The effect of dry and wet deposition of condensable vapors on secondary organic aerosols concentrations over the continental US

Abstract. The effect of dry and wet deposition of semi-volatile organic compounds (SVOCs) in the gas phase on the concentrations of secondary organic aerosol (SOA) is reassessed using recently derived water solubility information. The water solubility of SVOCs was implemented as a function of their volatility distribution within the WRF-Chem regional chemistry transport model, and simulations were carried out over the continental United States for the year 2010. Results show that including dry and wet removal of gas-phase SVOCs reduces annual average surface concentrations of anthropogenic and biogenic SOA by 48 and 63% respectively over the continental US. Dry deposition of gas-phase SVOCs is found to be more effective than wet deposition in reducing SOA concentrations (−40 vs. −8% for anthropogenics, and −52 vs. −11% for biogenics). Reductions for biogenic SOA are found to be higher due to the higher water solubility of biogenic SVOCs. The majority of the total mass of SVOC + SOA is actually deposited via the gas phase (61% for anthropogenics and 76% for biogenics). Results are sensitive to assumptions made in the dry deposition scheme, but gas-phase deposition of SVOCs remains crucial even under conservative estimates. Considering reactivity of gas-phase SVOCs in the dry deposition scheme was found to be negligible. Further sensitivity studies where we reduce the volatility of organic matter show that consideration of gas-phase SVOC removal still reduces average SOA concentrations by 31% on average. We consider this a lower bound for the effect of gas-phase SVOC removal on SOA concentrations. A saturation effect is observed for Henry's law constants above 108 M atm−1, suggesting an upper bound of reductions in surface level SOA concentrations by 60% through removal of gas-phase SVOCs. Other models that do not consider dry and wet removal of gas-phase SVOCs would hence overestimate SOA concentrations by roughly 50%. Assumptions about the water solubility of SVOCs made in some current modeling systems (H* = H* (CH3COOH); H* = 105 M atm−1; H* = H* (HNO3)) still lead to an overestimation of 35%/25%/10% compared to our best estimate.

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The effect of dry and wet deposition of condensable vapors on secondary organic aerosols concentrations over the continental US

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-14-13731-2014 ◽

2014 ◽

Vol 14 (9) ◽

pp. 13731-13767 ◽

Cited By ~ 8

Author(s):

C. Knote ◽

A. Hodzic ◽

J. L. Jimenez

Keyword(s):

Gas Phase ◽

Dry Deposition ◽

Wet Deposition ◽

Water Solubility ◽

Transport Model ◽

Chemistry Transport Model ◽

Dry And Wet Deposition ◽

Modeling Systems ◽

Sensitivity Studies ◽

Wet Removal

Abstract. The effect of dry and wet deposition of semi-volatile organic compounds (SVOC) in the gas-phase on the concentrations of secondary organic aerosol (SOA) is reassessed using recently derived water solubility information. The water solubility of SVOCs was implemented as a function of their volatility distribution within the regional chemistry transport model WRF-Chem, and simulations were carried out over the continental United States for the year 2010. Results show that including dry and wet removal of gas-phase SVOCs reduces annual average surface concentrations of anthropogenic and biogenic SOA by 48% and 63% respectively over the continental US Dry deposition of gas-phase SVOCs is found to be more effective than wet deposition in reducing SOA concentrations (−40% vs. −8% for anthropogenics, −52% vs. −11% for biogenics). Reductions for biogenic SOA are found to be higher due to the higher water solubility of biogenic SVOCs. The majority of the total mass of SVOC + SOA is actually deposited via the gas-phase (61% for anthropogenics, 76% for biogenics). A number of sensitivity studies shows that this is a robust feature of the modeling system. Other models that do not consider dry and wet removal of gas-phase SVOCs would hence overestimate SOA concentrations by roughly 50%. Assumptions about the water solubility of SVOCs made in some current modeling systems (H* = 105 M atm−1; H* = H* (HNO3)) still lead to an overestimation of 25% / 10% compared to our best estimate. A saturation effect is observed for Henry's law constants above 108 M atm−1, suggesting an upper bound of reductions in surface level SOA concentrations by 60% through removal of gas-phase SVOCs. Considering reactivity of gas-phase SVOCs in the dry deposition scheme was found to be negligible. Further sensitivity studies where we reduce the volatility of organic matter show that consideration of gas-phase SVOC removal still reduces average SOA concentrations by 31% on average. We consider this a lower bound for the effect of gas-phase SVOC removal on SOA concentrations.

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Plant assemblages in atmospheric deposition

10.5194/acp-2019-487 ◽

2019 ◽

Author(s):

Ke Dong ◽

Cheolwoon Woo ◽

Naomichi Yamamoto

Keyword(s):

Atmospheric Deposition ◽

Dry Deposition ◽

Wet Deposition ◽

High Throughput Sequencing ◽

Sampling Site ◽

Its2 Region ◽

Dry And Wet Deposition ◽

Custom Made ◽

Plant Assemblages ◽

Cloud Scavenging

Abstract. Plants disperse spores, pollen, and fragments into the atmosphere. The emitted plant particles return to the pedosphere by sedimentation (dry deposition) and/or by precipitation (wet deposition) and constitute part of the global cycle of substances. However, little is known regarding the taxonomic diversities and flux densities of plant particles deposited from the atmosphere. Here, plant assemblages were examined in atmospheric deposits collected in Seoul in South Korea. A custom-made automatic sampler was used to collect dry and wet deposition samples for which plant assemblages and quantities were determined using high-throughput sequencing and quantitative PCR with universal plant-specific primers targeting the internal transcribed spacer 2 (ITS2) region. Dry deposition was dominant for atmospheric deposition of plant particles (87 %). The remaining 13 % was deposited by precipitation, i.e., wet deposition, via rainout (in-cloud scavenging) and/or washout (below-cloud scavenging). Plant assemblage structures did not differ significantly between dry and wet deposition, indicating that washout, which is likely taxon-independent, predominated rainout, which is likely taxon-dependent, for wet deposition of atmospheric plant particles. A small number of plant genera were detected only in wet deposition, indicating that they might be specifically involved in precipitation through acting as nucleation sites in the atmosphere. Future interannual monitoring will control for the seasonality of atmospheric plant assemblages observed at our sampling site. Future global monitoring is also proposed to investigate geographical differences and investigate whether endemic species are involved in plant-mediated bioprecipitation in regional ecological systems.

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Subtropical subsidence and surface deposition of oxidized mercury produced in the free troposphere

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-8999-2017 ◽

2017 ◽

Vol 17 (14) ◽

pp. 8999-9017 ◽

Cited By ~ 9

Author(s):

Viral Shah ◽

Lyatt Jaeglé

Keyword(s):

Boundary Layer ◽

Dry Deposition ◽

Wet Deposition ◽

Transport Model ◽

Lower Troposphere ◽

Deposition Flux ◽

Surface Deposition ◽

Middle Troposphere ◽

Western Us ◽

Wet Deposition Flux

Abstract. Oxidized mercury (Hg(II)) is chemically produced in the atmosphere by oxidation of elemental mercury and is directly emitted by anthropogenic activities. We use the GEOS-Chem global chemical transport model with gaseous oxidation driven by Br atoms to quantify how surface deposition of Hg(II) is influenced by Hg(II) production at different atmospheric heights. We tag Hg(II) chemically produced in the lower (surface–750 hPa), middle (750–400 hPa), and upper troposphere (400 hPa–tropopause), in the stratosphere, as well as directly emitted Hg(II). We evaluate our 2-year simulation (2013–2014) against observations of Hg(II) wet deposition as well as surface and free-tropospheric observations of Hg(II), finding reasonable agreement. We find that Hg(II) produced in the upper and middle troposphere constitutes 91 % of the tropospheric mass of Hg(II) and 91 % of the annual Hg(II) wet deposition flux. This large global influence from the upper and middle troposphere is the result of strong chemical production coupled with a long lifetime of Hg(II) in these regions. Annually, 77–84 % of surface-level Hg(II) over the western US, South America, South Africa, and Australia is produced in the upper and middle troposphere, whereas 26–66 % of surface Hg(II) over the eastern US, Europe, and East Asia, and South Asia is directly emitted. The influence of directly emitted Hg(II) near emission sources is likely higher but cannot be quantified by our coarse-resolution global model (2° latitude  ×  2.5° longitude). Over the oceans, 72 % of surface Hg(II) is produced in the lower troposphere because of higher Br concentrations in the marine boundary layer. The global contribution of the upper and middle troposphere to the Hg(II) dry deposition flux is 52 %. It is lower compared to the contribution to wet deposition because dry deposition of Hg(II) produced aloft requires its entrainment into the boundary layer, while rain can scavenge Hg(II) from higher altitudes more readily. We find that 55 % of the spatial variation of Hg wet deposition flux observed at the Mercury Deposition Network sites is explained by the combined variation of precipitation and Hg(II) produced in the upper and middle troposphere. Our simulation points to a large role of the dry subtropical subsidence regions. Hg(II) present in these regions accounts for 74 % of Hg(II) at 500 hPa over the continental US and more than 60 % of the surface Hg(II) over high-altitude areas of the western US. Globally, it accounts for 78 % of the tropospheric Hg(II) mass and 61 % of the total Hg(II) deposition. During the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign, the contribution of Hg(II) from the dry subtropical regions was found to be 75 % when measured Hg(II) exceeded 250 pg m−3. Hg(II) produced in the upper and middle troposphere subsides in the anticyclones, where the dry conditions inhibit the loss of Hg(II). Our results highlight the importance the subtropical anticyclones as the primary conduits for the production and export of Hg(II) to the global atmosphere.

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Modeling of daytime HONO vertical gradients during SHARP 2009

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-27775-2012 ◽

2012 ◽

Vol 12 (10) ◽

pp. 27775-27819

Author(s):

K. W. Wong ◽

C. Tsai ◽

B. Lefer ◽

N. Grossberg ◽

J. Stutz

Keyword(s):

Boundary Layer ◽

Gas Phase ◽

Rural Areas ◽

Transport Model ◽

Vertical Transport ◽

Formation Mechanisms ◽

Aerosol Model ◽

Formation Pathway ◽

Sensitivity Studies ◽

Urban And Rural Areas

Abstract. Nitrous Acid (HONO) acts as a major precursor of the hydroxyl radical (OH) in the urban atmospheric boundary layer in the morning and throughout the day. Despite its importance, HONO formation mechanisms are not yet completely understood. It is generally accepted that conversion of NO2 on surfaces in the presence of water is responsible for the formation of HONO in the nocturnal boundary layer, although the type of surface on which the mechanism occurs is still under debate. Recent observations of higher than expected daytime HONO concentrations in both urban and rural areas indicate the presence of unknown daytime HONO source(s). Various formation pathways in the gas-phase and on aerosol and ground surfaces have been proposed to explain the presence of daytime HONO. However, it is unclear which mechanism dominates and, in the cases of heterogeneous mechanisms, on which surfaces they occur. Vertical concentration profiles of HONO and its precursors can help in identifying the dominant HONO formation pathways. In this study, daytime HONO and NO2 vertical profiles, measured in three different height intervals (20–70 m, 70–130 m and 130–300 m) in Houston, TX during the 2009 Study of Houston Atmospheric Radical Precursors (SHARP) are analyzed using a one-dimensional (1-D) chemistry and transport model. Model results with various HONO formation pathways suggested in the literature are compared to the the daytime HONO and HONO/NO2 ratios observed during SHARP. The best agreement of HONO and HONO/NO2 ratios between model and observations is achieved by including both a photolytic source of HONO at the ground and on the aerosol. Model sensitivity studies show that the observed diurnal variations of HONO/NO2 ratio are not reproduced by the model if there is only a photolytic HONO source on aerosol or in the gas-phase from NO2* + H2O. Further analysis of the formation and loss pathways of HONO shows a vertical dependence of HONO chemistry during the day. Photolytic HONO formation at the ground is the major formation pathway in the lowest 20 m, while a combination of gas-phase, photolytic formation on aerosol, and vertical transport is responsible for daytime HONO between 200–300 m a.g.l. HONO removal is dominated by vertical transport below 20 m and photolysis between 200–300 m a.g.l.

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Dry and wet deposition of inorganic nitrogen compounds to a tropical pasture site (Rondônia, Brazil)

Atmospheric Chemistry and Physics ◽

10.5194/acp-6-447-2006 ◽

2006 ◽

Vol 6 (2) ◽

pp. 447-469 ◽

Cited By ~ 85

Author(s):

I. Trebs ◽

L. L. Lara ◽

L. M. M. Zeri ◽

L. V. Gatti ◽

P. Artaxo ◽

...

Keyword(s):

Biomass Burning ◽

Dry Deposition ◽

Wet Deposition ◽

Particle Deposition ◽

Inorganic Nitrogen ◽

N Deposition ◽

Deposition Fluxes ◽

Dry And Wet Deposition ◽

Surface Atmosphere ◽

Nh3 Emission

Abstract. The input of nitrogen (N) to ecosystems has increased dramatically over the past decades. While total (wet + dry) N deposition has been extensively determined in temperate regions, only very few data sets of N wet deposition exist for tropical ecosystems, and moreover, reliable experimental information about N dry deposition in tropical environments is lacking. In this study we estimate dry and wet deposition of inorganic N for a remote pasture site in the Amazon Basin based on in-situ measurements. The measurements covered the late dry (biomass burning) season, a transition period and the onset of the wet season (clean conditions) (12 September to 14 November 2002) and were a part of the LBA-SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia – Smoke, Aerosols, Clouds, Rainfall, and Climate) 2002 campaign. Ammonia (NH3), nitric acid (HNO3), nitrous acid (HONO), nitrogen dioxide (NO2), nitric oxide (NO), ozone (O3), aerosol ammonium (NH4+) and aerosol nitrate (NO3-) were measured in real-time, accompanied by simultaneous meteorological measurements. Dry deposition fluxes of NO2 and HNO3 are inferred using the ''big leaf multiple resistance approach'' and particle deposition fluxes are derived using an established empirical parameterization. Bi-directional surface-atmosphere exchange fluxes of NH3 and HONO are estimated by applying a ''canopy compensation point model''. N dry and wet deposition is dominated by NH3 and NH4+, which is largely the consequence of biomass burning during the dry season. The grass surface appeared to have a strong potential for daytime NH3 emission, owing to high canopy compensation points, which are related to high surface temperatures and to direct NH3 emissions from cattle excreta. NO2 also significantly accounted for N dry deposition, whereas HNO3, HONO and N-containing aerosol species were only minor contributors. Ignoring NH3 emission from the vegetation surface, the annual net N deposition rate is estimated to be about −11 kgN ha-1 yr-1. If on the other hand, surface-atmosphere exchange of NH3 is considered to be bi-directional, the annual net N budget at the pasture site is estimated to range from −2.15 to −4.25 kgN ha-1 yr-1.

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Dispersion and dry and wet deposition of PAHs in an atmospheric environment

Water Science & Technology ◽

10.2166/wst.2006.055 ◽

2006 ◽

Vol 53 (2) ◽

pp. 215-224 ◽

Cited By ~ 10

Author(s):

N. Ozaki ◽

K. Nitta ◽

T. Fukushima

Keyword(s):

Dry Deposition ◽

Wet Deposition ◽

Fine Particles ◽

Atmospheric Environment ◽

Coarse Particles ◽

Atmospheric Concentration ◽

Dry And Wet Deposition ◽

Total Pahs ◽

Polycyclic Aromatic ◽

Rainy Days

The atmospheric concentration and dry and wet deposition were measured for particulate matter (PM) and polycyclic aromatic hydrocarbons (PAHs) from August to December in Higashi-Hiroshima City, Japan. PM concentration of fine particles (0.6–7 μm) was 5.7–75.1 μg m−3, and coarse particles (>7 μm) was 2.2–22.3 μg m−3. Total PAHs concentration of fine particles was 0.14–16.3 ng m−3, and coarse particles was 0.01–0.77 ng m−3. Their concentration increased on non-rainy days and decreased rapidly on rainy days. For seasonal fluctuations of PAHs, their concentrations decreased from summer to winter, and the rate of decrease was more distinct for fine particles. For total (dry+wet) depositions, the PM flux was 1.9–11.2 mg m−2 d−1, and the total PAHs flux was 1.9–97.2 ng m−3 d−1. From these measurements, the yearly total loading of PAHs was estimated for the particle phase. Total loading was 28 μg m−2 y−1 for the dry deposition and 52 mg m−2 y−1 for the wet deposition. The loading of the wet deposition was comparable to those of the dry deposition for all ring numbers.

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Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-5107-2017 ◽

2017 ◽

Vol 17 (8) ◽

pp. 5107-5118 ◽

Cited By ~ 33

Author(s):

Rachel F. Silvern ◽

Daniel J. Jacob ◽

Patrick S. Kim ◽

Eloise A. Marais ◽

Jay R. Turner ◽

...

Keyword(s):

Gas Phase ◽

Ammonium Sulfate ◽

Wet Deposition ◽

Transport Model ◽

Organic Aerosol ◽

Sulfate Aerosol ◽

Chemical Transport Model ◽

Thermodynamic Models ◽

So2 Emissions ◽

Mass Transfer Limitation

Abstract. Thermodynamic models predict that sulfate aerosol (S(VI) ≡ H2SO4(aq) + HSO4−+ SO42−) should take up available ammonia (NH3) quantitatively as ammonium (NH4+) until the ammonium sulfate stoichiometry (NH4)2SO4 is close to being reached. This uptake of ammonia has important implications for aerosol mass, hygroscopicity, and acidity. When ammonia is in excess, the ammonium–sulfate aerosol ratio R =  [NH4+] ∕ [S(VI)] should approach 2, with excess ammonia remaining in the gas phase. When ammonia is in deficit, it should be fully taken up by the aerosol as ammonium and no significant ammonia should remain in the gas phase. Here we report that sulfate aerosol in the eastern US in summer has a low ammonium–sulfate ratio despite excess ammonia, and we show that this is at odds with thermodynamic models. The ammonium–sulfate ratio averages only 1.04 ± 0.21 mol mol−1 in the Southeast, even though ammonia is in large excess, as shown by the ammonium–sulfate ratio in wet deposition and by the presence of gas-phase ammonia. It further appears that the ammonium–sulfate aerosol ratio is insensitive to the supply of ammonia, remaining low even as the wet deposition ratio exceeds 6 mol mol−1. While the ammonium–sulfate ratio in wet deposition has increased by 5.8 % yr−1 from 2003 to 2013 in the Southeast, consistent with SO2 emission controls, the ammonium–sulfate aerosol ratio decreased by 1.4–3.0 % yr−1. Thus, the aerosol is becoming more acidic even as SO2 emissions decrease and ammonia emissions stay constant; this is incompatible with simple sulfate–ammonium thermodynamics. A tentative explanation is that sulfate particles are increasingly coated by organic material, retarding the uptake of ammonia. Indeed, the ratio of organic aerosol (OA) to sulfate in the Southeast increased from 1.1 to 2.4 g g−1 over the 2003–2013 period as sulfate decreased. We implement a simple kinetic mass transfer limitation for ammonia uptake to sulfate aerosols in the GEOS-Chem chemical transport model and find that we can reproduce both the observed ammonium–sulfate aerosol ratios and the concurrent presence of gas-phase ammonia. If sulfate aerosol becomes more acidic as OA ∕ sulfate ratios increase, then controlling SO2 emissions to decrease sulfate aerosol will not have the co-benefit of suppressing acid-catalyzed secondary organic aerosol (SOA) formation.

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Modeling of daytime HONO vertical gradients during SHARP 2009

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-3587-2013 ◽

2013 ◽

Vol 13 (7) ◽

pp. 3587-3601 ◽

Cited By ~ 43

Author(s):

K. W. Wong ◽

C. Tsai ◽

B. Lefer ◽

N. Grossberg ◽

J. Stutz

Keyword(s):

Boundary Layer ◽

Gas Phase ◽

Rural Areas ◽

Transport Model ◽

Vertical Transport ◽

Formation Mechanisms ◽

Aerosol Model ◽

Formation Pathway ◽

Sensitivity Studies ◽

Urban And Rural Areas

Abstract. Nitrous acid (HONO) acts as a major precursor of the hydroxyl radical (OH) in the urban atmospheric boundary layer in the morning and throughout the day. Despite its importance, HONO formation mechanisms are not yet completely understood. It is generally accepted that conversion of NO2 on surfaces in the presence of water is responsible for the formation of HONO in the nocturnal boundary layer, although the type of surface on which the mechanism occurs is still under debate. Recent observations of higher than expected daytime HONO concentrations in both urban and rural areas indicate the presence of unknown daytime HONO source(s). Various formation pathways in the gas phase, and on aerosol and ground surfaces have been proposed to explain the presence of daytime HONO. However, it is unclear which mechanism dominates and, in the cases of heterogeneous mechanisms, on which surfaces they occur. Vertical concentration profiles of HONO and its precursors can help in identifying the dominant HONO formation pathways. In this study, daytime HONO and NO2 vertical profiles, measured in three different height intervals (20–70, 70–130, and 130–300 m) in Houston, TX, during the 2009 Study of Houston Atmospheric Radical Precursors (SHARP) are analyzed using a one-dimensional (1-D) chemistry and transport model. Model results with various HONO formation pathways suggested in the literature are compared to the the daytime HONO and HONO/NO2 ratios observed during SHARP. The best agreement of HONO and HONO/NO2 ratios between model and observations is achieved by including both a photolytic source of HONO at the ground and on the aerosol. Model sensitivity studies show that the observed diurnal variations of the HONO/NO2 ratio are not reproduced by the model if there is only a photolytic HONO source on aerosol or in the gas phase from NO2* + H2O. Further analysis of the formation and loss pathways of HONO shows a vertical dependence of HONO chemistry during the day. Photolytic HONO formation at the ground is the major formation pathway in the lowest 20 m, while a combination of gas-phase, photolytic formation on aerosol, and vertical transport is responsible for daytime HONO between 200–300 m a.g.l. HONO removal is dominated by vertical transport below 20 m and photolysis between 200–300 m a.g.l.

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Modeling organic aerosol composition at the puy de Dôme mountain (France) for two contrasted air masses with the WRF-Chem model

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-13395-2015 ◽

2015 ◽

Vol 15 (9) ◽

pp. 13395-13455 ◽

Cited By ~ 1

Author(s):

C. Barbet ◽

L. Deguillaume ◽

N. Chaumerliac ◽

M. Leriche ◽

A. Berger ◽

...

Keyword(s):

Gas Phase ◽

Dry Deposition ◽

Oxidation Rate ◽

Mass Concentration ◽

Transport Model ◽

Organic Aerosol ◽

Basis Set ◽

Formation Processes ◽

Aerosol Composition ◽

The Mean

Abstract. Simulations with the chemistry-transport model WRF-Chem are compared to aerosol measurements performed at the puy de Dôme station with a compact Time-of-Flight Aerosol Mass Spectrometer (cToF-AMS) for two episodes in autumn 2008 and in summer 2010. The WRF-Chem model is used with the Volatility Basis Set (VBS) approach dedicated to the formation of secondary organic aerosol (SOA). The model systematically underestimates the observed concentrations of organic aerosol (OA), with significant differences observed for the summer case. For this event, where high OA concentrations were observed (12.5 μg m-3 in average), simulated OA mass concentration is 2.0 μg m-3. For the autumn event, observed OA concentrations reached 2.6 μg m-3. The simulated concentrations reached only 0.6 μg m-3. During the summer event, several gas-phase volatile organic compounds (VOCs) were measured and were used to test the robustness of both emission/dry deposition and SOA formation processes. The concentrations of VOCs, and more specifically the anthropogenic ones, calculated by the model are estimated to be much lower than the observed ones. Hence, the emissions of all SOA precursors are multiplied by 2 in the model: this induces an increase of the mean OA mass concentration of 25% (+0.5 μg m-3) and 18% (+0.4 μg m-3), respectively for anthropogenic and biogenic VOCs. The dry deposition of gas-phase organic condensable vapours (OCVs) is also critical to predict the SOA mass concentrations: dividing the deposition factor by 2 leads to an increase of OA mass by an additional 12% (+0.2 μg m-3). However, these increases were not sufficient to explain the observed OA concentration, and the underestimation of the OA concentration levels seems to be principally related to a lack in the parameterization of SOA formation. Changing the oxidation rate of OCVs from 1.0 × 10-11 to 4.0 × 10-11 cm3 molecule−1 s-1, doubling the SOA yields for anthropogenic VOCs and finally doubling the SOA yields for biogenic VOCs results in an increase of the mean OA mass concentration by 56% (+1.1 μg m-3), 61% (+1.2 μg m-3) and 40% (+0.8 μg m-3), respectively. The consideration of both emission/dry deposition and SOA formation processes tests lead to a mean OA mass concentration of 10.7 μg m-3 for 2010, a value that is close to the observations. For 2008, modifying solely the oxidation rate of OCVs and the SOA yields is sufficient to reproduce the observed level of mean OA mass (a mass of 2.4 μg m-3 is obtained by the model whereas a mass of 2.6 μg m-3 was observed).

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GEM-MACH-PAH (rev2488): a new high-resolution chemical transport model for North American PAHs and benzene

10.5194/gmd-2017-324 ◽

2018 ◽

Author(s):

Cynthia H. Whaley ◽

Elisabeth Galarneau ◽

Paul A. Makar ◽

Ayodeji Akingunola ◽

Wanmin Gong ◽

...

Keyword(s):

High Resolution ◽

Spatial Variability ◽

Gas Phase ◽

North American ◽

Wet Deposition ◽

Transport Model ◽

Emission Factors ◽

Monitoring Networks ◽

Study Region ◽

Mobile Emissions

Abstract. Environment and Climate Change Canada’s online air quality forecasting model, GEMMACH, was extended to simulate atmospheric concentrations of benzene and seven polycyclic aromatic hydrocarbons (PAHs): phenanthrene, anthracene, fluoranthene, pyrene, benz(a)anthracene, chrysene, and benzo(a)pyrene (BaP). In the expanded model, benzene and PAHs are emitted from major point, area, and mobile sources, with emissions based on recent emission factors. Modelled PAHs undergo gas-particle partitioning (whereas benzene is only in the gas phase), atmospheric transport, oxidation, cloud processing, and dry and wet deposition. To represent PAH gas-particle partitioning, the Dachs-Eisenreich scheme was used, and we have improved gas-particle partitioning parameters based on an empirical analysis to get significantly better gas-particle partitioning results than the previous North American PAH model, AURAMS-PAH. Other added process parameterizations include the particle phase benzo(a)pyrene reaction with ozone via the Kwamena scheme and gas-phase scavenging of PAHs by snow via vapor sorption to the snow surface. The resulting GEM-MACH-PAH model was used to generate the first online model simulations of PAH emissions, transport, chemical transformation and deposition for a high resolution domain (2.5-km grid cell spacing) in North America, centered on the PAH-data-rich region of southern Ontario, Canada and the north-eastern United States. Model output for two seasons was compared to measurements from three monitoring networks spanning Canada and the U.S. Average summertime model results were found to be statistically indistinguishable from measurements of benzene and all seven PAHs. The same was true for the winter seasonal mean, except for BaP, which had a statistically significant positive bias.We present evidence that the benzo(a)pyrene results may be ameliorated via further improvements to PM and oxidant processes and transport. Our analysis focused on four key components to the prediction of atmospheric PAH levels: spatial variability; sensitivity to mobile emissions; gas-particle partitioning; and wet deposition. Spatial variability of PAHs/PM2.5 at 2.5-km resolution was found to be comparable to measurements. Predicted ambient surface concentrations of benzene and the PAHs were found to be critically dependent on mobile emission factors, indicating the mobile emissions sector has a significant influence on ambient PAH levels in the study region. PAH wet deposition was overestimated due to additive precipitation biases in the model and the measurements. Our overall performance evaluation suggests that GEM-MACHPAH can provide seasonal estimates for benzene and PAHs and be suitable for emissions scenario simulations.

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