Chemical transport models such as the Community Multiscale Air Quality (CMAQ) model provide useful guidance on air pollution control strategies. We evaluate the performance of a 12 km resolution CMAQ ...simulation with surface and aircraft observations of CO, O3, and NOx during the summer of 2002. When all data are considered, on average, modeled and observed CO total column contents (surface to 3,000 m) agreed to within 14% in the morning and 22% in the afternoon. Reducing the deposition velocity for CO improves model‐measurement agreement but did not eliminate the model bias. The majority of observed vertical profiles have a maximum near the surface. Although many observed spirals had a secondary maximum at the top of the boundary layer, indicating subgrid‐scale shallow convection. The model was not able to replicate these vertical structures. Water vapor profiles likewise showed greater vertical variability in the observations than in the model. General conclusions from these model‐measurement comparisons: total CO emissions estimates are either adequate or underestimated, but there is no evidence of gross error; NOx emissions from mobile sources may be overestimated while the lifetime of NOx may be underestimated in CMAQ 4.5.1 with CBIV, and vertical mixing in the model boundary layer may be too fast, but venting out of the boundary layer into the lower free troposphere may be too slow.
Key Points
Model‐measurement comparisons show CO emissions are either adequate or too low
NOx emissions from mobile sources may be overestimated
The lifetime of NOx may be underestimated in CMAQ 4.6.1 with CBIV
As part of Maryland Aerosol Research and CHaracterization (MARCH‐Atlantic) study, measurements of 24‐hr average elemental carbon (EC) aerosol concentration were made at Fort Meade, Maryland, USA, a ...suburban site within the Baltimore‐Washington corridor during July 1999, October 1999, January 2000, April 2000 and July 2000. Carbon monoxide (CO) and sulfur dioxide (SO2) were also measured nearly continuously over the period. Tight correlation between EC and CO in every month suggests common or proximate sources, likely traffic emissions. The EC versus CO slope varies in different seasons and generally increases with ambient temperature. The temperature dependence of EC/CO ratios suggests that EC source strength peaks in summer. By using the well established emission inventory for CO, and EC/CO ratio found in this study, EC emission over North America is estimated at 0.31±0.12 Tg yr−1, on the low end but in reasonable agreement with prior inventories based on emission factors and fuel consumption.
Upwind sources of NO
x
and SO
2 play a crucial role in the amount of O
3 and aerosols in the lower troposphere in the Mid-Atlantic US. This paper describes a novel method of clustering trace gas and ...aerosol profiles allowing for the quantification of the relationship between point sources and pollution levels. This improves our understanding of pollution origins and has the potential for prediction of episodes of poor air quality. A hierarchical clustering method was used to classify distinct chemical and meteorological events from over 200 aircraft vertical profiles in the lower troposphere. Profile measurements included O
3, SO
2, CO and particle scattering from June to August 1997–2003, in the Mid-Atlantic US (mostly in Maryland, Pennsylvania and Virginia). The clustering technique could discriminate distinct profile shapes including measurements made during the 2002 Canadian forest fires. Forty-eight-hour back trajectories were run for each profile and the integrated NO
x
and SO
2 point source emissions encountered by each trajectory were calculated using data from the EPA Clean Air Market Division's emissions database. There was a strong correlation between integrated NO
x
emissions and O
3 profiles, indicating that O
3 profiles are strongly influenced by and can be predicted with point source emissions. There is a prevalent concentration of SO
2 over the eastern US with mixing ratios decreasing smoothly from about 3.5
ppb near the surface to 0.2
ppb at 2400
m.
Chemically speciated fine particulate matter (PM
2.5) and trace gases (including NH
3, HNO
3, CO, SO
2, NO
y
) have been sampled at Fort Meade (FME: 39.10°N, 76.74°W; elevation 46
m MSL), Maryland, ...since July 1999. FME is suburban, located in the middle of the Baltimore–Washington corridor, and generally downwind of the highly industrialized Midwest. The PM
2.5 at FME is expected to be of both local and regional sources. Measurements over a 2-year period include eight seasonally representative months. The PM
2.5 shows an annual mean of 13
μg
m
−3 and primarily consists of sulfate, nitrate, ammonium, and carbonaceous material. Day-to-day and seasonal variations in the PM
2.5 chemical composition reflect changes of contribution from various sources. UNMIX, an innovative receptor model, is used to retrieve potential sources of the PM
2.5. A six-factor model, including regional sulfate, local sulfate, wood smoke, copper/iron processing industry, mobile, and secondary nitrate, is constructed and compared with reported source emission profiles. The six factors are studied further using an ensemble back trajectory method to identify possible source locations. Sources of local sulfate, mobile, and secondary nitrate are more localized around the receptor than those of other factors. Regional sulfate and wood smoke are more regional and associated with westerly and southerly transport, respectively. This study suggests that the local contribution to PM
2.5 mass can vary from <30% in summer to >60% in winter.
During the Aerosols99 trans‐Atlantic cruise from Norfolk, VA, to Cape Town, South Africa, daily ozonesondes were launched from the R/V Ronald H Brown between 17 January and 6 February 1999. A ...composite of tropospheric ozone profiles along the latitudinal transect shows 4 zones, nearly identical to the ozone distribution during a January‐February 1993 trans‐Atlantic cruise Weller et al., 1996. Sondes from the cruise and Ascension Island (8S, 14.5W), as well as the Earth‐Probe (EP)/TOMS satellite instrument, show elevated tropospheric ozone (>35 Dobson Units) throughout the south Atlantic in January 1999. Ozone layers associated with biomass burning north of the ITCZ (Intertropical Convergence Zone) are prominent at 0–5 km from 10–0N, but even higher ozone (100 ppbv, 5–15 km) occurred south of the ITCZ, where it was not burning‐an ozone “paradox” that contributes to a wave‐one zonal pattern in tropospheric ozone. Back trajectories, satellite observations and shipboard tracers suggest that the south Atlantic ozone results from a combination of interhemispheric transport, aged stratospheric‐upper tropospheric air, and possibly from ozone supplied by lightning nitric oxide.
Mount Washington, NH in the White Mountain National Forest, is flanked to the north-northeast and south by two Class I Wilderness areas, the Great Gulf and Presidential Range-Dry River Wildernesses, ...respectively. The Clean Air Act protects Class I Area natural resource values from air pollution. Aerosol sulfate, a fine particulate component that is often transported long distances, is a known contributor to visibility degradation and acidic deposition. We examined summertime fine particulate aerosol mass and sulfate, strong acidity and ammonium concentrations from 1988 to 2007 on Mount Washington at two elevations, 452 and 1540 m (msl). The former site is often within, and the latter at the interface of, the planetary boundary layer. Comparisons of sampling interval durations (10 and 24 h) and site vs. site are made. We also examine the extent to which aerosol sulfate is neutralized.
Ten hour (daytime) compared to 24 h samples have higher mass and aerosol sulfate concentrations, however paired samples are well correlated. Fine mass concentrations compared between the 452 m and 1540 m sites (standard temperature and pressure corrected) show a weak positive linear relationship with the later being approximately 32% lower. We attribute the lack of a strong correlation to the facts that the 1540 m site is commonly at the interface of and even above the regional planetary boundary layer in summer and that it can intercept different air masses relative to the 452 m site. Sulfate is ∼18% lower at the higher elevation site, but comprises a greater percentage of total fine mass; 42% compared to 37% for the high and low elevation site, respectively. Aerosol strong acidity was found to increase with increasing sulfate concentrations at both sites. Further the ratio of hydrogen to sulfate ion was greater in 24 h than 10 h samples at the higher elevation site likely due to overnight transport of fresh acidic aerosols.
Poor air quality episodes occur often in metropolitan Atlanta, GA. The primary focus of this research is to assess the capability of satellites as a tool in characterizing air quality in Atlanta. ...Results indicate that intracity PM
2.5
(particulate matter ≤2.5 μm in aerodynamic diameter) concentrations show similar patterns as other U.S. urban areas, with the highest concentrations occurring within the city. PM
2.5
and MODIS (Moderate Resolution ImagingSpectroradiometer) aerosol optical depth (AOD) have higher values in the summer than spring, yet MODIS AOD doubles in the summer unlike PM
2.5
. Most (80%) of the Ozone Monitoring Instrument aerosol index (AI) is below 0.5 with little differences between spring and summer. Using this value as a constraint of the carbonaceous aerosol signal in the urban area, aerosol transport events such as wildfire smoke associated with higher positive AI values can be identified. The results indicate that MODIS AOD is well correlated with PM
2.5
on a yearly and seasonal basis with correlation coefficients as high as 0.8 for Terra and 0.7 for Aqua. A possible alternative view of the PM
2.5
and AOD relationship is seen through the use of AOD thresholds. These probabilistic thresholds provide a means to describe the air quality index (AQI) through the use of multiyear AOD records for a specific area. The National Ambient Air Quality Standards (NAAQS) are used to classify the AOD into different AQI codes and probabilistically determine thresholds of AOD that represent most of a specific AQI category. For example, 80% of cases of moderate AQI days have AOD values between 0.5 and 0.6. The development of AOD thresholds provides a useful tool for evaluating air quality from the use of satellites in regions where there are sparse ground-based measurements of PM
2.5
.
Polycyclic aromatic hydrocarbons (PAHs), mutagenic compounds predominantly derived from combustion, have been used as markers of combustion sources to the atmosphere. Marine aerosol collected aboard ...the NOAA R/V Ronald Brown during the Aerosols99 and the Indian Ocean Experiment (INDOEX) projects was analyzed for PAHs to assess the continental impact of combustion‐derived particulate matter on the Atlantic and Indian Ocean atmospheres. PAH concentrations in the Atlantic and southern Indian Ocean atmospheres were consistent and low, ranging from <0.45 pg/m3 for coronene to 30 pg/m3 for 9, 10‐dimethylanthracene. PAH concentrations increased ten fold as the ship crossed the Intertropical Convergence Zone (ITCZ) into the northern Indian Ocean, indicating an increased anthropogenic influence. PAH concentrations over the northern Indian Ocean atmosphere were approximately an order of magnitude greater than those in the northern Atlantic Ocean atmosphere. PAH composition profiles over the northern Indian Ocean were specific to wind regimes and influenced by a combination of biomass and fossil fuel combustion. This was supported by significant correlations between select PAHs and organic carbon (OC), elemental carbon (EC), SO4−2 and K+ for particular wind regimes. Indeno1,2,3‐cdpyrene/EC ratios used as a combustion source marker suggest that fossil fuel combustion, rather than biomass burning, is the predominant source of PAHs to the Northern Hemisphere Indian Ocean atmosphere. Interestingly, fossil fuel consumption in the Indian sub‐continent is a fraction of that in Europe and the United States but the soot and PAH levels in the adjacent Northern Indian Ocean atmosphere are significantly greater than those in the Northern Atlantic atmosphere.
Observations of the mass and chemical composition of particles less than 2.5 μm in aerodynamic diameter (PM
2.5
), light extinction, and meteorology in the urban Baltimore-Washington corridor during ...July 1999 and July 2000 are presented and analyzed to study summertime haze formation in the mid-Atlantic region. The mass fraction of ammoniated sulfate (SO
4
2-
) and carbonaceous material in PM
2.5
were each ∼50% for cleaner air (PM
2.5
< 10 μg/m
3
) but changed to ∼60% and ∼20%, respectively, for more polluted air (PM
2.5
> 30 μg/m
3
). This signifies the role of SO
4
2-
in haze formation. Comparisons of data from this study with the Interagency Monitoring of Protected Visual Environments network suggest that SO
4
2−
is more regional than carbonaceous material and originates in part from upwind source regions. The light extinction coefficient is well correlated to PM
2.5
mass plus water associated with inorganic salt, leading to a mass extinction efficiency of 7.6 ± 1.7 m
2
/g for hydrated aerosol. The most serious haze episode occurring between July 15 and 19, 1999, was characterized by westerly transport and recirculation slowing removal of pollutants. At the peak of this episode, 1-hr PM
2.5
concentration reached ∼45 μg/m
3
, visual range dropped to ∼5 km, and aerosol water likely contributed to ∼40% of the light extinction coefficient.
The August 2003 North American electrical blackout provided a unique opportunity to quantify directly the contribution of power plants to regional haze and O3. Airborne observations over central ...Pennsylvania on August 15, 2003, ∼24 h into the blackout, revealed large reductions in SO2 (>90%), O3 (∼50%), and light scattered by particles (∼70%) relative to measurements outside the blackout region and over the same location when power plants were operating normally. CO and light absorbing particles were unaffected. Low level O3 decreased by ∼38 ppbv and the visual range increased by >40 km. This clean air benefit was realized over much of the eastern U.S. Reported SO2 and NOx emissions from upwind power plants were down to 34 and 20% of normal, respectively. The improvement in air quality provides evidence that transported emissions from power plants hundreds of km upwind play a dominant role in regional haze and O3 production.
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