Particulate Nitrate Photolysis Research Articles

Particulate Nitrate Photolysis as a Possible Driver of Rising Tropospheric Ozone

AbstractTropospheric ozone is an air pollutant and a greenhouse gas whose anthropogenic production is limited principally by the supply of nitrogen oxides (NOx) from combustion. Tropospheric ozone in the northern hemisphere has been rising despite the flattening of NOx emissions in recent decades. Here we propose that this sustained increase could result from the photolysis of nitrate particles (pNO3−) to regenerate NOx. Including pNO3− photolysis in the GEOS‐Chem atmospheric chemistry model improves the consistency with ozone observations. Our simulations show that pNO3− concentrations have increased since the 1960s because of rising ammonia and falling SO2 emissions, augmenting the increase in ozone in the northern extratropics by about 50% to better match the observed ozone trend. pNO3− will likely continue to increase through 2050, which would drive a continued increase in ozone even as NOx emissions decrease. More work is needed to better understand the mechanism and rates of pNO3− photolysis.

Impact of particulate nitrate photolysis on air quality over the Northern Hemisphere

We use the Community Multiscale Air Quality (CMAQv5.4) model to examine the potential impact of particulate nitrate (pNO3−) photolysis on air quality over the Northern Hemisphere. We estimate the photolysis frequency of pNO3− by scaling the photolysis frequency of nitric acid (HNO3) with an enhancement factor that varies between 10 and 100 depending on pNO3− and sea-salt aerosol concentrations and then perform CMAQ simulations without and with pNO3− photolysis to quantify the range of impacts on tropospheric composition. The photolysis of pNO3− produces gaseous nitrous acid (HONO) and nitrogen dioxide (NO2) over seawater thereby increasing atmospheric HONO and NO2 mixing ratios. HONO subsequently undergoes photolysis, producing hydroxyl radicals (OH). The increase in NO2 and OH alters atmospheric chemistry and enhances the atmospheric ozone (O3) mixing ratio over seawater, which is subsequently transported to downwind continental regions. Seasonal mean model O3 vertical column densities without pNO3− photolysis are lower than the Ozone Monitoring Instrument (OMI) retrievals, while the column densities with the pNO3− photolysis agree better with the OMI retrievals of tropospheric O3 burden. We compare model O3 mixing ratios with available surface observed data from the U.S., Japan, the Tropospheric Ozone Assessment Report – Phase II, and OpenAQ; and find that the model without pNO3− photolysis underestimates the observed data in winter and spring seasons and the model with pNO3− photolysis improves the comparison in both seasons, largely rectifying the pronounced underestimation in spring. Compared to measurements from the western U.S., model O3 mixing ratios with pNO3− photolysis agree better with observed data in all months due to the persistent underestimation of O3 without pNO3− photolysis. Compared to the ozonesonde measurements, model O3 mixing ratios with pNO3− photolysis also agree better with observed data than the model O3 without pNO3− photolysis.

Nitrous acid budgets in the coastal atmosphere: potential daytime marine sources

Abstract. Nitrous acid (HONO), a vital precursor of atmospheric hydroxyl radicals (OH), has been extensively investigated to understand its characteristics and formation mechanisms. However, discerning fundamental mechanisms across diverse environments remains challenging. This study utilizes measurements from Mount Lao, a coastal mountain in eastern China, and an observation-based chemical box model (OBM) to examine HONO budgets and their subsequent impacts on atmospheric oxidizing capacity. The model incorporates additional HONO sources, including direct emissions, heterogeneous conversions of NO2 on aerosol and ground surfaces, and particulate nitrate photolysis. The observed mean HONO concentration was 0.46 ± 0.37 ppbv. The updated model reproduced daytime HONO concentrations well during dust and photochemical pollution events. During dust events, daytime HONO formation was dominated by photo-enhanced heterogeneous reactions of NO2 on aerosol surfaces (> 50 %), whereas particulate nitrate photolysis (34 %) prevailed during photochemical pollution events. Nevertheless, the model uncovers a significant unidentified marine HONO source in a “sea case”, with its HONO production rate reaching up to 0.70 ppbv h−1 at noon. Without considering this unidentified source, an extraordinarily high photolysis coefficient of nitrate and/or a heterogeneous uptake coefficient of NO2 would be required to match observed HONO concentrations. This missing marine HONO source affected the peak O3 production rate and OH radical concentration by 36 % and 28 %, respectively, at the observation site. Given the limited HONO observation data in coastal and marine settings, the unidentified HONO source may cause an underestimation of the atmosphere's oxidizing capacity. This study highlights the necessity for further investigation of the role of HONO in atmospheric chemistry in coastal and marine environments.

Factors Influencing the Formation of Nitrous Acid from Photolysis of Particulate Nitrate.

Enhanced photolysis of particulate nitrate (pNO3) to form photolabile species, such as gas-phase nitrous acid (HONO), has been proposed as a potential mechanism to recycle nitrogen oxides (NOx) in the remote boundary layer ("renoxification"). This article presents a series of laboratory experiments aimed at investigating the parameters that control the photolysis of pNO3 and the efficiency of HONO production. Filters on which artificial or ambient particles had been sampled were exposed to the light of a solar simulator, and the formation of HONO was monitored under controlled laboratory conditions. The results indicate that the photolysis of pNO3 is enhanced, compared to the photolysis of gas-phase HNO3, at low pNO3 levels, with the enhancement factor reducing at higher pNO3 levels. The presence of cations (Na+) and halides (Cl-) and photosensitive organic compounds (imidazole) also enhance pNO3 photolysis, but other organic compounds such as oxalate and succinic acid have the opposite effect. The precise role of humidity in pNO3 photolysis remains unclear. While the efficiency of photolysis is enhanced in deliquescent particles compared to dry particles, some of the experimental results suggest that this may not be the case for supersaturated particles. These experiments suggest that both the composition and the humidity of particles control the enhancement of particulate nitrate photolysis, potentially explaining the variability in results among previous laboratory and field studies. HONO observations in the remote marine boundary layer can be explained by a simple box-model that includes the photolysis of pNO3, in line with the results presented here, although more experimental work is needed in order to derive a comprehensive parametrization of this process.

Nitrogen oxides in the free troposphere: implications for tropospheric oxidants and the interpretation of satellite NO2 measurements

Abstract. Satellite-based retrievals of tropospheric NO2 columns are widely used to infer NOx (≡ NO + NO2) emissions. These retrievals rely on model information for the vertical distribution of NO2. The free tropospheric background above 2 km is particularly important because the sensitivity of the retrievals increases with altitude. Free tropospheric NOx also has a strong effect on tropospheric OH and ozone concentrations. Here we use observations from three aircraft campaigns (SEAC4RS, DC3, and ATom) and four atmospheric chemistry models (GEOS-Chem, GMI, TM5, and CAMS) to evaluate the model capabilities for simulating NOx in the free troposphere and attribute it to sources. NO2 measurements during the Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) and Deep Convective Clouds and Chemistry (DC3) campaigns over the southeastern U.S. in summer show increasing concentrations in the upper troposphere above 10 km, which are not replicated by the GEOS-Chem, although the model is consistent with the NO measurements. Using concurrent NO, NO2, and ozone observations from a DC3 flight in a thunderstorm outflow, we show that the NO2 measurements in the upper troposphere are biased high, plausibly due to interference from thermally labile NO2 reservoirs such as peroxynitric acid (HNO4) and methyl peroxy nitrate (MPN). We find that NO2 concentrations calculated from the NO measurements and NO–NO2 photochemical steady state (PSS) are more reliable to evaluate the vertical profiles of NO2 in models. GEOS-Chem reproduces the shape of the PSS-inferred NO2 profiles throughout the troposphere for SEAC4RS and DC3 but overestimates NO2 concentrations by about a factor of 2. The model underestimates MPN and alkyl nitrate concentrations, suggesting missing organic NOx chemistry. On the other hand, the standard GEOS-Chem model underestimates NO observations from the Atmospheric Tomography Mission (ATom) campaigns over the Pacific and Atlantic oceans, indicating a missing NOx source over the oceans. We find that we can account for this missing source by including in the model the photolysis of particulate nitrate on sea salt aerosols at rates inferred from laboratory studies and field observations of nitrous acid (HONO) over the Atlantic. The median PSS-inferred tropospheric NO2 column density for the ATom campaign is 1.7 ± 0.44 × 1014 molec. cm−2, and the NO2 column density simulated by the four models is in the range of 1.4–2.4 × 1014 molec. cm−2, implying that the uncertainty from using modeled NO2 tropospheric columns over clean areas in the retrievals for stratosphere–troposphere separation is about 1 × 1014 molec. cm−2. We find from GEOS-Chem that lightning is the main primary NOx source in the free troposphere over the tropics and southern midlatitudes, but aircraft emissions dominate at northern midlatitudes in winter and in summer over the oceans. Particulate nitrate photolysis increases ozone concentrations by up to 5 ppbv (parts per billion by volume) in the free troposphere in the northern extratropics in the model, which would largely correct the low model bias relative to ozonesonde observations. Global tropospheric OH concentrations increase by 19 %. The contribution of the free tropospheric background to the tropospheric NO2 columns observed by satellites over the contiguous U.S. increases from 25 ± 11 % in winter to 65 ± 9 % in summer, according to the GEOS-Chem vertical profiles. This needs to be accounted for when deriving NOx emissions from satellite NO2 column measurements.

Formation mechanisms and atmospheric implications of summertime nitrous acid (HONO) during clean, ozone pollution and double high-level PM2.5 and O3 pollution periods in Beijing

Nitrous acid (HONO) is a key precursor of the hydroxyl radicals (OH) and has a significant impact on air quality. Nowadays, the source of HONO is still controversial due to its complex formation mechanisms, which is widely explored in extensive field and laboratory studies. In this study, the pollution characteristics and source contribution of HONO under different air quality conditions in summer in Beijing were analyzed. The observation periods were classified as three typical periods: clean, ozone pollution, and double high pollution (co-occurrence of high PM2.5 and O3 concentrations). The average concentrations of observed HONO were 0.38 ± 0.35 ppb, 0.21 ± 0.18 ppb, 0.26 ± 0.20 ppb and 0.54 ± 0.45 ppb during the whole, clean, ozone and double high periods, respectively. The elevated HONO levels at night were attributed to vehicle emissions and the RH-dependent heterogeneous conversion of NO2 to HONO. The average emission ratio (HONO/NOx) was 0.85 % ± 0.38 %, and the mean value of calculated nocturnal NO2 to HONO conversion frequency was 0.0076 ± 0.0031 h−1. Based on daytime HONO budget analysis, the largest potential source of HONO was the homogeneous reaction of NO and OH (0.33 and 0.34 ppb h−1), followed by the unknown source (0.11 and 0.21 ppb h−1) during clean and ozone periods, while the unknown source (0.49 ppb h−1) played the predominant role during double high period. The unknown sources of HONO could be attributed to the photo-enhanced heterogeneous conversion of NO2 and the photolysis of particulate nitrate. Furthermore, the photolysis of ozone (0.17, 0.34 and 0.44 ppb h−1) was the major contributor to primary OH during three typical periods. HONO photolysis contributed considerable amounts of primary OH (0.32 ppb h−1) during double high period. These results are helpful to further understand the linkage between HONO and air quality variation.

Exploring HONO formation and its role in driving secondary pollutants formation during winter in the North China Plain

Daytime HONO photolysis is an important source of atmospheric hydroxyl radicals (OH). Knowledge of HONO formation chemistry under typical haze conditions, however, is still limited. In the Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain in 2018, we investigated the wintertime HONO formation and its atmospheric implications at a rural site Gucheng. Three different episodes based on atmospheric aerosol loading levels were classified: clean periods (CPs), moderately polluted periods (MPPs) and severely polluted periods (SPPs). Correlation analysis revealed that HONO formation via heterogeneous conversion of NO2 was more efficient on aerosol surfaces than on ground, highlighting the important role of aerosols in promoting HONO formation. Daytime HONO budget analysis indicated a large missing source (with an average production rate of 0.66 ± 0.26, 0.97 ± 0.47 and 1.45 ± 0.55 ppbV/hr for CPs, MPPs and SPPs, respectively), which strongly correlated with photo-enhanced reactions (NO2 heterogeneous reaction and particulate nitrate photolysis). Average OH formation derived from HONO photolysis reached up to (0.92 ± 0.71), (1.75 ± 1.26) and (1.82 ± 1.47) ppbV/hr in CPs, MPPs and SPPs respectively, much higher than that from O3 photolysis (i.e., (0.004 ± 0.004), (0.006 ± 0.007) and (0.0035 ± 0.0034) ppbV/hr). Such high OH production rates could markedly regulate the atmospheric oxidation capacity and hence promote the formation of secondary aerosols and pollutants.

The seasonal variations and potential sources of nitrous acid (HONO) in the rural North China Plain

Nitrous acid (HONO), an essential precursor of hydroxyl radicals (OH) in the troposphere, plays an integral role in atmospheric photochemistry. However, potential HONO sources remain unclear, particularly in rural areas, where long-term (including seasonal) measurements are scarce. HONO and related parameters were measured at a rural site in the North China Plain (NCP) during the winter of 2017 and summer and autumn of 2020. The mean HONO level was higher in winter (1.79 ± 1.44 ppbv) than in summer (0.67 ± 0.50 ppbv) and autumn (0.83 ± 0.62 ppbv). Source analysis revealed that the heterogeneous conversion (including photo-enhanced conversion) of NO2 on the ground surface dominated the daytime HONO production in the three seasons (43.1% in winter, 54.3% in summer, and 62.0% in autumn), and the homogeneous reaction of NO and OH contributed 37.8, 12.2, and 28.4% of the daytime HONO production during winter, summer, and autumn, respectively. In addition, the total contributions of other sources (direct vehicle emissions, particulate nitrate photolysis, NO2 uptake and its photo-enhanced reaction on the aerosol surface) to daytime HONO production were less than 5% in summer and autumn and 12.0% in winter. Unlike winter and autumn, an additional HONO source was found in summer (0.45 ± 0.21 ppbv h−1, 31.4% to the daytime HONO formation), which might be attributed to the HONO emission from the fertilized field. Among the primary radical sources (photolysis of HONO, O3, and formaldehyde), HONO photolysis was dominant, with contributions of 82.6, 49.3, and 63.2% in winter, summer, and autumn, respectively. Our findings may aid in understanding HONO formation in different seasons in rural areas and may highlight the impact of HONO on atmospheric oxidation capacity.

Direct Constraints on Secondary HONO Production in Aged Wildfire Smoke From Airborne Measurements Over the Western US

AbstractNitrous acid (HONO) mixing ratios measured in aged wildfire smoke plumes were higher than expected from known homogeneous chemical reactions. In a representative smoke plume, intercepted hours to days downwind of the source, the missing HONO source was highly correlated to particulate nitrate photolysis and NO2 reactive uptake to particles. Using a multilinear regression involving these two sources, we could explain the missing HONO production in this plume (R2 = 0.77). The resulting fit parameters from this plume had good explanatory power (R2 = 0.64) for missing HONO production in other fire plumes. The mean enhancement factor for particulate nitrate photolysis relative to gas‐phase nitric acid photolysis was 63 and the mean NO2 reactive uptake coefficient to submicron aerosol surface area forming HONO was 4.9 × 10−4. Given the likelihood of other neglected secondary HONO sources, these values are upper‐limits, suggesting a need to revisit HONO formation mechanisms in aged wildfire smoke.

An investigation into the chemistry of HONO in the marine boundary layer at Tudor Hill Marine Atmospheric Observatory in Bermuda

Abstract. Here we present measurement results of temporal distributions of nitrous acid (HONO) along with several chemical and meteorological parameters during the spring and the late summer of 2019 at Tudor Hill Marine Atmospheric Observatory in Bermuda. Large temporal variations in HONO concentration were controlled by several factors including local pollutant emissions, air mass interaction with the island, and long-range atmospheric transport of HONO precursors. In polluted plumes emitted from local traffic, power plant, and cruise ship emissions, HONO and nitrogen oxides (NOx) existed at substantial levels (up to 278 pptv and 48 ppbv, respectively), and NOx-related reactions played dominant roles in daytime formation of HONO. The lowest concentration of HONO was observed in marine air, with median concentrations at ∼ 3 pptv around solar noon and < 1 pptv during the nighttime. Considerably higher levels of HONO were observed during the day in the low-NOx island-influenced air ([NO2] < 1 ppbv), with a median HONO concentration of ∼ 17 pptv. HONO mixing ratios exhibited distinct diurnal cycles that peaked around solar noon and were lowest before sunrise, indicating the importance of photochemical processes for HONO formation. In clean marine air, NOx-related reactions contribute to ∼ 21 % of the daytime HONO source, and the photolysis of particulate nitrate (pNO3) can account for the missing source assuming a moderate enhancement factor of 29 relative to gaseous nitric acid photolysis. In low-NOx island-influenced air, the contribution from both NOx-related reactions and pNO3 photolysis accounts for only ∼ 48 % of the daytime HONO production, and the photochemical processes on surfaces of the island, such as the photolysis of nitric acid on the forest canopy, might contribute significantly to the daytime HONO production. The concentrations of HONO, NOx, and pNO3 were lower when the site was dominated by the aged marine air in the summer and were higher when the site was dominated by North American air in the spring, reflecting the effects of long-range transport on the reactive nitrogen chemistry in background marine environments.

Photochemical evolution of the 2013 California Rim Fire: synergistic impacts of reactive hydrocarbons and enhanced oxidants

Abstract. Large wildfires influence regional atmospheric composition, but chemical complexity challenges model predictions of downwind impacts. Here, we elucidate key connections within gas-phase photochemistry and assess novel chemical processes via a case study of the 2013 California Rim Fire plume. Airborne in situ observations, acquired during the NASA Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) mission, illustrate the evolution of volatile organic compounds (VOCs), oxidants, and reactive nitrogen over 12 h of atmospheric aging. Measurements show rapid formation of ozone and peroxyacyl nitrates (PNs), sustained peroxide production, and prolonged enhancements in oxygenated VOCs and nitrogen oxides (NOx). Observations and Lagrangian trajectories constrain a 0-D puff model that approximates plume photochemical history and provides a framework for evaluating process interactions. Simulations examine the effects of (1) previously unmeasured reactive VOCs identified in recent laboratory studies and (2) emissions and secondary production of nitrous acid (HONO). Inclusion of estimated unmeasured VOCs leads to a 250 % increase in OH reactivity and a 70 % increase in radical production via oxygenated VOC photolysis. HONO amplifies radical cycling and serves as a downwind NOx source, although impacts depend on how HONO is introduced. The addition of initial HONO (representing primary emissions) or particulate nitrate photolysis amplifies ozone production, while heterogeneous conversion of NO2 suppresses ozone formation. Analysis of radical initiation rates suggests that oxygenated VOC photolysis is a major radical source, exceeding HONO photolysis when averaged over the first 2 h of aging. Ozone production chemistry transitions from VOC sensitive to NOx sensitive within the first hour of plume aging, with both peroxide and organic nitrate formation contributing significantly to radical termination. To simulate smoke plume chemistry accurately, models should simultaneously account for the full reactive VOC pool and all relevant oxidant sources.

Atmospheric measurements at Mt. Tai – Part I: HONO formation and its role in the oxidizing capacity of the upper boundary layer

Abstract. A comprehensive field campaign, with measurements of HONO and related parameters, was conducted in summer 2018 at the foot (150 m a.s.l.) and the summit (1534 m a.s.l.) of Mt. Tai (Shandong province, China). At the summit station, high HONO mixing ratios were observed (mean ± 1σ: 133 ± 106 pptv, maximum: 880 pptv), with a diurnal noontime peak (mean ± 1σ: 133 ± 72 pptv at 12:30 local time). Constraints on the kinetics of aerosol-derived HONO sources (NO2 uptake on the aerosol surface and particulate nitrate photolysis) were performed and discussed, which enables a better understanding of the interaction of HONO and aerosols, especially in the polluted North China Plain. Various evidence of air mass transport from the ground to the summit level was provided. Furthermore, daytime HONO formation from different paths and its role in radical production were quantified and discussed. We found that the homogeneous reaction NO + OH could only explain 8.0 % of the daytime HONO formation, resulting in strong unknown sources (Pun). Campaigned-averaged Pun was about 290 ± 280 pptv h−1, with a maximum of about 1800 pptv h−1. Aerosol-derived HONO formation mechanisms were not the major sources of Pun at the summit station. Their contributions to daytime HONO formation varied from negligible to moderate (similar to NO + OH), depending on the chemical kinetic parameters used. Coupled with sensitivity tests on the kinetic parameters used, the NO2 uptake on the aerosol surface and particulate nitrate photolysis contributed 1.5 %–19 % and 0.6 %–9.6 % of the observed Pun, respectively. Based on synchronous measurements at the foot and the summit station, an amount of field evidence was proposed to support the finding that the remaining majority (70 %–98 %) of Pun was dominated by the rapid vertical transport from the ground to the summit level and heterogeneous formation on the mountain surfaces during transport. HONO photolysis at the summit level initialized daytime photochemistry and still represented an essential OH source in the daytime, with a contribution of about one-quarter of O3. We provided evidence that ground-derived HONO played a significant role in the oxidizing capacity of the upper boundary layer through the enhanced vertical air mass exchange driven by mountain winds. The follow-up impacts should be considered in regional chemistry transport models.

Atmospheric measurements at Mt. Tai – Part II: HONO budget and radical (RO<sub><i>x</i></sub> + NO<sub>3</sub>) chemistry in the lower boundary layer

Abstract. In the summer of 2018, a comprehensive field campaign, with measurements on HONO and related parameters, was conducted at the foot (150 m a.s.l.) and the summit of Mt. Tai (1534 m a.s.l.) in the central North China Plain (NCP). With the implementation of a 0-D box model, the HONO budget with six additional sources and its role in radical chemistry at the foot station were explored. We found that the model default source, NO + OH, could only reproduce 13 % of the observed HONO, leading to a strong unknown source strength of up to 3 ppbv h−1. Among the additional sources, the NO2 uptake on the ground surface dominated (∼ 70 %) nighttime HONO formation, and its photo-enhanced reaction dominated (∼ 80 %) daytime HONO formation. Their contributions were sensitive to the mixing layer height (MLH) used for the parameterizations, highlighting the importance of a reasonable MLH for exploring ground-level HONO formation in 0-D models and the necessity of gradient measurements. A ΔHONO/ΔNOx ratio of 0.7 % for direct emissions from vehicle exhaust was inferred, and a new method to quantify its contribution to the observations was proposed and discussed. Aerosol-derived sources, including the NO2 uptake on the aerosol surface and the particulate nitrate photolysis, did not lead to significant HONO formation, with their contributions lower than NO + OH. HONO photolysis in the early morning initialized the daytime photochemistry at the foot station. It was also a substantial radical source throughout the daytime, with contributions higher than O3 photolysis to OH initiation. Moreover, we found that OH dominated the atmospheric oxidizing capacity in the daytime, while modeled NO3 appeared to be significant at night. Peaks of modeled NO3 time series and average diurnal variation reached 22 and 9 pptv, respectively. NO3-induced reactions contribute 18 % of nitrate formation potential (P(HNO3)) and 11 % of the isoprene (C5H8) oxidation throughout the whole day. At night, NO3 chemistry led to 51 % and 44 % of P(HNO3) or the C5H8 oxidation, respectively, implying that NO3 chemistry could significantly affect nighttime secondary organic and inorganic aerosol formation in this high-O3 region. Considering the severe O3 pollution in the NCP and the very limited NO3 measurements, we suggest that besides direct measurements of HOx and primary HOx precursors (O3, HONO, alkenes, etc.), NO3 measurements should be conducted to understand the atmospheric oxidizing capacity and air pollution formation in this and similar regions.

Exploration of the atmospheric chemistry of nitrous acid in a coastal city of southeastern China: results from measurements across four seasons

Abstract. Because nitrous acid (HONO) photolysis is a key source of hydroxyl (OH) radicals, identifying the atmospheric sources of HONO is essential to enhance the understanding of atmospheric chemistry processes and improve the accuracy of simulation models. We performed seasonal field observations of HONO in a coastal city of southeastern China, along with measurements of trace gases, aerosol compositions, photolysis rate constants (J), and meteorological parameters. The results showed that the average observed concentration of HONO was 0.54 ± 0.47 ppb. Vehicle exhaust emissions contributed an average of 1.45 % to HONO, higher than the values found in most other studies, suggesting an influence from diesel vehicle emissions. The mean conversion frequency of NO2 to HONO in the nighttime was the highest in summer due to water droplets evaporating under high-temperature conditions. Based on a budget analysis, the rate of emission from unknown sources (Runknown) was highest around midday, with values of 4.51 ppb h−1 in summer, 3.51 ppb h−1 in spring, 3.28 ppb h−1 in autumn, and 2.08 ppb h−1 in winter. Unknown sources made up the largest proportion of all sources in summer (81.25 %), autumn (73.99 %), spring (70.87 %), and winter (59.28 %). The photolysis of particulate nitrate was probably a source in spring and summer while the conversion from NO2 to HONO on BC enhanced by light was perhaps a source in autumn and winter. The variation of HONO at night can be exactly simulated based on the HONO / NOx ratio, while the J(NO3-_R) × pNO3- should be considered for daytime simulations in summer and autumn, or 1/4× (J(NO3-_R) × pNO3-) in spring and winter. Compared with O3 photolysis, HONO photolysis has long been an important source of OH except for summer afternoons. Observation of HONO across four seasons with various auxiliary parameters improves the comprehension of HONO chemistry in southeastern coastal China.

Particulate nitrate photolysis in the atmosphere

Particulate nitrate photolysis can lead to the formation of secondary inorganic and organic aerosols that affect climate, air quality, and human health.

Nitrate Photolysis in Mixed Sucrose-Nitrate-Sulfate Particles at Different Relative Humidities.

Atmospheric particles can be viscous. The limitation in diffusion impedes the mass transfer of oxidants from the gas phase to the particle phase and hinders multiphase oxidation processes. On the other hand, nitrate photolysis has been found to be effective in producing oxidants such as OH radicals within the particles. Whether nitrate photolysis can effectively proceed in viscous particles and how it may affect the physicochemical properties of the particle have not been much explored. In this study, we investigated particulate nitrate photolysis in mixed sucrose-nitrate-sulfate particles as surrogates of atmospheric viscous particles containing organic and inorganic components as a function of relative humidity (RH) and the molar fraction of sucrose to the total solute (FSU) with an in situ micro-Raman system. Sucrose suppressed nitrate crystallization, and high photolysis rate constants (∼10-5 s-1) were found, irrespective of the RH. For FSU = 0.5 and 0.33 particles under irradiation at 30% RH, we observed morphological changes from droplets to the formation of inclusions and then likely "hollow" semisolid particles, which did not show Raman signal at central locations. Together with the phase states of inorganics indicated by the full width at half-maxima (FWHM), images with bulged surfaces, and size increase of the particles in optical microscopic imaging, we inferred that the hindered diffusion of gaseous products (i.e., NOx, NOy) from nitrate photolysis is a likely reason for the morphological changes. Atmospheric implications of these results are also presented.

Seasonality of nitrous acid near an industry zone in the Yangtze River Delta region of China: Formation mechanisms and contribution to the atmospheric oxidation capacity

HONO along with other trace gases and actinic flux were continuously measured in selected months from December 2015 to November 2016 near a major industrial zone of the Yangtze River Delta (YRD) region of China. The seasonalities of HONO and its formation mechanisms were explored. A box model was developed to assess the role of HONO chemistry in the atmospheric oxidative capacity in the study area. The model was constrained by in-situ observations to minimize simulation uncertainties. Our measurement results demonstrate that HONO mean concentration and diurnal profile varied noticeably between different seasons with the maximum (1.32 ± 0.92 ppbv) observed in winter, coincided with high mass loading of fine particles (PM2.5 = 114 ± 59.6 μg m−3). The averaged nighttime NO2 to HONO conversion ratios (hr−1) were (1.83 ± 0.73)% in spring, (2.91 ± 1.06)% in summer, (0.72 ± 0.44)% in autumn, and (0.84 ± 0.45)% in winter. The homogeneous reaction of OH + NO was responsible for a large portion of HONO production throughout the year but was relatively weaker compared to other mechanisms in winter. Heterogeneous reactions on ground surfaces were much stronger than those on aerosol surfaces, dominating nighttime HONO production, except in summer. Primary HONO emissions were only responsible for a small portion of the HONO budget. Particulate nitrate photolysis was a significant source of HONO and it showed very little variation in different seasons. OH budget analysis shows that O3 photolysis was a major contributor to the OH production in the study area while HONO photolysis provided an extra boost to the photochemical reactivity in the early morning, particularly during summer. However, HONO chemistry played a more important role in autumn and winter by dominating OH production throughout the day. Accordingly, effective control of NOx emissions dominated by industries can significantly reduce HONO production and thus weaken the atmospheric oxidative capacity in the study area.

Production of Formate via Oxidation of Glyoxal Promoted by Particulate Nitrate Photolysis.

Particulate nitrate photolysis can produce oxidants (i.e., OH, NO2, and NO2-/HNO2) in aqueous droplets and may play a potential role in increased atmospheric oxidative capacity. Our earlier works have reported on the SO2 oxidation promoted by nitrate photolysis to produce sulfate. Here, we used glyoxal as a model precursor to examine the role of particulate nitrate photolysis in the formation of secondary organic aerosol (SOA) from particle-phase oxidation of glyoxal by OH radicals. Particles containing sodium nitrate and glyoxal were irradiated at 300 nm. Interestingly, typical oxidation products of oxalic acid, glyoxylic acid, and higher-molecular-weight products reported in the literature were not found in the photooxidation process of glyoxal during nitrate photolysis in the particle phase. Instead, formic acid/formate production was found as the main oxidation product. At glyoxal concentration higher than 3 M, we found that the formic acid/formate production rate increases significantly with increasing glyoxal concentration. Such results suggest that oxidation of glyoxal at high concentrations by OH radicals produced from nitrate photolysis in aqueous particles may not contribute significantly to SOA formation since formic acid is a volatile species. Furthermore, recent predictions of formic acid/formate concentration from the most advanced chemical models are lower than ambient observations at both the ground level and high altitude. The present study reveals a new insight into the production of formic acid/formate as well as a sink of glyoxal in the atmosphere, which may partially narrow the gap between model predictions and field measurements in both species.

Production of HONO from NO<sub>2</sub> uptake on illuminated TiO<sub>2</sub> aerosol particles and following the illumination of mixed TiO<sub>2</sub>∕ammonium nitrate particles

Abstract. The rate of production of HONO from illuminated TiO2 aerosols in the presence of NO2 was measured using an aerosol flow tube system coupled to a photo-fragmentation laser-induced fluorescence detection apparatus. The reactive uptake coefficient of NO2 to form HONO, γNO2→HONO, was determined for NO2 mixing ratios in the range 34–400 ppb, with γNO2→HONO spanning the range (9.97 ± 3.52) × 10−6 to (1.26 ± 0.17) × 10−4 at a relative humidity of 15 ± 1 % and for a lamp photon flux of (1.63 ± 0.09) ×1016 photons cm−2 s−1 (integrated between 290 and 400 nm), which is similar to midday ambient actinic flux values. γNO2→HONO increased as a function of NO2 mixing ratio at low NO2 before peaking at (1.26 ± 0.17) ×10-4 at ∼ 51 ppb NO2 and then sharply decreasing at higher NO2 mixing ratios rather than levelling off, which would be indicative of surface saturation. The dependence of HONO production on relative humidity was also investigated, with a peak in production of HONO from TiO2 aerosol surfaces found at ∼ 25 % RH. Possible mechanisms consistent with the observed trends in both the HONO production and reactive uptake coefficient were investigated using a zero-dimensional kinetic box model. The modelling studies supported a mechanism for HONO production on the aerosol surface involving two molecules of NO2, as well as a surface HONO loss mechanism which is dependent upon NO2. In a separate experiment, significant production of HONO was observed from illumination of mixed nitrate/TiO2 aerosols in the absence of NO2. However, no production of HONO was seen from the illumination of nitrate aerosols alone. The rate of production of HONO observed from mixed nitrate/TiO2 aerosols was scaled to ambient conditions found at the Cape Verde Atmospheric Observatory (CVAO) in the remote tropical marine boundary layer. The rate of HONO production from aerosol particulate nitrate photolysis containing a photocatalyst was found to be similar to the missing HONO production rate necessary to reproduce observed concentrations of HONO at CVAO. These results provide evidence that particulate nitrate photolysis may have a significant impact on the production of HONO and hence NOx in the marine boundary layer where mixed aerosols containing nitrate and a photocatalytic species such as TiO2, as found in dust, are present.

Photochemical reactions on aerosols at West Antarctica: A molecular case-study of nitrate formation among sea salt aerosols

Environmental implications of climate change are complex and exhibit regional variations both within and between the polar regions. The increase of solar UV radiation flux over Antarctica due to stratospheric ozone depletion creates the optimal conditions for photochemical reactions on the snow. Modeling, laboratory, and indirect field studies suggest that snowpack process release gases to the atmosphere that can react on sea salt particles in remote regions such as Antarctica, modifying aerosol composition and physical properties of aerosols. Here, we present evidence of photochemical processing in West Antarctica aerosols using microscopic and chemical speciation of individual atmospheric particles. Individual aerosol particles collected at the Brazilian module Criosfera 1 were analyzed by scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) combined with computer-controlled scanning electron microscopy (CCSEM) with energy-dispersive X-ray (EDX) microanalysis. The displacement of chlorine relative to sodium was observed over most of the sea salt particles. Particles with a chemical composition consistent with NaCl-NO3 contributed up to 30% of atmospheric particles investigated. Overall, this study provides evidence that the snowpack and particulate nitrate photolysis should be considered in dynamic partition equilibrium in the troposphere. These findings may assist in reducing modeling uncertainties and present new insights into the aerosol chemical composition in the polar environment.