOA concentration by 1.4 µg m -3 , equating to a 21% decrease in the absolute mean bias. Most of the increased SOA mass was formed from nonoxygenated IVOC VCP precursors, followed by SOA formed from traditional VOC precursors and oxygenated IVOC precursors, with little SOA formed from siloxanes. Improvements were additionally seen in simulated formaldehyde and ozone concentrations. 665 Deleted: Because formaldehyde is a major product of VOC oxidation, the temperature-dependence of both SOA and formaldehyde may suggest that additional missing chemistry may be Deleted: , detailed chemistry of oxygenated precursors,

Moved (insertion) [2]
Deleted: F Deleted: As mobile-source emissions decline and VCP emissions become the dominant source of aerosol in urban areas, formaldehyde should be considered an important indicator for SOA production from VCPs. ¶