Multicomponent Remote Sensing of Vehicle Exhaust by Dispersive Absorption Spectroscopy. 2. Direct On-Road Ammonia Measurements MARC M. BAUM,* EILEEN S. KIYOMIYA, SASI KUMAR, AND ANASTASIOS M. LAPPAS Departm ent of Chem istry, Oak Crest Institute of Science, 2275 East Foothill Boulevard, Pasadena, California 91107 VADYM A. KAPINUS Division of Chem istry and Chem ical Engineering, California Institute of Technology, Pasadena, California 91125 HARRY C. LORD III Air Instrum ents & Measurem ents, Inc., 13300 Brooks Dr., Suite A, Baldwin Park, California 91706 Remote sensing was employed for the first time to measure NH 3 levels in the exhaust of on-road light duty motor vehicles. The sensor also measured the concentration of CO 2 , CO, hydrocarbons, and NO, among other pollutants, in the emitted exhaust. Field measurements were conducted at a Los Angeles freeway on-ramp; vehicles traveled at cruise speeds between 20 and 25 m s -1 (45- 55 mi h -1 ). Mean fleet NH 3 levels of 44.7 ( 4.1 ppm were observed. These emissions exhibited a highly skewed distribution: 50.1% of the emitted NH 3 was contributed by 10% of the sampled fleet. The pollutant distribution among high NH 3 emitters is analyzed to identify the conditions that lead to three-way catalyst malfunction and, hence, NH 3 formation. In contradiction with previous reports, we found that high NH 3 emissions could not be attributed to vehicles running under rich air-fuel conditions. We estimate a mean fleet NH 3 mass emission rate of 667 ( 57 mg L -1 (E r ) 94 ( 8 mg km -1 ). These findings could have significant implications on air quality in the South Coast Air Basin (SoCAB) of California, since they support the hypothesis that emissions from motor vehicles constitute a dominant regional source of NH 3 , between 20 and 27% of total daily emissions. As NH 3 is the predominant atmospheric base, tropospheric levels play a key role in the buffering capacity of the atmosphere and, hence, the formation of fine aerosol. Our results could explain the ubiquitous distribution of ammonium fine particles observed during fall stagnation conditions in the SoCAB. Introduction Atmosphericammonia (NH3)emissionsfrom anthropogenic sourcesare increasinglycomingunder scrutinyasassociated environmentalimpacts are becomingbetter understood (1). Ammonia,the primaryatmosphericbase,playsa crucialrole in determiningthe acid-neutralizingcapacityoftropospheric air masses (2, 3). Airborne sulfuric and nitric acid, produced byatmosphericoxidation ofsulfurdioxide(SO2)and nitrogen dioxide (NO2), respectively (4), react with gaseous NH3 to afford ammonium bisulfate (NH4HSO4), ammonium sulfate [(NH4)2SO4],and ammonium nitrate (NH4NO3)fine particles (aerosolfraction with a mass median aerodynamic diameter of 2.5 μm or less, PM2.5)(5-9). Secondary air pollution particles are well-known to degrade visibility via efficient light scattering (10-22), leading to the dense haze common to many polluted urban atmospheres. As compared to free, gas-phase NH3, NH4 + aerosol has a higher chance for long-range transport as its removal rate from the atmosphere is 5-10 times slower (23). Fine aerosol is of special concern because smaller particles are able to penetrate more deeplyinto the lungand maylead to adverse health effects. Numerous epidemiological studies have examined air pollutant concentrations in relation to health statistics and have concluded that fine combustion-source pollution, common to many urban and industrial environ- ments,isan important riskfactorforcardiopulmonarydisease and mortality (24). However, atmospheric NH3 also reduces ambient levels of acidic fine aerosol (pH < 4.3), which is suspected to be more deleterious to human health than neutral aerosol of the same size distribution (25-27). The quantitative ecological impacts resulting from an- thropogenicNH3 emissionsare stillriddled with uncertainties, butevidenceismounting(28, 29)that dryand wet deposition of NH3 and its salts can adversely affect terrestrial (30-39) and aquatic (40-42) ecosystems. Deposited NH3 disturbs soil nutrient balance (35) and can contribute toward soil acidification following nitrification (38). Atmospheric nitro- gen deposition, thought to inhibit mechanisms used by needles of some conifers to protect themselves from frost injury(30), has been suggested (43) as one ofthe factors that may have triggered forest dieback in Europe. Atmospheric nitrogen inputs to aquatic ecosystems are a concern due to the potential for eutrophication (1, 41). Global inventories cite excreta from domestic and wild animals, the use of synthetic N-fertilizers, oceans, and biomassburningastheprincipalsources(in decreasingorder) of the estimated 54 million tons N of NH 3 emitted globally in 1990 (23, 44). However, Cass and Fraser reported (45)that NH3 emissions from light duty motor vehicles (LDMVs) equipped with three-waycatalysts (TWCs)maycompete with agricultural sources on a regional scale, such as the South Coast Air Basin (SoCAB) of California. High NH3 concentra- tions, frequently exceeding corresponding nitric oxide (NO) levels, emitted in the exhaust of modern LDMVs were first reported by Baum (46, 47) and later confirmed by remote sensing(48).The same remote sensingsystem was employed to determine NH 3 levels, among other pollutants, in the exhaust of 2091 vehicles as they drove up a Southern California freewayon-ramp.For the first time,NH3 emissions from on-road vehicles were measured on a car-by-car basis. Preliminarydata from this studysuggest that NH3 emissions by vehicles in the SoCAB could be significantly higher than previously suspected (49). The purpose of this paper is to provide a full account of these results. Engine operating conditions and emission profiles of high NH 3 emitters will be discussed in terms of TWC performance, as will the implicationsofour measurementson air pollution modeling and control in the SoCAB. *Corresponding author phone: (626) 817-0883; fax: (626) 817- 0884; e-mail: m.baum@oak-crest.org. Environ. Sci. Technol. 2001, 35, 3735-3741 10.1021/es002046y CCC: $20.00  2001 American Chemical Society VOL. 35, NO. 18, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3735 Published on Web 08/09/2001