In order to study particulate matter transport and transformation in the Megacity environment, fine particulate carbon was measured simultaneously at two supersites, suburban T1 and rural T2, downwind of Mexico City during the MILAGRO field campaign in March 2006. Organic carbon (OC), element carbon (EC), and total carbon (TC=OC+EC) were determined in near real-time using a Sunset semi-continuous OCEC field analyzer. The semi-empirical EC tracer method was used to derive primary organic carbon (POC) and secondary organic carbon (SOC). Diurnal variations of primary and secondary carbon were observed at T1 and T2, which resulted from boundary layer inversion and impacted by local traffic patterns. The majority of organic carbon particles at T1 and T2 were secondary. The SOC TC % (SOC%=SOC/TC×100%) at T1 ranged from 0.5–93.8% with an average of 63.5±17.2%. The SOC TC % at T2 ranged from 9.3–98.1% with an average of 67.4±12.4%. The average EC to PM 2.5 percentage (EC PM %=EC/PM 2.5 ×100%) and OC PM % were 6.0% and 20.0% over the whole sampling time at T1. The POC to PM percentage (POC PM %) and SOC PM % were 3.7% and 16.3%, respectively at the same site. The maximum EC PM % was 21.2%, and the maximum OC PM % was 57.2% at T1. The maximum POC PM % was 12.9%, and the maximum SOC PM % was 49.7% at T1. Comparison of SOC and POC at T1 and T2 showed similar characteristics under favorable meteorological conditions, which indicated that transport from T1 towards T2 took place. Strong correlations between EC and carbon monoxide (CO) and odd nitrogen species (NO and NO x ) were observed at T1. This indicated that EC had nearby sources, such as local traffic emissions. The EC/CO ratio derived by linear regression analysis, with units of μg C/m 3 and μg/m 3 , respectively, was 0.004 at T1. Correlations were also seen between OC and SOC vs. the sum of oxidants, such as O 3 and NO 2 , suggesting the secondary nature of carbons observed at T1.