Aerosol Optical Extinction during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) 2014 Summertime Field Campaign, Colorado U.S.A.

. Summertime aerosol optical extinction (β ext ) was measured in the Colorado Front Range and Denver Metropolitan Area as part of the Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) campaign during July-August 2014. An Aerodyne Cavity Attenuated Phase Shift particle light extinction monitor (CAPS-PM ex ) was deployed to measure dry, β ext of submicron aerosols at λ=632 nm at 1 Hz. Data from a suite of gas-phase instrumentation were used to interpret β ext behavior 20 under various categories of air masses and sources. Extinction enhancement ratios relative to CO (Δβ ext /ΔCO) were significantly increased in highly aged air masses compared to fresh air masses by 50-60%. The resulting increase in Δβ ext /ΔCO under highly aged air masses was accompanied by formation of secondary organic aerosols (SOA). In addition, the impacts of aerosol composition on β ext in air masses under the influence of urban, natural oil and gas operations (O&G), and agriculture and livestock operations were evaluated. Estimated non-refractory mass extinction efficiency (MEE) values for different air 25 mass types ranged from 1.83-3.30 m 2 g -1 , with the minimum and maximum values observed in agriculture and urban + O&G influenced air masses, respectively. The mass distribution for organic, nitrate, and sulfate aerosols presented distinct profiles in different air mass types. During Aug. 11-12, regional influence of a biomass burning event was observed, increasing the background β ext by 10-15 Mm -1 and the estimated MEE and Δβ ext /ΔCO values in the Front Range. standard deviations uncertainties ext x y values of 0.3-0.7, relatively aged, intermediately aged, and fresh NO x -containing air masses, respectively. on air Δβ ext /ΔCO the the lowest values of 0.16 ± 0.005 Mm -1 ppbv -1 and 0.15 ± 0.004 Mm -1 ppbv -1 and intermediately aged air ext value -1


Introduction
The aerosol optical extinction coefficient (β ext ) represents the attenuation of light due to aerosol absorption and scattering of solar radiation.For a population of aerosol particles, β ext depends on aerosol size, composition, particle number concentration, shape, and morphology (Bohren and Huffman, 1998).Atmospheric aerosols have important implications on climate.They modify the Earth's radiative energy budget directly through absorption and scattering of light and indirectly through changing cloud characteristics (e.g., cloud droplet number concentration, cloud droplet size, cloud reflectivity, or lifetime) (Ramanathan et al., 2001;Seinfeld and Pandis, 2006;Langridge et al., 2011).In addition, aerosols with diameters between 0.1 to 1 µm are the main contributors to visibility degradation in anthropogenically polluted areas and on regional scales due to their direct interactions Published by Copernicus Publications on behalf of the European Geosciences Union.
with solar radiation (Malm, 1989;Hobbs, 2000;Ying et al., 2004).For example, it has been observed that the important anthropogenic contributors to light scattering in the Colorado Rocky Mountains are particulate matter from the urban emissions (Levin et al., 2009).The Denver metropolitan area has also experienced seasonal air pollution and visibility degradation in the past.The wintertime pollution in Denver when trapped closer to the surface due to the low inversion layer causes a greyish-brown cloud referenced to as the "Denver brown cloud."The composition of the Denver brown cloud and contribution of different chemical species to the observed β ext during the wintertime have been investigated in the 1970s to late 1980s (Groblicki et al., 1981;Wolff et al., 1981;Neff, 1997).These studies concluded that, among all the measured aerosol species, elemental carbon, ammonium sulfate, and ammonium nitrate contributed the most (37.7, 20.2, and 17.2 %, respectively) to wintertime optical extinction in the visible range.Previous measurements of summertime particle composition in the Colorado Front Range were conducted during the northern Front Range Air Quality Study (NFRAQS) between 17 July and 31 August 1996 at several urban and rural sites.The major components of PM 2.5 mass were identified to be carbonaceous and inorganic aerosols, with carbonaceous aerosols contributing about 46 % of PM 2.5 mass.The 24 h average measurements of PM 2.5 organic carbon, nitrate, and sulfate particles were observed to be 4.2, 0.9-1.2µg m −3 , and 1.4-1.5, respectively (Watson et al., 1998).In response to the wintertime haze episodes observed in the region, the State of Colorado has implemented a visibility standard based on total optical extinction of 76 Mm −1 at 550 nm, averaged during a 4 h period when ambient relative humidity (RH) is less than 70 % (Ely et al., 1993).Total optical extinction measurements are provided by the Colorado Department of Public Health and Environment's transmissometer installed in downtown Denver.The average total extinction values of August 2001-2014, ranging from 40 to 80 Mm −1 , reveal no significant trend in summertime extinction and visibility in the region since 2001.
The meteorological influence on air quality and visibility in the Front Range can also be important (e.g., Vu et al., 2016).Typically during the day, easterly upslope flow transports emissions from local sources westward, while during the night the flow reverses and downslope drainage flow through Platte River valley sets in.Occasionally, a synoptic scale cyclone, called the Denver cyclone, is established when drainage flow of air masses is prevented due to propagation of a vortex that develops east of the Rocky Mountains, contributing to transport and mixing of air masses in a cyclonic flow pattern (Crook et al., 1990;Reddy et al., 1995).
With a twofold increase in natural-resource extraction wells since 2005 to about 24 000 active oil and natural gas (O&G) production wells in 2012, northeastern Colorado has experienced extensive fossil fuel production within the past decade (Scamehorn, 2002;Pétron et al., 2014).This includes increases in fossil fuel production from coal bed methane, tight sand and shale natural gas, shale oil, and the associated gases.The emissions from these processes have several environmental impacts, such as greenhouse emissions of methane and emissions of non-methane hydrocarbons, that impair air quality.Emissions from diesel trucks, drilling rigs, power generators, phase separators, dehydrators, storage tanks, compressors, and pipes used in O&G operations also contribute to the regional burden of volatile organic compounds (VOCs), nitrogen oxides, and particulate matter (i.e., black carbon and primary organic carbon) (Gilman et al., 2013).One of the major air quality issues the Colorado Front Range faces is the exceedance of the 8 h National Ambient Air Quality Standard (NAAQS) standard for ozone (75 ppbv) during the summertime.In 2007, the Denver metropolitan area and the northern parts of the Colorado Front Range were classified as nonattainment areas due to the summertime elevated levels of ozone (www.colorado.gov/cdphe/attainment).Summertime impacts of O&G emissions on the formation of secondary pollutants and visibility reduction in the Front Range have not been explored previously.In addition to local point and area sources in the Front Range, biomass burning (BB) emissions from wildfires in the region may also act as a source of aerosols, contributing to regional haze (Park et al., 2003).
During July-August 2014, airborne measurements were conducted over the Colorado Front Range as part of the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) to characterize the influence of sources, photochemical processing, and transport on atmospheric gaseous and aerosol pollutants in the area.This paper will discuss the role of local aerosol sources in the Front Range and regional wildfires on aerosol optical extinction in the absence of the Denver cyclone by investigating chemical and optical properties of aerosols in different air masses.

FRAPPÉ field campaign
In situ measurements were conducted aboard the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) C-130 aircraft during 20 July-18 August 2014.Flight tracks of the C-130, color coded with different trace gases, are presented in Fig. 1a-c.In the current analysis, airborne data were limited to those obtained only in the boundary layer of the Colorado Front Range (i.e., altitudes below 2500 m above sea level (a.s.l.) east of the foothills and below 5500 m a.s.l. with easterly winds over the foothills and the Continental Divide) to capture the influence of local sources such as power plants, O&G, agriculture, livestock, and urban emissions.Occasionally, when air masses with the mountain-valley circulation patterns were sampled, data from higher altitudes (< 4000 m a.s.l.) over the Denver metropolitan area were also considered.

Instrumentation and methodology
The NSF/NCAR C-130 aircraft carried an extensive collection of instruments for the characterization of the diverse atmospheric pollutants in the Colorado Front Range.The relevant instrumentations used in the current analysis are described below.(The data produced by these instruments are available at http://www-air.larc.nasa.gov/cgi-bin/ArcView/discover-aq.co-2014?C130=1).
The extinction coefficient (β ext ) at 632 nm was measured using a cavity attenuated phase shift particle light extinction monitor (CAPS-PM ex ) (Aerodyne Research Inc., Billerica, MA).The CAPS-PM ex utilizes high-reflectivity mirrors at two ends of a 26 cm long, near-confocal cavity.Within the optical cell cavity, the highly reflective mirrors create an effective path length of approximately 2 km.In the particlefree sampling mode, the light-emitting diode (LED) light output is directed to the first mirror, while a small fraction passes through the second mirror to the photodiode detector, producing a slightly distorted waveform of the square-wave modulated by the LED, whereas in the aerosol sampling mode the detector detects a greater distorted waveform, characterized by a phase shift.The vacuum photodiode, which is located behind the second reflective mirror, detects and measures that phase shift when the square wave becomes distorted due to interactions with sampled air under a relatively long effective pathlength.The observed phase shift is then related to aerosol β ext as follows: where cotθ o is the phase shift from the particle-free sampling mode, cotθ is the phase shift during the aerosol sampling mode, f is the frequency, and c is the speed of light.The estimated uncertainty in β ext is 10 %, while the 3σ detection limit for 1 s data under the conditions of particle-free air encountered during FRAPPÉ was ∼ 1.5 Mm −1 (Massoli et al., 2010;Petzold et al., 2013).Measurements of the baseline were conducted through the system's internal filter unit regularly, at 5 min intervals.The filter period, which lasted for 1 min, included 10 s of flush time, then 40 s of filter sampling, followed by another 10 s of flush time.Although, for the majority (72 %) of the data, consecutive baseline values had shifted by less than 0.5 Mm −1 , baseline values were interpolated for a more accurate estimation of optical extinction.CAPS-PM ex data, obtained at 1 Hz, were averaged to the aerosol mass spectrometer's averaging time of 15 s.The measured β ext includes the combined effects of scattering and absorption of light by aerosols at 632 nm; given relatively high single-scattering albedo values of aerosols downwind of urban environments (Langridge et al., 2012), β ext is expected to be dominated by the scattering coefficient.As discussed in Sects.3.3 and 3.4, in urban or biomass-burning-influenced air masses, contribution of absorption by black carbon (BC) to β ext could be more significant.It is worth mentioning that anthropogenic gases such as nitrogen dioxide have minimal effect on the measured β ext at 632 nm since regular baseline corrections based on sampled filtered air were applied to the data.Given the average mixing ratio of NO 2 , the parameterization by Groblicki et al. (1981) for estimating NO 2 absorption at 550 nm, and the factor-of-10-smaller value of NO 2 absorption cross section at 632 nm compared to 550 nm (Schneider et al., 1987), we estimated the average contribution of NO 2 absorption at 632 nm to be ∼ 0.1 Mm −1 , indicating a minor contribution to total extinction at 632 nm.
The CAPS-PM ex shared a common inlet with a compact aerosol mass spectrometer (mAMS; Aerodyne Inc., Billerica, MA) coupled with a time-of-flight (TOFwerk, Thun, Switzerland) detector to measure particle mass distribution and non-refractory submicron aerosol composition (NR-PM 1 ) of organics, nitrate, sulfate, chloride, and ammonium.The mAMS inlet was characterized to have a 50 % transmission of ∼ 800 nm (physical diameter) particles.Aerosol concentrations from the mAMS were corrected for vaporizer bounce using composition-dependent collection efficiencies (Middlebrook et al., 2012).The estimated uncertainty for all aerosol species is ∼ 30 % (Bahreini et al., 2009).Both instruments sampled particles through a secondary diffuser mounted inside a NCAR HIAPER (High-performance Instrumented Airborne Platform for Environmental Research) modular inlet (HIMIL), mounted facing forward, under the C-130 aircraft.Given the total flow rate within the inlet and assuming particle density of 1500 kg m −3 , ambient temperature of 20 • C, and ambient pressure of 70 KPa, 2 µm particles were estimated to be transmitted by 50%, making the inlet nominally a PM 2 inlet.Residence time in the inlet was estimated to be ∼ 0.56 s.Ambient aerosol size distributions were measured aboard the C-130 by a Passive Cavity Aerosol Spectrometer Probe (PCASP).Estimated extinction values using Mie calculations with a nominal refractive index of 1.5 and the measured PCASP size distributions indicated that particles smaller than 800 nm captured > 92 % of PM 2 extinction values, confirming that the majority of the extinction signal originated from aerosols in the size range of the mAMS.We note that the calculated extinction coefficients were not highly sensitive to the choice of refractive index; only a 4 % decrease in the slope of scattering from PM 0.8 vs. PM 2 was observed by increasing the refractive index from 1.48 to 1.52.
Based on the ambient RH and temperature and the temperature within the CAPS-PM ex extinction cell, and assuming that aerosols had equilibrated to the conditions within the measurement cell, the CAPS-PM ex measurements during the flights discussed here represent extinction values at an average RH of 20 ± 7 % (range of 15-30 %).Additionally, β ext values were normalized for standard temperature and pressure (273 K and 1 atm) conditions.
The relationship between the primarily emitted nitrogen oxides (NO x ) and the higher oxidized species of nitrogen captures the transformation of NO x in the atmosphere upon aging (Kleinman et al., 2007;Langridge et al., 2012).Thus, measurements of nitric oxide (NO), nitrogen dioxide (NO 2 ), nitric acid (HNO 3 ), particulate phase nitrate (NO − 3 ), alkyl nitrates (ANs), peroxy acetyl nitrate (PAN), and peroxy propionyl nitrate (PPN) were used to calculate the ratio of primary nitrogen oxides (NO x = NO + NO 2 ) to NO y (NO y = NO x + HNO 3 + NO − 3 + ANs + PAN + PPN) in order to track the extent of photochemical aging in an air mass with nonzero emissions of NO x (Kleinman et al., 2007;DeCarlo et al., 2010).A ratio that yields a value close to 1 represents air masses that are relatively fresh, whereas a ratio closer to 0 represents more-aged air masses.NO and NO 2 were measured using the NCAR two-channel chemiluminescence instrument (Ridley and Grahek, 1990).A chemical ionization mass spectrometer (CIMS) coupled with a quadrupole detector was operated to measure HNO 3 , using SF − 5 as the reagent ion (Huey et al., 1998;Huey, 2007).ANs were measured using thermal-dissociation laser-induced fluorescence (TD-LIF) (Thornton et al., 2000;Day et al., 2002).PAN and PPN species were measured using the CIMS Instrument by Georgia Tech and NCAR (PAN-CIGAR; Slusher et al., 2004;Zheng et al., 2011), with I − as the reagent ion.
The impacts of different pollution sources on sampled air masses were characterized by considering several auxiliary gas-phase tracers.Carbon monoxide, the tracer for combustion emissions, was measured by vacuum ultra-violet fluorescence with the estimated uncertainty of 3 % (Gerbig et al., 1999).Ethane (C 2 H 6 ), used to identify influence of O&G emissions, was measured using the Compact Atmospheric Multispecies Spectrometer (CAMS), employing infrared spectrometry (Richter et al., 2015).The Aerodyne dual quantum cascade trace gas monitor for ammonia (NH 3 ) equipped with a mid-infrared absorption spectrometer was used to identify emissions of agriculture and livestock operations (Ellis et al., 2010).The influence of biomass burning was identified using the measurements of hydrogen cyanide from the NCAR Trace Organic Gas Analyzer (TOGA), a fast gas chromatograph coupled with a quadrupole mass spectrometer (GC-MS) set to selected ion-monitoring mode for quantification (Apel et al., 2015) and acetonitrile (CH 3 CN) by proton-transfer reaction mass spectrometry (PTR-MS), a high-sensitivity instrument with fast time response that employs a quadrupole mass spectrometer to measure volatile organic compounds (de Gouw and Warneke, 2007;Karl et al., 2009).A Passive Cavity Aerosol Spectrometer Probe (PCASP) was available as the only instrument to measure ambient aerosol size distributions in the size range of 0.1-3 µm (Rosenberg et al., 2012).

Results and discussion
3.1 Urban aerosol optical extinction characterization in different photochemical aging regimes NO x / NO y ratios were observed to be highest over the city, representing freshly emitted plumes from vehicular traffic (Fig. S1 in the Supplement).Away from the city center, NO x / NO y values decrease, representing the relative evolution of fresh air masses containing NO x .Figure 2 shows the scatterplot of β ext vs. CO, color coded with the NO x / NO y ratio, on 2-3, 7-8, 15-16, and 18 August (i.e., excluding days with the influences of the Denver cyclone and biomass burning events).Measurements here focused on air masses impacted by urban sources only, as defined by enhancement of CO over the background (105 ppbv, as defined by the mode in the frequency distribution of CO in the Front Range boundary layer) while C 2 H 6 / CO < 20 pptv ppbv −1 (Warneke et al., 2007;Borbon et al., 2013).The extinction enhancement ratios β ext / CO in two aging regimes, categorized by NO x / NO y ratio values, were analyzed by weighted linear orthogonal distance regression (ODR) fits, with the slopes representing the enhancement ratios.In obtaining Orthogonal distance linear regression fits to extinction (Mm −1 ) vs. CO (ppbv) for fresh (blue fit line) and aged air masses (red fit line).
these fits, weights represented standard deviations equal to the uncertainties in CO (3 %) and β ext (10 %).Uncertainties in the slope values of ODR fits throughout the manuscript represent the estimated propagated uncertainties, in this case the square root of the quadratic sum of the relative uncertainties in the ODR fit (1σ ), CO mixing ratio, and β ext coefficient.NO x / NO y values of < 0.5 and > 0.5 represent relatively aged and fresh NO x -containing air masses, respectively.Different trends in β ext / CO were seen in the two aging regimes, with a lower value of 0.13 ± 0.014 Mm −1 ppbv −1 observed in the fresh air masses.
On the other hand, the relatively aged air masses showed a higher β ext / CO value of 0.20 ± 0.025 Mm −1 ppbv −1 , indicating about a 54 % increase in the extinction enhancement ratio due to photochemical aging.The correlation coefficient r values were 0.92 and 0.85 for relatively fresh and aged air masses, respectively.The most dominant component of the non-refractory aerosols in urban plumes was organic aerosol (OA), with a contribution of 74 % of NR-PM 1 mass.The high OA contribution combined with the observed significant increase in the enhancement ratio of OA to CO with aging (from 0.021 ± 0.009 to 0.11 ± 0.01 µg m −3 ppbv −1 ) suggests that the bulk of the aged urban aerosol mass during the daytime in the Front Range was secondary organic aerosol (SOA).Although carrying out positive matrix factorization analysis on the measured OA mass spectra during FRAPPÉ is beyond the scope of this paper, such analysis in the future would be conducive to confirming the large contribution of SOA to the measured OA.Since NO − 3 / CO and SO 2− 4 / CO enhancement ratios did not increase with photochemical aging and demonstrated poor overall correlation coefficients (r < 0.35 for NO − 3 / CO and r < 0.29 for SO 2− 4 / CO), the increase in the enhancement ratio of the aerosol optical extinction coefficient with CO was likely also driven by SOA formation.

Impacts of source and aerosol composition on aerosol optical extinction
Analysis of the average composition of NR-PM 1 in the Northern Front Range, in the absence of the Denver cyclone, revealed significantly higher concentrations of organic aerosols relative to inorganic anions in the urban and urban + O&G-influenced air masses, with a fractional contribution of ∼ 74 % (Fig. 3).On average, similar concentrations of non-refractory aerosol sulfate and chloride were observed in the different air masses, while concentration of nitrate aerosols increased by a factor of ∼ 2-3 in agriculturally influenced air masses compared to the other air mass types with the exception of urban + O&G air masses.
Aerosol optical extinction values under the influence of different sources were further analyzed using auxiliary gasphase data.As mentioned in Sect.3.1, urban emissions were classified by enhancement of CO over the background (105 ppbv, as defined by the mode in the frequency distribution of CO in the Front Range boundary layer) while C 2 H 6 / CO < 20 pptv ppbv −1 .O&G and agricultural emissions were classified using enhancements of C 2 H 6 over 2500 pptv and those of ammonia over 5 ppbv, respectively, while all other tracers were at background level.A fourth air mass classification used in this analysis, urban + O&G, was based on air masses where both urban and O&G classifications were satisfied.The background mixing ratios for each gas tracer were determined by the mode of the frequency distribution of the tracer's mixing ratio observed in each flight.The impacts of sources and aerosol composition on extinction were explored by considering correlation coefficients of linear least-squares regression fits to the scatterplots of aerosol extinction vs. the mass concentration of the three dominant aerosol species (OA, nitrate aerosols, and sulfate aerosols) in urban, O&G-, urban + O&G-, and agriculturally influenced air masses.
Figure 4 shows the correlation coefficient (r) values of extinction vs. aerosol species mass concentration, in different air mass types as characterized above.The scatterplots of β ext vs. OA under conditions of urban, O&G, and ur-ban + O&G air masses presented correlation coefficients of r = 0.46, 0.72, and 0.46, respectively.This observation suggests that O&G emissions are important for organic aerosol contribution to β ext .On the other hand, in urban plumes, the correlation between β ext and OA was lower than in O&G plumes, while, as demonstrated in Fig. 2, β ext and CO were strongly correlated under conditions of both fresh and aged air masses.These observations suggest that species other than OA, e.g., black carbon, that are co-emitted with CO are also important in driving β ext in urban air masses.The correlation between β ext vs. OA was weakest in plumes with agricultural emissions (r = 0.085), suggesting OA had little impact on β ext in these plumes.The correlation coefficients for β ext vs. aerosol nitrate mass were strongest under the influence of O&G, urban + O&G, and agriculture and livestock emissions (r = 0.75, 0.75, and 0.90, respectively) and weakest in the urban plumes (r = 0.18).Aerosol nitrate formation depends on ambient conditions (temperature and relative humidity), relative mixing ratios of nitric acid and ammonia, and aerosol composition and pH (Seinfeld and Pandis, 2006;Weber et al., 2016).With uniform concentrations of sulfate aerosol and small contribution of chloride and dust components to the Front Range fine aerosol mass, variability in aerosol pH was not expected to be high.Furthermore, there was no specific trend in temperature or relative humidity in different plume types.On the other hand, mixing ratios of ammonia were observed to be variable in the different air masses, with average values of 1.41 ± 1.2, 2.75 ± 1.88, 8.21 ± 2.06, and 5.47 ± 1.81 ppbv in urban, O&G, agriculture, and urban + O&G plumes, respectively.These observations suggest that ammonia emissions that are colocated with O&G-related activities in the Front Range play a significant role in controlling β ext in these air masses by enhancing the partitioning of nitric acid to the condensed phase.In fact, the average aerosol inorganic nitrate fraction over total inorganic nitrate (aerosol nitrate / [HNO 3 + aerosol nitrate]) in agriculture and O&G plumes were 0.25 ± 0.09 and 0.11 ± 0.10, respectively, compared to 0.070 ± 0.071 in urban plumes.β ext was poorly correlated with sulfate aerosols in the region under the influence of all sources (r = 0.30, 0.37, 0.07, 0.23 for urban, O&G, agriculture, and urban + O&G, respectively), suggesting a low impact of sulfate aerosol and its precursors on β ext in the region.
Due to the higher hygroscopicity of inorganic salts compared to organics, contribution of sulfate and nitrate aerosols to the ambient β ext could be higher than what is discussed above.However, under the average ambient conditions encountered during FRAPPÉ (average RH ∼ 44 ± 17 %), the increase in ambient β ext due to aerosol hygroscopicity is not expected to be significant (∼ 20 %) given the high organic fraction of 64-74 % in urban, O&G-, or urban + O&Ginfluenced plumes (Massoli et al., 2009).In agriculturally influenced plumes, the influence of nitrate aerosol on ambient β ext will be greater because of the lower organic fraction and higher nitrate mass in these plumes, re-emphasizing the role of nitrate aerosol in β ext for such emissions.

Mass extinction efficiency
Mass extinction efficiency (MEE) is a function of the diameter of the particle, wavelength of attenuated light, and aerosol refractive index (Seinfeld and Pandis, 2006).To further asses the impacts of aerosol sources on β ext , MEE values, i.e., the ratio of the observed β ext to NR-PM 1 mass, in different air masses were estimated.For this analysis, MEE values were determined as the slope of the weighted linear ODR fits of β ext against NR-PM 1 mass, with weights representing standard deviations equal to the uncertainties in NR-PM 1 mass (30 %) and β ext (10 %).As indicated in Fig. 5a-d, MEE values under the urban, O&G, agriculture, and urban + O&G influence were ∼ 1.51 ± 0.49 m 2 g −1 (r = 0.40), 1.62 ± 0.51 m 2 g −1 (r = 0.79), 2.27 ± 0.83 m 2 g −1 (r = 0.83), and 2.14 ± 0.68 m 2 g −1 (r = 0.73), respectively.The highest average MEE value was observed in agricultural plumes although, considering the uncertainties in the fitted slopes, the MEE values were not significantly different.The overall MEE value in the Front Range, i.e., MEE observed for aerosols in all air mass types but in the absence of biomass burning, was 2.24 ± 0.71 m 2 g −1 (r = 0.80).Based on the values of the intercepts of the ODR fits in Fig. 5, it appears that at background levels of NR-PM 1 mass there was a background extinction value of ∼ 2 Mm −1 in all, except agricultural, plumes.This observation could be explained by optical extinction due to the presence of refractory aerosol species, such as black carbon or dust, which are not accounted for in NR-PM 1 mass.A high degree of correlation between β ext and CO (Fig. 2) in urban plumes and low average concentrations of some of the dust components (e.g., calcium and magnesium) throughout the region support the non-negligible contribution of BC to β ext in the Front Range.
As seen in Fig. S2, different aerosol mass distributions were observed under different air mass types.For the mass distribution analysis, d va (vacuum aerodynamic diameter) was converted to d p (physical diameter) by dividing d va by the overall mass-weighted effective density (ρ), assuming ρ = 1.25 g cm −3 for OA, assuming ρ = 1.75 g cm −3 for ammonium sulfate and ammonium nitrate, and assuming that particles sampled by the mAMS were internally mixed (Jayne et al., 2000;Seinfeld and Pandis, 2006).Next we examine the similarity of MEE values observed in the Colorado Front Range to previous measurements.MEE is the sum of the mass absorption and scattering efficiencies (MAE and MSE, respectively), which both depend on particle size, refractive index, and wavelength of light (Seinfeld and Pandis, 2006).For typical urban air masses, mass distributions were dominated by organic aerosols in the size range of d p = 150-500 nm (Fig. S2a).This is consistent with previous observations for urban aerosol volume distributions with modes at the size range of d p ∼ 200-500 nm (Seinfeld and Pandis, 2006).In O&G air masses (Fig. S2b), individual mass distributions were broader, with modes for all species shifted to larger sizes (d p ∼ 200-550 nm).In agriculturally influenced air masses nitrate aerosols presented a significant mode in the size range of d p ∼ 250-400 nm, while OA species were concentrated on smaller sizes (d p ∼ 100-200 nm; Fig. S2c).The mass distributions in urban + O&G plumes were more variable.Occasionally, the distribution was dominated by OA in the smaller size range (∼ 90-110 nm), but it also included contributions from sulfates and nitrates in the larger size range (∼ 225-275 and ∼ 430-550 nm) (Fig. S2d), while other times the mass distribution had significantly higher contribution from OA in the size range of ∼ 225-350 nm, showing a clear shift and OA growth to larger sizes (Fig. S2e).
Keeping in mind that in the presence of absorbing species, MEE is higher than MSE, in the absence of estimates of MEE in other regions we present estimates of MSE from previous studies for comparison with the current MEE estimates in the Front Range.PM 2.5 scattering efficiencies at 550 nm in several ground-based studies at urban commercial/residential sites have typically been measured to be in the range of 2-3 m 2 g −1 (e.g., Chow et al. 2002;Hand and Malm, 2007).In such studies, the main aerosol sources contributing to the observed PM 1 MSE were the automotive emissions and combustion processes.Although the contribution of elemental or black carbon to PM 1 mass during FRAPPÉ is unknown, similar to these previous studies, OA contributed the most to the NR-PM 1 mass in the Front Range, and in comparison the observed average MEE value (2.24 ± 0.71 m 2 g −1 ) is consistent with the previous estimates of MSE.

Impacts of biomass burning emissions on optical extinction
During 11 and 12 August, several wildfires were observed at Rocky Mountain National Park, near Tonahutu Creek Trail, about 95 km NW of Denver and Grand Mesa, Uncompah- were analyzed to assess the regional impact of wildfire emissions in the Front Range to PM 2.5 during the BB and non-BB days.The time series of PM 2.5 mass concentrations at the sites described above during days preceding and following the wildfires show increases in mass concentration for PM 2.5 during the days of BB (Fig. S4).The mean PM 2.5 mass concentrations during 09:00-19:00 local time at Denver, La Casa, Denver-CAMP, and Denver-I25 during non-BB days were 5.61 ± 2.02, 6.01 ± 3.52, and 7.28 ± 2.91 µg m −3 , while mean mass concentrations increased to 9.47 ± 2.05, 11.51 ± 3.04, and 14.08 ± 4.68 µg m −3 , respectively, during the BB days.As seen in Fig. 6, the average daytime PM 2.5 mass concentration on BB days increased by 75-98 % compared to the non-BB days, confirming the regional influence of wildfires on the Front Range aerosol loadings.In addition to scattering of light by smoke particles, BB emissions of BC and brown carbon (BrC) can lead to significant absorption of the solar radiation in the visible and UV region; at 632 nm absorption by BrC is minimal (Lack et al., 2012;May et al., 2014).MEE values were analyzed for days with and without the BB influence, using weighted linear ODR fit analysis, as explained previously.As seen in Fig. 7, average MEE on 11-12 August was ∼ 63 % greater compared to days without the influence of BB (3.65 ± 1.16 m 2 g −1 vs. 2.24 ± 0.71 m 2 g −1 ).Additionally, during 11-12 August, the background value of airborne β ext was higher at 4.00 ± 0.71 Mm −1 compared to 0.25 ± 0.11 Mm −1 on days without the BB influence, suggesting the additional contribution to β ext from the wildfires.Although the AMS does not detect refractory materials such as BC due to the relatively low temperature of its vaporizer (600 • C), it is likely that on 11-12 August BC emissions from the fires had resulted in elevated extinction values on a regional scale, resulting in higher MEE.The observed increase in MEE on 11-12 August suggests that regional BB emissions have at least a comparable impact on aerosol optical extinction and visibility in the Front Range relative to the local sources.

Conclusions
Airborne aerosol optical extinction (632 nm) and submicron non-refractory aerosol composition were measured during the summer in the Colorado Front Range to understand sources and processes that impact summertime visibility in the area.In assessing the role of atmospheric processing on β ext , the β ext / CO enhancement ratio increased in aged urban air masses by ∼ 54 %.The increase in β ext / CO in the aged air masses was accompanied by a factor of ∼ 5 increase in OA / CO, indicating that secondary formation of organic aerosols had significant impacts on the evolution of urban β ext in the Front Range.Correlation between β ext and organic, nitrate, and sulfate aerosol mass under urban, O&G, agriculture, and urban + O&G mixed-source influence was analyzed by linear regression fits.β ext best correlated with organic aerosols in the O&G emissions and best correlated with nitrate aerosols under the O&G and agriculture influences.Correlation with sulfate was poor for all air mass types.Estimated average non-refractory mass extinction efficiency values for different air mass types ranged from 1.51 ± 0.49 to 2.27 ± 0.83 m 2 g −1 , with the minimum and maximum average values observed in urban and agriculture air masses, respectively.Finally, aerosol components emitted from wildfires during the days of 11 and 12 August increased β ext background values by ∼ 4 Mm −1 and resulted in higher average MEE values by about 63 % compared to non-biomass-burning days, indicating that summertime visibility in the Front Range may equally be impacted by regional wildfires in addition to local sources.

Figure 1 .
Figure 1.C-130 flight tracks in the Colorado Front Range, color coded with observed mixing ratios of (a) CO, (b) ethane, and (c) ammonia.The yellow arrow indicates the Denver metropolitan area.To the west of the Denver metropolitan area are the Rocky Mountain foothills depicted by the topographic color scheme.

Figure 7 .
Figure7.Orthogonal distance linear regression fits to extinction (Mm −1 ) vs. total NR-PM 1 mass (µg m −3 ).Data points are color coded with the average HCN mixing ratio for non-biomass-burning and biomass burning days.