GAW Report No. 205 - IGAC Project

CHAPTER 5 – NORTH AMERICAstories found anywhere in the world. Between 1970 and the present, SoCAB VOC and NO Xemissions have declined markedly despite a substantial increase in commerce and vehicle traffic[Cox et al., 2009]. Peak O 3 levels that exceeded 600 ppbv in the 1960’s have not reached 200ppbv since 1998.Figure 3 - The Los Angeles Megacity defined as the South Coast Air Basin (outline). The colours indicate the topographyof the basin (blues for low elevation, brown for high elevation).The Pacific Ocean lies to the south and westThis improvement in air quality has been accomplished despite several unfavourablecircumstances that make SoCAB particularly susceptible to high air pollution concentrations. Thelarge population lives in a basin bounded by the Pacific Ocean on the west and by mountains onthe other three sides, which prevent efficient horizontal ventilation of the area. Low inversionheights associated with a persistent high pressure system and the adjacent Pacific marineenvironment limits the vertical mixing within the basin, and the land-sea breeze system recirculatespolluted air within the basin. These topographic and meteorological features allowemissions to accumulate over several days during episodes of relatively stagnant airflow. Duringthe summer ozone season, May through October, clear skies and high temperatures dominate,which speed photochemical production of O 3 and other photochemical products. Privateautomobiles on extensive freeway systems provide the primary transportation in the area. Thistransportation system accounts for a large fraction of the emissions in the urban area.Over the past three decades ambient concentrations of key pollutants in the SoCAB regionhave decreased substantially despite a doubling of the population and tripling of vehicle use.Figure 4 compares the temporal trends of four pollutant concentrations. These data are presentedin units that correspond to US EPA National Ambient Air Quality Standards (NAAQS)(http://www.epa.gov/air/criteria.html), which are presently: O 3 - 75 ppbv (8-hr); CO - 9 ppmv (8-hr);NO 2 - 53 ppbv (1-yr); PM 2.5 - 35 µg/m 3 (24-hr). The numbers in parentheses give the averagingperiod. The O 3 data are 3-yr averages of the 4 th highest annual maxima, the CO data are annualmaxima, and the PM 2.5 are annual 98 th percentiles. Ozone concentrations apparently peaked inthe late 1960s. It is evident that there has been an impressive decline in ozone concentrations aswell as other air pollutants since then. Although it still violates the NAAQS for O 3 and PM 2.5 , theLos Angeles basin is in compliance with the NAAQS for nitrogen dioxide and carbon monoxide, aswell as sulphur dioxide and lead. It is fair to say that this megacity has gone from being one of themost polluted in the world 50 years ago to presently one of the “least polluted” cities of its size.Estimates are that many thousands of lives have been saved in LAMC from improvements in airquality [Hall et al., 2008].174