Earth’sFuture10.1002/2013EF000174Figure 2.Correlation between ambient concentrations of CO and PM2.5during2007–2011.between CO and reactive nitrogenspecies, NOy′, in Islamabad during2007–2011. A linear regressionof hourly average CO and NO,and CO and NOy′was performed,which shows a significant(p-value≤0.01) correlation betweenCO and NO concentrations([CO]=10.13[NO]+511.3;r2=0.76)and a significant (p-value≤0.01) cor-relation between CO and NOy′con-centrations ([CO]=9.84[NOy′]+256.8;r2=0.78). From this ratio analysis,relative background concentrationsmay be determined by examin-ing the intercept of the regres-sion lines. The regression curvesreveal a background CO con-centration of∼300 to∼600 ppbv in the Islamabad urban area. This is similar to Raleigh, NC, USA,urban site value of 470±52 ppbv [Aneja et al., 1997]; however, CO background concentration inNew Delhi, India, has been observed as approximately 1693 ppbv [Aneja et al., 2001]. Moreover, rel-ative source strengths like mobile sources versus point sources may also be suggested by exam-ining the slope of the regression lines, and compared with emissions inventory.Klimont et al. and ECCAD (Emissions of atmospheric Compounds & Compilation of Ancillary Data, 2014,, hereinafter referred to as ECCAD, onlinereport, 2014) have provided an emissions inventory (developed for the year 2010) for CO, SO2, and NOx.Table 1 compares and contrasts the emissions from this inventory by examining the relationship betweenambient CO and NOx, and between ambient SO2and NOxfor 2007–2011 in Islamabad, Pakistan. It alsocompares and contrasts with CO and NOxrelationship observed in Denver, CO, USA [Parrish et al., 1991];Boulder, CO, USA [Goldan et al., 1995]; Raleigh, NC, USA [Aneja et al., 1997]; and New Delhi, India [Anejaet al., 2001]. Based on ratio analysis of CO and NOx,Parrish et al.  reported values of 8.4, 7.8, and 10.2for mobile sources in the Eastern United States, Pennsylvania area, and Western United States, respec-tively. Given the average ratio of about 10 (i.e., the slope of the regression line) in Islamabad, it appearsthat mobile sources contribute more to the concentrations of CO and NOxthan point sources.Monthly averages of SO2concentration (1μg m−3SO2=0.38 ppbv) are plotted in Figure 4. Sulfur dioxideconcentrations are below Pakistan’s 24 h average NEQS value of 120μg m−3during the measurement(a)(b)Figure 3.Correlation between (a) CO and NO, and (b) CO and NOy′during 2007–2011.RASHEED ET AL.