One solution for less air pollution in cities is 1) Build more houses in the suburbs
2) Less public transit options
3) More walkability in cities
4) Place stores farther from residential areas

I believe more walkability in cities

So  if  the sum of the enthalpies of the reactants is greater than the products, the  reaction  will be  exothermic.  if  the products side has a larger enthalpy, the  reaction  is  endothermic. you may wonder why  endothermic reactions, which soak up energy or enthalpy from the environment, even happen