Molecular composition of fresh and aged aerosols from residential wood combustion and gasoline car with modern emission mitigation technology

Emissions from road traffic and residential heating contribute to urban air pollution. Advances in emission reduction technologies may alter the composition of emissions and affect their fate during atmospheric processing. Here, emissions of a gasoline car and a wood stove, both equipped with modern emission mitigation technology, were photochemically aged in an oxidation flow reactor to the equivalent of one to five days of photochemical aging. Fresh and aged exhausts were analyzed by ultrahigh resolution mass spectrometry. The gasoline car equipped with a three-way catalyst and a gasoline particle filter emitted minor primary fine particulate matter (PM2.5), but aging led to formation of particulate low-volatile, oxygenated and highly nitrogen-containing compounds, formed from volatile organic compounds (VOCs) and gases incl. NOx, SO2, and NH3. Reduction of the particle concentration was also observed for the application of an electrostatic precipitator with residential wood combustion but with no significant effect on the chemical composition of PM2.5. Comparing the effect of short and medium photochemical exposures on PM2.5 of both emission sources indicates a similar trend for formation of new organic compounds with increased carbon oxidation state and nitrogen content. The overall bulk compositions of the studied emission exhausts became more similar by aging, with many newly formed elemental compositions being shared. However, the presence of particulate matter in wood combustion results in differences in the molecular properties of secondary particles, as some compounds were preserved during aging.

Reactive oxidized nitrogen speciation and partitioning in urban and rural New York State

This study examined reactive oxidized nitrogen (NO_y) speciation and partitioning at one urban site, Queens College (QC) in New York City, and one rural site, Pinnacle State Park (PSP) in Addison, New York (NY) from September 2016 to August 2018 and June 2016 to September 2018, respectively. Oxides of nitrogen (NO_x), nitric acid (HNO₃), particle nitrate (pNO₃), peroxy nitrates (PNs), alkyl nitrates (ANs), and NO_y measurements were made at both sites. Across all seasons at QC, the median NO_x, HNO₃, pNO₃, PNs, ANs, and NO_y concentrations were 10.99, 0.49, 0.24, 0.62, 0.94, and 13.95 parts per billion (ppb), respectively. All-season median percent contributions of NO_x, HNO₃, pNO₃, PNs, and ANs to the total NO_y at QC were 77, 4, 2, 5, and 7%, respectively. Therefore, the sum of the individual NO_y species (NO_yi ≈ NO_x + HNO₃ + pNO₃ + PNs + ANs) accounted for 95% of the total NO_y at QC, which was well within measurement uncertainties. At PSP, the median NO_x, HNO₃, pNO₃, PNs, ANs, and NO_y concentrations were 0.65, 0.16, 0.12, 0.13, 0.18, and 1.56 ppb, respectively, over all seasons. The median percent contributions of NO_x, HNO₃, pNO₃, PNs, and ANs to NO_y over all seasons at PSP were 42, 10, 8, 9, and 12%, respectively. NO_yi comprised 81% of NO_y across all seasons at PSP, and deviations from 100% closure were generally within measurement uncertainties. Since both datasets yielded NO_y budget closure results that were either fully or largely explained by the measurement uncertainties, the observed NO_yi is likely representative of ambient NO_y in urban and rural New York. The results have implications for understanding the fate of NO_x emissions and their impact on local and regional air quality in urban and rural New York State.Implications: Continuous speciated and total reactive oxidized nitrogen (NO_y) measurements were made in urban and rural New York from 2016 to 2018. Different NO_y species have contrasting effects on the chemistry that impacts ozone (O₃) and fine particulate matter (PM_2.5) formation and concentrations. Since O₃ and PM_2.5 are regulated pollutants that have proven difficult to control, the results have implications for current and future air quality policy.

Long-term variations of C1-C5 alkyl nitrates and their sources in Hong Kong

Investigating the long-term trends of alkyl nitrates (RONO₂) is of great importance for evaluating the variations of photochemical pollution. Mixing ratios of C₁-C₅ RONO₂ were measured in autumn Hong Kong from 2002 to 2016, and the average level of 2-butyl nitrate (2-BuONO₂) always ranked first. The C₁-C₄ RONO₂ all showed increasing trends (p < 0.05), and 2-BuONO₂ had the largest increase rate. The enhancement in C₃ RONO₂ was partially related to elevated propane, and dramatic decreases (p < 0.05) in both nitrogen monoxide (NO) and nitrogen dioxide (NO₂) also led to the increased RONO₂ formation. In addition, an increase of hydroxyl (OH) and hydroperoxyl (HO₂) radicals (p < 0.05) suggested enhanced atmospheric oxidative capacity, further resulting in the increases of RONO₂. Source apportionment of C₁-C₄ RONO₂ specified three typical sources of RONO₂, including biomass burning emission, oceanic emission, and secondary formation, of which secondary formation was the largest contributor to ambient RONO₂ levels. Mixing ratios of total RONO₂ from each source were quantified and their temporal variations were investigated. Elevated RONO₂ from secondary formation and biomass burning emission were two likely causes of increased ambient RONO₂. By looking into the spatial distributions of C₁-C₅ RONO₂, regional transport from the Pearl River Delta (PRD) was inferred to build up RONO₂ levels in Hong Kong, especially in the northwestern part. In addition, more serious RONO₂ pollution was found in western PRD region. This study helps build a comprehensive understanding of RONO₂ pollution in Hong Kong and even the entire PRD.

Observations of C1-C5 alkyl nitrates in the Yellow River Delta, northern China: Effects of biomass burning and oil field emissions

Alkyl nitrates (RONO₂) are important reservoirs of nitrogen oxides and play key roles in the tropospheric chemistry. Two phases of intensive campaigns were conducted during February-April and June-July of 2017 at a rural coastal site and in open oil fields of the Yellow River Delta region, northern China. C₁-C₅ alkyl nitrates showed higher concentration levels in summer than in winter-spring (p < 0.01), whilst their parent hydrocarbons showed an opposite seasonal variation pattern. The C₃-C₅ RONO₂ levels in the oil fields were significantly higher than those in the ambient rural air. Alkyl nitrates showed well-defined diurnal variations, elucidating the effects of in-situ photochemical production and regional transport of aged polluted plumes. Backward trajectory analysis and fire maps revealed the significant contribution of biomass burning to the observed alkyl nitrates and hydrocarbons. A simplified sequential reaction model and an observation-based chemical box model were deployed to diagnose the formation mechanisms of C₁-C₅ RONO₂. The C₃-C₅ RONO₂ were mainly produced from the photochemical oxidation of their parent hydrocarbons (i.e., C₃-C₅ alkanes), whilst C₁-C₂ RONO₂ compounds have additional sources. In addition to parent hydrocarbons, longer alkanes with >4 carbon atoms were also important precursors of alkyl nitrates in the oil fields. This study demonstrates the significant effects of oil field emissions and biomass burning on the volatile organic compounds and alkyl nitrate formation, and provides scientific support for the formulation of control strategies against photochemical air pollution in the Yellow River Delta region.

Impact of Front Range sources on reactive nitrogen concentrations and deposition in Rocky Mountain National Park

Human influenced atmospheric reactive nitrogen (RN) is impacting ecosystems in Rocky Mountain National Park (ROMO). Due to ROMO's protected status as a Class 1 area, these changes are concerning, and improving our understanding of the contributions of different types of RN and their sources is important for reducing impacts in ROMO. In July-August 2014 the most comprehensive measurements (to date) of RN were made in ROMO during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ). Measurements included peroxyacetyl nitrate (PAN), C1-C5 alkyl nitrates, and high-time resolution NOx, NOy, and ammonia. A limited set of measurements was extended through October. Co-located measurements of a suite of volatile organic compounds provide information on source types impacting ROMO. Specifically, we use ethane as a tracer of oil and gas operations and tetrachloroethylene (C2Cl4) as an urban tracer to investigate their relationship with RN species and transport patterns. Results of this analysis suggest elevated RN concentrations are associated with emissions from oil and gas operations, which are frequently co-located with agricultural production and livestock feeding areas in the region, and from urban areas. There also are periods where RN at ROMO is impacted by long-range transport. We present an atmospheric RN budget and a nitrogen deposition budget with dry and wet components. Total deposition for the period (7/1-9/30) was estimated at 1.58 kg N/ha, with 87% from wet deposition during this period of above average precipitation. Ammonium wet deposition was the dominant contributor to total nitrogen deposition followed by nitrate wet deposition and total dry deposition. Ammonia was estimated to be the largest contributor to dry deposition followed by nitric acid and PAN (other species included alkyl nitrates, ammonium and nitrate). All three species are challenging to measure routinely, especially at high time resolution.

Photochemical Formation of C1-C5 Alkyl Nitrates in Suburban Hong Kong and over the South China Sea

Alkyl nitrates (RONO₂) are important reservoirs of atmospheric nitrogen, regulating nitrogen cycling and ozone (O₃) formation. In this study, we found that propane and n-butane were significantly lower at the offshore site (WSI) in Hong Kong ( p < 0.05), whereas C₃-C₄ RONO₂ were comparable to the suburban site (TC) ( p > 0.05). Stronger oxidative capacity at WSI led to more efficient RONO₂ formation. Relative incremental reactivity (RIR) was for the first time used to evaluate RONO₂-precursor relationships. In contrast to a consistently volatile organic compounds (VOC)-limited regime at TC, RONO₂ formation at WSI switched from VOC-limited regime during O₃ episodes to VOC and nitrogen oxides (NO _x) colimited regime during nonepisodes. Furthermore, unlike the predominant contributions of parent hydrocarbons to C₄-C₅ RONO₂, the production of C₁-C₃ RONO₂ was more sensitive to other VOCs like aromatics and carbonyls, which accounted for ∼40-90% of the productions of C₁-C₃ alkylperoxy (RO₂) and alkoxy radicals (RO) at both sites. This resulted from the decomposition of larger RO₂/RO and the change of OH abundance under the photochemistry of other VOCs. This study advanced our understanding of the photochemical formation of RONO₂, particularly the relationships between RONO₂ and their precursors, which were not confined to the parent hydrocarbons.

Seasonal variations of C1-C4 alkyl nitrates at a coastal site in Hong Kong: Influence of photochemical formation and oceanic emissions

Five C₁-C₄ alkyl nitrates (RONO₂) were measured at a coastal site in Hong Kong in four selected months of 2011 and 2012. The total mixing ratios of C₁-C₄ RONO₂ (Σ₅RONO₂) ranged from 15.4 to 143.7 pptv with an average of 65.9 ± 33.0 pptv. C₃-C₄ RONO₂ (2-butyl nitrate and 2-propyl nitrate) were the most abundant RONO₂ during the entire sampling period. The mixing ratios of C₃-C₄ RONO₂ were higher in winter than those in summer, while the ones of methyl nitrate (MeONO₂) were higher in summer than those in winter. Source analysis suggests that C₂-C₄ RONO₂ were mainly derived from photochemical formation along with biomass burning (58.3-71.6%), while ocean was a major contributor to MeONO₂ (53.8%) during the whole sampling period. The photochemical evolution of C₂-C₄ RONO₂ was investigated, and found to be dominantly produced by the parent hydrocarbon oxidation. The notable enrichment of MeONO₂ over C₃-C₄ RONO₂ was observed in a summer episode when the air masses originating from the South China Sea (SCS) and MeONO₂ was dominantly derived from oceanic emissions. In order to improve the accuracy of ozone (O₃) prediction in coastal environment, the relative contribution of RONO₂ from oceanic emissions versus photochemical formation and their coupling effects on O₃ production should be taken into account in future studies.

Free amino acid concentrations and nitrogen isotope signatures in Pinus massoniana (Lamb.) needles of different ages for indicating atmospheric nitrogen deposition

Free amino acid concentrations and nitrogen (N) isotopic composition in new current-year (new), mature current-year (middle-aged) and previous-year (old) Masson pine (Pinus massoniana Lamb.) needles were determined to indicate atmospheric N deposition in Guiyang (SW China). In different areas, free amino acids (especially arginine) concentrations in new and middle-aged needles were higher than in old needles, and the variation of free amino acids (especially arginine) concentrations in new and middle-aged needles was also greater than in old needles. This indicate that free amino acids in new and middle-aged needles may be more sensitive to N deposition compared to old needles. Moreover, concentrations of total free amino acids, arginine, histidine, γ-aminobutyric acid and alanine in middle-aged needles exhibited a strong relationship with N deposition (P < 0.05). Needle δ¹⁵N values showed a strong gradient from central Guiyang to the rural area, with more positive δ¹⁵N (especially in old needles) in the city center (0-5 km) and more negative δ¹⁵N (especially in old needles) in rural area (30-35 km). These suggest that N deposition in the urban center may be dominated by ¹⁵N-enriched NO_x-N from traffic exhausts, while it is dominated by isotopically light atmospheric NH_x-N from agriculture in rural area. Soil δ¹⁵N decreased slightly with distance from the city center, and the difference in δ¹⁵N values between the soil and needles (especially for old needles) increased significantly with the distance gradient, indicating that atmospheric N deposition may be an important N source for needles. This study provides novel evidence that free amino acids in needles and age-dependent needle δ¹⁵N values are useful indicators of atmospheric N deposition.

Cavity enhanced spectroscopy for measurement of nitrogen oxides in the Anthropocene: results from the Seoul tower during MAPS 2015

Cavity enhanced spectroscopy, CES, is a high sensitivity direct absorption method that has seen increasing utility in the last decade, a period also marked by increasing requirements for understanding human impacts on atmospheric composition. This paper describes the current NOAA six channel cavity ring-down spectrometer (CRDS, the most common form of CES) for measurement of nitrogen oxides and O₃. It further describes the results from measurements from a tower 300 m above the urban area of Seoul in late spring of 2015. The campaign demonstrates the performance of the CRDS instrument and provides new data on both photochemistry and nighttime chemistry in a major Asian megacity. The instrument provided accurate, high time resolution data for N₂O₅, NO, NO₂, NO_yand O₃, but suffered from large wall loss in the sampling of NO₃, illustrating the requirement for calibration of the NO₃inlet transmission. Both the photochemistry and nighttime chemistry of nitrogen oxides and O₃were rapid in this megacity. Sustained average rates of O₃buildup of 10 ppbv h⁻¹during recurring morning and early afternoon sea breezes led to a 50 ppbv average daily O₃rise. Nitrate radical production rates,P(NO₃), averaged 3–4 ppbv h⁻¹in late afternoon and early evening, much greater than contemporary data from Los Angeles, a comparable U. S. megacity. TheseP(NO₃) were much smaller than historical data from Los Angeles, however. Nighttime data at 300 m above ground showed considerable variability in high time resolution nitrogen oxide and O₃, likely resulting from sampling within gradients in the nighttime boundary layer structure. Apparent nighttime biogenic VOC oxidation rates of several ppbv h⁻¹were also likely influenced by vertical gradients. Finally, daytime N₂O₅mixing ratios of 3–35 pptv were associated with rapid daytimeP(NO₃) and agreed well with a photochemical steady state calculation.

A theoretical study of the mechanism of the atmospherically relevant reaction of chlorine atoms with methyl nitrate, and calculation of the reaction rate coefficients at temperatures relevant to the troposphere

Computed rate coefficients of the atmospherically important Cl + CH₃ONO₂ → HCl + CH₂ONO₂ reaction reported for the first time.

Real time in situ detection of organic nitrates in atmospheric aerosols

A novel instrument is described that quantifies total particle-phase organic nitrates in real time with a detection limit of 0.11 microg m(-3) min(-1), 45 ppt min(-1) (-ONO(2)). Aerosol nitrates are separated from gas-phase nitrates with a short residence time activated carbon denuder. Detection of organic molecules containing -ONO(2) subunits is accomplished using thermal dissociation coupled to laser induced fluorescence detection of NO(2). This instrument is capable of high time resolution (seconds) measurements of particle-phase organic nitrates, without interference from inorganic nitrate. Here we use it to quantify organic nitrates in secondary organic aerosol generated from high-NO(x) photooxidation of limonene, alpha-pinene, Delta-3-carene, and tridecane. In these experiments the organic nitrate moiety is observed to be 6-15% of the total SOA mass.

Response of an aerosol mass spectrometer to organonitrates and organosulfates and implications for atmospheric chemistry

Organonitrates (ON) are important products of gas-phase oxidation of volatile organic compounds in the troposphere; some models predict, and laboratory studies show, the formation of large, multifunctional ON with vapor pressures low enough to partition to the particle phase. Organosulfates (OS) have also been recently detected in secondary organic aerosol. Despite their potential importance, ON and OS remain a nearly unexplored aspect of atmospheric chemistry because few studies have quantified particulate ON or OS in ambient air. We report the response of a high-resolution time-of-flight aerosol mass spectrometer (AMS) to aerosol ON and OS standards and mixtures. We quantify the potentially substantial underestimation of organic aerosol O/C, commonly used as a metric for aging, and N/C. Most of the ON-nitrogen appears as NO(x)+ ions in the AMS, which are typically dominated by inorganic nitrate. Minor organonitrogen ions are observed although their identity and intensity vary between standards. We evaluate the potential for using NO(x)+ fragment ratios, organonitrogen ions, HNO(3)+ ions, the ammonium balance of the nominally inorganic ions, and comparison to ion-chromatography instruments to constrain the concentrations of ON for ambient datasets, and apply these techniques to a field study in Riverside, CA. OS manifests as separate organic and sulfate components in the AMS with minimal organosulfur fragments and little difference in fragmentation from inorganic sulfate. The low thermal stability of ON and OS likely causes similar detection difficulties for other aerosol mass spectrometers using vaporization and/or ionization techniques with similar or larger energy, which has likely led to an underappreciation of these species.

Measurements of NOx, acyl peroxynitrates, and NOy with automatic interference corrections using a NO2 analyzer and gas phase titration

NO(2) analyzers are much more valuable if they can also measure NO since the two (NO+NO(2)=NO(x)) are often found together. NO can be quantitatively converted to NO(2) by reaction with ozone and subsequent thermal decomposition of the N(2)O(5) that may form from further oxidation. The conversion of NO, along with decomposition of N(2)O(5) and removal of the remaining unreacted ozone with a heated chamber, allows for quantitative determination of NO(x) using a NO(2) analyzer and the determination of decomposed acyl peroxynitrates. Ambient tests are performed to demonstrate these methods.

A compact broadband cavity enhanced absorption spectrometer for detection of atmospheric NO2 using light emitting diodes

A compact and low power detector has been developed for the in situ measurement of atmospheric NO(2) using broadband cavity enhanced absorption spectroscopy. Absorption by the O(2)-O(2) collisional pair was used to determine the cavity mirror reflectivity, thus enabling the retrieval of absolute absorber concentrations by differential spectral fitting techniques. Quantitative amounts of ambient NO(2) (between 3 and 32 parts per billion) were retrieved from spectra recorded in the presence of ambient aerosol with statistical uncertainties approaching 100 ppt for a 60 s averaging period. The instrument's response was compared to that of a commercial chemiluminescence detector and was found to agree to within 6%.

Small scale structure in the atmosphere: implications for chemical composition and observational methods

Non-linearities in chemical processes are recognised as being important in a number of areas of atmospheric science. In this paper we show simulations using an idealised plume model which describes the relaxation of an urban plume into the background atmosphere. As might be anticipated, the initial conditions of NOx, O3 and VOCs within the plume and background are important in determining the chemistry downstream of the source, but crucially for this study, the rate of mixing (on timescales appropriate to the real atmosphere) is found to alter the composition of the atmosphere significantly. The model shows that NO3 chemistry can play a major role in the oxidation of biogenic VOCs present in the background atmosphere. In addition, the reaction of hydrocarbons with NO3 potentially has important implications for NOy speciation because a significant fraction of organic nitrates thus formed are sufficiently long-lived to leave the planetary boundary layer. A particularly critical result of the model is that under certain NOx conditions, O3 surface deposition can be significantly inhibited, with consequent effects on the O3 budget.