During the summer of 2004, extensive wildfires burned in Alaska and western Canada; the fires were the largest on record for Alaska. Smoke from these fires was observed over the continental United ...States in satellite images, and a variety of chemical tracers associated with the fires were sampled by aircrafts deployed during the International Consortium for Atmospheric Research on Transport and Transformation field experiment. Several recent studies have quantified the impacts of the long‐range transport of pollution associated with these fires on tropospheric CO and O3 levels over the eastern United States. This study quantifies the episodic impact of this pollution transport event on surface‐level fine particulate matter (PM2.5) concentrations over the eastern United States during mid‐July 2004, through the complementary use of remotely sensed, aloft, and surface measurements, in conjunction with a comprehensive regional atmospheric chemistry‐transport model. A methodology is developed to assimilate MODIS aerosol optical depths in the model to represent the impacts of the fires. The resultant model predictions of CO and PM2.5 distributions are compared extensively with corresponding surface and aloft measurements. On the basis of the model calculations, a 0.12Tg enhancement in tropospheric PM2.5 mass loading over the eastern United States is estimated on 19 July 2004 due to the fires. This amount is significantly larger (approximately a factor of 8) than the total daily anthropogenic fine particulate matter emissions for the continental United States. Analysis of measured and modeled PM2.5 surface‐level concentrations suggests that the transport of particulate matter pollution associated with the fires resulted in a 24–42 % enhancement in median surface‐level PM2.5 concentrations across the eastern United States during 19–23 July 2004.
Aerosol direct effects (ADEs), i.e., scattering and absorption of incoming solar radiation, reduce radiation reaching the ground and the resultant photolysis attenuation can decrease ozone (O
) ...formation in polluted areas. One the other hand, evidence also suggests that ADE-associated cooling suppresses atmospheric ventilation, thereby enhancing surface-level O
. Assessment of ADE impacts is thus important for understanding emission reduction strategies that seek co-benefits associated with reductions in both particuate matter and O
levels. This study quantifies the impacts of ADEs on tropospheric ozone by using a two-way online coupled meteorology and atmospheric chemistry model, WRF- CMAQ, using a process analysis methodology. Two mani-festations of ADE impacts on O3 including changes in atmospheric dynamics (ᐃDynamics) and changes in photolysis rates (∆Photolysis) were assessed separately through multiple scenario simulations for January and July of 2013 over China. Results suggest that ADEs reduced surface daily maxima 1 h O
(DM1O
) in China by up to 39μgm
through the combination of ∆Dynamics and ∆Photolysis in January but enhanced surface DM1O
by up to 4μgm
in July. Increased O
in July is largely attributed to ∆Dynamics, which causes a weaker O
sink of dry deposition and a stronger O
source of photochemistry due to the stabilization of the at-mosphere. Meanwhile, surface OH is also enhanced at noon in July, though its daytime average values are reduced in January. An increased OH chain length and a shift towards more volatile organic compound (VOC)-limited conditions are found due to ADEs in both January and July. This study suggests that reducing ADEs may have the potential risk of increasing O
in winter, but it will benefit the reduction in maxima O
in summer.
Stratospheric intrusion and trans-Pacific transport have been recognized as a potential source of tropospheric ozone over the US. The state-of-the-science Community Multiscale Air Quality (CMAQ) ...modeling system has recently been extended for hemispheric-scale modeling applications (referred to as H-CMAQ). In this study, H-CMAQ is applied to study the stratospheric intrusion and trans-Pacific transport during April 2010. The results will be presented in two companion papers. In this Part 1 paper, model evaluation for tropospheric ozone (O
) is presented. Observations at the surface, by ozonesondes and airplane, and by satellite across the Northern Hemisphere are used to evaluate the model performance for O
. H-CMAQ is able to capture surface and boundary layer (defined as surface to 750hPa) O
with a normalized mean bias (NMB) of -10%; however, a systematic underestimation with an NMB up to -30% is found in the free troposphere (defined as 750-250hPa). In addition, a new air mass characterization method is developed to distinguish influences of stratosphere-troposphere transport (STT) from the effects of photochemistry on O
levels. This method is developed based on the ratio of O
and an inert tracer indicating stratospheric O
to examine the importance of photochemistry, and sequential intrusion from upper layer. During April 2010, on a monthly average basis, the relationship between surface O
mixing ratios and estimated stratospheric air masses in the troposphere show a slight negative slope, indicating that high surface O
values are primarily affected by other factors (i.e., emissions), whereas this relationship shows a slight positive slope at elevated sites, indicating that STT has a possible impact at elevated sites. STT shows large day-to-day variations, and STT impacts can either originate from the same air mass over the entire US with an eastward movement found during early April, or stem from different air masses at different locations indicated during late April. Based on this newly established air mass characterization technique, this study can contribute to understanding the role of STT and also the implied importance of emissions leading to high surface O
. Further research focused on emissions is discussed in a subsequent paper (Part 2).
Fate of ozone in marine environments has been receiving increased attention due to the tightening of ambient air quality standards. The role of deposition and halogen chemistry is examined through ...incorporation of an enhanced ozone deposition algorithm and inclusion of halogen chemistry in a comprehensive atmospheric modeling system. The enhanced ozone deposition treatment accounts for the interaction of iodide in seawater with ozone and increases deposition velocities by 1 order of magnitude. Halogen chemistry includes detailed chemical reactions of organic and inorganic bromine and iodine species. Two different simulations are completed with the halogen chemistry: without and with photochemical reactions of higher iodine oxides. Enhanced deposition reduces mean summer-time surface ozone by ∼3% over marine regions in the Northern Hemisphere. Halogen chemistry without the photochemical reactions of higher iodine oxides reduces surface ozone by ∼15% whereas simulations with the photochemical reactions of higher iodine oxides indicate ozone reductions of ∼48%. The model without these processes overpredicts ozone compared to observations whereas the inclusion of these processes improves predictions. The inclusion of photochemical reactions for higher iodine oxides leads to ozone predictions that are lower than observations, underscoring the need for further refinement of the halogen emissions and chemistry scheme in the model.
Excess deposition (including both wet and dry deposition) of nitrogen and sulfur are detrimental to ecosystems. Recent studies have investigated the spatial patterns and temporal trends of nitrogen ...and sulfur wet deposition, but few studies have focused on dry deposition due to the scarcity of dry deposition measurements. Here, we use long-term model simulations from the coupled WRF-CMAQ model covering the period from 1990 to 2010 to study changes in spatial distribution as well as temporal trends in total (TDEP), wet (WDEP) and dry deposition (DDEP) of total inorganic nitrogen (TIN) and sulfur (TSO
). We first evaluate the model's performance in simulating WDEP over the U.S. by comparing the model results with observational data from the U.S. National Atmospheric Deposition Program. The coupled model generally underestimates the WDEP of both TIN (including both the oxidized nitrogen deposition-TNO
, and the reduced nitrogen deposition-NH
) and TSO
, with better performance in the eastern U.S. than the western U.S. TDEP of both TIN and TSO
show significant decreases over the U.S., especially in the east due to the large emission reductions that occurred in that region. The decreasing trends of TIN TDEP are caused by decrease of TNO
, and the increasing trends of TIN deposition over the Great Plains and Tropical Wet Forests regions are caused by increases in NH
emissions although it should be noted that these increasing trends are not significant. TIN WDEP shows decreasing trends throughout the U.S., except for the Marine West Coast Forest region. TIN DDEP shows significant decreasing trends in the region of Eastern Temperate Forests, Northern Forests, Mediterranean California and Marine West Coast Forest, and significant increasing trends in the region of Tropical Wet Forests, Great Plains and Southern Semi-arid Highlands. For the other three regions (North American Deserts, Temperate Sierras and Northwestern Forested Mountains), the decreasing or increasing trends were not significant. Both the WDEP and DDEP of TSO
have decreases across the U.S., with a larger decreasing trend in the DDEP than that in the WDEP. Across the U.S. during the 1990-2010 period, DDEP of TIN accounted for 58-65% of TDEP of TIN. TDEP of TIN over the U.S. was dominated by deposition of TNO
during the first decade, which then shifts to reduced nitrogen (NH
) dominance after 2003 resulting from combination of NO
emission reductions and NH
emission increases. The sulfur DDEP is usually higher than the sulfur WDEP until recent years, as the sulfur DDEP has a larger decreasing trend than WDEP.
The Community Multiscale Air Quality (CMAQ) modeling system is extended to simulate ozone, particulate matter, and related precursor distributions throughout the Northern Hemisphere. Modelled ...processes were examined and enhanced to suitably represent the extended space and time scales for such applications. Hemispheric scale simulations with CMAQ and the Weather Research and Forecasting (WRF) model are performed for multiple years. Model capabilities for a range of applications including episodic long-range pollutant transport, long-term trends in air pollution across the Northern Hemisphere, and air pollution-climate interactions are evaluated through detailed comparison with available surface, aloft, and remotely sensed observations. The expansion of CMAQ to simulate the hemispheric scales provides a framework to examine interactions between atmospheric processes occurring at various spatial and temporal scales with physical, chemical, and dynamical consistency.
The Community Multiscale Air Quality (CMAQ) model version 5.3 (CMAQ53), released to the public in August 2019 and followed by version 5.3.1 (CMAQ531) in December 2019, contains numerous science ...updates, enhanced functionality, and improved computation efficiency relative to the previous version of the model, 5.2.1 (CMAQ521). Major science advances in the new model include a new aerosol module (AERO7) with significant updates to secondary organic aerosol (SOA) chemistry, updated chlorine chemistry, updated detailed bromine and iodine chemistry, updated simple halogen chemistry, the addition of dimethyl sulfide (DMS) chemistry in the CB6r3 chemical mechanism, updated M3Dry bidirectional deposition model, and the new Surface Tiled Aerosol and Gaseous Exchange (STAGE) bidirectional deposition model. In addition, support for the Weather Research and Forecasting (WRF) model's hybrid vertical coordinate (HVC) was added to CMAQ53 and the Meteorology-Chemistry Interface Processor (MCIP) version 5.0 (MCIP50). Enhanced functionality in CMAQ53 includes the new Detailed Emissions Scaling, Isolation and Diagnostic (DESID) system for scaling incoming emissions to CMAQ and reading multiple gridded input emission files. Evaluation of CMAQ531 was performed by comparing monthly and seasonal mean daily 8 h average (MDA8) O
and daily PM
values from several CMAQ531 simulations to a similarly configured CMAQ521 simulation encompassing 2016. For MDA8 O
, CMAQ531 has higher O
in the winter versus CMAQ521, due primarily to reduced dry deposition to snow, which strongly reduces wintertime O
bias (2-4 ppbv monthly average). MDA8 O
is lower with CMAQ531 throughout the rest of the year, particularly in spring, due in part to reduced O
from the lateral boundary conditions (BCs), which generally increases MDA8 O
bias in spring and fall ( 0.5 μg m
). For daily 24 h average PM
, CMAQ531 has lower concentrations on average in spring and fall, higher concentrations in summer, and similar concentrations in winter to CMAQ521, which slightly increases bias in spring and fall and reduces bias in summer. Comparisons were also performed to isolate updates to several specific aspects of the modeling system, namely the lateral BCs, meteorology model version, and the deposition model used. Transitioning from a hemispheric CMAQ (HCMAQ) version 5.2.1 simulation to a HCMAQ version 5.3 simulation to provide lateral BCs contributes to higher O
mixing ratios in the regional CMAQ simulation in higher latitudes during winter (due to the decreased O
dry deposition to snow in CMAQ53) and lower O
mixing ratios in middle and lower latitudes year-round (due to reduced O
over the ocean with CMAQ53). Transitioning from WRF version 3.8 to WRF version 4.1.1 with the HVC resulted in consistently higher (1.0-1.5 ppbv) MDA8 O
mixing ratios and higher PM
concentrations (0.1-0.25 μg m
) throughout the year. Finally, comparisons of the M3Dry and STAGE deposition models showed that MDA8 O
is generally higher with M3Dry outside of summer, while PM
is consistently higher with STAGE due to differences in the assumptions of particle deposition velocities to non-vegetated surfaces and land use with short vegetation (e.g., grasslands) between the two models. For ambient NH
, STAGE has slightly higher concentrations and smaller bias in the winter, spring, and fall, while M3Dry has higher concentrations and smaller bias but larger error and lower correlation in the summer.
Dynamic evaluation of the fully coupled Weather Research and Forecasting (WRF)– Community Multi-scale Air Quality (CMAQ) model ozone simulations over the contiguous United States (CONUS) using two ...decades of simulations covering the period from 1990 to 2010 is conducted to assess how well the changes in observed ozone air quality are simulated by the model. The changes induced by variations in meteorology and/or emissions are also evaluated during the same timeframe using spectral decomposition of observed and modeled ozone time series with the aim of identifying the underlying forcing mechanisms that control ozone exceedances and making informed recommendations for the optimal use of regional-scale air quality models. The evaluation is focused on the warm season's (i.e., May–September) daily maximum 8-hr (DM8HR) ozone concentrations, the 4th highest (4th) and average of top 10 DM8HR ozone values (top10), as well as the spectrally-decomposed components of the DM8HR ozone time series using the Kolmogorov-Zurbenko (KZ) filter. Results of the dynamic evaluation are presented for six regions in the U.S., consistent with the National Oceanic and Atmospheric Administration (NOAA) climatic regions. During the earlier 11-yr period (1990–2000), the simulated and observed regional average trends are not statistically significant. During the more recent 2000–2010 period, all observed trends are statistically significant and WRF-CMAQ captures the observed downward trend in the Southwest and Midwest but under-predicts the downward trends in observations for the other regions. Observational analysis reveals that it is the magnitude of the long-term forcing that dictates the maximum ozone exceedance potential; there is a strong linear relationship between the long-term forcing and the 4th highest or the average of the top10 ozone concentrations in both observations and model output. This finding indicates that improving the model's ability to reproduce the long-term component will also enable better simulation of ozone extreme values that are of interest to regulatory agencies.
•Dynamic evaluation of decadal-scale WRF-CMAQ ozone simulations for Contiguous U.S.•WRF-CMAQ is better at predicting ozone changes than absolute concentration levels.•Spectral decomposition shows ozone exceedances depend on magnitude of long-term forcing.•Trends in ozone exceedances are significant during the second decade (2000–2010).•Better simulation of long-term forcing results in better prediction of ozone extremes.
This study analyzes simulated regional-scale ozone burdens both near the surface and aloft, estimates process contributions to these burdens, and calculates the sensitivity of the simulated ...regional-scale ozone burden to several key model inputs with a particular emphasis on boundary conditions derived from hemispheric or global-scale models. The Community Multiscale Air Quality (CMAQ) model simulations supporting this analysis were performed over the continental US for the year 2010 within the context of the Air Quality Model Evaluation International Initiative (AQMEII) and Task Force on Hemispheric Transport of Air Pollution (TF-HTAP) activities. CMAQ process analysis (PA) results highlight the dominant role of horizontal and vertical advection on the ozone burden in the mid-to-upper troposphere and lower stratosphere. Vertical mixing, including mixing by convective clouds, couples fluctuations in free-tropospheric ozone to ozone in lower layers. Hypothetical bounding scenarios were performed to quantify the effects of emissions, boundary conditions, and ozone dry deposition on the simulated ozone burden. Analysis of these simulations confirms that the characterization of ozone outside the regional-scale modeling domain can have a profound impact on simulated regional-scale ozone. This was further investigated by using data from four hemispheric or global modeling systems (Chemistry - Integrated Forecasting Model (C-IFS), CMAQ extended for hemispheric applications (H-CMAQ), the Goddard Earth Observing System model coupled to chemistry (GEOS-Chem), and AM3) to derive alternate boundary conditions for the regional-scale CMAQ simulations. The regional-scale CMAQ simulations using these four different boundary conditions showed that the largest ozone abundance in the upper layers was simulated when using boundary conditions from GEOS-Chem, followed by the simulations using C-IFS, AM3, and H-CMAQ boundary conditions, consistent with the analysis of the ozone fields from the global models along the CMAQ boundaries. Using boundary conditions from AM3 yielded higher springtime ozone columns burdens in the middle and lower troposphere compared to boundary conditions from the other models. For surface ozone, the differences between the AM3-driven CMAQ simulations and the CMAQ simulations driven by other large-scale models are especially pronounced during spring and winter where they can reach more than 10 ppb for seasonal mean ozone mixing ratios and as much as 15 ppb for domain-averaged daily maximum 8 h average ozone on individual days. In contrast, the differences between the C-IFS-, GEOS-Chem-, and H-CMAQ-driven regional-scale CMAQ simulations are typically smaller. Comparing simulated sur face ozone mixing ratios to observations and computing seasonal and regional model performance statistics revealed that boundary conditions can have a substantial impact on model performance. Further analysis showed that boundary conditions can affect model performance across the entire range of the observed distribution, although the impacts tend to be lower during summer and for the very highest observed percentiles. The results are discussed in the context of future model development and analysis opportunities.
PDF
