Volatile organic compounds (VOCs) degrade air quality and are emitted to urban air from many sources, including fossil fuel combustion (Warneke et al., 2012), the use of volatile chemical products (VCPs; McDonald et al., 2018), industrial processes (Zhang et al., 2004), residential heating and wood burning (e.g., McDonald et al., 2000; Coggon et al., 2016), cooking (Wernis et al., 2022), and urban vegetation (e.g., Churkina et al., 2017). Each source emits a diverse set of molecules that react alongside nitrogen oxides (NO + NO2= NOx) to form ozone. Recent fieldwork in major US metropolitan areas has characterized the distribution of urban VOCs to assess the chemical fingerprint of understudied emission sources, such as VCPs (Gkatzelis et al., 2021b; Peng et al., 2022). These studies have shown that these sources emit oxygenated VOCs (oVOCs) that react to form ozone and secondary organic aerosol (SOA). Models have been updated to better describe the emissions and chemistry of select oVOCs, including alcohols, siloxanes, glycols, and furanoids (Coggon et al., 2021; Pye et al., 2023; Qin et al., 2021).

Many oVOCs are emitted to urban air that have not been well studied or incorporated into air quality models (Karl et al., 2018). For example, McDonald et al. (2018) showed that C > 5 aldehydes measured in Los Angeles, CA, could not be explained by emissions inventories that contain VCPs, fossil fuels, or biogenic sources. Cooking is a source of oVOCs that is rich in aldehydes and fatty acids (Klein et al., 2016a). Cooking VOCs have been extensively characterized in the laboratory (Bastos and Pereira, 2010; Klein et al., 2016a, b; Schauer et al., 1999; Zhao and Zhao, 2018), and it has been shown that cooking is a key activity controlling the budget of VOCs measured in indoor air (Arata et al., 2021; Klein et al., 2019, 2016a). Numerous studies have shown that cooking is a ubiquitous and important component of organic aerosol in urban areas (Hayes et al., 2013; Robinson et al., 2006; Robinson et al., 2018; Shah et al., 2018; Slowik et al., 2010; Zhang et al., 2019), but only a few studies have been conducted to characterize cooking VOCs in ambient datasets (e.g., Peng et al., 2022; Wernis et al., 2022).

Cooking emissions result from the combustion and high-temperature decomposition of food and oils (Bastos and Pereira, 2010; Umano and Shibamoto, 1987). During heating, fatty acids undergo thermal oxidation to produce emissions of aldehydes, ketones, alcohols, acids, and other products of depolymerization (Bastos and Pereira, 2010; Schauer et al., 1999). The use of spices emits monoterpenes and their derivatives (Klein et al., 2016a). Studies that have speciated VOCs from a variety of Western cooking styles (e.g., charbroiling, grilling, and frying) and ingredients (e.g., oils, meats, and vegetables) show that aliphatic C1–C11 aldehydes account for a large fraction of VOCs measured by gas chromatography and mass spectrometry (e.g., Bastos and Pereira, 2010; Klein et al., 2016b; Peng et al., 2017; Schauer et al., 1999). For example, Klein et al. (2016b) reported that aldehydes represent > 60 % of the VOC mass emitted from frying or charbroiling meats and vegetables.

Ambient observations have shown that long-chain aldehydes are present in urban air at significant mixing ratios (e.g., nonanal ∼ 100–200 ppt (parts per trillion); Bowman et al., 2003; Wernis et al., 2022). Recent studies have used hexanal and nonanal as markers for cooking emissions in Atlanta, GA, and Livermore, CA (Peng et al., 2022; Wernis et al., 2022). These species correlated with morning and evening meal preparation, and it was suspected that restaurant emissions were a driving factor of the observed temporal variability in Livermore (Wernis et al., 2022). In addition to cooking, certain long-chain aldehydes are known to be emitted from diesel exhaust (e.g., hexanal; Gentner et al., 2013), and some are produced from the emission and ozonolysis of oils and fatty acids present in human skin, at the surface of ocean waters, or from other surfaces that contain unsaturated lipids (Kruza et al., 2017; Liu et al., 2021; Wang et al., 2022; Kilgour et al., 2024).

The high abundance of aliphatic aldehydes from cooking suggests that they may be useful markers to constrain cooking VOC emissions in urban areas. The utility of aldehydes as cooking markers relies on the characterization of their sources in the atmosphere as well as careful characterization of the measurement techniques used to detect these species. Short-chain aldehydes (C < 5) are unlikely to serve as useful markers because they are produced in the atmosphere from the OH oxidation of primary organic molecules and are also directly emitted from fossil fuel emissions and biomass burning (Gentner et al., 2013; Koss et al., 2018; de Gouw et al., 2018). Long-chain aliphatic aldehydes (C > 6) have the potential to be useful markers for cooking in urban areas, but their primary sources, spatial distributions, and abundances in the atmosphere remain uncertain. Some long-chain aldehydes may be emitted from mobile sources (e.g., Gentner et al., 2013), while others may be emitted from surface ozone chemistry (e.g., Liu et al., 2021). No field measurements have reported the spatial distribution of long-chain aldehydes to determine likely sources in urban air.

This study evaluates the contribution of commercial and residential cooking emissions to urban VOCs using mobile laboratory and ground site observations made in Las Vegas, NV, with supplemental observations made Los Angeles, CA, and Boulder, CO. Mobile laboratory measurements show that long-chain aliphatic aldehydes are significantly enhanced downwind of restaurants and exhibit spatial distributions in urban regions of Las Vegas that closely match restaurant density. These measurements also show that the chemical footprint of VOCs observed in entertainment and dining districts closely matches the fingerprint observed in restaurant plumes. We conduct a source apportionment analysis to determine the extent to which cooking emissions impact urban VOCs relative to other important anthropogenic sources, such as motor vehicles and VCPs, and compare these observations to commonly used emissions inventories.

The analysis described in Sect. 3.2 provides a perspective on the key VOCs emitted from commercial cooking. Other VOCs are also likely associated with cooking, but they are not resolved by a simple correlation analysis due to the presence of other important sources along the Las Vegas Strip, such as ethanol from VCPs or monoterpenes from fragranced consumer products. Moreover, emissions from residential cooking may also contribute to regional VOC mixing ratios. Here, we discuss the PMF results to determine the emissions profile and the contribution of cooking to total VOC emissions measured in Las Vegas.

Figures 8 and 9 show the PMF solution for the data collected at the Jerome Mack ground site. Figure 8 shows the time series and factor profiles, while Fig. 9 shows the average diurnal profiles. We present a five-factor solution: VOCs are apportioned to (1) a mobile source factor, (2) a VCP-dominated factor, (3) a cooking-dominated factor, (4) a regional background plus secondary oxidation processes factor, and (5) a local solvent source factor. A full description of the PMF results is provided in the Supplement.

The mobile source factor is largely composed of ethanol (EOH) and C6–C10 aromatics (Fig. S8). The VCP-dominated factor is primarily composed of ethanol but also contains D5-siloxane, monoterpenes, and acetone, which are common ingredients in consumer products. Both factors resemble the solution presented by Gkatzelis et al. (2021b) for New York City. The mobile source factor had the highest correlation to CO measured at the ground site (R2= 0.72), which is consistent with the expectation that mobile sources are important contributors to the variability in CO in urban areas (McDonald et al., 2013). The VCP-dominated factor was also correlated to CO (R2= 0.67) and likely results from the coincidental emission of VCPs and mobile sources over similar temporal and spatial scales. Similar behavior was proposed to drive correlations between VCP and mobile source emissions observed in other cities, such as New York City, Boulder (CO), and Toronto (ON) (Coggon et al., 2018; Gkatzelis et al., 2021a; Gkatzelis et al., 2021b).

The VCP-dominated factor derived here has two key differences from the factor derived by Gkatzelis et al. (2021b). First, the VCP-dominated factor from New York City contained a series of other VCP markers, including parachlorobenzotrifluoride (PCBTF) and D4-siloxane. These molecules are largely associated with construction activity and are expected to be emitted from the application of industrial coatings and adhesives (Gkatzelis et al., 2021a; Stockwell et al., 2021). PCBTF was attributed to the VCP-dominated factor at the Jerome Mack ground site but was also associated with the local solvent factor which appeared to come from a point source located near the ground site (Fig. 8). Second, the VCP-dominated factor reported by Gkatzelis et al. (2021b) also contained methyl ethyl ketone, which is a prominent solvent in consumer and industrial VCPs. Methyl ethyl ketone was excluded from this analysis due to a water cluster interference produced within the Vocus drift tube.

The oxidation factor is largely composed of multiple oxygenated carbon-containing masses, which agrees with secondary factors resolved by PMF in other cities (Gkatzelis et al., 2021b; Peng et al., 2022). At the Jerome Mack site, this factor also contained VOCs that are emitted during daytime hours from biogenic sources (e.g., isoprene and monoterpenes) due to emissions from urban vegetation. In general, isoprene and monoterpenes had relatively low mixing ratios over the analyzed sampling period (< 150 ppt). For comparison, measurements in Los Angeles during RECAP-CA show that average isoprene mixing ratios exceeded 2 ppb (Coggon et al., 2024). This difference highlights that urban vegetation emissions in Las Vegas are significantly lower than other cities.

PMF of the Jerome Mack data resolves a factor that is enriched in aldehydes, which we interpret as the cooking-dominated factor. This factor includes contributions from octanal, nonanal, acetic acid, acrolein, and higher-carbon aldehydes and acids, consistent with the cooking emissions observed along the Las Vegas Strip and downwind of restaurants. Figure S12 compares the PMF profile to the VOC / nonanal profiles resolved by the mobile laboratory and shows that the two profiles agree for overlapping species. This agreement supports the PMF resolution of mass associated with important cooking VOCs. The PMF factor also includes ethanol, monoterpenes, and acetone and propanal, which were not resolved by the mobile laboratory analysis. Cooking emits significant amounts of ethanol and monoterpenes to indoor air (Arata et al., 2021; Klein et al., 2016a) and is a dominant source of total VOC emissions in residential indoor air (Arata et al., 2021; Klein et al., 2019). These species represent ∼ 22 % of the cooking-dominated factor.

Figure 9 shows daily mass concentrations for each factor. Gkatzelis et al. (2021b) found that ∼ 53 % of mobile source emissions and ∼ 50 % of VCP emissions are associated with molecules that cannot be resolved by PTR-ToF-MS (e.g., alkanes and small alkenes). The mobile source and VCP-dominated factors shown in Figs. 9 and 10 have been adjusted by the following equation to account for this unresolved mass: 1Mtotal=Ma, where Mtotal is the adjusted PMF factor, M is factor mass, and a is the fraction of the factor mass estimated by Gkatzelis et al. (2021b) to be measured by PTR-ToF-MS for VCP (0.5) and mobile source (0.47) emissions. No adjustments are applied to the cooking-dominated, solvent, or oxidation factors, as it is assumed that PTR-ToF-MS measures the key VOCs from these sources. This may not account for mass that has been previously reported from cooking emissions, such as alkanes or alkenes (Schauer et al., 1999).

For the mobile source, VCP-dominated, and cooking-dominated factors, mass concentrations are highest at night when the nocturnal boundary layer is shallow. In the daytime, the boundary layer rapidly rises to as high as 4 km (Langford et al., 2022), and VOC concentrations decrease in response. In contrast, the chemical oxidation + daytime emissions factor increases during daytime hours, further supporting that this factor is largely driven by secondary processes.

Figure 10a shows the fraction (fi) that each primary factor contributes to total VOC mass resolved by PMF at the Jerome Mack ground site, which is calculated as follows: 2fi=Mi∑Mi, where Mi is the mass concentration of the VCP-dominated, mobile source, and cooking-dominated factors. Here, the denominator represents the total anthropogenic emissions resolved by PMF. The local solvent factor is excluded from this analysis because it is not representative of regional VOC concentrations. Figure 10b shows the average factor contribution over the entire analysis period.

Figure 10a shows that each factor contributes to the total anthropogenic emissions at different times of the day depending on the emission patterns. The VCP-dominated factor is the largest contributor to total VOC emissions in Las Vegas and constitutes 40 %–80 % of the primary VOCs resolved by PMF. These concentrations are largely driven by the high emissions of solvents, such as ethanol and acetone, consistent with observations from New York City (Gkatzelis et al., 2021b). Emissions inventories indicate that these molecules are primarily emitted from the personal care product sector (see Sect. 5). VCP emissions exhibit the highest relative abundances early in the day (∼ 11:00 LT) and then decrease in relative abundance throughout the day. This behavior is similar to the diurnal pattern of personal care product emissions observed in cities such as Boulder, CO, where the mixing ratios of D5-siloxane from deodorants and hair products peak during morning hours and then decay as personal care products evaporate (Coggon et al., 2018). During evening and rush hour periods, mobile sources constitute ∼ 30 %–40 % of the total primary VOC mixing ratios, but they then decrease during midday due to a large enhancement of VCPs and lower emissions from mobile sources. Over the entire dataset, VCPs and mobile sources are estimated to represent 54 % and 25 % of the total anthropogenic VOCs, respectively (Fig. 10b).

The cooking-dominated factor represents 10 %–30 % of the total primary VOC mass resolved by PMF, depending on the time of day. The relative fraction of the cooking-dominated factor peaks in the mid-afternoon as well as in the evening and night, when activity along the Las Vegas Strip is highest. Similar behavior has been observed in the relative abundance of primary cooking organic aerosol in cities such as Los Angeles (Hayes et al., 2013). The cooking-dominated factor is estimated to represent as much as 21 % of the total anthropogenic VOCs over the entire dataset.

The fraction of cooking VOCs estimated here (21 %) is specific to a site downwind of the Las Vegas Strip where restaurants are abundant. These impacts are likely to vary across urban areas based on ground site locations, restaurant density, and differences in the proportions of fossil fuel and VCP emissions. Cooking emissions are commonly resolved from the source apportionment of organic aerosol measurements in US cities (e.g., Hayes et al., 2013; Lyu et al., 2019; Zhang et al., 2019; Xu et al., 2015). Far fewer studies have estimated the impact of cooking on the outdoor VOC burden in US urban areas. Recently, cooking emissions were identified from the source apportionment of thermal desorption aerosol gas chromatograms and shown to be present at significant mixing ratios in Livermore, CA (Wernis et al., 2022). Similarly, source apportionment of PTR-ToF-MS data from Atlanta, GA, showed that cooking emissions mixed with biomass burning were responsible for 6 %–15 % of the reported VOC carbon, which included contributions from fossil fuel, VCPs, and biogenic sources (Peng et al., 2022). These proportions are similar to those reported here, suggesting that cooking VOCs represent a significant fraction of total anthropogenic VOCs in other US cities. Tables S1 and S2 summarize the cooking profile resolved by the PMF analysis. This profile could be used for comparison against measurements of cooking VOCs in other urban areas.

The PMF results shown in Fig. 10b are compared against the distribution of anthropogenic VOCs reported in emissions inventories used to model air quality (Fig. 10 c, d). Figure 10c shows emissions reported in the 2020 National Emissions Inventory (NEI) for Clark County, NV. The NEI is a benchmark for determining US emissions standards, and its methodology is fully described by the US Environmental Protection Agency (US Environmental Protection Agency, 2023). The NEI for Clark County includes emissions for mobile sources (e.g., on- and off-road vehicles), fossil fuel evaporative sources (e.g., gasoline stations), solvent evaporative sources (e.g., VCPs), and miscellaneous point and area sources. Cooking emissions in the NEI predominantly result from commercial sources with minor contributions from residential backyard barbecuing.

Figure 10d shows the distribution of VOCs represented by the FIVE-VCP-NEI17NRT inventory described by He et al. (2024). The FIVE-VCP inventory was developed following the methods prescribed by McDonald et al. (2018) and was recently used to determine VCP impacts on air quality in US cities (e.g., Coggon et al., 2021; Qin et al., 2021). He et al. (2024) updated FIVE-VCP to include 2017 NEI emissions (NEI17) and revised VOC emissions with near-real-time (NRT) adjustment factors to account for COVID-19 impacts on various emission sectors. Mobile source emissions are determined from fuel sales and on- and off-road emission factors. VCP emissions are estimated based on the mass balance of the chemical product industry for 2010 and then adjusted to 2021 emissions based on the long-term declining trends in VCP emissions reported by Kim et al. (2022) and economic scaling factors reported by He et al. (2024). Other sectors are from the NEI17 and are similarly updated with near-real-time adjustment factors. Cooking emissions in FIVE-VCP-NEI17NRT are the same as those used in the 2017 NEI.

The 2020 NEI and FIVE-VCP-NEI17NRT inventories both indicate that VCPs are the dominant VOC emission sources in Clark County. Fossil fuel emissions are the next largest source, although differences between the two inventories are evident. In the NEI, total fossil fuel emissions (= on- and off-road emissions + evaporative emissions) are 15 % lower than VCP emissions. In FIVE-VCP-NEI17NRT, fossil fuel emissions are 57 % lower than VCPs. These differences are reflected in previous comparisons between the NEI and FIVE-VCP (Coggon et al., 2021; McDonald et al., 2018). The PMF solution shows that fossil fuels are ∼ 54 % lower than VCPs, which is most consistent with FIVE-VCP-NEI17NRT. Zhu et al. (2023) show that FIVE-VCP speciation agrees well with VOCs primarily emitted from fossil fuel and VCPs reported during SUNVEx and RECAP-CA. Aldehydes, ethanol, and monoterpenes were underestimated which may point to the importance of missing emission sources, such as cooking.

The fraction of total cooking VOC emissions represented in both inventories is significantly lower than the fraction resolved by PMF. Commercial sources dominate the cooking emissions in the 2017 and 2020 NEI emissions and are estimated based on food consumption estimates and emission factors derived from laboratory studies (e.g., Schauer et al., 1999). The differences between the PMF results and what is reported in the inventories may be partially explained by the spatial scale of the datasets: the inventories represent county-level emissions estimates, whereas the observations are specific to a site strongly influenced by the Las Vegas Strip. Restaurant statistics indicate that the Las Vegas Strip and downtown regions of Las Vegas have ∼ 550 restaurants (Fig. 1). In July 2021, approximately 106 000 tourists visited Las Vegas every day (LVCVA, 2024). Assuming that the tourism population dominates in this region, this would suggest that there are ∼ 530 restaurants per 100 000 people within the entertainment districts. In contrast, there are ∼ 5000 restaurants and 2.4 million people (including tourists) in Clark County, which equates to ∼ 210 restaurants per 100 000 people county-wide. This suggests that the ratio of cooking / VCP emissions along the Las Vegas Strip may be more than twice as high as that in Clark County. These differences do not account for other factors that may affect emissions, such as the types of cooking conducted in each region.

Despite the potential variability in emission patterns between datasets, Fig. 10 shows that there is a significant disconnect between the cooking emissions resolved by PMF and those represented by inventories. These differences highlight the need for further analysis. It is possible that the emission factors and/or the consumption of oils, meats, and other foods are different from what is reflected in laboratory studies.

Mobile laboratory and ground site measurements were analyzed to determine the importance of cooking emissions on the urban VOC composition in Las Vegas, NV. PTR-ToF-MS data show that cooking is a significant source of long-chain aldehydes to urban air. Measurements of octanal and nonanal are found to be useful markers to evaluate cooking emissions due to their abundance in restaurant plumes and local enhancements in areas with high restaurant density. A comparison of the mass spectra downwind of restaurants with those obtained in regions with significant commercial cooking show similar distributions in aldehydes and fatty acids known to be emitted from laboratory cooking experiments.

Based on a PMF analysis, it is estimated that cooking emissions represent as much as 20 % of the anthropogenic VOCs emitted to the atmosphere in Las Vegas, NV. It is expected that the relative importance of cooking emissions in other cities will vary based on the regional restaurant density and the magnitude of other anthropogenic emissions, including VCPs and mobile sources. More work is needed to quantify cooking in other urban areas. Measurements from this study (Fig. S12) in Pasadena, CA, and those conducted previously in Livermore, CA, and Atlanta, GA, show that long-chain aldehydes are ubiquitous in urban air (∼ 100–200 ppt) and modulated by commercial and residential cooking (Wernis et al., 2022; Peng et al., 2022). The source apportionment profiles determined here may be compared against other urban environments to evaluate cooking in other cities.

The VOCs emitted from cooking are reactive and may contribute to the formation of ozone, secondary organic aerosol, and other pollutants such as peroxyacyl nitrates (Bowman et al., 2003). A review of VOC emissions inventories show that total cooking emissions (i.e., residential + commercial cooking) are likely underrepresented in air quality models. Spatial patterns of long-chain aldehydes suggest that more work is needed to quantify the magnitude of emissions from commercial cooking, which are also important sources of primary urban SOA (Robinson et al., 2018; Shah et al., 2018). PMF results in Las Vegas suggest that cooking emissions may be as important to urban VOCs as mobile sources in regions with significant restaurant activity.