Mutual promotion between aerosol particle liquid water and particulate nitrate enhancement leads to severe nitrate-dominated particulate matter pollution and low visibility

As has been the case in North America and western Europe, the SO2 emissions have substantially reduced in the North China Plain (NCP) in recent years. Differential rates of reduction in SO2 and NOx concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate (pNO−3) over the NCP. In this study, we observed a polluted episode with the particulate nitrate mass fraction in nonrefractory PM1 (NR-PM1) being up to 44 % during wintertime in Beijing. Based on this typical pNO−3-dominated haze event, the linkage between aerosol water uptake and pNO−3 enhancement, further impacting on visibility degradation, has been investigated based on field observations and theoretical calculations. During haze development, as ambient relative humidity (RH) increased from ∼10 % to 70 %, the aerosol particle liquid water increased from ∼1 µg m−3 at the beginning to ∼75 µg m−3 in the fully developed haze period. The aerosol liquid water further increased the aerosol surface area and volume, enhancing the condensational loss of N2O5 over particles. From the beginning to the fully developed haze, the condensational loss of N2O5 increased by a factor of 20 when only considering aerosol surface area and volume of dry particles, while increasing by a factor of 25 when considering extra surface area and volume due to water uptake. Furthermore, aerosol liquid water favored the thermodynamic equilibrium of HNO3 in the particle phase under the supersaturated HNO3 and NH3 in the atmosphere. All the above results demonstrated that pNO−3 is enhanced by aerosol water uptake with elevated ambient RH during haze development, in turn facilitating the aerosol take-up of water due to the hygroscopicity of particulate nitrate salt. Such mutual promotion between aerosol particle liquid water and particulate nitrate enhancement can rapidly degrade air quality and halve visibility within 1 d. Reduction of nitrogen-containing gaseous precursors, e.g., by control of traffic emissions, is essential in mitigating severe haze events in the NCP

Constraining emissions of volatile organic compounds from western US wildfires with WE-CAN and FIREX-AQ airborne observations

The impact of biomass burning (BB) on the atmospheric burden of volatile organic compounds (VOCs) is highly uncertain. Here we apply the GEOS-Chem chemical transport model (CTM) to constrain BB emissions in the western US at ~25 km resolution. Across three BB emission inventories widely used in CTMs, the total of 14 modeled BB VOC emissions in the western US agree with each other within 30&ndash;40 %. However, emissions for individual VOC differ by up to a factor of 5 (i.e., lumped &ge; C4 alkanes), driven by the regionally averaged emission ratios (ERs) among inventories. We further evaluate GEOS-Chem simulations with aircraft observations made during WE-CAN (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen) and FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality) field campaigns. Despite being driven by different global BB inventories or applying various injection height assumptions, GEOS-Chem simulations underpredict observed vertical profiles by a factor of 3&ndash;7. The model shows small-to-no bias for most species in low/no smoke conditions. We thus attribute the negative model biases mostly to underestimated BB emissions in these inventories. Tripling BB emissions in the model reproduces observed vertical profiles for primary compounds, i.e., CO, propane, benzene, and toluene. However, it shows no-to-less significant improvements for oxygenated VOCs, particularly formaldehyde, formic acid, acetic acid, and lumped &ge; C3 aldehydes, suggesting the model is missing secondary sources of these compounds in BB-impacted environments. The underestimation of primary BB emissions in inventories is likely attributable to underpredicted amounts of effective dry matter burned, rather than errors in fire detection, injection height, or ERs. We cannot rule out potential sub-grid uncertainties (i.e., not being able to fully resolve fire plumes) in the nested GEOS-Chem which could explain the model negative bias partially, though the back-of-the-envelope calculation and evaluation using longer-term ground measurements help increase the argument of the dry matter burned underestimation. The ERs of the 14 BB VOCs implemented in GEOS-Chem account for about half of the total 161 measured VOCs (~75 versus 150 ppb ppm-1). This reveals a significant amount of missing reactive organic carbon in widely-used BB emission inventories. Considering both uncertainties in effective dry matter burned and unmodeled VOCs, we infer that BB contributed up to 10 % in 2019 and 45 % in 2018 (240 and 2040 GgC) of the total VOC primary emission flux in the western US during these two fire seasons, compared to only 1&ndash;10 % in the standard GEOS-Chem.</p

Nighttime and daytime dark oxidation chemistry in wildfire plumes: an observation and model analysis of FIREX-AQ aircraft data

Wildfires are increasing in size across the western US, leading to increases in human smoke exposure and associated negative health impacts. The impact of biomass burning (BB) smoke, including wildfires, on regional air quality depends on emissions, transport, and chemistry, including oxidation of emitted BB volatile organic compounds (BBVOCs) by the hydroxyl radical (OH), nitrate radical (NO3), and ozone (O3). During the daytime, when light penetrates the plumes, BBVOCs are oxidized mainly by O3 and OH. In contrast, at night or in optically dense plumes, BBVOCs are oxidized mainly by O3 and NO3. This work focuses on the transition between daytime and nighttime oxidation, which has significant implications for the formation of secondary pollutants and loss of nitrogen oxides () and has been understudied. We present wildfire plume observations made during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality), a field campaign involving multiple aircraft, ground, satellite, and mobile platforms that took place in the United States in the summer of 2019 to study both wildfire and agricultural burning emissions and atmospheric chemistry. We use observations from two research aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailed chemical box model, including updated phenolic mechanisms, to analyze smoke sampled during midday, sunset, and nighttime. Aircraft observations suggest a range of NO3 production rates (0.1&ndash;1.5&thinsp;ppbv&thinsp;h&minus;1) in plumes transported during both midday and after dark. Modeled initial instantaneous reactivity toward BBVOCs for NO3, OH, and O3 is 80.1&thinsp;%, 87.7&thinsp;%, and 99.6&thinsp;%, respectively. Initial NO3 reactivity is 10&ndash;104 times greater than typical values in forested or urban environments, and reactions with BBVOCs account for &gt;97&thinsp;% of NO3 loss in sunlit plumes (jNO2 up to ), while conventional photochemical NO3 loss through reaction with NO and photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH and O3 (11&thinsp;%&ndash;43&thinsp;%, 54&thinsp;%&ndash;88&thinsp;% for alkenes; 18&thinsp;%&ndash;55&thinsp;%, 39&thinsp;%&ndash;76&thinsp;%, for furans, respectively), but phenolic oxidation is split between NO3, O3, and OH (26&thinsp;%&ndash;52&thinsp;%, 22&thinsp;%&ndash;43&thinsp;%, 16&thinsp;%&ndash;33&thinsp;%, respectively). Nitrate radical oxidation accounts for 26&thinsp;%&ndash;52&thinsp;% of phenolic chemical loss in sunset plumes and in an optically thick plume. Nitrocatechol yields varied between 33&thinsp;% and 45&thinsp;%, and NO3 chemistry in BB plumes emitted late in the day is responsible for 72&thinsp;%&ndash;92&thinsp;% (84&thinsp;% in an optically thick midday plume) of nitrocatechol formation and controls nitrophenolic formation overall. As a result, overnight nitrophenolic formation pathways account for 56&thinsp;%&plusmn;2&thinsp;% of NOx loss by sunrise the following day. In all but one overnight plume we modeled, there was remaining NOx (13&thinsp;%&ndash;57&thinsp;%) and BBVOCs (8&thinsp;%&ndash;72&thinsp;%) at sunrise. &nbsp;</p

Secondary organic aerosols from anthropogenic volatile organic compounds contribute substantially to air pollution mortality

Anthropogenic secondary organic aerosol (ASOA), formed from anthropogenic emissions of organic compounds, constitutes a substantial fraction of the mass of submicron aerosol in populated areas around the world and contributes to poor air quality and premature mortality. However, the precursor sources of ASOA are poorly understood, and there are large uncertainties in the health benefits that might accrue from reducing anthropogenic organic emissions. We show that the production of ASOA in 11 urban areas on three continents is strongly correlated with the reactivity of specific anthropogenic volatile organic compounds. The differences in ASOA production across different cities can be explained by differences in the emissions of aromatics and intermediate- and semi-volatile organic compounds, indicating the importance of controlling these ASOA precursors. With an improved model representation of ASOA driven by the observations, we attribute 340&thinsp;000&nbsp;PM2.5-related premature deaths per year to ASOA, which is over an order of magnitude higher than prior studies. A sensitivity case with a more recently proposed model for attributing mortality to PM2.5 (the Global Exposure Mortality Model) results in up to 900&thinsp;000 deaths. A limitation of this study is the extrapolation from cities with detailed studies and regions where detailed emission inventories are available to other regions where uncertainties in emissions are larger. In addition to further development of institutional air quality management infrastructure, comprehensive air quality campaigns in the countries in South and Central America, Africa, South Asia, and the Middle East are needed for further progress in this area. &nbsp;</p

Gas-to-Particle Partitioning of Major Oxidation Products from Monoterpenes and Real Plant Emissions

Secondary organic aerosol (SOA), formed through the oxidation of volatile organic compounds (VOCs) in the atmosphere, play a key role in climate change and air quality. Due to thousands of individual compounds involved in SOA formation, the chemical characterization of organic aerosols (OA) remains a huge analytical challenge. Defining the fundamental parameters that distribute these organic molecules between the gas and particle phases is essential, as atmospheric lifetime and their impacts change drastically depending on their phase state. In this work, an instrument called aerosol collection module (ACM) was redeveloped and automated to allow a better characterization of SOA originating from the oxidation of biogenic precursors. An inter-comparison of the ACM to different aerosol chemical characterization techniques was performed with a focus on the partitioning of major biogenic oxidation products between the gas- and particle-phase. In particular, the ACM, the collection thermal desorption unit (TD) and the chemical analysis of aerosol on-line (CHARON) are different aerosol sampling inlets utilizing a Proton-Transfer-Reaction Timeof- Flight Mass Spectrometer (PTR-ToF-MS). These techniques were deployed at the atmosphere simulation chamber SAPHIR to study SOA formation and aging from different monoterpenes (β-pinene, limonene) and real plant emissions ($\textit{Pinus sylvestris L.}$). The capabilities of the PTR-based techniques were compared among each other and to results from an Aerodyne Aerosol Mass Spectrometer (AMS) and a Scanning Mobility Particle Sizer (SMPS). Gas-to-particle partitioning values were determined based on the saturation mass concentration (C*) of individual ions by performing simultaneous measurement of their signal in the gas- and particle-phase. Despite significant differences in the aerosol collection and desorption methods of the PTR based techniques, the determined chemical composition was comparable, i.e. the same major contributing ions were found by all instruments for the different chemical systems studied. These ions could be attributed to known products expected from the oxidation of the examined monoterpenes. Averaged over all experiments, the total aerosol mass recovery compared to an SMPS was 80 ± 10%, 51 ± 5% and 27 ± 3% for CHARON, ACM and TD, respectively. Comparison to the oxygen to carbon ratios (O:C) obtained by AMS showed that all PTR based techniques observed lower O:C ratios indicating a loss of molecular oxygen either during aerosol sampling or detection. Differences in total mass recovery and O:C between the three instruments was found to result predominately from differences in the electric field strength (V cm$^{-1}$) to buffer gas density (molecules cm$^{-3}$) (E/N) ratio in the drifttube reaction ionization chambers of the PTR-ToF-MS instruments and from dissimilarities in the collection/desorption of aerosols. A method to identify and exclude ions affected by thermal dissociation during desorption and ionic dissociation in the ionization chamber of the PTRMS was developed and tested. Determined species were mapped onto the two dimensional volatility basis set (2D-VBS) and results showed a decrease of the C* with increasing oxidation state. For compounds measured [...

Gas-to-Particle Partitioning of Major Oxidation Products from Monoterpenes and Real Plant Emissions

Secondary organic aerosol (SOA), formed through the oxidation of volatile organic compounds (VOCs) in the atmosphere, play a key role in climate change and air quality. Due to thousands of individual compounds involved in SOA formation, the chemical characterization of organic aerosols (OA) remains a huge analytical challenge. Defining the fundamental parameters that distribute these organic molecules between the gas and particle phases is essential, as atmospheric lifetime and their impacts change drastically depending on their phase state. In this work, an instrument called aerosol collection module (ACM) was redeveloped and automated to allow a better characterization of SOA originating from the oxidation of biogenic precursors. An inter-comparison of the ACM to different aerosol chemical characterization techniques was performed with a focus on the partitioning of major biogenic oxidation products between the gas- and particle-phase. In particular, the ACM, the collection thermal desorption unit (TD) and the chemical analysis of aerosol on-line (CHARON) are different aerosol sampling inlets utilizing a Proton-Transfer-Reaction Timeof- Flight Mass Spectrometer (PTR-ToF-MS). These techniques were deployed at the atmosphere simulation chamber SAPHIR to study SOA formation and aging from different monoterpenes (β-pinene, limonene) and real plant emissions ($\textit{Pinus sylvestris L.}$). The capabilities of the PTR-based techniques were compared among each other and to results from an Aerodyne Aerosol Mass Spectrometer (AMS) and a Scanning Mobility Particle Sizer (SMPS). Gas-to-particle partitioning values were determined based on the saturation mass concentration (C*) of individual ions by performing simultaneous measurement of their signal in the gas- and particle-phase. Despite significant differences in the aerosol collection and desorption methods of the PTR based techniques, the determined chemical composition was comparable, i.e. the same major contributing ions were found by all instruments for the different chemical systems studied. These ions could be attributed to known products expected from the oxidation of the examined monoterpenes. Averaged over all experiments, the total aerosol mass recovery compared to an SMPS was 80 ± 10%, 51 ± 5% and 27 ± 3% for CHARON, ACM and TD, respectively. Comparison to the oxygen to carbon ratios (O:C) obtained by AMS showed that all PTR based techniques observed lower O:C ratios indicating a loss of molecular oxygen either during aerosol sampling or detection. Differences in total mass recovery and O:C between the three instruments was found to result predominately from differences in the electric field strength (V cm$^{-1}$) to buffer gas density (molecules cm$^{-3}$) (E/N) ratio in the drifttube reaction ionization chambers of the PTR-ToF-MS instruments and from dissimilarities in the collection/desorption of aerosols. A method to identify and exclude ions affected by thermal dissociation during desorption and ionic dissociation in the ionization chamber of the PTRMS was developed and tested. Determined species were mapped onto the two dimensional volatility basis set (2D-VBS) and results showed a decrease of the C* with increasing oxidation state. For compounds measured [...