Are sesquiterpenes a good source of secondary organic cloud condensation nuclei (CCN)? Revisiting β-caryophyllene CCN

Secondary organic aerosol (SOA) was formed in an environmental reaction chamber from the ozonolysis of β-caryophyllene (β-C) at low concentrations (5 ppb or 20 ppb). Experimental parameters were varied to characterize the effects of hydroxyl radicals, light and the presence of lower molecular weight terpene precursor (isoprene) for β-C SOA formation and cloud condensation nuclei (CCN) characteristics. Changes in β-C SOA chemicophysical properties (e.g., density, volatility, oxidation state) were explored with online techniques to improve our predictive understanding of β-C CCN activity. In the absence of OH scavenger, light intensity had negligible impacts on SOA oxidation state and CCN activity. In contrast, when OH reaction was effectively suppressed (> 11 ppm scavenger), SOA showed a much lower CCN activity and slightly less oxygenated state consistent with previously reported values. Though there is significant oxidized material present (O / C > 0.25), no linear correlation existed between the mass ratio ion fragment 44 in the bulk organic mass (<i>f</i><sub>44</sub>) and O / C for the β-C-O<sub>3</sub> system. No direct correlations were observed with other aerosol bulk ion fragment fraction (<i>f</i><sub><i>x</i></sub>) and κ as well. A mixture of β-C and lower molecular weight terpenes (isoprene) consumed more ozone and formed SOA with distinct characteristics dependent on isoprene amounts. The addition of isoprene also improved the CCN predictive capabilities with bulk aerosol chemical information. The β-C SOA CCN activity reported here is much higher than previous studies (κ < 0.1) that use higher precursor concentration in smaller environmental chambers; similar results were only achieved with significant use of OH scavenger. Results show that aerosol formed from a mixture of low and high molecular weight terpene ozonolysis can be hygroscopic and can contribute to the global biogenic SOA CCN budget

Cloud condensation nuclei (CCN) activity of aliphatic amine secondary aerosol

Aliphatic amines can form secondary aerosol via oxidation with atmospheric radicals (e.g., hydroxyl radical and nitrate radical). The particle can contain both secondary organic aerosol (SOA) and inorganic salts. The ratio of organic to inorganic materials in the particulate phase influences aerosol hygroscopicity and cloud condensation nuclei (CCN) activity. SOA formed from trimethylamine (TMA) and butylamine (BA) reactions with hydroxyl radical (OH) is composed of organic material of low hygroscopicity (single hygroscopicity parameter, κ, ≤ 0.25). Secondary aerosol formed from the tertiary aliphatic amine (TMA) with N_2O_5 (source of nitrate radical, NO_3) contains less volatile compounds than the primary aliphatic amine (BA) aerosol. As relative humidity (RH) increases, inorganic amine salts are formed as a result of acid–base reactions. The CCN activity of the humid TMA–N_2O_5 aerosol obeys Zdanovskii, Stokes, and Robinson (ZSR) ideal mixing rules. The humid BA + N_2O_5 aerosol products were found to be very sensitive to the temperature at which the measurements were made within the streamwise continuous-flow thermal gradient CCN counter; κ ranges from 0.4 to 0.7 dependent on the instrument supersaturation (ss) settings. The variance of the measured aerosol κ values indicates that simple ZSR rules cannot be applied to the CCN results from the primary aliphatic amine system. Overall, aliphatic amine aerosol systems' κ ranges within 0.2 < κ < 0.7. This work indicates that aerosols formed via nighttime reactions with amines are likely to produce hygroscopic and volatile aerosol, whereas photochemical reactions with OH produce secondary organic aerosol of lower CCN activity. The contributions of semivolatile secondary organic and inorganic material from aliphatic amines must be considered for accurate hygroscopicity and CCN predictions from aliphatic amine systems

New particle formation from photooxidation of diiodomethane (CH_2I_2)

Photolysis of CH_2I_2 in the presence of O_3 has been proposed as a mechanism leading to intense new particle formation in coastal areas. We report here a comprehensive laboratory chamber study of this system. Rapid homogeneous nucleation was observed over three orders of magnitude in CH_2I_2 mixing ratio, down to a level of 15 ppt (∼4 × 10^8 molec. cm^(−3)) comparable to the directly measured total gas-phase iodine species concentrations in coastal areas. After the nucleation burst, the observed aerosol dynamics in the chamber was dominated by condensation of additional vapors onto existing particles and particle coagulation. Particles formed under dry conditions are fractal agglomerates with mass fractal dimension, D_f ∼ 1.8–2.5. Higher relative humidity (65%) does not change the nucleation or growth behavior from that under dry conditions, but results in more compact and dense particles (D_f ∼ 2.7). On the basis of the known gas-phase chemistry, OIO is the most likely gas-phase species to produce the observed nucleation and aerosol growth; however, the current understanding of this chemistry is very likely incomplete. Chemical analysis of the aerosol using an Aerodyne Aerosol Mass Spectrometer reveals that the particles are composed mainly of iodine oxides but also contain water and/or iodine oxyacids. The system studied here can produce nucleation events as intense as those observed in coastal areas. On the basis of comparison between the particle composition, hygroscopicity, and nucleation and growth rates observed in coastal nucleation and in the experiments reported here, it is likely that photooxidation of CH_2I_2, probably aided by other organic iodine compounds, is the mechanism leading to the observed new particle formation in the west coast of Ireland

Laboratory Characterization of PM Emissions from Combustion of Wildland Biomass Fuels

[1] Particle emissions from open burning of southwestern (SW) and southeastern (SE) U.S. fuel types during 77 controlled laboratory burns are presented. The fuels include SW vegetation types: ceanothus, chamise/scrub oak, coastal sage scrub, California sagebrush, manzanita, maritime chaparral, masticated mesquite, oak savanna, and oak woodland, as well as SE vegetation types: 1 year, 2 year rough, pocosin, chipped understory, understory hardwood, and pine litter. The SW fuels burned at higher modified combustion efficiency (MCE) than the SE fuels resulting in lower particulate matter mass emission factor. Particle mass distributions for six fuels and particle number emission for all fuels are reported. Excellent mass closure (slope = 1.00, r2 = 0.94) between ions, metals, and carbon with total weight was obtained. Organic carbon emission factors inversely correlated (R2 = 0.72) with average MCE, while elemental carbon (EC) had little correlation with average MCE (R2 = 0.10). The EC/total carbon ratio sharply increased with MCE for MCEs exceeding 0.94. The average levoglucosan and total polycyclic aromatic hydrocarbon (PAH) emissions factors ranged from 25 to 1272 mg/kg fuel and 1.8 to 11.3 mg/kg fuel, respectively. No correlation between average MCE and emissions of PAHs/levoglucosan was found. Additionally, PAH diagnostic ratios were observed to be poor indicators of biomass burning. Large fuel type and regional dependency were observed in the emission rates of ammonium, nitrate, chloride, sodium, and potassium

Cloud condensation nuclei (CCN) activity of aliphatic amine secondary aerosol

Aliphatic amines can form secondary aerosol via oxidation with atmospheric radicals (e.g., hydroxyl radical and nitrate radical). The particle can contain both secondary organic aerosol (SOA) and inorganic salts. The ratio of organic to inorganic materials in the particulate phase influences aerosol hygroscopicity and cloud condensation nuclei (CCN) activity. SOA formed from trimethylamine (TMA) and butylamine (BA) reactions with hydroxyl radical (OH) is composed of organic material of low hygroscopicity (single hygroscopicity parameter, κ, ≤ 0.25). Secondary aerosol formed from the tertiary aliphatic amine (TMA) with N_2O_5 (source of nitrate radical, NO_3) contains less volatile compounds than the primary aliphatic amine (BA) aerosol. As relative humidity (RH) increases, inorganic amine salts are formed as a result of acid–base reactions. The CCN activity of the humid TMA–N_2O_5 aerosol obeys Zdanovskii, Stokes, and Robinson (ZSR) ideal mixing rules. The humid BA + N_2O_5 aerosol products were found to be very sensitive to the temperature at which the measurements were made within the streamwise continuous-flow thermal gradient CCN counter; κ ranges from 0.4 to 0.7 dependent on the instrument supersaturation (ss) settings. The variance of the measured aerosol κ values indicates that simple ZSR rules cannot be applied to the CCN results from the primary aliphatic amine system. Overall, aliphatic amine aerosol systems' κ ranges within 0.2 < κ < 0.7. This work indicates that aerosols formed via nighttime reactions with amines are likely to produce hygroscopic and volatile aerosol, whereas photochemical reactions with OH produce secondary organic aerosol of lower CCN activity. The contributions of semivolatile secondary organic and inorganic material from aliphatic amines must be considered for accurate hygroscopicity and CCN predictions from aliphatic amine systems

Coupling Field and Laboratory Measurements to Estimate the Emission Factors of Identified and Unidentified Trace Gases for Prescribed Fires

An extensive program of experiments focused on biomass burning emissions began with a laboratory phase in which vegetative fuels commonly consumed in prescribed fires were collected in the southeastern and southwestern US and burned in a series of 71 fires at the US Forest Service Fire Sciences Laboratory in Missoula, Montana. The particulate matter (PM2.5) emissions were measured by gravimetric filter sampling with subsequent analysis for elemental carbon (EC), organic carbon (OC), and 38 elements. The trace gas emissions were measured by an open-path Fourier transform infrared (OP-FTIR) spectrometer, proton-transfer-reaction mass spectrometry (PTRMS), proton-transfer ion-trap mass spectrometry (PIT-MS), negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS), and gas chromatography with MS detection (GC-MS). 204 trace gas species (mostly non-methane organic compounds (NMOC)) were identified and quantified with the above instruments. Many of the 182 species quantified by the GC-MS have rarely, if ever, been measured in smoke before. An additional 153 significant peaks in the unit mass resolution mass spectra were quantified, but either could not be identified or most of the signal at that molecular mass was unaccounted for by identifiable species. In a second, field phase of this program, airborne and ground-based measurements were made of the emissions from prescribed fires that were mostly located in the same land management units where the fuels for the lab fires were collected. A broad variety, but smaller number of species (21 trace gas species and PM2.5) was measured on 14 fires in chaparral and oak savanna in the southwestern US, as well as pine forest understory in the southeastern US and Sierra Nevada mountains of California. The field measurements of emission factors (EF) are useful both for modeling and to examine the representativeness of our lab fire EF. The lab EF/field EF ratio for the pine understory fuels was not statistically different from one, on average. However, our lab EF for smoldering compounds emitted from the semiarid shrubland fuels should likely be increased by a factor of similar to 2.7 to better represent field fires. Based on the lab/field comparison, we present emission factors for 357 pyrogenic species (including unidentified species) for 4 broad fuel types: pine understory, semiarid shrublands, coniferous canopy, and organic soil. To our knowledge this is the most comprehensive measurement of biomass burning emissions to date and it should enable improved representation of smoke composition in atmospheric models. The results support a recent estimate of global NMOC emissions from biomass burning that is much higher than widely used estimates and they provide important insights into the nature of smoke. 31-72% of the mass of gas-phase NMOC species was attributed to species that we could not identify. These unidentified species are not represented in most models, but some provision should be made for the fact that they will react in the atmosphere. In addition, the total mass of gas-phase NMOC divided by the mass of co-emitted PM2.5 averaged about three (range similar to 2.0-8.7). About 35-64% of the NMOC were likely semivolatile or of intermediate volatility. Thus, the gas-phase NMOC represent a large reservoir of potential precursors for secondary formation of ozone and organic aerosol. For the single lab fire in organic soil about 28% of the emitted carbon was present as gas-phase NMOC and similar to 72% of the mass of these NMOC was unidentified, highlighting the need to learn more about the emissions from smoldering organic soils. The mass ratio of total NMOC to NOx as NO ranged from 11 to 267, indicating that NOx-limited O-3 production would be common in evolving biomass burning plumes. The fuel consumption per unit area was 7.0 +/- 2.3 Mg ha(-1) and 7.7 +/- 3.7 Mg ha(-1) for pine-understory and semiarid shrubland prescribed fires, respectively

PM PEM’s On-Road Investigation – With and Without DPF Equipped Engines

Regulatory agencies are in the process of implementing an in-use testing program for heavy-duty diesel vehicles that will include testing with portable emissions measurement systems (PEMS) under in-use driving conditions. An important aspect of this regulation is the Measurement Allowance program where EPA, CARB, and the Engine Manufacturers Association (EMA) are working together to systematically evaluate various sources of error for gaseous and PM measurements with PEMS in comparison with laboratory measurements. This error is then accounted for in the regulatory standards as a “Measurement Allowance”. A comprehensive program has already been conducted for the gas-phase measurement allowance, with the PM measurement allowance program about to begin. The main objective of this work was to provide preliminary measurements from PM PEMS to assess the accuracy of PM measurements under in-use conditions and provide a basis for the development of the more comprehensive Measurement Allowance program. The MASC utilized the University of California, Riverside (UCR) Bourns College of Engineering – Center for Environmental Research and Technology’s (CE-CERT) Mobile Emissions Laboratory (MEL) to perform the initial in-use PM PEMS evaluation. For this program, PM PEMS were directly compared with the MEL over a series of different onroad driving conditions. Prior to the on-road testing, MEL underwent a 40CFR Part 1065 selfaudit focused on PM sampling. In-use measurements were made from three different Class 8 tractors representing three different engine manufactures. One truck had a 2000 Caterpillar engine without a DPF and the other two were equipped with OEM DPFs, one from Cummins and the other from Volvo. Each of the 2007 vehicles was modified to vary their emission levels using regeneration, ECM recalibrations, and a DPF bypass. The on-road driving courses included segments near sea level, in coastal regions, in desert regions, and on longer uphill inclines. The goal was to test the vehicle at or slightly above the Not-To-Exceed (NTE) threshold to investigate sources of error for the PM instruments at levels where their performance is most critical. The bsPM level varied from 0.1 g/hp-h to 0.0003 g/hp-h over the different vehicles and operating conditions where one vehicle had high EC, one had high OC, and another had a substantial amount of sulfate. In addition to varying composition and bsPM level, one of the vehicles showed a significant reduction in particle size thus challenging the PM PEMS measurement systems. PM measurements in real-time were made with a variety of different PM instruments from manufacturers preparing for the PM Measurement Allowance program, including a Horiba OBSTRPM system, a Sensors SemtechDS PPMD (QCM), and an AVL Photoacoustic MicroSoot Sensor, as well as other commercially available instruments such as a Dekati DMM and TSI Dustrak. These measurements were directly compared with gravimetric PM mass measurements that were collected with the MEL under 1065 compliant sampling conditions. Measurements were made under conditions where NTE events would be expected (e.g., uphill driving segments) and for varying durations to provide a range of mass loadings. The results of this study are expected to be an important component of PM Measurement Allowance program development

PM PEM’s Pre-Measurement Allowance – On-Road Evaluation and Investigation

Regulatory agencies are in the process of implementing an in-use testing program for heavy-duty diesel vehicles that will include testing with portable emissions measurement systems (PEMS) under in-use driving conditions. An important aspect of this regulation is the Measurement Allowance program where EPA, CARB, and the Engine Manufacturers Association (EMA) are working together to systematically evaluate various sources of error for gaseous and PM measurements with PEMS in comparison with laboratory measurements. This error is then accounted for in the regulatory standards as a “Measurement Allowance”. A comprehensive program has already been conducted for the gas-phase measurement allowance, with the PM measurement allowance program about to begin. The main objective of this work was to provide preliminary measurements from PM PEMS to assess the accuracy of PM measurements under in-use conditions and provide a basis for the development of the more comprehensive Measurement Allowance program. The MASC utilized the University of California, Riverside (UCR) Bourns College of Engineering – Center for Environmental Research and Technology’s (CE-CERT) Mobile Emissions Laboratory (MEL) to perform the initial in-use PM PEMS evaluation. For this program, PM PEMS were directly compared with the MEL over a series of different on-road driving conditions. Prior to the on-road testing, MEL underwent a 40CFR Part 1065 self-audit focused on PM sampling. In-use measurements were made from a class 8 truck whose in-use PM emissions levels averaged 0.043 g/hp-h, which is near the 0.03 g/hp-h in-use threshold level. The goal was to test a vehicle at or slightly above the Not-To-Exceed (NTE) threshold to investigate sources of error for the PM instruments at levels where their performance is most critical. The on-road driving courses included segments near sea level, in coastal regions, in desert regions, and on longer uphill inclines. PM measurements in real-time were made with a variety of different PM instruments from manufacturers preparing for the PM Measurement Allowance program, as well as other commercially available instruments such as a Dekati DMM and TSI Dustrak. These measurements were directly compared with gravimetric PM mass measurements that were collected with the MEL under 1065 compliant sampling conditions. Measurements were made under conditions where NTE events would be expected (e.g., uphill driving segments) and for varying durations to provide a range of mass loadings. Comparisons of the performance of the instruments and PM mass measurements from 0.02 to 0.1 g/hp-h are be presented. The results of this study are expected to be an important component of PM Measurement Allowance program development

PM PEM’s On-Road Investigation – With and Without DPF Equipped Engines

Regulatory agencies are in the process of implementing an in-use testing program for heavy-duty diesel vehicles that will include testing with portable emissions measurement systems (PEMS) under in-use driving conditions. An important aspect of this regulation is the Measurement Allowance program where EPA, CARB, and the Engine Manufacturers Association (EMA) are working together to systematically evaluate various sources of error for gaseous and PM measurements with PEMS in comparison with laboratory measurements. This error is then accounted for in the regulatory standards as a “Measurement Allowance”. A comprehensive program has already been conducted for the gas-phase measurement allowance, with the PM measurement allowance program about to begin. The main objective of this work was to provide preliminary measurements from PM PEMS to assess the accuracy of PM measurements under in-use conditions and provide a basis for the development of the more comprehensive Measurement Allowance program. The MASC utilized the University of California, Riverside (UCR) Bourns College of Engineering – Center for Environmental Research and Technology’s (CE-CERT) Mobile Emissions Laboratory (MEL) to perform the initial in-use PM PEMS evaluation. For this program, PM PEMS were directly compared with the MEL over a series of different onroad driving conditions. Prior to the on-road testing, MEL underwent a 40CFR Part 1065 selfaudit focused on PM sampling. In-use measurements were made from three different Class 8 tractors representing three different engine manufactures. One truck had a 2000 Caterpillar engine without a DPF and the other two were equipped with OEM DPFs, one from Cummins and the other from Volvo. Each of the 2007 vehicles was modified to vary their emission levels using regeneration, ECM recalibrations, and a DPF bypass. The on-road driving courses included segments near sea level, in coastal regions, in desert regions, and on longer uphill inclines. The goal was to test the vehicle at or slightly above the Not-To-Exceed (NTE) threshold to investigate sources of error for the PM instruments at levels where their performance is most critical. The bsPM level varied from 0.1 g/hp-h to 0.0003 g/hp-h over the different vehicles and operating conditions where one vehicle had high EC, one had high OC, and another had a substantial amount of sulfate. In addition to varying composition and bsPM level, one of the vehicles showed a significant reduction in particle size thus challenging the PM PEMS measurement systems. PM measurements in real-time were made with a variety of different PM instruments from manufacturers preparing for the PM Measurement Allowance program, including a Horiba OBSTRPM system, a Sensors SemtechDS PPMD (QCM), and an AVL Photoacoustic MicroSoot Sensor, as well as other commercially available instruments such as a Dekati DMM and TSI Dustrak. These measurements were directly compared with gravimetric PM mass measurements that were collected with the MEL under 1065 compliant sampling conditions. Measurements were made under conditions where NTE events would be expected (e.g., uphill driving segments) and for varying durations to provide a range of mass loadings. The results of this study are expected to be an important component of PM Measurement Allowance program development