  Quantifying the impact of climate change on future air quality is a
challenging subject in air quality studies. An ANN model is employed to
simulate hourly O3 concentrations. The model is developed based on hourly
monitored values of temperature, solar radiation, nitrogen monoxide, and
nitrogen dioxide which are monitored during summers (June, July, and August) of
2009-2012 at urban air quality stations in Tehran, Iran. Climate projections by
HadCM3 GCM over the study area, driven by IPCC SRES A1B, A2, and B1 emission
scenarios, are downscaled by LARS-WG5 model over the periods of 2015-2039 and
2040-2064. The projections are calculated by assuming that current emissions
conditions of O3 precursors remain constant in the future. The employed O3
metrics include the number of days exceeding one-hour (1-hr) (120 ppb) and
eight-hour (8-hr) (75 ppb) O3 standards and the number of days exceeding 8-hr
Air Quality Index (AQI). The projected increases in solar radiation and
decreases in precipitation in future summers along with summertime daily
maximum temperature rise of about 1.2 and 3 celsius in the first and second
climate periods respectively are some indications of more favorable conditions
for O3 formation over the study area in the future. Based on pollution
conditions of the violation-free summer of 2012, the summertime exceedance days
of 8-hr O3 standard are projected to increase in the future by about 4.2 days
in the short term and about 12.3 days in the mid-term. Similarly, based on
pollution conditions of the polluted summer of 2010 with 58 O3 exceedance days,
this metric is projected to increase about 4.5 days in the short term and about
14.1 days in the mid-term. Moreover, the number of Unhealthy and Very Unhealthy
days in 8-hr AQI is also projected to increase based on pollution conditions of
both summers.
