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1. Modeling Diurnal Variation of Biogenic SOA Formation via Multiphase Reaction of Biogenic Hydrocarbons

   https://doi.org/10.5194/acp-2022-327  

   Han, S. ; Jang, M. ( January 2022 , Atmospheric chemistry and physics discussion)

   The daytime oxidation of biogenic hydrocarbons is attributed to both OH radicals and O3, while nighttime chemistry is dominated by the reaction with O3 and NO3 radicals. Here, the diurnal pattern of Secondary Organic Aerosol (SOA) originating from biogenic hydrocarbons was intensively evaluated under varying environmental conditions (temperature, humidity, sunlight intensity, NOx levels, and seed conditions) by using the UNIfied Partitioning Aerosol phase Reaction (UNIPAR) model, which comprises multiphase gas-particle partitioning and in-particle chemistry. The oxidized products of three different hydrocarbons (isoprene, α-pinene, and β-caryophyllene) were predicted by using near explicit gas mechanisms for four different oxidation paths (OH, O3,more »NO3, and O(3P)) during day and night. The gas mechanisms implemented the Master Chemical Mechanism (MCM v3.3.1), the reactions that formed low volatility products via peroxy radical (RO2) autoxidation, and self- and cross-reactions of nitrate-origin RO2. In the model, oxygenated products were then classified into volatility-reactivity base lumping species, which were dynamically constructed under varying NOx levels and aging scales. To increase feasibility, the UNIPAR model that equipped mathematical equations for stoichiometric coefficients and physicochemical parameters of lumping species was integrated with the SAPRC gas mechanism. The predictability of the UNIPAR model was demonstrated by simulating chamber-generated SOA data under varying environments day and night. Overall, the SOA simulation decoupled to each oxidation path indicated that the nighttime isoprene SOA formation was dominated by the NO3-driven oxidation, regardless of NOx levels. However, the oxidation path to produce the nighttime α-pinene SOA gradually transited from the NO3-initiated reaction to ozonolysis as NOx levels decreased. For daytime SOA formation, both isoprene and α-pinene were dominated by the OH-radical initiated oxidation. The contribution of the O(3P) path to all biogenic SOA formation was negligible in daytime. Sunlight during daytime promotes the decomposition of oxidized products via photolysis and thus, reduces SOA yields. Nighttime α-pinene SOA yields were significantly higher than daytime SOA yields, although the nighttime α-pinene SOA yields gradually decreased with decreasing NOx levels. For isoprene, nighttime chemistry yielded higher SOA mass than daytime at the higher NOx level (isoprene/NOx > 5 ppbC/ppb). The daytime isoprene oxidation at the low NOx level formed epoxy-diols that significantly contributed SOA formation via heterogeneous chemistry. For isoprene and α-pinene, daytime SOA yields gradually increased with decreasing NOx levels. The daytime SOA produced more highly oxidized multifunctional products and thus, it was generally more sensitive to the aqueous reactions than the nighttime SOA. β-Caryophyllene, which rapidly oxidized and produced SOA with high yields, showed a relatively small variation in SOA yields from changes in environmental conditions (i.e., NOx levels, seed conditions, and diurnal pattern), and its SOA formation was mainly attributed to ozonolysis day and night. To mimic the nighttime α-pinene SOA formation under the polluted urban atmosphere, α-pinene SOA formation was simulated in the presence of gasoline fuel. The simulation suggested the growth of α-pinene SOA in the presence of gasoline fuel gas by the enhancement of the ozonolysis path under the excess amount of ozone, which is typical in urban air. We concluded that the oxidation of the biogenic hydrocarbon with O3 or NO3 radicals is a source to produce a sizable amount of nocturnal SOA, despite of the low emission at night.« less
2. Formation of condensable organic vapors from anthropogenic and biogenic volatile organic compounds (VOCs) is strongly perturbed by NO_<i>x</i> in eastern China

   https://doi.org/10.5194/acp-21-14789-2021  

   Liu, Yuliang ; Nie, Wei ; Li, Yuanyuan ; Ge, Dafeng ; Liu, Chong ; Xu, Zhengning ; Chen, Liangduo ; Wang, Tianyi ; Wang, Lei ; Sun, Peng ; et al ( January 2021 , Atmospheric Chemistry and Physics)

   Abstract. Oxygenated organic molecules (OOMs) are the crucial intermediates linkingvolatile organic compounds (VOCs) to secondary organic aerosols (SOAs) in theatmosphere, but comprehensive understanding of the characteristics of OOMsand their formation from VOCs is still missing. Ambient observations ofOOMs using recently developed mass spectrometry techniques are stilllimited, especially in polluted urban atmospheres where VOCs and oxidants areextremely variable and complex. Here, we investigate OOMs, measured by anitrate-ion-based chemical ionization mass spectrometer at Nanjing ineastern China, through performing positive matrix factorization on binnedmass spectra (binPMF). The binPMF analysis reveals three factors aboutanthropogenic VOC (AVOC) daytime chemistry, three isoprene-relatedfactors, three factors about biogenicmore »VOC (BVOC) nighttime chemistry, andthree factors about nitrated phenols. All factors are influenced by NOxin different ways and to different extents. Over 1000 non-nitro moleculeshave been identified and then reconstructed from the selected solution ofbinPMF, and about 72 % of the total signals are contributed bynitrogen-containing OOMs, mostly regarded as organic nitrates formed throughperoxy radicals terminated by nitric oxide or nitrate-radical-initiatedoxidations. Moreover, multi-nitrates account for about 24 % of the totalsignals, indicating the significant presence of multiple generations,especially for isoprene (e.g., C5H10O8N2 andC5H9O10N3). Additionally, the distribution of OOMconcentration on the carbon number confirms their precursors are driven by AVOCsmixed with enhanced BVOCs during summer. Our results highlight the decisiverole of NOx in OOM formation in densely populated areas, and we encouragemore studies on the dramatic interactions between anthropogenic and biogenicemissions.« less
3. Explicit modeling of isoprene chemical processing in polluted air masses in suburban areas of the Yangtze River Delta region: radical cycling and formation of ozone and formaldehyde

   https://doi.org/10.5194/acp-21-5905-2021  

   Zhang, Kun ; Huang, Ling ; Li, Qing ; Huo, Juntao ; Duan, Yusen ; Wang, Yuhang ; Yaluk, Elly ; Wang, Yangjun ; Fu, Qingyan ; Li, Li ( January 2021 , Atmospheric Chemistry and Physics)

   Abstract. In recent years, ozone pollution has become one of the most severeenvironmental problems in China. Evidence from observations have showedincreased frequency of high O3 levels in suburban areas of the YangtzeRiver Delta (YRD) region. To better understand the formation mechanism oflocal O3 pollution and investigate the potential role of isoprenechemistry in the budgets of ROx (OH+HO2+RO2) radicals,synchronous observations of volatile organic compounds (VOCs), formaldehyde(HCHO), and meteorological parameters were conducted at a suburban site ofthe YRD region in 2018. Five episodes with elevated O3 concentrationsunder stagnant meteorological conditions were identified; anobservation-based model (OBM) with the Master Chemical Mechanism was appliedtomore »analyze the photochemical processes during these high O3 episodes.The high levels of O3, nitrogen oxides (NOx), and VOCs facilitatedstrong production and recycling of ROx radicals with the photolysis ofoxygenated VOCs (OVOCs) being the primary source. Our results suggest thatlocal biogenic isoprene is important in suburban photochemical processes.Removing isoprene could drastically slow down the efficiency of ROx recyclingand reduce the concentrations of ROx. In addition, the absence of isoprenechemistry could further lead to a decrease in the daily average concentrationsof O3 and HCHO by 34 % and 36 %, respectively. Therefore, thisstudy emphasizes the importance of isoprene chemistry in the suburbanatmosphere, particularly with the participation of anthropogenic NOx.Moreover, our results provide insights into the radical chemistry thatessentially drives the formation of secondary pollutants (e.g., O3 andHCHO) in suburban areas of the YRD region.« less
4. PTR-TOF-MS eddy covariance measurements of isoprene and monoterpene fluxes from an eastern Amazonian rainforest

   https://doi.org/10.5194/acp-20-7179-2020  

   Sarkar, Chinmoy ; Guenther, Alex B. ; Park, Jeong-Hoo ; Seco, Roger ; Alves, Eliane ; Batalha, Sarah ; Santana, Raoni ; Kim, Saewung ; Smith, James ; Tóta, Julio ; et al ( January 2020 , Atmospheric Chemistry and Physics)

   Abstract. Biogenic volatile organic compounds (BVOCs) are important components of the atmosphere due to their contribution to atmospheric chemistry and biogeochemical cycles. Tropical forests are the largest source of the dominant BVOC emissions (e.g. isoprene and monoterpenes). In this study, we report isoprene and total monoterpene flux measurements with a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) using the eddy covariance (EC) method at the Tapajós National Forest (2.857∘ S, 54.959∘ W), a primary rainforest in eastern Amazonia. Measurements were carried out from 1 to 16 June 2014, during the wet-to-dry transition season. During the measurement period, the measured daytime (06:00–18:00 LT) average isoprene mixingmore »ratios and fluxes were 1.15±0.60 ppb and 0.55±0.71 mg C m−2 h−1, respectively, whereas the measured daytime average total monoterpene mixing ratios and fluxes were 0.14±0.10 ppb and 0.20±0.25 mg C m−2 h−1, respectively. Midday (10:00–14:00 LT) average isoprene and total monoterpene mixing ratios were 1.70±0.49 and 0.24±0.05 ppb, respectively, whereas midday average isoprene and monoterpene fluxes were 1.24±0.68 and 0.46±0.22 mg C m−2 h−1, respectively. Isoprene and total monoterpene emissions in Tapajós were correlated with ambient temperature and solar radiation. Significant correlation with sensible heat flux, SHF (r2=0.77), was also observed. Measured isoprene and monoterpene fluxes were strongly correlated with each other (r2=0.93). The MEGAN2.1 (Model of Emissions of Gases and Aerosols from Nature version 2.1) model could simulate most of the observed diurnal variations (r2=0.7 to 0.8) but declined a little later in the evening for both isoprene and total monoterpene fluxes. The results also demonstrate the importance of site-specific vegetation emission factors (EFs) for accurately simulating BVOC fluxes in regional and global BVOC emission models.« less
5. Measurement report: Leaf-scale gas exchange of atmospheric reactive trace species (NO₂, NO, O₃) at a northern hardwood forest in Michigan

   https://doi.org/10.5194/acp-20-11287-2020  

   Wang, Wei ; Ganzeveld, Laurens ; Rossabi, Samuel ; Hueber, Jacques ; Helmig, Detlev ( January 2020 , Atmospheric Chemistry and Physics)

   Abstract. During the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) campaign from 21 July to 3 August 2016,field experiments on leaf-level trace gas exchange of nitric oxide (NO), nitrogen dioxide (NO2), and ozone (O3) were conducted for thefirst time on the native American tree species Pinus strobus (eastern white pine), Acer rubrum (redmaple), Populus grandidentata (bigtooth aspen), and Quercus rubra (red oak) in a temperate hardwood forest inMichigan, USA. We measured the leaf-level trace gas exchange rates andinvestigated the existence of an NO2 compensation point, hypothesizedbased on a comparison of a previously observed average diurnal cycle ofNOx (NO2+NO) concentrations withmore »that simulated using amulti-layer canopy exchange model. Known amounts of trace gases wereintroduced into a tree branch enclosure and a paired blank referenceenclosure. The trace gas concentrations before and after the enclosures weremeasured, as well as the enclosed leaf area (single-sided) and gas flow rate to obtain the trace gas fluxes with respect to leaf surface. There was nodetectable NO uptake for all tree types. The foliar NO2 and O3uptake largely followed a diurnal cycle, correlating with that of the leafstomatal conductance. NO2 and O3 fluxes were driven by theirconcentration gradient from ambient to leaf internal space. The NO2 loss rate at the leaf surface, equivalently the foliar NO2 deposition velocity toward the leaf surface, ranged from 0 to 3.6 mm s−1 for bigtooth aspen and from 0 to 0.76 mm s−1 for red oak, both of which are∼90 % of the expected values based on the stomatalconductance of water. The deposition velocities for red maple and white pineranged from 0.3 to 1.6 and from 0.01 to 1.1 mm s−1, respectively, and were lower than predicted from the stomatal conductance, implying amesophyll resistance to the uptake. Additionally, for white pine, theextrapolated velocity at zero stomatal conductance was 0.4±0.08 mm s−1, indicating a non-stomatal uptake pathway. The NO2compensation point was ≤60 ppt for all four tree species andindistinguishable from zero at the 95 % confidence level. This agrees withrecent reports for several European and California tree species butcontradicts some earlier experimental results where the compensation pointswere found to be on the order of 1 ppb or higher. Given that the sampledtree types represent 80 %–90 % of the total leaf area at this site, theseresults negate the previously hypothesized important role of a leaf-scaleNO2 compensation point. Consequently, to reconcile these findings,further detailed comparisons between the observed and simulated in- and above-canopy NOx concentrations and the leaf- and canopy-scaleNOx fluxes, using the multi-layer canopy exchange model withconsideration of the leaf-scale NOx deposition velocities as well asstomatal conductances reported here, are recommended.« less