NH4+, NO3−, and SO42− in roadside and rural size-resolved particles and transformation of NO2/SO2 to nanoparticle-bound NO3−/SO42−

This study investigates ammonium, nitrate, and sulfate (NH4+, NO3−, and SO42−) in size-resolved particles (particularly nano (PM0.01–0.056)/ultrafine (PM0.01–0.1)) and NOx/SO2 collected near a busy road and at a rural site. The average (mass) cumulative fraction of secondary inorganic aerosols (SO42−+NO3−+NH4+) in nano or ultrafine particles at the roadside was found to be three to four times that at the rural site. The above three secondary inorganic aerosol species were present in similar cumulative fractions in particles of size 1–18 μm at both sites; however, dissimilar fractions were observed for Cl−, Na+, and K+. The nitrogen ratios (NRs: NR = NO3−−N/(NO3−−N + NO2–N)), sulfur ratios (SRs: SR = SO42−−S/(SO42−−S + SO2–S)), dNR/DP (derivative of NR with respect to DP (particle diameter)), and dSR/DP (derivative of SR with respect to DP) at the roadside were higher than those at the rural site for nano/ultrafine particles. At both sites (particularly the roadside), the nanoparticles had significantly higher dNR/DP and dSR/DP values than differently sized particles, implying that NO3−/SO42− (from NO2/SO2 transformation or NO3−/SO42− deposition) were present on these particles.