Future land use and land cover influences on regional biogenic emissions and air quality in the United States

A regional modeling system was applied with inputs from global climate and chemistry models to quantify the effects of global change on future biogenic emissions and their impacts on ozone and biogenic secondary organic aerosols (BSOA) in the US. Biogenic emissions in the future are influenced by projected changes in global and regional climates and by variations in future land use and land cover (LULC). The modeling system was applied for five summer months for the present-day case (1990–1999, Case 1) and three future cases covering 2045–2054. Individual future cases were: present-day LULC (Case 2); projected-future LULC (Case 3); and future LULC with designated regions of tree planting for carbon sequestration (Case 4). Results showed changing future meteorology with present-day LULC (Case 2) increased average isoprene and monoterpene emission rates by 26% and 20% due to higher temperature and solar insolation. However when LULC was changed together with climate (Case 3), predicted isoprene and monoterpene emissions decreased by 52% and 31%, respectively, due primarily to projected cropland expansion. The reduction was less, at 31% and 14% respectively, when future LULC changes were accompanied by regions of tree planting (Case 4). Despite the large decrease in biogenic emission, future average daily maximum 8-h (DM8H) ozone was found to increase between +8 ppbv and +10 ppbv due to high future anthropogenic emissions and global chemistry conditions. Among the future cases, changing LULC resulted in spatially varying future ozone differences of ?5 ppbv to +5 ppbv when compared with present-day case. Future BSOA changed directly with the estimated monoterpene emissions. BSOA increased by 8% with current LULC (Case 2) but decreased by 45%–28% due to future LULC changes. Overall, the results demonstrated that on a regional basis, changes in LULC can offset temperature driven increases in biogenic emissions, and, thus, LULC projection is an important factor to consider in the study of future regional air quality.     

Seasonal ozone behavior along an elevation gradient in the Colorado Front Range Mountains

Ambient surface ozone was monitored for one year at a series of seven sites along an elevation gradient from 1600 m to 3500 m above sea level (ASL) in Boulder County, Colorado. Spatial variability of ozone, quantified as the root mean squared deviation of hourly ozone per kilometer horizontal separation, decreased with elevation and distance from local sources, validating the assumption that (except at the City of Boulder (BO) site) the results of the study are representative of the Colorado Front Range. The northern hemisphere (NH) tropospheric spring ozone peak was clearly apparent in late April and early May and affected ozone at all elevations. Ozone consistently increased with elevation during winter, with a mean monthly rate of 1.5 ppbv per 100 m elevation. In summer, this monotonic increase in ozone with elevation was not observed; instead mean monthly ozone increased in two steps, by ～15 ppbv between 1610 m and 1940 m ASL and by ～10 ppbv between 3350 m and 3530 m ASL to a maximum of 60 ppbv. The amplitude of the diurnal ozone cycle decreased with increasing elevation. Average summertime diurnal swings in ozone concentration had a magnitude of 29 ppbv at 1610 m ASL, and 7–16 ppbv at the mid-elevation sites. In winter a diurnal cycle was observed only at the BO site, ozone concentrations at the remaining six locations changed on a multi-day timescale, indicating regional background behavior as the primary factor for wintertime ozone. Even the highest elevation site was influenced by transported urban air pollution in summer, indicated by the average 5 ppbv diurnal increase in ozone. Ozone exposure at the mid- to high-elevation sites in many instances approached and exceeded the 8-h National Ambient Air Quality Standard of 75 ppbv. The elevated ozone levels along this transect were interpreted to be caused by the confounding effects of the high elevation of these sites, increased ozone in long-range transported air, and anthropogenic ozone production in air transported from the nearby urban and suburban areas east of the Colorado Front Range Mountains.     

Measurement and analysis of atmospheric concentrations of isoprene and its reaction products in central Texas

A field experiment was conducted in August 1998 to investigate the concentrations of isoprene and isoprene reaction products in the surface and mixed layers of the atmosphere in Central Texas. Measured near ground-level concentrations of isoprene ranged from 0.3 (lower limit of detection – LLD) to 10.2 ppbv in rural regions and from 0.3 to 6.0 ppbv in the Austin urban area. Rural ambient formaldehyde levels ranged from 0.4 ppbv (LLD) to 20.0 ppbv for 160 rural samples collected, while the observed range was smaller at Austin (0.4–3.4 ppbv) for a smaller set of samples (37 urban samples collected). Methacrolein levels did not vary as widely, with rural measurements from 0.1 ppbv (LLD) to 3.7 ppbv and urban concentrations varying between 0.2 and 5.7 ppbv. Isoprene flux measurements, calculated using a simple box model and measured mixed-layer isoprene concentrations, were in reasonable agreement with emission estimates based on local ground cover data. Ozone formation attributable to biogenic hydrocarbon oxidation was also calculated. The calculations indicated that if the ozone formation occurred at low VOC/NO_x ratios, up to 20 ppbv of ozone formed could be attributable to biogenic photooxidation. In contrast, if the biogenic hydrocarbon reaction products were formed under low NO_x conditions, ozone production attributable to biogenics oxidation would be as low as 1 ppbv. This variability in ozone formation potentials implies that biogenic emissions in rural areas will not lead to peak ozone levels in the absence of transport of NO_x from urban centers or large rural NO_x sources.     

Enhanced ozone over western North America from biomass burning in Eurasia during April 2008 as seen in surface and profile observations

During April 2008, as part of the International Polar Year (IPY), a number of ground-based and aircraft campaigns were carried out in the North American Arctic region (e.g., ARCTAS, ARCPAC). The widespread presence during this period of biomass burning effluent, both gaseous and particulate, has been reported. Unusually high ozone readings for this time of year were recorded at surface ozone monitoring sites from northern Alaska to northern California. At Barrow, Alaska, the northernmost point in the United States, the highest April ozone readings recorded at the surface (hourly average values >55 ppbv) in 37 years of observation were measured on April 19, 2008. At Denali National Park in central Alaska, an hourly average of 79 ppbv was recorded during an 8-h period in which the average was over 75 ppbv, exceeding the ozone ambient air quality standard threshold value in the U.S. Elevated ozone (>60 ppbv) persisted almost continuously from April 19–23 at the monitoring site during this event. At a coastal site in northern California (Trinidad Head), hourly ozone readings were >50 ppbv almost continuously for a 35-h period from April 18–20. At several sites in northern California, located to the east of Trinidad Head, numerous occurrences of ozone readings exceeding 60 ppbv were recorded during April 2008. Ozone profiles from an extensive series of balloon soundings showed lower tropospheric features at ～1–6 km with enhanced ozone during the times of elevated ozone amounts at surface sites in western Canada and the U.S. Based on extensive trajectory calculations, biomass burning in regions of southern Russia was identified as the likely source of the observed ozone enhancements. Ancillary measurements of atmospheric constituents and optical properties (aerosol optical thickness) supported the presence of a burning plume at several locations. At two coastal sites (Trinidad Head and Vancouver Island), profiles of a large suite of gases were measured from airborne flask samples taken during probable encounters with burning plumes. These profiles aided in characterizing the vertical thickness of the plumes, as well as confirming that the plumes reaching the west coast of North America were associated with biomass burning events.     

A review of surface ozone in the polar regions

Surface ozone records from ten polar research stations were investigated for the dependencies of ozone on radiative processes, snow-photochemisty, and synoptic and stratospheric transport. A total of 146 annual data records for the Arctic sites Barrow, Alaska; Summit, Greenland; Alert, Canada; Zeppelinfjellet, Norway; and the Antarctic stations Halley, McMurdo, Neumayer, Sanae, Syowa, and South Pole were analyzed. Mean ozone at the Northern Hemisphere (NH) stations (excluding Summit) is ∼5 ppbv higher than in Antarctica. Statistical analysis yielded best estimates for the projected year 2005 median annual ozone mixing ratios, which for the Arctic stations were 33.5 ppbv at Alert, 28.6 ppbv at Barrow, 46.3 ppbv ppb at Summit and 33.7 ppbv at Zeppelinfjellet. For the Antarctic stations the corresponding ozone mixing ratios were 21.6 ppbv at Halley, 27.0 ppbv at McMurdo, 24.9 ppbv at Neumayer, 27.2 ppbv at Sanae, 29.4 ppbv at South Pole, and 25.8 ppbv at Syowa. At both Summit (3212 m asl) and South Pole (2830 m asl), annual mean ozone is higher than at the lower elevation and coastal stations. A trend analysis revealed that all sites in recent years have experienced low to moderate increases in surface ozone ranging from 0.02 to 0.26 ppbv yr⁻¹, albeit none of these changes were found to be statistically significant trends. A seasonal trend analysis showed above-average increases in ozone during the spring and early summer periods for both Arctic (Alert, Zeppelinfjellet) and Antarctic (McMurdo, Neumayer, South Pole) sites. In contrast, at Barrow, springtime ozone has been declining. All coastal stations experience springtime episodes with rapid depletion of ozone in the boundary layer, attributable to photochemically catalyzed ozone depletion from halogen chemistry. This effect is most obvious at Barrow, followed by Alert. Springtime depletion episodes are less pronounced at Antarctic stations. At South Pole, during the Antarctic spring and summer, photochemical ozone production yields frequent episodes with enhanced surface ozone. Other Antarctic stations show similar, though less frequent spring and summertime periods with enhanced ozone. The Antarctic data provide evidence that austral spring and summertime ozone production in Antarctica is widespread, respectively, affects all stations at least through transport events. This ozone production contributes to a several ppbv enhancement in the annual mean ozone over the Antarctic plateau; however, it is not the determining process in the Antarctic seasonal ozone cycle. Although Summit and South Pole have many similarities in their environmental conditions, this ozone production does not appear to be of equal importance at Summit. Amplitudes of diurnal, summertime ozone cycles at these polar sites are weaker than at lower latitude locations. Amplitudes of seasonal ozone changes are larger in the Southern Hemisphere (by ∼5 ppbv), most likely due to less summertime photochemical ozone loss and more transport of ozone-rich air to the Arctic during the NH spring and summer months.     

What is causing high ozone at Summit,Greenland?

Causes for the unusually high and seasonally anomalous ozone concentrations at Summit, Greenland were investigated. Surface data from continuous monitoring, ozone sonde data, tethered balloon vertical profiling data, correlation of ozone with the radionuclide tracers ⁷Be and ²¹⁰Pb, and synoptic transport analysis were used to identify processes that contribute to sources and sinks of ozone at Summit. Northern Hemisphere (NH) lower free troposphere ozone mixing ratios in the polar regions are ∼20 ppbv higher than in Antarctica. Ozone at Summit, which is at 3212 m above sea level, reflects its altitude location in the lower free troposphere. Transport events that bring high ozone and dry air, likely from lower stratospheric/higher tropospheric origin, were observed ∼40% of time during June 2000. Comparison of ozone enhancements with radionuclide tracer records shows a year-round correlation of ozone with the stratospheric tracer ⁷Be. Summit lacks the episodic, sunrise ozone depletion events, which were found to reduce the annual, median ozone at NH coastal sites by up to ∼3 ppbv. Synoptic trajectory analyses indicated that, under selected conditions, Summit encounters polluted continental air with increased ozone from central and western Europe. Low ozone surface deposition fluxes over long distances upwind of Summit reduce ozone deposition losses in comparison to other NH sites, particularly during the summer months. Surface-layer photochemical ozone production does not appear to have a noticeable influence on Summit's ozone levels.     

Background ozone levels of air entering the west coast of the US and assessment of longer-term changes

An analysis of surface ozone measurements at a west coast site in northern California (Trinidad Head) demonstrates that this location is well situated to sample air entering the west coast of the US from the Pacific Ocean. During the seasonal maximum in the spring, this location regularly observes hourly average ozone mixing ratios 50 ppbv in air that is uninfluenced by the North American continent. Mean daytime values in the spring exceed 40 ppbv. A location in southern California (Channel Islands National Park) demonstrates many of the characteristics during the spring as Trinidad Head in terms of air flow patterns and ozone amounts suggesting that background levels of ozone entering southern California from the Pacific Ocean are similar to those in northern California. Two inland locations (Yreka and Lassen Volcanic National Park) in northern California with surface ozone data records of 20 years or more are more difficult to interpret because of possible influences of local or regional changes. They show differing results for the long-term trend during the spring. The 10-year ozone vertical profile measurements obtained with weekly ozonesondes at Trinidad Head show no significant longer-term change in tropospheric ozone.     

Modelling study of the impact of isoprene and terpene biogenic emissions on European ozone levels

The impact of biogenic volatile organic compound (BVOC) emissions on European ozone distributions has not yet been evaluated in a comprehensive way. Using the CHIMERE chemistry-transport model the variability of surface ozone levels from April to September for 4 years (1997, 2000, 2001, 2003) resulting from biogenic emissions is investigated. It is shown that BVOC emissions increased on average summer daily ozone maxima over Europe by 2.5 ppbv (5%). The impact is most significant in Portugal (up to 15 ppbv) and in the Mediterranean region (about 5 ppbv), being smaller in the northern part of Europe (1.3 ppbv north of 47.5°N). The average impact is rather similar for the three summers (1997, 2000, 2001), but is much larger during the extraordinarily hot summer of 2003. Here, the biogenic contribution to surface ozone doubles compared to other years at some locations. Interaction with anthropogenic NO_x emissions is found to be a key process for ozone production of biogenic precursors. Comparing the impact of the state-of-the-art BVOC emission inventory compiled within the NatAir project and an earlier, widely used BVOC inventory derived from Simpson et al. [1999. Inventorying emissions from nature in Europe. Journal of Geophysical Research 104(D7), 8113–8152] on surface ozone shows that ozone produced from biogenic precursors is less in central and northern Europe but in certain southern areas much higher e.g. Iberian Peninsula and the Mediterranean Sea. The uncertainty in the regionally averaged impact of BVOC on ozone build-up in Europe is estimated to be ±50%.     

Ozone patterns for three metropolitan statistical areas in North Carolina, USA

As part of an effort by the state of North Carolina to develop a State Implementation Plan (SIP) for 1-h peak ozone control, a network of ozone stations was established to monitor surface ozone concentrations across the state. Between 19 and 23 ozone stations made continuous surface measurements between 1993 and 1995 surrounding three major metropolitan statistical areas (MSAs): Raleigh/Durham (RDU), Charlotte/Mecklenburg (CLT), and Greensboro/High Point/Winston-Salem (GSO). Statistical averages of the meteorological and ozone data were performed at each Metropolitan Statistical Area (MSA) to study trends and/or relationships on high ozone days (days in which one of the MSA sites measured an hourly ozone concentration90.0 ppbv). County emission maps of precursor gases, wind roses, total area averages of ozone, total downwind averages of ozone deviations, upwind averages of ozone, and a modified delta ozone analysis were all obtained and analyzed. The results of this study show a reduction in the delta ozone relative to an earlier study at RDU, but no average significant change at CLT (no comparison can be made for GSO). The statistical data analyses in this study are used to quantify the importance of local contributions and regional transport, to ozone air pollution in the MSAs.     

Single-source impact analysis using three-dimensional air quality models

Isolating the effects of an individual emissions source on secondary air pollutants such as ozone and some components of particulate matter must incorporate complex nonlinear processes, be sensitive to small emissions perturbations, and account for impacts that may occur hundreds of kilometers away. The ability to evaluate these impacts is becoming increasingly important for efficient air quality management. Here, as part of a recent compliance enforcement action for a violation of the Clean Air Act and as an evaluation of ozone response to single-source emissions plumes, two three-dimensional regional photochemical air quality models are used to assess the impact on ozone from approximately 2000 to 3000 excess t/month of nitrogen oxides emitted from a single power plant in Ohio. Periods in May, July, and August are evaluated. Two sensitivity methods are applied: the "brute-force" (B-F) method and the decoupled direct method (DDM). Using DDM, maximum 1-hr averaged ozone concentrations are found to increase by up to 1.8, 1.3, and 2.2 ppbv during May, July, and August episodes, respectively, and concentration increases greater than 0.5 ppbv occur in Ohio, Pennsylvania, Maryland, New York, West Virginia, Virginia, and North and South Carolina. B-F results for the August episode show a maximum 1-hr averaged ozone concentration increase of 2.3 ppbv. Significant localized decreases are also simulated, with a maximum of 3.6 ppbv in Ohio during the August episode and decreases of 0.50 ppbv and greater in Ohio, Pennsylvania, Maryland, West Virginia, and Virginia. Maximum increases are compared with maximum decreases for the August period using second-order DDM and are found, in aggregate, to be greater in magnitude by 42%. When evaluated during hours when ozone concentrations exceed 0.060 ppm, the maximum increases in ozone are higher than decreases by 82%. The spatial extent of ozone increase in both cases is about triple that of reduction.     

The sensitivity of modeled ozone to the temporal distribution of point, area, and mobile source emissions in the eastern United States

Ozone remains one of the most recalcitrant air pollution problems in the US. Hourly emissions fields used in air quality models (AQMs) generally show less temporal variability than corresponding measurements from continuous emissions monitors (CEM) and field campaigns would imply. If emissions control scenarios to reduce emissions at peak ozone forming hours are to be assessed with AQMs, the effect of emissions' daily variability on modeled ozone must be understood. We analyzed the effects of altering all anthropogenic emissions' temporal distributions by source group on 2002 summer-long simulations of ozone using the Community Multiscale Air Quality Model (CMAQ) v4.5 and the Carbon Bond IV (CBIV) chemical mechanism with 12 km resolution. We find that when mobile source emissions were made constant over the course of a day, 8-h maximum ozone predictions changed by ±7 parts per billion by volume (ppbv) in many urban areas on days when ozone concentrations greater than 80 ppbv were simulated in the base case. Increasing the temporal variation of point sources resulted in ozone changes of +6 and −6 ppbv, but only for small areas near sources. Changing the daily cycle of mobile source emissions produces substantial changes in simulated ozone, especially in urban areas at night; results suggest that shifting the emissions of NO_x from day to night, for example in electric powered vehicles recharged at night, could have beneficial impacts on air quality.     

Measurements of ozone and nonmethane hydrocarbons at Chichi-jima island,a remote island in the western Pacific: long-range transport of polluted air from the Pacific rim region

Chichi-jima island is located in the Pacific about 1000 km from the Japanese main island and is an ideal remote observatory from which to assess the long-range transport of polluted air from East Asia. The ozone concentration was measured from August 1997 to August 1998. Owing to the air mass change, the seasonal variation of ozone shows a distinct character: low concentration (about 13 ppbv) for the maritime air mass during the summer, and high concentration (about 40 ppbv) for the continental air mass during the winter. To assess the contribution of the long-range transport of polluted air during winter, nonmethane hydrocarbons were also measured in December 1999. Using backward trajectory analysis, the transport time of the air mass from the source area in the Pacific rim region was calculated for each sample. The concentration of hydrocarbons shows a clear negative correlation against the transport time. This analysis clearly shows the transport of polluted air, emitted in East Asia, to the Pacific during the winter. The plots of suitable hydrocarbon pairs showed that the decrease of hydrocarbon concentrations during winter is mainly caused by the mixing with clean background air.     

Examination of the impact of photoexcited NO2 chemistry on regional air quality

Impact of the excited nitrogen dioxide (NO₂¹) chemistry on air quality in the U.S. is examined using the Community Multiscale Air Quality (CMAQ) model for a summer month. Model simulations were conducted with and without the NO₂¹ chemistry. The largest impact of the NO₂¹ chemistry in the eastern U.S. occurred in the northeast and in the western U.S. occurred in Los Angeles. While the single largest daily maximum 8-h ozone (O₃) increased by 9 ppbv in eastern U.S. and 6 ppbv in western U.S., increases on most days were much lower. No appreciable change in model performance statistics for surface-level O₃ predictions relative to measurements is noted between simulations with and without the NO₂¹ chemistry. Based on model calculations using current estimates of tropospheric emission burden, the NO₂¹ chemistry can increase the monthly mean daytime hydroxyl radicals (OH) and nitrous acid (HONO) by a maximum of 28% and 100 pptv, respectively.     

Natural emissions for regional modeling of background ozone and particulate matter and impacts on emissions control strategies

Natural emissions adopted in current regional air quality modeling are updated to better describe natural background ozone and PM concentrations for North America. The revised natural emissions include organosulfur from the ocean, NO from lightning, sea salt, biogenic secondary organic aerosol (SOA) precursors, and pre-industrial levels of background methane. The model algorithm for SOA formation was also revised. Natural background ozone concentrations increase by up to 4 ppb in annual average over the southeastern US and Gulf of Mexico due to added NO from lightning while the revised biogenic emissions produced less ozone in the central and western US. Natural PM_2.5 concentrations generally increased with the revised natural emissions. Future year (2018) simulations were conducted for several anthropogenic emission reduction scenarios to assess the impact of the revised natural emissions on anthropogenic emission control strategies. Overall, the revised natural emissions did not significantly alter the ozone responses to the emissions reductions in 2018. With revised natural emissions, ozone concentrations were slightly less sensitive to reducing NOx in the southeastern US than with the current natural emissions due to higher NO from lightning. The revised natural emissions have little impact on modeled PM_2.5 responses to anthropogenic emission reductions. However, there are substantial uncertainties in current representations of natural sources in air quality models and we recommend that further study is needed to refine these representations.     

A long-term study of background ozone concentrations in the central Mediterranean—diurnal and seasonal variations on the island of Gozo

A four and a half year study of ozone concentrations in the Central Mediterranean was carried out between January 1997 and August 2001 on a background monitoring station located on the island of Gozo midway between Southern Europe and North Africa.Seasonal and diurnal variations of background ozone are documented. They show the existence of seasonal cycles with a primary maximum in spring followed by a secondary, more variable maximum in summer which indicates that photochemically produced ozone is being transported over the Mediterranean to the rural island of Gozo. Although peak ozone concentrations seldom exceeded 100 ppbv during summer, the background ozone-mixing ratios (as monthly averages) are some of the highest values which can be found at low latitude sites throughout the world. An increasing trend in the annual background ozone concentration from 48.2 ppbv in 1997 to 52.2 ppbv in 2000 is observed. During wintertime the average ozone mixing-ratio (as monthly averages) of 44 ppbv in December is approximately twice as high as on the European continent. This may imply that on Malta, due to higher average ozone concentrations between autumn and spring (the main growing season), crop damage of high economic value may occur.     

The variability of free tropospheric ozone over Beltsville,Maryland (39N, 77W) in the summers 2004–2007

Ozone profiles are often used to investigate day-to-day and year-to-year variability in origins of free tropospheric ozone. With this in mind, more than 50 ozonesonde launches were conducted in Beltsville, MD, during the summers of 2004 through 2007. Budgets of free tropospheric ozone were calculated for each ozone profile in the four summers using a laminar identification (LID) method and unusual episodes were analyzed with respect to meteorological variables. The laminar method showed that stratosphere-to-troposphere transport (ST) accounted for greater than 50% of the free tropospheric ozone column on 17% of days sampled, a more pronounced influence than regional convective and lightning (RCL) sources. The ST origins were confirmed with trajectories, and tracers (water vapor and potential vorticity). The amount of free tropospheric ozone from ST and RCL sources varied from year-to-year (up to 13%) and can be explained by differences in mean meteorological patterns. On average, almost 30% of the free tropospheric column was attributed to ST influence, about twice as much as RCL, although the LID method may not capture weeks-old lightning influences as in a chemical model. The prevalence of ST ozone in summertime Beltsville soundings was similar to six sounding sites in the IONS-04 campaign [Thompson, A.M., et al., 2007b. Intercontinental Transport Experiment Ozonesonde Network Study (IONS, 2004): 1. Summertime upper tropospheric/lower stratosphere ozone over northeastern North America. J. Geophys. Res. 112, D12S12; Thompson, A.M., et al., 2007c. Intercontinental Transport Experiment Ozonesonde Network Study (IONS, 2004): 2. Tropospheric ozone budgets and variability over northeastern North America. J. Geophys. Res. 112, D12S13.] and to statistics from a 30 year climatology of European soundings [Collette, A., Ancellet, G., 2005. Impact of vertical transport processes on the tropospheric ozone layering above Europe. Part II: Climatological analysis of the past 30 years. Atmos. Environ. 39, 5423–5435]. The Beltsville record also demonstrated the value of soundings for air quality forecasting in an urban area. The 22 nighttime soundings collected over Beltsville in 2004–2007 can be divided into distinct polluted and unpolluted subsets, the former 20 ppbv higher in residual layer ozone (1 km) than the latter. These distinctions propagated to daytime differences of 10 ppbv at the surface in the Washington, DC, area, with the high-ozone residual layers leading to non-attainment of the National Ambient Air Quality Standard for ozone. More frequent ozone observations aloft appear essential for better understanding ozone variability and for enabling air quality modelers to achieve more accurate ozone forecasts.     

Variations and sources of ambient formaldehyde for the 2008 Beijing Olympic games

As the host city of the 2008 Olympic games, Beijing implemented a series of air pollution control measures before and during the Olympic games. Ambient formaldehyde (HCHO) concentrations were measured using a fluorometric instrument based on a diffusion scrubber and the Hantzsch reaction; hydrocarbons were simultaneously measured using gas chromatography–mass spectrometry (GC–MS). Meteorological parameters, CO, O₃, and NO₂ concentrations were measured by standard commercial instrumentation. In four separate periods: (a) before the vehicle plate number control (3–19 July); (b) during the Olympic Games (8–24 August); (c) during the Paralympic Games (6–17 September) and (d) after the vehicle control was ceased (21–28 September), the average HCHO mixing ratios were 7.31 ± 2.67 ppbv, 5.54 ± 2.41 ppbv, 8.72 ± 2.48 ppbv, and 6.42 ± 2.79 ppbv, while the total non-methane hydrocarbons (NMHCs) measured were 30.41 ± 18.08 ppbv, 18.12 ± 9.38 ppbv, 30.50 ± 13.37 ppbv, and 33.33 ± 15.85 ppbv, respectively. Both HCHO and NMHC levels were the lowest during the Olympic games, and increased again during the Paralympic games even with the same vehicle control measures operative. Similar diurnal HCHO and O₃ patterns indicated that photo-oxidation of NMHCs may be the major source of HCHO. The diurnal profile of total NMHCs was very similar to that of NO₂ and CO: morning and evening peaks appeared in rush hours, indicating even after strict vehicle control, automobile emission may still be the dominant source of the HCHO precursors. The contributions of HCHO, alkanes, alkenes, and aromatics to OH loss rates were also calculated. HCHO contributed 22 ± 3% to the total VOCs and 24 ± 1% to the total OH loss rate. HCHO was not only important in term of abundance, but also important in chemical reactivity in the air.     

An examination of oxidant amounts on secondary organic aerosol formation and aging

The effect of HO_x radicals (OH and HO₂) and ozone (O₃) on aerosol formation and aging has been studied. Experiments were performed in presence as well as in absence of oxygen in a flow-through chamber at 299 K for three organic precursor gases, isoprene, α-pinene and m-xylene. The HO_x source was the UV photolysis of humidified air or nitrogen and was measured with a GTHOS (Ground-based Tropospheric Hydrogen Oxides Sensor). The precursor gases concentration was monitored with an online GC-FID. The aerosol mass was then quantified by a Tapered Element Oscillating Microbalance (TEOM). Typical oxidant mixing ratios were (0–4.5) ppm for O₃, 200 pptv for OH and 3 ppbv for HO₂. A simple kinetics model is used to infer the aerosol production mechanism. In the present of O₃ (or O₂), the SOA yields were 0.46, 0.036 and 0.12 for α-pinene with an initial concentration of 100 ppbv (RH = 37%), isoprene with an initial concentration of 177 ppbv (RH = 50%) and m-xylene with an initial concentration of 100 ppbv (RH = 37%), respectively. When the chosen precursor gases reacted with HO_x in the absence of O₃, the maximum SOA yields were significantly increased by factors of 1.6 for isoprene 1.1 for α-pinene, and 3 for m-xylene respectively. The comparison of the calculated and measured potential aerosol mass concentrations as function of time shows that presence of ozone or oxygen can influence the aerosol yield and the absence of ozone or oxygen in the system resulted in high concentrations of its organic aerosol products.     

Sources and photodecomposition of formaldehyde and acetaldehyde in Rome ambient air

Atmospheric levels of formaldehyde and acetaldehyde as well as their diurnal and seasonal variations were investigated from 1994 to 1997 in downtown Rome during sunny and wind calm days. Hourly concentrations of formaldehyde ranged from 8 to 28 ppbV in summer and 7 to 17 ppbv in winter; acetaldehyde concentrations varied correspondingly within the 3–18 and 2–7 ppbv intervals. Percentages of both aldehydes photochemically produced were estimated through a simple relationship based upon the comparison of individual ratios of formaldehyde and acetaldehyde to toluene in ambient air and automobile emission. Photochemical production was found to weigh upon atmospheric levels for 80–90% in summer days. It dropped below 35% in the winter period, when direct emission from traffic largely predominated. Photochemical summer source was more efficient for acetaldehyde than for formaldehyde, especially in the early morning. The importance of formaldehyde as the major source of hydroxyl radicals in Rome was also assessed.     

Examining the temperature dependence of ethanol (E85) versus gasoline emissions on air pollution with a largely-explicit chemical mechanism

The increased use of ethanol in transportation fuels warrants an investigation of its consequences. An important component of such an investigation is the temperature dependence of ethanol and gasoline exhaust chemistry. We use the Master Chemical Mechanism (MCM, version 3.1, LEEDS University) with the SMVGEAR II chemical ordinary differential solver to provide the speed necessary to simulate complex chemistry to examine such effects. The MCM has over 13,500 organic reactions and 4600 species. SMVGEAR II is a sparse-matrix Gear solver that reduces the computation time significantly while maintaining any specified accuracy. Although we use a box model for this study, we determine and demonstrate in a separate study that the speed of the MCM with SMVGEAR II allows the MCM to be modeled in 3-dimensions. We also verified the accuracy of the model in comparison with smog chamber data. We then use the model with species-resolved tailpipe emissions data for E85 (15% gasoline, 85% ethanol fuel blend) and gasoline vehicles to compare the impact of each on nitrogen oxides, organic gases, and ozone as a function of ambient temperature and background concentrations, using Los Angeles in 2020 as a base case. We use two different emissions sets – one is a compilation of exhaust and evaporative data taken near 24 °C and the other from exhaust data taken at ?7 °C – to determine how atmospheric chemistry and emissions are affected by temperature. We include diurnal effects by examining two day scenarios. We find that, accounting for chemistry and dilution alone, the average ozone concentrations through the range of temperatures tested are higher with E85 than with gasoline by ～7 part per billion volume (ppbv) at higher temperatures (summer conditions) to ～39 ppbv at low temperatures and low sunlight (winter conditions) for an area with a high nitrogen oxide (NOx) to non-methane organic gas (NMOG) ratio. The results suggest that E85's effect on health through ozone formation becomes increasingly more significant relative to gasoline at colder temperatures due to the change in exhaust emission composition at lower temperatures. Acetaldehyde and formaldehyde concentrations are also much higher with E85 at cold temperatures, which is a concern because both are considered to be carcinogens. These could have implications for wintertime use of E85. Peroxy acetyl nitrate (PAN), another air pollutant of concern, increases with E85 by 0.3–8 ppbv. The sensitivity of the results to box size, initial background concentrations, background emissions, and water vapor were also examined.