Forgot your password?
New user?
New Window Opens on the Secret Life of Microbes: Scientists Develop First Microbial Profiles of Ecosystems
CAS announces the release of SciFinder Scholar for Mac OS X
Press Releases
Press Releases
| Check out our New Publishers' Select for Free Articles |
Atmospheric Environment (v.40, #23)
On-road particulate matter (PM2.5) and gaseous emissions in the Shing Mun Tunnel, Hong Kong by Y. Cheng; S.C. Lee; K.F. Ho; P.K.K. Louie (pp. 4235-4245).
Real-world emission factors of PM2.5 (particle's aerodynamic diameter of less than 2.5μm), NO x (the sum of NO2 and NO), and CO from mixed vehicles in the Shing Mun Tunnel in Hong Kong have been determined. A total of 16 runs were made during varying times that encompassed different traffic fractions of diesel-fueled vehicles. During the study, a total of 27,310 vehicles traversed the tunnel, comprising approximately 50% diesel-fueled vehicles, 41% gasoline-fueled vehicles, and 9% LPG-vehicles. The PM2.5 emission factors, derived from the DRI (Desert Research Institute, USA) particulate sampler, ranged from 0.066 to 0.190g veh−1 km−1. Result from a t-test showed no statistical significance for PM2.5 difference in summer and winter. The PM2.5 emission factors were found to correlate well with NO x ( R2=0.65) and both had a positive relationship with the proportion of diesel-fueled vehicles. Two DustTrak air monitors were also used to monitor PM2.5 masses in the tunnel. The mean emission factor derived from DustTrak, 0.150±0.045g veh−1 km−1, had reasonable agreement with the filter method, 0.131±0.037g veh−1 km−1, demonstrating that the DustTrak air monitor can be used in tunnel studies, but needs careful calibrations with the filter method. The emission factors for PM2.5 mass and NO x were obtained for on-road diesel-fueled vehicles based on the linear regression analysis and they were 0.257±0.031 and 1.931±0.309g veh−1 km−1, respectively.
Keywords: Emission factor; Tunnel; PM; 2.5; NO; x; CO
A statistical model for characterizing common air pollutants in air-conditioned offices by L.T. Wong; K.W. Mui; P.S. Hui (pp. 4246-4257).
Maintaining acceptable indoor air quality (IAQ) for a healthy environment is of primary concern, policymakers have developed different strategies to address the performance of it based on proper assessment methodologies and monitoring plans. It could be cost prohibitive to sample all toxic pollutants in a building. In search of a more manageable number of parameters for cost-effective IAQ assessment, this study investigated the probable correlations among the 12 indoor environmental parameters listed in the IAQ certification scheme of the Hong Kong Environment Protection Department (HKEPD) in 422 Hong Kong offices. These 12 parameters consists of nine indoor air pollutants: carbon dioxide (CO2), carbon monoxide (CO), respirable suspended particulates (RSP), nitrogen dioxide (NO2), ozone (O3), formaldehyde (HCHO), total volatile organic compounds (TVOC), radon (Rn), airborne bacteria count (ABC); and three thermal comfort parameters: temperature ( T), relative humidity (RH) and air velocity ( V). The relative importance of the correlations derived, from largest to smallest loadings, was ABC, Rn, CO, RH, RSP, CO2, TVOC, O3, T, V, NO2 and HCHO.Together with the mathematical expressions derived, an alternative sampling protocol for IAQ assessment with the three ‘most representative and independent’ parameters namely RSP, CO2 and TVOC measured in an office environment was proposed. The model validity was verified with on site measurements from 43 other offices in Hong Kong. The measured CO2, RSP and TVOC concentrations were used to predict the probable levels of the other nine parameters and good agreement was found between the predictions and measurements. This simplified protocol provides an easy tool for performing IAQ monitoring in workplaces and will be useful for determining appropriate mitigation measures to finally honor the certification scheme in a cost-effective way.
Keywords: Indoor air quality; Air-conditioned offices; Sampling protocol; Representative pollutants
Modelling the temperature-induced blow-off and blow-on artefacts in filter-sorbent measurements of semivolatile substances by Elisabeth Galarneau; Terry F. Bidleman (pp. 4258-4268).
A common method of sampling semivolatile organic compounds (SVOCs) is to pull air through a filter to collect the particulate fraction followed by a gas-trapping sorbent (e.g. polyurethane foam or XAD resin) to collect the vapour fraction. This sampling method is prone to artefacts that may misrepresent the particle/gas distribution due to volatilisation from, and sorption to, the particles collected on the filter and the filter material itself. Such artefacts could be due to temperature variations as well as changes in atmospheric concentrations of sorbing species and in the mass of particles on the filter over the sampling period.Mathematical simulations of the changing equilibrium between particle and gas SVOCs during air sampling show that, in the absence of other artefacts, partition coefficients based on 24-h samples will match those in air at the end of the sampling period under warm conditions as well as cool conditions for the more volatile SVOCs. However, to reflect the average conditions of the sampling period, 24-h samples should be started near one of the two times that the average daily temperature occurs. If conditions are such that the equilibration needs of the collected particles cannot be met at all times during the sampling period, samples should be started at the morning occurrence of the average temperature. For the conditions in a mid-latitude urban area simulated in this study, 24-h samples started in early morning can yield temperature-induced blow-on and blow-off errors in estimated partition coefficients of up to 0.4log Kp units.
Keywords: Air sampling; Artefact (artifact); Blow-off; Particle/gas partitioning; Semivolatile organic compounds
Diurnal and seasonal trends in total gaseous mercury flux from three urban ground surfaces by M.C. Mark C. Gabriel; D.G. Derek G. Williamson; Hong Zhang; Steve Brooks; Steve Lindberg (pp. 4269-4284).
Total gaseous mercury flux measurements were carried out over three urban ground surfaces for 1 year in Tuscaloosa, AL, USA. The objective was to provide insight into the characteristics of gaseous mercury flux from urban surface covers. Bare soil, grass, and pavement surfaces were sampled as the most representative terrestrial surfaces throughout Tuscaloosa. Measurements were quantified over diurnal and seasonal periods and relationships were developed between flux from each surface and major meteorological parameters. Averaging data over the entire year, fluxes from each surface were as follows: bare soil (6.48ng/m−2h), pavement (0.02ng/m−2h), and grass (0.28ng/m−2h). Pavement and many grass fluxes were small and arguably indistinguishable from chamber blanks. The soil surface displayed the largest difference between evening and daytime flux, particularly during the spring and summer seasons (i.e., evening low (12ng/m−2h) to daytime high (30ng/m−2h) during summer). The grass surface showed the largest amount of atmospheric deposition, mainly during the spring and fall periods (up to −2.31ng/m−2h), with pavement showing somewhat less (up to −1.05ng/m−2h). Bare soil showed very little to negligible deposition. Diurnal flux variance was greater than seasonal flux variance for all surfaces. The regression results demonstrate that despite the highly dissimilar physical and geochemical make-up of pavement, bare soil, and grass, each surface displayed similar responses to time series change in meteorological parameters. However, each surface may be seasonally controlled or limited by different sets of meteorological parameters.
Keywords: Terrestrial surface; Urban areas; Mercury flux; Seasonal; Diurnal
Particle size characterization and emission rates during indoor activities in a house by Tareq Hussein; Thodoros Glytsos; Ondracek Jakub OndráÄ?ek; Dohanyosova Pavla Dohányosová; V. Zdimal VladimÃr ŽdÃmal; Hameri Kaarle Hämeri; Mihalis Lazaridis; J. Smolik Jiřà SmolÃk; Markku Kulmala (pp. 4285-4307).
Characterization and emission rates of indoor aerosols have been of great interest. However, few studies have presented quantitative determinations of aerosol particle emissions during indoor activities. In the current study we presented and investigated the physical characteristics and size-fractionated emission rates of indoor aerosol particles during different activities in a house (naturally ventilated) located in Prague, Czech Republic. We utilized a multi-compartment and size-resolved indoor aerosol model (MC-SIAM) to investigate the indoor-to-outdoor relationship of aerosol particles and also to estimate their emission rates. When the windows and the main door were closed for several hours and there were minor indoor activities that did not produce significant amounts of aerosol particles, the particle number concentration showed similar levels at different indoor locations. As expected, the natural ventilation did not provide a controlled indoor-to-outdoor relationship of aerosol particles. As previous studies have emphasized, cooking and tobacco smoking activities are major sources indoors; the total particle number concentration was, respectively, as high as 1.8×105 and 3.6×104cm−3 with emission rates around 380 and 36cm−3s−1. During intensive cooking activities the outdoor aerosol particle concentrations were also affected even though windows were closed. It seems that a simple model is not able to describe the fate of indoor aerosols within a multi-compartment construction; instead, a numerical and dynamic model with a multi-compartment approach is needed. Based on the indoor aerosol model simulations, the deposition rate was comparable to previous studies with friction velocity between 10–30cms−1 and surface area to volume ratio around 2.9–3.1m−1. The penetration factor was equivalent to G3 filter standards and the ventilation rate varied between 0.6–1.2h−1. Based on the emission rate analysis, aerosol particles produced during tobacco smoking and incense stick burning remain airborne for a longer time than cooking particles. It seems that aerosol particles emitted during tobacco smoking and incense stick burning undergo different processes; therefore, there is a need for a combined physical–chemical indoor aerosol model to better describe the evolution of indoor aerosol particles due to different activities.
Keywords: Particle number size distribution; Penetration; Deposition; Natural ventilation; Modeling; Indoor sources
Vertical atmospheric structure estimated by heat island intensity and temporal variations of methane concentrations in ambient air in an urban area in Japan by Masahide Aikawa; Takatoshi Hiraki; Jiro Eiho (pp. 4308-4315).
The vertical atmospheric structure was studied and evaluated based on the distribution and variation of the air temperature in an urban area in Japan. A difference was observed in the annual mean diurnal variation of the air temperature between the urban site and a suburban site. The maximum and minimum temperatures were1.64∘C at 1:00 and1.17∘C at 15:00, respectively, resulting in an estimated intrinsic heat island intensity of 0.47(=1.64–1.17)∘C. The height of the temperature inversion layer was approximately 90m above the ground, based on the intrinsic heat island intensity in an area where no vertical air temperature was available. The temporal variations of the methane concentrations in ambient air and the contribution of automobile emissions were estimated and well accounted for by the postulated temperature inversion layer.
Keywords: Urban heat island; Air temperature; Air pollution; Methane; Urban area; Japan
Measurement of fine particulate and gas-phase species during the New Year's fireworks 2005 in Mainz, Germany by Frank Drewnick; Silke S. Hings; Joachim Curtius; Gunter Eerdekens; Jonathan Williams (pp. 4316-4327).
The chemical composition and chemically resolved size distributions of fine aerosol particles were measured at high time resolution (5min) with a time-of-flight aerosol mass spectrometer (TOF-AMS) during the New Year's 2005 fireworks in Mainz, central Germany. In addition, particle number concentrations and trace gas concentrations were measured using a condensation particle counter (CPC) and a proton transfer reaction mass spectrometer (PTR-MS). The main non-refractory components of the firework aerosol were potassium, sulfate, total organics and chloride. Increased trace gas mixing ratios of methanol, acetonitrile, acetone and acetaldehyde were observed. Aerosol nitrate and ammonium concentrations were not significantly affected by the fireworks as well as the measured aromatic trace gases. The sub-micron aerosol concentrations peaked about 20min after midnight with total mass concentrations larger than 600μgm−3. The trace gas concentrations peaked about 30min later. Using the sulfur-to-potassium concentration ratio measured in another fireworks aerosol, it was for the first time possible to estimate the relative ionization efficiency of aerosol potassium, measured with the TOF-AMS. Here we found a value of RIEK=2.9.
Keywords: Aerosol; Fine particles; Fireworks; TOF-AMS; PTR-MS
Influences of relative humidity on aerosol optical properties and aerosol radiative forcing during ACE-Asia by S.-C. Soon-Chang Yoon; Jiyoung Kim (pp. 4328-4338).
In situ measurements at Gosan, South Korea, and onboard C-130 aircraft during ACE-Asia were analyzed to investigate the influence of relative humidity (RH) on aerosol optical properties and radiative forcing. The temporal variation of aerosol chemical composition at the Gosan super-site was highly dependent on the air mass transport pathways and source region. RH in the springtime over East Asia were distributed with very high spatial and temporal variation. The RH profile onboard C-130 aircraft measurements exhibits a mixed layer height of about 2km. Aerosol scattering coefficient ( σsp) under ambient RH was greatly enhanced as compared with that at dry RH (RH<40%). From the aerosol optical and radiative transfer modeling studies, we found that the extinction and scattering coefficients are greatly enhanced with RH. Single scattering albedo with RH is also sensitively changed in the longer wavelength. Asymmetry parameter ( g) is gradually increased with RH although g decreases with wavelength at a given RH. Aerosol optical depth (AOD) at 550nm and RH of 50% increased to factors 1.24, 1.51, 2.16, and 3.20 at different RH levels 70, 80, 90, and 95%, respectively. Diurnal-averaged aerosol radiative forcings for surface, TOA, and atmosphere were increased with RH because AOD was increased with RH due to hygroscopic growth of aerosol particles. This result implies that the hygroscopic growth due to water-soluble or hydrophilic particles in the lower troposphere may significantly modify the magnitude of aerosol radiative forcing both at the surface and TOA. However, the diurnal-averaged radiative forcing efficiencies at the surface, TOA, and atmosphere were decreased with increasing RH. The decrease of the forcing efficiency with RH results from the fact that increasing rate of aerosol optical depth with RH is greater than the increasing rate of aerosol radiative forcing with RH.
Keywords: Aerosol; Hygroscopic growth; Radiative forcing; Forcing efficiency; Relative humidity; Optical properties
Spatial and temporal distribution of pesticide air concentrations in Canadian agricultural regions by Yuan Yao; Ludovic Tuduri; Tom Harner; Pierrette Blanchard; Don Waite; Laurier Poissant; Clair Murphy; Wayne Belzer; Fabien Aulagnier; Y.-F Yi-Fan Li; Ed Sverko (pp. 4339-4351).
The Canadian Pesticide Air Sampling Campaign was initiated in 2003 to assess atmospheric levels of pesticides, especially currently used pesticides (CUPs) in agricultural regions across Canada. In the first campaign during the spring to summer of 2003, over 40 pesticides were detected. The spatial and temporal distribution of pesticides in the Canadian atmosphere was shown to reflect the pesticide usage in each region. Several herbicides including triallate, bromoxynil, MCPA, 2,4-D, dicamba, trifluralin and ethalfluralin were detected at highest levels at Bratt's Lake, SK in the prairie region. Strong relationships between air concentrations and dry depositions were observed at this site. Although no application occurred in the Canadian Prairies in 2003, high air concentrations of lindane ( γ-hexachlorocyclohexane) were still observed at Bratt's Lake and Hafford, SK. Two fungicides (chlorothalonil and metalaxyl) and two insecticides (endosulfan and carbofuran) were measured at highest levels at Kensington, PEI. Maximum concentrations of chlorpyrifos and metolachlor were found at St. Anicet, QC. The southern Ontario site, Egbert showed highest concentration of alachlor. Malathion was detected at the highest level at the west coast site, Abbotsford, BC. In case of legacy chlorinated insecticides, high concentrations of DDT, DDE and dieldrin were detected in British Columbia while α-HCH and HCB were found to be fairly uniform across the country. Chlordane was detected in Ontario, Québec and Prince Edward Island. This study demonstrates that the sources for the observed atmospheric occurrence of pesticides include local current pesticide application, volatilization of pesticide residues from soil and atmospheric transport. In many instances, these data represent the first measurements for certain pesticides in a given part of Canada.
Keywords: Pesticides; Currently used pesticides; Canadian atmosphere; Spatial and temporal distribution; Atmospheric transport
K-model description of probabilistic long-range atmospheric transport in the Northern Hemisphere by Bent Lauritzen; Alexander Baklanov; Alexander Mahura; Torben Mikkelsen; Havskov Sorensen Jens Havskov Sørensen (pp. 4352-4369).
Predictions obtained from a simple Eulerian K-model are compared to a full-year ensemble average of long-range transport and deposition data, obtained from a numerical weather prediction model coupled to a 3D Lagrangian dispersion model. Five release sites located in different climate regions in the Northern Hemisphere were investigated. In all cases, the K-model is found qualitatively to account for the gross structure of the mean deposition patterns, and as such, it is providing a simple means for a first qualitative description of site-specific, averaged long-range atmospheric dispersion and deposition. An embedded stochastic gamma model can be used to describe the large-scale spatial fluctuations around the mean values. The parameters of both the K-model and the stochastic gamma distribution model have been obtained through maximum likelihood estimation. For 1μm diameter aerosols, the effective deposition length is found to be of the order of 500km. We argue that the K-model may be used as a practical tool for qualitative probabilistic safety assessment, where it is suggested that climatological data of e.g. mean wind can be used to estimate the model parameters.
Keywords: Long-range transport; Pollutants; K-model; Deposition; Risk assessment
Acidity effects on the formation of α-pinene ozone SOA in the presence of inorganic seed by Nadine M. Czoschke; Myoseon Jang (pp. 4370-4380).
A 2m3 chamber was used to explore the effect of relative humidity (RH) and inorganic composition on aerosol yield due to aerosol phase heterogeneous processes in the α-pinene ozone oxidation system. The inorganic compositions considered range from pure aqueous sulfuric acid to a completely neutralized acid, ammonium sulfate. The heterogeneous organic mass (OMH) and the organic mass from partitioning (OMP) were decoupled from the total organic mass to consider the effect of acidity parameters solely on the aerosol mass fraction they impact the most (OMH). An experiment with aqueous ammonium sulfate was preformed in a 25m3 chamber to determine the distribution of α-pinene ozone products for an explicit multiproduct model used in the decoupling process. Both RH and inorganic seed composition were found to influence heterogeneous aerosol yield, due to the acid catalytic nature of the reactions that lead to this mass. The highest heterogeneous aerosol yields were found under conditions of low RH and strongly acidic inorganic seed compositions. A multiple linear regression model is presented and analyzed to show that both RH and inorganic seed composition significantly influence the production of aerosol mass through heterogeneous processes. The results presented here aid our understanding of the development of heterogeneous mass in the presence of inorganic aerosols and will be useful for modeling heterogeneous aerosol mass.
Keywords: SOA formation; Inorganic seed; Sulfuric acid; Relative humidity; Molar yield
Inhalation intake of ambient air pollution in California's South Coast Air Basin by Julian D. Marshall; Patrick W. Granvold; Abigail S. Hoats; Thomas E. McKone; Elizabeth Deakin; William W Nazaroff (pp. 4381-4392).
Reliable estimates of inhalation intake of air pollution and its distribution among a specified population are important for environmental epidemiology, health risk assessment, urban planning, and environmental policy. We computed distributional characteristics of the inhalation intake of five pollutants for a group of ∼25,000 people (∼29,000 person-days) living in California's South Coast Air Basin. Our approach incorporates four main inputs: temporally resolved information about people's location (latitude and longitude), microenvironment, and activity level; temporally and spatially explicit model determinations of ambient concentrations; stochastically determined microenvironmental adjustment factors relating the exposure concentration to the ambient concentration; and, age-, gender-, and activity-specific breathing rates. Our study is restricted to pollutants of outdoor origin, i.e. it does not incorporate intake in a microenvironment from direct emissions into that microenvironment. Median estimated inhalation intake rates (μgd−1) are 53 for benzene, 5.1 for 1,3-butadiene, 8.7×10−4 for hexavalent chromium in fine particulate matter (Cr-PM2.5), 30 for diesel fine particulate matter (DPM2.5), and 68 for ozone. For the four primary pollutants studied, estimated median intake rates are higher for non-whites and for individuals in low-income households than for the population as a whole. For ozone, a secondary pollutant, the reverse is true. Accounting for microenvironmental adjustment factors, population mobility and temporal correlations between pollutant concentrations and breathing rates affects the estimated inhalation intake by 40% on average. The approach presented here could be extended to quantify the impact on intakes and intake distributions of proposed changes in emissions, air quality, and urban infrastructure.
Keywords: Diesel particulate matter; Environmental justice; Exposure analysis; Mobility; Ozone; Geographic information system (GIS)
Modeling residential exposure to secondhand tobacco smoke by Neil E. Klepeis; William W. Nazaroff (pp. 4393-4407).
We apply a simulation model to explore the effect of a house's multicompartment character on a nonsmoker's inhalation exposure to secondhand tobacco smoke (SHS). The model tracks the minute-by-minute movement of people and pollutants among multiple zones of a residence and generates SHS pollutant profiles for each room in response to room-specific smoking patterns. In applying the model, we consider SHS emissions of airborne particles, nicotine, and carbon monoxide in two hypothetical houses, one with a typical four-room layout and one dominated by a single large space. We use scripted patterns of room-to-room occupant movement and a cohort of 5000 activity patterns sampled from a US nationwide survey. The results for scripted and cohort simulation trials indicate that the multicompartment nature of homes, manifested as inter-room differences in pollutant levels and the movement of people among zones, can cause substantial variation in nonsmoker SHS exposure.
Keywords: Indoor air quality; Multiple compartments; Human activity patterns; Particles; Nicotine
Mitigating residential exposure to secondhand tobacco smoke by N.E. Neil E. Klepeis; W.W. William W. Nazaroff (pp. 4408-4422).
In a companion paper, we used a simulation model to explore secondhand tobacco smoke (SHS) exposures for typical conditions in residences. In the current paper, we extend this analysis to evaluate the effectiveness of physical mitigation approaches in reducing nonsmokers’ exposure to airborne SHS particulate matter in a hypothetical 6-zone house. Measures investigated included closing doors or opening windows in response to smoking activity, modifying location patterns to segregate the nonsmoker and the active smoker, and operating particle filtration devices. We first performed 24 scripted simulation trials using hypothetical patterns of occupant location. We then performed cohort simulation trials across 25 mitigation scenarios using over 1000 pairs of nonsmoker and smoker time-location patterns that were selected from a survey of human activity patterns in US homes. We limited cohort pairs to cases where more than 10 cigarettes were smoked indoors at home each day and the nonsmoker was at home for more than two thirds of the day. We evaluated the effectiveness of each mitigation approach by examining its impact on the simulated frequency distribution of residential SHS particle exposure. The two most effective strategies were the isolation of the smoker in a closed room with an open window, and a ban on smoking whenever the nonsmoker was at home. The use of open windows to supply local or cross ventilation, or the operation of portable filtration devices in smoking rooms, provided moderate exposure reductions. Closed doors, by themselves, were not effective.
Keywords: Exposure mitigation; Doors; Windows; Filtration; Smoker segregation
Trends in cloud water sulfate and nitrate as measured at two mountain sites in the Eastern United States: by James B. Anderson; Ralph E. Baumgardner Jr.; Sandra E. Grenville (pp. 4423-4437).
Air pollutant emissions in the US generally peaked in 1970, the year that the Clean Air Act was passed, and have declined since, except for oxides of nitrogen (NO x), which have remained steady or slowly increased. In 1995 Phase 1 of the Clean Air Act Amendments (CAAA) of 1990 went into effect resulting in a sharp drop in sulfur dioxide (SO2) emissions in most areas. Pollutant concentrations measured in precipitation and ambient air generally reflected the changes in emissions in most areas of the eastern US. Only the southern Appalachian Mountain region did not see an appreciable improvement in precipitation acidity over the last decade.Previous studies of cloud chemistry in the eastern US found no pattern of temporal trends, possibly because of large year-to-year variation in meteorology. In this paper, we present spatial and temporal trends in SO42− and NO3− concentrations of cloud water samples collected in warm seasons only at two mountain sites (Whiteface Mountain, NY and Whitetop Mountain, VA). This analysis is based on a combined cloud chemistry data-set collected by EPA's Mountain Cloud Chemistry Program (MCCP) (1986–1989) and Mountain Acid Deposition Program (MADPro) (1994–1999). Sample concentrations were (1) normalized by liquid water content (to reduce within-cloud variation) and (2) segregated into 90° arrival quadrants based on 36h back trajectory analysis (to diminish between-cloud or meteorological variation). For each quadrant at the two sites, annual (12 month) county emissions of SO2 and NO x were compiled out to 600, 1000, and1600km, and these values were compared with the annual means (warm season only) of normalized SO42− and NO3− concentrations in hourly samples of cloud water (segregated by back-trajectory) collected at each site.For the period 1987–1999, Quadrant 3 (SW) for Whiteface Mt. and Quadrants 3 (SW) and 4 (NW) for Whitetop Mt. had the highest SO2 emissions and showed the largest decline in SO2 emissions after the CAAA was implemented. These same quadrants which had the largest decrease in emissions showed significant declines in cloud water SO42− over the time period. NO x emissions were highest in Quadrant 3 for Whiteface Mt. and in Quadrants 1 and 4 for Whitetop Mt. Only in Quadrant 1 at Whitetop Mt. did NO x emissions decrease during the study period (1987–1999). Cloud water NO3− showed no consistent pattern at either mountain site with some quadrants having higher cloud water NO3− values after Phase 1 of the CAAA and other quadrants having little change in cloud water NO3− values.
Keywords: Cloud chemistry; Cloud liquid water content; Back trajectory